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Limiting the soil degradational impacts of wildfire in pine and eucalyptus forests in Portugal
Applied
SOl43-6228(96)00022-7
Geo,qraph~. Vol. 16. No. 4. pp. 337-3.5.5. 1996 Copyright 0 1996 Eisevier Science Ltd Printed in Great Britain. All rights reserved 014%6228/96 $lS.Oll + 0.00
Limiting the soil degradational impacts of wildfire in pine and eucalyptus forests in Portugal A comparison practices
of alternative post-fire management
Richard A Shakesby
Department ofGeography, University of Wales Swansea, Singleton Park, Swanserl SA2 RPP: UK
David J Boakes Department ofGeography, University of Wales Swansea. Singleton Park, Swansea SA2 8PP UK
Celeste de 0 A Coelho Departamento Portugal
de Ambiente e Ordenamento,
Universidude de Aveiro. 3810 Aveiro.
A J Bento Gonqalves Nticleo de Investiga@o Cientzjcica de Inc&dios Bat-veto. 58, R/Ch, 3000 Coimbru, Portugal
Florestais, FLUC, Av. Bissaya
Rory P D Walsh Department of Geography, University of Wales Swansea, Singleton Park, Swanserr SA2 8Pe UK
In newly burnt and unburnt pine and eucalyptus forest in Portugal, overland flow and soil losses were monitored to assess the impacts of the following post-fire treatments: application of different quantities of logging litter; rip-ploughing compared with minimum tillage prior to planting eucalyptus seedlings; and clearance of pine needles and vegetation. Eucalyptus logging litter reduced soil losses by up to 95 per cent. The impact of pine logging litter was equivocal, but removal of pine needles increased soil losses elevenfold. Implications for soil longevity, soil quality and land management strategy are discussed. Copyright 0 1996 Elsevier Science Ltd 337
Soil conservation and forest wildfires in Portugal: R A Shakesbyet al.
In the last few decades, forest wildfires have increased in frequency and extent throughout the Mediterranean basin for a variety of reasons, including: planting of highly flammable tree species (notably pine and eucalyptus); a decline in traditional practices like grazing and coppicing that previously reduced the accumulation of flammable litter and undergrowth; increasing recreational use of forests; and a succession of unusually dry years (World Meteorological Organization, 1995). Such fires not only damage trees and destroy undergrowth and litter but can also lead to detrimental soil changes, including reduced organic matter content and aggregate stability (Giovannini et al., 1988) increased soil erodibility (Imeson and Vis, 1984) enhanced soil water repellency (DeBano, 1991) and decreased infiltration capacity. These changes result in substantial increases in overland flow and soil losses, peaking immediately following fire and declining rapidly thereafter (see, for example, Swanson, 198 1; Diaz-Fierros et al., 1987; Morris and Moses, 1987; Sevink et al., 1989; Imeson et al., 1992). Despite considerable research into the short-term erosional impacts of fire and forest fire prevention (such as Viegas, 1990, 1994; Sala and Rubio, 1994) conservational implications of alternative post-fire land management practices and the longer-term implications of differing fire/land management scenarios for soil and forest sustainability have received little attention. This paper addresses these two research gaps with respect to eucalyptus and pine forests in the Agueda Basin, Portugal, where fires have increased in frequency since the early 1980s. In this area, there are three main forms of post-fire land use practices following manual logging of the scorched timber: (1) eucalyptus regrowth from the stumps; (2) pine seedling regeneration in muto (scrub vegetation); and (3) planting eucalyptus seedlings on ground prepared by rip-ploughing. The ways in which these land use practices are implemented have important repercussions for soil erosion. Ground cover on bare burnt soils is clearly critical for limiting erosion, yet logging litter is often left at roadsides. Logging of pine too soon after fire can prevent the development of a ‘carpet’ of fallen scorched needles. Rip-ploughing is known to be erosionally very damaging (Shakesby et al., 1994) yet the conservational benefit of any alternative ground preparation technique has not been investigated. Using the results of bounded plot studies of post-fire erosion, this study assesses the impacts of applying different quantities of post-fire logging litter, the protection provided by a pine needle cover, and the possible benefits of minimum tillage for planting eucalyptus seedlings compared with rip-ploughing. The conflict between the soil conservational properties of logging litter and its flammability and tendency to encourage pest infestation and implications of fire and/or fire and rip-ploughing for soil longevity and quality into the medium to long term (50-l 00 years) are then considered.
Study area and land management
background
The study sites are located near Falgorosa (40” 32’ N, 8” 22’ W) and Lourizela (40” 38’ N, 8” 19’ W) in newly burnt and adjacent unburnt Eucalyptus gl~bulus and Pinus pinaster forest in the Agueda Basin (Figure f). The area is a deeply dissected schist upland (up to c. 500m) in the foothills of the Caramulo Mountains. Slopes are generally convexorectilinear, typically with 20” rectilinear sections. On steep slopes, soils are shallow (only rarely reaching 40cm in depth) stony Umbric Leptosols (Pereira and Fitzpatrick, 1995) comprising mainly sand (59-80 per cent) and silt (18-40 per cent), with little clay ( l-2 per cent) (Terry, 1992). The long-term mean annual rainfall varies with altitude from 1300 to 1900 mm, but was below average for all but two years between 1980 and 1993 (Ferreira et al., 1995). Most rainfall results from winter depressions, the period from July to September being mainly dry. Until the 1920s the area was largely under pasture with patches of forest, but land degradation led to legislation for widespread planting of P. pinaster. This species, which 338
Soil consenwtiun
nnd.fr,rest
wil&res
in Portugal:
R A Shakesby et 01.
I Datelastburnt
.
Location of bounded plols (E I eucalyptus regrowth
July 1986
I w
ME - mature eucalyptus P - pine regrowth MP - mature pine)
July 1991
m .
August 1992
A
Automatic raingauge
Settlement
A
Storage raingauge
Figure 1 Extent and timing of selected fires, and location of raingauges and bounded plot sites in the Agueda Basin
reaches maturity in about 25-35 years, was supplanted, following a large fire in 1986, as the dominant forest species by faster-growing E. globulus, which ‘matures’ (that is, provides a commercial crop) in only 9-l 2 years. A number of smaller fires have occurred in the region since 1986. This paper focuses on pine burnt in July 1991 and eucalyptus burnt in August 1992. Severe understorey fires (sensu Wells et al., 1979) are typical, with all surface organic material (litter, undergrowth, and so on) being consumed, but with tree trunks and canopies only being scorched. The flammability of pine and eucalyptus litter, legislation against prescribed burning (Rego et al., 1990), arson (Walsh et al., 1988) and a series of unusually dry years have increased fire frequency.
Research design and methodology A bounded plot approach post-fire land management
was adopted in assessing the impact on soil loss of different strategies. In all, 16 bounded plots at 4 sites (E, ME. P and MP
Soil conservation and forest wildfires in Portugal: R A Shakesby et al.
in Figure 1) were monitored from November 1992 to July 1994. Two ‘control’ plots in mature unburnt forest (ME and MP on Figure 1) provided an indication of pre-bum erosion levels. Two plots at each burnt site (E and P on Figure I) were left untreated for the entire period; the remaining ten plots were subject to the following three treatment types at the end of June 1993 (Figures 2 and 3).
(1) The first set of treatments compared the impacts on soil loss of applications of varying
(2)
amounts of post-fire logging litter. Quantities and types of logging litter differ for eucalyptus and pine. Assessments were made of the highest amounts of litter available following clear-felling by selecting sites in burnt and logged mature eucalyptus and pine forests where all litter had been left on the slopes. Replicated areas were marked out and the litter collected and weighed. Mean litter weights per unit area and percentage cover were much higher for eucalyptus (respectively, 4.6 kg mm2 and 89.5 per cent) than for pine (1.8 kg mm2 and 8.3 per cent). To test the impact of different quantities of available litter in reducing soil loss, 100 per cent (termed ‘high’ treatment), 50 per cent (‘medium’) and 10 per cent (‘low’) of the mean weight per unit area found in the squares were applied as plot litter treatments. (The last category was considered appropriate only for eucalyptus, where it still provided an appreciable ground cover.) Replication of low litter for eucalyptus and high litter treatment plots for pine was possible within the research design (Figures 2 and 3). To assess the conservational impact of a post-fire cover of pine needles fallen from the scorched tree canopy, all needles from a burnt pine plot were removed and all
j!-,
I
August 1992
Fire
-
I
November 1992
Pre-treatment phase
June 1993
TREATMENT
Post-treatment phase July 1994 Figure 2 340
Research design: eucalyptus
bounded plots
Soil conservarion and forest wildfires in Portugal: R A Shakesbv et 01.
July 1991 November 1992
Pre-treatment phase
June 1993
TREATMENT
fl
Post-treatment phase
Mature
July 1994
L-J Figure 3
Research design: pine bounded plots
vegetation cut at soil level. The collected needles represented a 38 per cent ground cover. (The impact of eucalyptus leaves was not considered as either they do not fall before logging or they tend to be blown off exposed slopes.) for planting (3) High soil losses resulting from ground preparation by rip-ploughing eucalyptus seedlings have been reported in previous work (Shakesby et ctl., 1994; Coelho et al., 1995a, 1995b). The possible soil conservational benefits of minimum tillage compared with rip-ploughing were investigated by planting on and around a burnt pine plot at the pine site eucalyptus seedlings at a typical density for the region and restricting ground disturbance to c. 50cm diameter around each seedling. A ripploughed plot at the burnt eucalyptus site provided comparative data.
The bounded plots were comparatively small (8 m x 2 m) in response to the restricted downslope distances between forest tracks, the small size of tree stands of a single treatment and the practical limitations of recording soil losses and overland flow accurately for larger areas during high-magnitude events. Each plot was linked to a sediment trap, tipping bucket flow recorder and a series of large collecting tanks (Shakesby et al., 1991; Walsh er ul., 1995). Overland flow was mostly recorded storm by storm. Ideally, soil losses would have been monitored on the same basis, but logistical constraints involved in sediment trap changes and cleaning precluded a higher frequency and resulted in periods of unequal length. Soil losses were monitored over a combination
Soil conservation
and forest wildfires in Portugal: R A Shakesby et al.
of seven long (49-108 days) and two short (9 and 22 days) periods. The pre-treatment phase (November 1992-June 1993) comprised five measurement periods and the posttreatment phase (July 1993-July 1994) four measurement periods. Rainfall was measured using standard raingauges at the plot sites. Daily and hourly rainfall intensities were extrapolated from weekly autographic records at Falgueirinho (altitude 460 m) and Castanheira (200 m) in the study area (Figure I). From previous work, it was known that vegetation recovery and the development of a stone cover are factors affecting soil loss considerably during the first few years after disturbance (Shakesby er ul., 1994). Plot vegetation and stone ( > 0.5 cm) covers were estimated in June 1993 and July 1994 from mosaics of vertical photographs (taken from a 3-m-high movable frame) and from field surveys. Vegetation cover was determined directly from the mosaics but stone cover was estimated using Graftik Optilab image analysis of quartz stones on representative plot areas and total stone cover was calibrated by field surveys (details in Coelho er al., 1995b).
1
2 1201
560
100
6 6718
Figure 4
Daily rainfall for the nine monitoring periods at the pine bounded plot site. (Daily rainfall for the eucalyptus site has a similar pattern of rainfall events but with lower totals) 342
Results Rair$dl
and overland flow
Daily rainfall over the monitoring period for the burnt pine plots is given in Figure 4 (rainfall at the eucalyptus site is similar but with smaller totals) and summary rainfall characteristics at both sites are shown in Table 1. Rainfall at the nearest long-term weather station (Campia, altitude 470 m, c.20 km northeast of the study area) in the rainfall years (October-September) 1992/3 and 1993/4 was respectively 22.8 per cent below and 12.6 per cent above the long-term average. The post-treatment phase (July 1993-July 1994) was much wetter than the pre-treatment phase (November 1992-June 1993) with twice as many daily falls >lO mm, 3-5 times as many days with >25 mm and about 2-4 times as many hours with intensities >lO mm (Table I). Overland flow at both burnt-plot sites was higher in the post-treatment than pretreatment phase (Tables 2 and 3) reflecting the rainfall differences between the sites. Application of large amounts of eucalyptus litter may have reduced the amount of overland flow increase in the post-treatment phase, explaining why the high and medium eucalyptus treatment plots have smaller post-treatment/pre-treatment ratios (1.1% 1.66) of overland flow percentages than those for low and untreated (2.06-4.93) plots. For pine. there is no discernible relationship between the treatments and their ratios.
Soil loss from individual plots at the burnt eucalyptus and pine sites in the nine monitoring periods is shown in Figures 5 and 6 and is expressed in terms of soil loss in grammes. unitrain erosion rate (gmm-’ of rainfall), and unit overland flow erosion rate (g mm-’ of overland flow). The latter two measures are helpful in comparing soil loss in periods differing in length and rainfall amount. Eucalyptus litter impact. In the pre-treatment phase, eucalyptus soil losses vary both between periods and between plots. For example, in period 4, with the highest losses and rainfall, loss from untreated plot A (1819g) is over seven times that from untreated plot B (243 g). In the post-treatment phase, low and untreated plot losses for individual periods are higher than in all (for untreated B plot) or all but one (untreated A and low A and B) of the pre-treatment periods (Figure 5).
Table 1 Rainfall at the plot sites during the pre- and post-treatment Falgorosa
Period Pre-treatment. periods 1-S Post-treatment. periods 6-9
(eucalyptus)” Days >25 mm
periods
Lourizela
Daily max. (mm)
Days >lO mm
678.4
39.6
24
5
2
147 I .8
63.5
so
17
9
Total (mm)
monitoring
Hours >lO mm
(pine)h
Daily max. (mm)
Days >lO mm
919.1
70.4
29
6
2027.0
8X.X
60
31
Total (mm)
Days >25 mm
Hours >lO mm IO 23
~’Altitude I IOm; mean annual rainfall 1301 mm “Altitude 320 m: mean annual rainfall 1782 mm
343
Soil conservation and forest wildfires in Portugal: R A Shakesby et al. Table 2 Summary
of overland
tlow in pre- and post-treatment
phases: eucalyptus Overland flow (mm)
Period
Rainfall (mm)
Untreated A
Low A
Untreated B
Low B
Medium
High
(a) Pre-treatment (periods 2-5)b
644.9
55.4 (8.6)
39.9 (6.2)
63.3 (9.8)
26.4 (4.1)
104.7 (16.2)
76.9 (11.9)
(b) Post-treatment (periods 6-9)
1471.8
392.3 (26.7)
188.5 (12.8)
362.8 (24.7)
297.1 (20.2)
280.8 (19.1)
283.8 (19.3)
2.06
2.52
4.93
(b) %/(a) 8
3.10
“Ovedand flow as a percentage of rainfall is given in parentheses bThe calculations relate only to the periods in the pre-treatment and post-treatment Source: after Walsh et al. (1994)
1.18
1.62
phases indicated
Considering unit-rain and unit overland flow erosion rates, apart from period 5, posttreatment values are similar to or higher than pre-treatment ones. The clear exception is the high treatment plot, where all post-treatment values are lower than pre-treatment ones. For both unit erosion rates, period 5 has the highest values, despite having the least rainfall (18.7 mm in 3 days following a 16-day virtually dry warm interlude). Soil losses at the ‘control’ plot in unburnt eucalyptus forest were negligible (totalling only 56.9g, August 1992-May 1994). Pine litter and needle removal impacts. As with eucalyptus plots, soil losses from burnt pine plots (Figure 6) vary considerably between periods and plots. Unlike the eucalyptus plots, post-treatment soil losses and unit erosion rates for the burnt pine plots tend to be similar to or lower than pre-treatment ones for all except the bare treatment plot (soil loss and unit-rain erosion rate only). Similarly to the eucalyptus plots, lowest rainfall in period 5 corresponds to the highest (or second highest) unit-rain and unit overland flow erosion rates. The mature pine plot soil loss was negligible (63.3 g, July 1992-May 1994).
Table 3 Summary
of overland
flow in pre- and post-treatment
phases: pine
Overland flow (mmy
Period
Rainfall (mm)
(a) Pre-treatment (periods 2 + 5)b
351.2
(b) Post-treatmenth (periods 7-9)
1352.5
(b) %/(a) CL
Untreated A
Untreated B
Min. tillage
Medium
High B
Bare
49.0 (14.0)
26.2 (7.5)
22.7 (6.5)
42.4 (12.1)
27.4 (7.8)
22.7 (6.5)
(I 1.7)
396.3 (29.3)
198.1 (14.6)
218.4 (16.1)
258.4 (19.1)
239.4 (17.7)
186.1 (13.8)
306.4 (22.7)
2.27
2.12
2.09
1.95
2.48
1.58
“Overland flow as a percentage of rainfall is given in parentheses “The calculations relate only to the periods in the pre-treatment and post-treatment Source: after Walsh et al. (1994) 344
High A
phases indicated
41.2
I .94
Soil conservation and forest wildfires in Portugal: R A Shake.& et al.
Unit-rain erosion rate (g/mm of rain)
Soil loss (9)
Unit overland flow erosion rate (g/mm of overland flow)
Untreated B
Low A
Medium
High
El -
Pre treatment phase (periods 1-5)
Figure 5
N
Nodata
(gmm-’ of rainfall) and unit overland flow (gmm-’ for the eucalyptus bounded plots
Soil loss (g) and unit-rain
overland flow) erosion rates
Post-treatment phase (periods 6-Q)
of
Minimum tillage and rip-ploughing. Results for the minimum tillage plot (Figure 6) are similar to those for the untreated B pine plot. Soil loss from the rip-ploughed plot in periods 7 and 8 was 4649.4 g, giving a unit-rain erosion rate of 4.8 g mm-‘, which is intermediate between the two untreated eucalyptus plots for these periods. Vegetation and stone cover The average eucalyptus plot vegetation cover changed little during the monitoring period (2.8 per cent, June 1993; 6.0 per cent, July 1994). Stone cover, however, more 345
Soil conservation
and forest wildfhs
in Portugal:
R A Shakesby er al.
Soil loss
Unit-rain erosion fate
Unit overland ftow erosian rate
(g)
(g/mm of rain]
(g/mm of overland Ilow)
-1
Medium
High A
High B
Bare
Minimum tiflage
. cl
S;i Pre-treatmenl phase
(periods
1-5)
Post-treatment phase (periods 6-9)
6 Soil loss (g) and unit-rain 1.gmm-’ of rainfall) and unit overland overland flow) erosion rates for the pine bounded plots
Figure
146
N
Nodata
flow (gmnm’
of
than doubled between these dates, from 17.4 per cent to 42.2 per cent. For pine. vegetation cover rose from 13.0 per cent in June 1993 to 3 1.7 per cent (excluding the bare treatment plot) by July 1994. The higher pine figures reflect the greater time since fire, more conducive local conditions for vegetative growth (northeast-facing for pine and deeper soils compared with southwest-facing and shallower soil for eucalyptus) and the allelopathic tendencies of eucalyptus (chemical inhibition of native plant germination and growth). Pine plot stone cover rose from 18.5 per cent in June 1993 to 44.1 per cent in July 1994.
Interpretation
of the results
Interpreting differences in treatment impacts is complicated by the inherent, sometimes large, pre-treatment inter-plot soil loss differences and by progressive changes in plot responses as vegetation recovered and stone cover developed. A better assessment of the treatment impacts on soil losses was made by viewing ratios of post- to pre-treatmentphase soil losses for untreated plots as the ‘control’ or ‘expected’ change in soil losses between the two phases; ratios for treated plots were then compared with the mean ratio for the appropriate (pine or eucalyptus) pair of untreated plots (Tuhks 4 and 5).
Eucul_yptus litter upplicution Post-fire soil losses were reduced substantially in burnt eucalyptus forest by applications of logging litter (Tuhle 4). Even the 8.8 per cent ground cover (10 per cent by weight of available eucalyptus litter) of the low-treatment plots reduced soil losses by about half (46 per cent and 64 per cent) compared with untreated plots. Thus, instead of an increase in soil losses of 3.4 and 7 times, as occurred for untreated plots from pre-treatment to posttreatment phase, losses increased only 1.9 and 2.8 times for the low-treatment plots and the 89.5 per cent cover of the high-treatment plot reduced soil losses to only 5 per cent of the expected amounts indicated by untreated plot losses. The relatively small increase in overland flow following treatment (Table 2) can only partly explain this soil loss decline. most being the result of litter protecting against rainsplash and trapping sediment transported by overland flow.
Table 4 Effects of different post-fire treatments
Plot treatment Untreated A Untreakd B Isw A Low B Medium High Rip-ploughed‘
on soil loss at burnt eucalyptus
Pre-treatment loss (t ha-‘)
Post-treatment loss (t ha-’ )
plots
Impact of treatment in post-treatment phase“
Percentage soil loss change due to treatment”
1.92
6.57
0.46 2.23
3.24 4.13
0.36
+I
1.3I
3.65
0.53
-46
1.17 I.58
I59 0.42 2.73
0.26 0.05
-73 -9s
I .o
(Post-/pre-treatment-phaatt “Impact of treiltmrnt (IT)
0
hoi1 low for treated plot)
= (Post-/pre-treatment-phase
soil losses for untreated plo1\)/2
“Calculakd for treated plot as (IT X 100) - 100 ‘The rip-ploughed plot was monitored only for 21/10/93-4/S/93
(periods 7 and 8)
347
Soil conservation and forest wildfires in Portugal: R A Shakesby et al. ‘&ble 5 Effects of different post-fire treatments
on soil loss at burnt pine plots Impact of treatment in post-treatment phase”
Plot treatment
Pre-treatment loss (t ha-‘)
Post-treatment loss (t ha-‘)
Untreated A Untreated B
1.52 0.86
I .09 0.56
1.0
Minimum Medium High A High B Bare
0.86 4.55 2.24 2.5 1 0.87
0.46 2.01 1.14 1.35 6.79
0.77 0.64 0.73 0.78 11.2
tillage
(Post-/pre-treatment-phase Ympact of treatment
0 -23 -36 -27 -22 + 1020
soil loss for treated plot)
(IT) = (Post-/pre-treatment-phase
hCalculated
Percentage soil loss change due to treatmentb
soil losses for untreated
plots)/2
for treated plot as (IT X 100) - 100
Although fire is conventionally, considered as inducing or intensifying soil hydrophobicity (DeBano, 1991), in the Agueda Basin unburnt and burnt forest soils when dry are similarly hydrophobic (Doerr et al., 1996). However, since the burnt soils dry out more rapidly, drought-enhanced soil hydrophobicity (Burch et al., 1989; Soto et al., 1994) becomes more rapidly established than in mature forest, leading to increased overland flow and erosion in rainstorms following droughts (Terry and Shakesby, 1993; Walsh et al., 1995). This explains the high unit erosion rates in period 5. Pine litter application At the burnt pine site, although perhaps partly masked by rapid vegetation recovery (in the third year after fire; post-treatment phase) not apparent at the eucalyptus plots (in the second year after fire; post-treatment phase), the absence of a clear impact for the medium and high pine litter treatments (Table 5) is attributed to their lower application densities (0.9 kg rnw2 and 1.8 kgm-’ for pine compared with 2.3 kg mm2 and 4.6 kg mm2 for eucalyptus), the associated sparse cover (4.8 and 8.3 per cent) and the rigidity of pine twigs and branches, which makes them less effective at damming sediment. That the percentage soil loss change differs little between minimum tillage (where treatment involved ground disturbance and no added litter) and treated plots supports the view that adding pine litter had little impact on soil losses. Pine needle cover The importance of the pine needle ‘carpet’, as suggested by others (see, for example, Connaughton, 1935; Grigal andMcCol1, 1975; Megahan and Molitor, 1975; White and Wells, 1982), is confirmed for Agueda Basin pine forests by the elevenfold increase in post-treatment soil loss at the bare treatment plot. This increase may understate bare soil losses immediately after fire due to (1) the binding effect of needles incorporated in the soil and erosion of many needles prior to treatment; (2) the stabilizing effect of roots of cut plants; and (3) the growth of moss. Minimum
tillage versus rip-ploughing
Minimum tillage has been shown to be a very advantageous post-fire practice for limiting soil losses when planting eucalyptus seedlings because (1) the post- to pre-treatment ratio 348
Soil conservarion and fores
wildfiresin Porrugal:R A Shake& et al.
for the minimum tillage plot differed little from those of the other pine litter treatments, suggesting no marked soil loss increase, and (2) this treatment produced an order of magnitude less sediment than the rip-ploughed plot at Falgorosa in the post-treatment phase and two orders of magnitude less than losses recorded for plots installed immediately following rip-ploughing (see, for example, Shakesby et al., 1994; Coelho et al., 1995a). Whether this treatment is acceptable from a silvicultural viewpoint, however, remains to be assessed. Post-fire recovery at untreated plots That untreated eucalyptus unit-rain erosion rates did not decline in the second year after burning is significant in two respects. First, this pattern is contrary to that usually reported, of highest soil losses within a year of fire with a rapid decline thereafter (Swanson, 1981; Diaz-Fierros et al., 1987; Vega and Diaz-Fierros, 1987; Lavee et al., 1991). In this study, rainstorm timing and magnitude-frequency may have been partly responsible. Thus the higher frequency of intense daily and hourly falls (Table I; Figure 4) may explain the much higher untreated and low-treatment eucalyptus soil losses of the post-treatment phase (Table 4). Secondly, it suggests that even for thin soils, preferential loss of fines (Thomas, 1996) and resulting surface stone lags do not necessarily lead rapidly to post-fire ‘soil exhaustion’ (see also Sala et al., 1994). By the third year after fire, however, previous work (Shakesby et al., 1994) indicates that untreated eucalyptus unit-rain erosion rates decline sharply towards mature forest levels. Percentage overland flow, however, can remain high (still 5 per cent six years after fire) (Walsh et al., 1995), suggesting that by this stage eucalyptus soil loss attenuation results more from reduced sediment supply, mainly through surface stabilization by vegetation (particularly moss, but also grasses and herbs), rather than from reduced overland flow. The comparatively high rates of erosion in the pre-treatment phase for the burnt pine (second year after fire) suggest that the post-fire erosion peak lasts two years, as is the case for eucalyptus. Pine overland flow percentages decline rapidly to mature forest levels (5 per cent by three years and 1 per cent by six years; see Walsh et al., 1995).
Implications Erosion limitation and fire hazard properties
of logging litter
To limit soil erosion after fire, the results indicate that logging should be carried out differently for pine and eucalyptus. Pine logging litter is less important in reducing soil losses than needles. Thus delaying timber clearance until after needlefall (12-15 weeks after fire) would be a simple but very effective conservational management practice. Since in reality clearance tends not to precede needlefall, this recommendation would not involve any change to current practice. For eucalyptus, however, the logging litter is important in soil loss reduction. To maximize its soil conservational benefit, post-fire logging should be completed as soon as feasible, although this may not be easily implemented elsewhere in Portugal, where eucalyptus stands represent a long-term family asset rather than a commercial crop (Coelho et al., 1995b). Logging litter, however, is also flammable. After wildfire, logging litter, together with leaf litter and ground cover vegetation, rapidly reach critical amounts for renewed wildfire (Vega et al., 1994). Recent research has advocated the reintroduction of traditional labourintensive fuel load reduction practices for Mediterranean forests, such as prescribed burning, grazing and coppicing (Vega et al., 1987; Diaz-Fierros et al., 1990; Naveh, 1990; Rego et al., 1990; Botelho et al., 1994), which have declined with depopulation and other socioeconomic changes. The response to recent increased fire frequency in the Agueda
Soil conservation and forest wildfires in Portugal: R A Shakesby et al.
Basin, however, has been to focus on different following:
fire prevention
measures,
including
the
(1) a publicity campaign stressing the damaging aspects of fire; (2) forest road improvement to encourage people to remain in remote forest areas, providing early warning of fire; (3) establishing a fire watchtower network; (4) clearing undergrowth from 30-m strips either side of selected forest roads; (5) national legislation stipulating that all logging litter should be removed within 15 days of clear-felling (in part to reduce the risk of pest infestation), with a maximum penalty of 6 million Escudos (c. &26000). This last measure is of most relevance to this investigation. For pine, there is no conflict as it is the needle ‘carpet’ (not included in the legislation) that is important for soil conservation rather than the logging litter (which can encourage pest infestation). For eucalyptus, however, there is a direct conflict between the fire hazard and soil conservational aspects of its litter. Two points help to resolve this problem. First, only 10 per cent by weight (0.46 kg mm2) of available litter can halve soil losses, and this need not present a significant fire hazard or increase pest infestation risk appreciably. Secondly, once the bark (a significant proportion of the litter) becomes moulded to the soil surface it presents no major fire hazard, as it remains damp for much of any dry period. Leaving the bark is also advantageous as its nutrient content is high (Hopmans er al., 1993). Post-fire management,
soil longevity und soil quality
An important question arising from this work is the significance of the soil losses for soil longevity and soil quality. Data are available from this and earlier work (Shakesby et al., 1993; Coelho et al., 1995a) to determine soil losses for different post-fire management scenarios, but extrapolating to the longer term (50-100 years) is difficult, not only because of time since fire and treatment type, but also because rainstorm timing and magnitudefrequency strongly influence soil losses. A simple approach incorporating the effect of variation of rainfall amount on soil loss is presented in Table 6. Soil losses for years 1 and 2 (eucalyptus) and 2 and 3 (pine) after fire have been calculated from unit-rain erosion rates for three post-fire rainfall scenarios: the mean, maximum (‘wettest’) and minimum (‘driest’) annual rainfall totals for the site (as predicted from Campia records, 193 l-94). Soil losses for eucalyptus for the wettest rainfall scenario are calculated as three times those in the driest years, reaching 6.73 t ha-’ and 9.47 t ha-’ in years 1 and 2 respectively for the most erosive plot. For pine, corresponding estimates reach 14.37 t ha-’ in year 2 and 2.88 t ha-’ in year 3. Thus, using the figures in Table 6, two consecutive exceptionally wet post-fire years would result in an estimated highest eucalyptus soil loss of 18.94 t ha-‘. The calculated maximum soil loss for pine in year 2 of 14.37 t ha-’ implies that for two exceptionally wet years immediately after fire, the highest soil losses would be greater than for the eucalyptus site. With two exceptionally dry years after fire, eucalyptus cumulative soil losses as low as 0.88-5.90 t ha-’ are implied. These estimates are, however, crude because unit-rain erosion rates derived from the two monitored phases are assumed to be typical, though as Figures 5 and 6 show, they vary considerably from period to period. If the rain in years 1 and 2 of the monitoring programme had been transposed, the relative unit-rain erosion rates are likely to have been rather different. Other points also render the assessment of soil longevity difficult. First, bounded plots represent only ‘top-of-slope’ locations. In middle and footslope locations, upslope overland flow and sediment input could either lead to higher erosion rates (because of greater generated overland flow depths) or lower erosion rates (because sediment from upslope would partly replace eroded sediment) than those recorded at the plots. The 350
Table 6 Post-fire soil losses based on plot data estimated for different rainfall scenarios Year 1 Rainfall scenario Eucalyptus Actual Average wettest Driest
Year 2
Rainfall (mm)
Soil loss range (t ha-‘)
Rainfall (mm)
Soil loss range (t ha-’ )
678.4 I30 I .o
0.46-2.23 0.87-4.13
1472.0
2 120.0 660.0
1.42-6.73 0.44-2. IO
1301.0 2120.0 660.0
3.24-6.57 2.86-5.81 4.66-9.47 1.4.5-2.9.5
919.2 1782.2 2900.0 860.0
0.86-4.55 I .67-8.83 2.72- 14.37 0.82-4.26
Year 3 Rainfall
(mm)
Soil loss range (t ha-‘)
2027.3 1782.2 2900.0 860.0
0.56-2.0 I o.so- 1.77 0.8 I-2.88 0.24-0.85
Pine
Actual Average Wettest Driest
Now: Rainfall estimates based on established relationship of Castanheira and Falgueirinho raingauges in the study area with Campia long-term rainfall records. Wettest and driest annual rainfall estimates have been rounded to the nearest IOmm. Soil loss ranges are based on all plots using unit-rain erosion rates for the pretreatment phase but for the post-treatment phase the ranges exclude values calculated for pine bare treatment and all eucalyptus litter treatment plots
former view is supported by an average 4.5 mm of ground lowering measured in a midslope newly burnt eucalyptus stand with 1193 mm of rainfall (Shakesby et rrl., 1993). Assuming 1.5t ha-’ represents 1 mm soil depth (Morgan 1987), this gives a loss of c. 67 t ha-‘. This is an order of magnitude higher than the plot results in the present study and suggests that the erosional effect of uninterrupted overland flow could be larger than is suggested by them. Secondly, erosion caused directly by logging and rip-ploughing operations is not represented in the plot losses but could well far outweigh them. Although manual logging of the type that occurs in the Agueda Basin causes little on-site erosion (Lal, 1981), excavating and using forest roads can increase soil losses considerably (Reid and Dunne, 1984; Trimble, 1988). More importantly, the rip-ploughing process itself causes considerable downslope soil transfer. Thirdly, little is known about commercial forest soil renewal rates. Estimates for natural conditions range from, for example,
Soil conservation and forest wildfires in Portugal: R A Shakesby et al
(FAO, 1979; Kardell et al., 1986), so that ground preparation involving uprooting the old stumps and planting of seedlings would be needed at least every 30-40 years. If this involved downslope rip-ploughing, then losses would be likely to be an order of magnitude higher than fire-induced losses in the first year (Coelho et al., 1995a). Even with negligible soil renewal rates, these fire-induced soil losses represent net ground lowering after fire of little more than l-2 mm (assuming 15 t ha-’ represents 1 mm of lowering). (Downslope rip-ploughing every 3-4 clear-felling cycles would add about another 3-4 mm of lowering over a 30-40-year period.) These amounts do not appear to present a medium-term threat of substantial soil depth reduction. However, such losses may be highly detrimental to soil quality for the following reasons. First, the sediment removed is not only rich in fines but also in organic matter and nutrients. Whilst losses of nitrogen and potassium can be expected to be replenished relatively quickly from the air and bedrock respectively, the same is not true of phosphorus. Thomas (1996) estimates that, for a 3-cm-thick soil at the burnt eucalyptus site, 55 per cent of available phosphorus was removed in just two years after the August 1992 fire. Even if soil depth were 10 cm, calculations show that 19 per cent of available phosphorus would be removed. Since weathering of schist yields little or no phosphorus, successive fires and accompanying soil losses on thin schist soils in the Agueda Basin would severely deplete them of this vital nutrient for plant growth. Secondly, preferential loss of the water-retaining fines and organic matter (Coelho et al., 1995a; Thomas, 1996) would make establishing eucalyptus seedlings difficult.
Conclusions Simple post-fire management practices can limit soil losses substantially in the wet pine and eucalyptus forests of north-central Portugal, but attaining worthwhile soil loss reduction with minimal disruption to established post-fire forest practices relies on an appreciation of the quantities, soil conservational qualities and timing of litter availability of different tree species. Thus for eucalyptus, clear-felling after fire is best carried out as soon as possible and litter (particularly bark) should be left on-site. Conversely, for pine, where litter quantities are typically much less and the rigid twigs and branches are comparatively ineffective at reducing soil losses (and harbour pests), the best feasible post-fire management is to delay logging until needlefall from the scorched tree canopy has occurred. The resulting needle ‘carpet’ is highly effective at reducing soil loss by some 90 per cent. Whilst leaving large amounts of eucalyptus litter on site can provide good soil protection, it conflicts directly with fire prevention and pest control regulations. As a compromise, since the bark is the most effective part of the litter at limiting erosional losses and is comparatively rich in nutrients, existing regulations should be modified from removing all litter (bark and timber) to removing just the timber component. Minimum tillage is a far less erosive ground preparation method for planting eucalyptus seedlings than rip-ploughing, but its applicability is questionable: it could only be used to plant seedlings in pine areas (as pine does not regrow from sawn stumps), unless eucalyptus stumps can be killed without having to uproot them by rip-ploughing. Despite near-homogeneity as regards slope angle, soil texture and soil depth at each site, soil loss and overland flow at the plot scale varied as much because of inherent plot variations as treatment impacts. Sufficient plot replication to eliminate this effect was impractical. The solution adopted here, which was to view the response of untreated plots during the post-treatment compared with pre-treatment phase as the expected behaviour of plots for the rainfall patterns experienced, and to compare treated plot changes with these responses, was largely successful. It worked particularly well for eucalyptus plots, where
Soil conservation
and forest wildfires in Portugal: R A Shakesbv et al.
treatment impact was set against a rising trend in soil losses at the untreated plots, but was less successful for pine plots where, because of more advanced post-fire vegetation regrowth, soil losses declined sharply in the post-treatment phase, despite higher rainfall. Scaling up rates of soil loss from the plot data, even for worst-case scenarios, does not suggest imminent, serious soil degradation, but this may be misleading for three reasons. First, these data do not tally with substantial ground lowering rates measured previously in middle and footslope locations, where overland flow was unimpeded by plot boundaries. Secondly, the pine plot results relate to thicker and potentially, therefore, more easily eroded soils than at the eucalyptus site but do not include the critical early post-fire months when losses might have been very large. Thirdly, erosion selectively removes organic-rich fines containing important nutrients, notably phosphorus, which is not easily replaced by weathering.
Acknowledgements We thank particularly Antonio J D Ferreira, Stefan Doerr and Andrew D Thomas for invaluable assistance with fieldwork, data collation and discussion of results. We thank Guy Lewis and Nicola Jones for drawing the diagrams. Financial support was provided by an EU research contract (EV5V-0041) and we thank Dr P Balabanis and Mr D Peter of the European Commission for their interest and support. We also thank two anonymous referees for their very helpful comments.
References 12. Alexander, E B (1988) Strategies for determining soil-loss tolerance’. Environmentol Management 79 l-796 Bennett, H H (1939) Soil consenution McGraw-Hill, New York Botelho, H S. Vega, J A, Femandes, P and Rego, F M C (1994) ‘Prescribed fire behavior and fuel consumption in northern Portugal and Galiza maritime pine stands’, in Proceedings of’ the Second International Cor?f>rence on Forest Fire Research, Coimbra, Portugal. November 1994, Vol. I, B 16,pp 343-353 Burch, G I. Moore, I D and Burns, J (1989) ‘Soil hydrophobic effects on infiltration and catchment runoff‘. Hydrological Processes 3, 2 I J-222 Coelho. C de 0 A, Shakesby, R A and Walsh, R P D (1995a) Effects qf Forest Fires and Post:fire Lund MrmaKement Practice on Soil Erosion and Stream Dynamics, Agueda Basin, Portugal Directorate General for Science, Research and Development, European Commission, Environment and Research Programme. Soil and Groundwater Research Report V, EUR 15689 EN Coelho, C de 0 A, Shakesby, R A, Gonzales del Tanago, M, Teman, L, Walsh, R P D and Williams, A G (I 995b) IBERLIM: Land Management and Erosion Limitation in the Iberiun Peninsula Final Report to the EC in fulfilment of Project EV5V-0041 ‘Land Management Practice and Erosion Limitation in Contrasting Wildfire and Gullied Locations in the Iberian Peninsula’ (unpublished) Connaughton, C A (1935) ‘Forest fire and accelerated erosion’, Journal of Forestry 33, 75 l-752 DeBano, L F (I 99 I) ‘The effects of fire on soil properties’, Generrrl Technical Report, United States Department of Agriculture, Forest Service INT-280, 151-156 Diaz-Fierros, F, Benito, R E and Perez, M R (1987) ‘Evaluation of the USLE for the prediction of erosion in burnt forest areas in Galicia (N.W. Spain)‘, Catena 14, 189-199 Diaz-Fierros, F, Benito, E, Vega, J A, Castelao, A, Soto, B, Perez, R and Taboada, T (1990) ‘Solute loss and soil erosion in burnt soil from Galicia (NW Spain)‘, in Goldammer, J G and Jenkins, M J (eds) Fire in Ecosytem Dyamics: Mediterranean and Northern Perspectives SPB Academic Publishing, The Hague, pp l-20 Doerr. S, Shakesby, R A and Walsh, R P D (1996) ‘Soil hydrophobicity variations with depth and particle size fraction in burned and unburned Eucalyptus globulus and Pinus pinaster forest terrain in the Agueda Basin. Portugal’. Catena 27, 25-47 FAO ( 1979) Eucalypts for Planting FAO Forestry Series No 11, FAO, Rome Ferreira, A J D, Coelho, C de 0 A, Gotqalves, A J B, Shakesby, R A and Walsh, R P D (1995) ‘Impact of climate change on slope and catchment hydrology in forest areas, central Portugal’, in Proceedings of the Conference on Erosion and Land Degradation in the Mediterranean, IGU Study Group on Erosion and Desertification in Regions of Mediterranean Climate, University of Aveiro, 14-18 June 1995, pp 261-274 353
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Giovannini. G, Lucchesi, S and Giachetti, M (1988) ‘Effects of heating on some physical and chemical parameters related to soil aggregation and erodibility’, Soil Science 146, 255-261 Grigal, D F and McCall, J G (1975) ‘Litter fall after wildfire in virgin forests of northeastern Minnesota’, Canadian
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Hopmans, P, Stewart, H T L and Flinn, D W (1993) ‘Impacts of harvesting on nutrients in a eucalypt ecosystem in southeastern Australia’, Forest Ecology and Management 59, 29-51 Imeson, A C and Vis, M (1984) ‘Seasonal variations in soil erodibility under different cover types in Luxembourg’, Journal of Soil Science 35, 323-33 I Imeson, A C, Verstraten, J M. van Mulligen, E J and Sevink, .I (1992) ‘The effects of fire and water repellency on infiltration and runoff under Mediterranean type forest’, Catena 19, 345-362 Kardell, L, Steen, E and Fabiao. A (1986) ‘Eucalyptus in Portugal: a threat or a promise’?‘, Ambio 15(l), 6-l 3 La], R (1981) ‘Deforestation of tropical rainforest and hydrological problems’, in Lal, R and Russell. E W (eds) Tropical Agricultural Hydrology Chichester, Wiley, pp 13 l-l 40 Lavee, H, Benyamini, Y, Kutiel, P and Segev, M (1991) ‘Infiltration, runoff and erosion processes as influenced by forest fires in the Carmel Mountains, Israel’. in Sala, M and Rubio, J L (eds) European Societv,for Soil Conservation Cortference on Soil Erosion and Degrudation 3-7 September 1991, Barcelona and Valencia, Spain, p 20
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Morgan, R P C (1987) ‘Sensitivity of European soils to ultimate physical degradation’, in Barth, H and L’Hermite, P (eds) Scientific Basis for Soil Protecrion in the European Community Amsterdam, Elsevier Applied Science, pp 147-157 Morris, S E and Moses, T A (1987) ‘Forest fire and the natural soil regime in the Colorado Front Range’, Annals of the Association
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Naveh, Z (1990) ‘Fire in the Mediterranean: a landscape ecological perspective’, in Goldammer, J G and Jenkins, M J (eds) Fire in Ecosystem Dvtamics: Mediterranean and Northern Perspectives SPB Academic Publishing, The Hague, pp l-20 Pereira, V and Fitzpatrick, E A (1995) ‘Cambisols and related soils in north-central Portugal: their genesis and classification’, Geoderma 66. 185-2 I2 Pimentel, D et al. ( 1976) ‘Land degradation: effects on food and energy resources’, Scietrce 194, 149-15.5 Rego, E Botelho, H and Torres, F (1990) ‘Fuel management in Portuguese forest by prescribed fire and grazing‘, in Proceedings of the International Conference on Forest Fire Research, 19-22 November 1990. Coimbra, Portugal. 8.02, pp 1-8 Reid. L M and Dunne, T (1984) ‘Sediment production from forest road surfaces’, Water Resources Research 20. 1753-1761 Sala, M and Rubio. J L (eds) (1994) Soil Erosion as a Consequence of Forest Fires Geoforma Ediciones, Logroho, Spain Sala, M, Soler. M and Pradas. M (1994) ‘Temporal and spatial variations in runoff and erosion in burnt soils’. in Proceedings of the Second International Conference on Forest Fire Research, November 1994. Coimbra. Portugal. Vol II, D34, pp 1123-l 134 Sevink. J, Imeson, A C and Verstraten, J (I 989) ‘Humus form development and hillside runoff and the effects of fire and management under Mediterranean forest in NE-Spain’. Carena 16, 461-475 Shakesby. R A, Walsh, R P D and Coelho, C 0 A (1991) ‘New developments in techniques for measuring soil erosion in burned and unburned forested carchments. Portugal’, Zeit.schrjft fiir GrurnorI,/to/o,~ie, Supplementband 83. 161-174 Shakesby. R A, Coelho, C de 0 A, Ferreira, A D, Terry, J P and Walsh, R P D (1993) ‘Wildfire impacts on soil erotion and hydrology in wet Mediterranean forest, Portugal’. Internationa/ Journal of Wildland Fire 3. 95-l
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Shakesby, R A. Coelho, C de 0 A, Ferreira, A D. Terry, J P and Walsh. R P D (1994) ‘Fire, post-burn land management practice and soil erosion response curves in eucalyptus and pine forests, north-central Portugal’, in Sala, M and Rubio, J L (eds) Soil Erosion and Degradation as a Consequence of Forest Fires Geoforma, Logroho, Spain, pp I1 l-132 Soto, B, Basanta, E, Benito. E, Perez, R and Diaz-Fierros, F (1994) ‘Runoff and erosion from burnt soils in Northwest Spain’, in Sala, M and Rubio, J L (eds) Soil Erosion and Degradution as a Consequence ofForest Fires Geoforma, Logrofio, Spain, pp 91-98 Swanson, F J (1981) ‘Fire and geomorphic processes’, in Mooney, H A, Bonnicksen, T M, Christiansen, N L. Lotrun. J E and Reiners, WA (eds) Fire Regimes and Ecasystem Properties: Proceedinp ofthe Conference, I I-IS December 1978, United States Department of Agriculture Forest Service, General Technical Report WO-26, US Government Printing Office, Washington, DC, pp 401-420 Terry, J P ( 1992) Rainsplash Detachment and Soil Erosion in the Agueda Basin, Portugal: Effects of Forest Fire and Lund Management Changes Unpublished PhD thesis, University of Wales
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Thomas, A D ( 1996) The Eflects Fire and Different Logging Techniques on Nutrient Losses in Overland F/m from Eucalyptus and Pine Forests in Northern Portugal Unpublished PhD thesis, University of Wales Trimble, S W (1988) ‘The impact of organisms on overall erosion rates within catchments in temperate regions’. in Viles, H A (ed) Biogeomorphologv Blackwell, Oxford. pp 83-142 Vega J A. Bara, S, Alonso. M, Fonturbel, T and Garcia, P (1987) ‘Preliminary results of a study on short term effects of prescribed fire in pine stands in NW Spain’, Ecologic Meditrrranecr 13(d),177- I88 Vega, J A and Diaz-F&rob, F (1987) ‘Wildfire effects on soil erosion’, G~ologicr Medirerwwrr 1j(4).
110-125 Vega. J A et a/. (1994) ‘Forest fire prevention through prescribed burning: an international cooperative project carried out in the European S.T.E.P. program’, in Proceedings of the Second International Confirrncr on Forest Fire Research. Coimbra. Portugal, November 1994. Vol I, AOI, pp 75-84 Viegas, D X (ed) (1990) International Conference on Forest Fire Resrarch, Procerdin~s, 19-22 November I9YO. Coimbra, Portugal Viegas. D X (ed) ( 1994) Second International Crmference on Forest Fire Resecwch. Prowrdin,~.~. 2 l-21 November 1994. Coimbra, Portugal Walsh. R P D, Shakesby. R A and Coelho, C 0 A (1988) ‘The erosional impact of forest fire in the Agueda drainage basin. Portugal: background to the project and experimental design’. Swtrnsea Grogrcpher 25. X8-98 Walsh, R P D. Boakes, D J. Coelho, C de 0 A, Gorqalves, A J B and Shakesby, R A (1994) ‘Impact of fireInduced hydrophobicity and post-fire forest litter on overland flow in northern and central Portugal’. in Procwdings of the Second Interncrtional Conference on Forest Fire Resrcrrch. Coimbra. Portugal, 2 I-21 November 1994, Vol 11. D38, pp 1149-I 159 Walsh. R P D. Boakeh. D J, Coelho. C de 0 A, Ferreira. A D, Shakesby. R A and Thomas, A D (199.5) ‘Post-fire land USCmanagement and runoff responses to rainstorms in Portugal’. in McGregor. D F M and Thompson. D (cds) Geonlor/,ho/oR?. trnd Ltrnd Manirgement in a Chan,ying Enr,ironment Wiley, Chichester. pp 283-308 Wells. C G. Campbell, R E, DeBano. L F. Lewis. C E, Fredriksen, R L. Franklin, E C. Froelich, R C and Dunn. P C ( 1979) ,!?&a of Fire on Soil; a Stcrte-ok-kno,~/edRe Rel,iekv General Technical Report WO-7. US Department of Agriculture, Forest Service. Washington. DC White. W D and Wells, S G (1982) ‘Forest-fire devegetation and drainage basin adjustments in mountainous terrain’. in Rhodes. D D and Williams. G P (eds) Adju.stment.s qf the Flrrvial System: Prowedinp of the IOth Ge~,rilor/~ho/o,~~ S~m/~o.~ium. Binghamton, Allen and Unwm. Boston, pp 19Y%223 World Meteorological Organization ( 1995) The G/o/m/ Climtrte System Rel?rn. Geneva, World Metcorolopical Organization. pp 44-46
(Re\?sed nwnuscript
recei\,ed 26 July I YY6J
355
SOl43-6228(96)00022-7
Geo,qraph~. Vol. 16. No. 4. pp. 337-3.5.5. 1996 Copyright 0 1996 Eisevier Science Ltd Printed in Great Britain. All rights reserved 014%6228/96 $lS.Oll + 0.00
Limiting the soil degradational impacts of wildfire in pine and eucalyptus forests in Portugal A comparison practices
of alternative post-fire management
Richard A Shakesby
Department ofGeography, University of Wales Swansea, Singleton Park, Swanserl SA2 RPP: UK
David J Boakes Department ofGeography, University of Wales Swansea. Singleton Park, Swansea SA2 8PP UK
Celeste de 0 A Coelho Departamento Portugal
de Ambiente e Ordenamento,
Universidude de Aveiro. 3810 Aveiro.
A J Bento Gonqalves Nticleo de Investiga@o Cientzjcica de Inc&dios Bat-veto. 58, R/Ch, 3000 Coimbru, Portugal
Florestais, FLUC, Av. Bissaya
Rory P D Walsh Department of Geography, University of Wales Swansea, Singleton Park, Swanserr SA2 8Pe UK
In newly burnt and unburnt pine and eucalyptus forest in Portugal, overland flow and soil losses were monitored to assess the impacts of the following post-fire treatments: application of different quantities of logging litter; rip-ploughing compared with minimum tillage prior to planting eucalyptus seedlings; and clearance of pine needles and vegetation. Eucalyptus logging litter reduced soil losses by up to 95 per cent. The impact of pine logging litter was equivocal, but removal of pine needles increased soil losses elevenfold. Implications for soil longevity, soil quality and land management strategy are discussed. Copyright 0 1996 Elsevier Science Ltd 337
Soil conservation and forest wildfires in Portugal: R A Shakesbyet al.
In the last few decades, forest wildfires have increased in frequency and extent throughout the Mediterranean basin for a variety of reasons, including: planting of highly flammable tree species (notably pine and eucalyptus); a decline in traditional practices like grazing and coppicing that previously reduced the accumulation of flammable litter and undergrowth; increasing recreational use of forests; and a succession of unusually dry years (World Meteorological Organization, 1995). Such fires not only damage trees and destroy undergrowth and litter but can also lead to detrimental soil changes, including reduced organic matter content and aggregate stability (Giovannini et al., 1988) increased soil erodibility (Imeson and Vis, 1984) enhanced soil water repellency (DeBano, 1991) and decreased infiltration capacity. These changes result in substantial increases in overland flow and soil losses, peaking immediately following fire and declining rapidly thereafter (see, for example, Swanson, 198 1; Diaz-Fierros et al., 1987; Morris and Moses, 1987; Sevink et al., 1989; Imeson et al., 1992). Despite considerable research into the short-term erosional impacts of fire and forest fire prevention (such as Viegas, 1990, 1994; Sala and Rubio, 1994) conservational implications of alternative post-fire land management practices and the longer-term implications of differing fire/land management scenarios for soil and forest sustainability have received little attention. This paper addresses these two research gaps with respect to eucalyptus and pine forests in the Agueda Basin, Portugal, where fires have increased in frequency since the early 1980s. In this area, there are three main forms of post-fire land use practices following manual logging of the scorched timber: (1) eucalyptus regrowth from the stumps; (2) pine seedling regeneration in muto (scrub vegetation); and (3) planting eucalyptus seedlings on ground prepared by rip-ploughing. The ways in which these land use practices are implemented have important repercussions for soil erosion. Ground cover on bare burnt soils is clearly critical for limiting erosion, yet logging litter is often left at roadsides. Logging of pine too soon after fire can prevent the development of a ‘carpet’ of fallen scorched needles. Rip-ploughing is known to be erosionally very damaging (Shakesby et al., 1994) yet the conservational benefit of any alternative ground preparation technique has not been investigated. Using the results of bounded plot studies of post-fire erosion, this study assesses the impacts of applying different quantities of post-fire logging litter, the protection provided by a pine needle cover, and the possible benefits of minimum tillage for planting eucalyptus seedlings compared with rip-ploughing. The conflict between the soil conservational properties of logging litter and its flammability and tendency to encourage pest infestation and implications of fire and/or fire and rip-ploughing for soil longevity and quality into the medium to long term (50-l 00 years) are then considered.
Study area and land management
background
The study sites are located near Falgorosa (40” 32’ N, 8” 22’ W) and Lourizela (40” 38’ N, 8” 19’ W) in newly burnt and adjacent unburnt Eucalyptus gl~bulus and Pinus pinaster forest in the Agueda Basin (Figure f). The area is a deeply dissected schist upland (up to c. 500m) in the foothills of the Caramulo Mountains. Slopes are generally convexorectilinear, typically with 20” rectilinear sections. On steep slopes, soils are shallow (only rarely reaching 40cm in depth) stony Umbric Leptosols (Pereira and Fitzpatrick, 1995) comprising mainly sand (59-80 per cent) and silt (18-40 per cent), with little clay ( l-2 per cent) (Terry, 1992). The long-term mean annual rainfall varies with altitude from 1300 to 1900 mm, but was below average for all but two years between 1980 and 1993 (Ferreira et al., 1995). Most rainfall results from winter depressions, the period from July to September being mainly dry. Until the 1920s the area was largely under pasture with patches of forest, but land degradation led to legislation for widespread planting of P. pinaster. This species, which 338
Soil consenwtiun
nnd.fr,rest
wil&res
in Portugal:
R A Shakesby et 01.
I Datelastburnt
.
Location of bounded plols (E I eucalyptus regrowth
July 1986
I w
ME - mature eucalyptus P - pine regrowth MP - mature pine)
July 1991
m .
August 1992
A
Automatic raingauge
Settlement
A
Storage raingauge
Figure 1 Extent and timing of selected fires, and location of raingauges and bounded plot sites in the Agueda Basin
reaches maturity in about 25-35 years, was supplanted, following a large fire in 1986, as the dominant forest species by faster-growing E. globulus, which ‘matures’ (that is, provides a commercial crop) in only 9-l 2 years. A number of smaller fires have occurred in the region since 1986. This paper focuses on pine burnt in July 1991 and eucalyptus burnt in August 1992. Severe understorey fires (sensu Wells et al., 1979) are typical, with all surface organic material (litter, undergrowth, and so on) being consumed, but with tree trunks and canopies only being scorched. The flammability of pine and eucalyptus litter, legislation against prescribed burning (Rego et al., 1990), arson (Walsh et al., 1988) and a series of unusually dry years have increased fire frequency.
Research design and methodology A bounded plot approach post-fire land management
was adopted in assessing the impact on soil loss of different strategies. In all, 16 bounded plots at 4 sites (E, ME. P and MP
Soil conservation and forest wildfires in Portugal: R A Shakesby et al.
in Figure 1) were monitored from November 1992 to July 1994. Two ‘control’ plots in mature unburnt forest (ME and MP on Figure 1) provided an indication of pre-bum erosion levels. Two plots at each burnt site (E and P on Figure I) were left untreated for the entire period; the remaining ten plots were subject to the following three treatment types at the end of June 1993 (Figures 2 and 3).
(1) The first set of treatments compared the impacts on soil loss of applications of varying
(2)
amounts of post-fire logging litter. Quantities and types of logging litter differ for eucalyptus and pine. Assessments were made of the highest amounts of litter available following clear-felling by selecting sites in burnt and logged mature eucalyptus and pine forests where all litter had been left on the slopes. Replicated areas were marked out and the litter collected and weighed. Mean litter weights per unit area and percentage cover were much higher for eucalyptus (respectively, 4.6 kg mm2 and 89.5 per cent) than for pine (1.8 kg mm2 and 8.3 per cent). To test the impact of different quantities of available litter in reducing soil loss, 100 per cent (termed ‘high’ treatment), 50 per cent (‘medium’) and 10 per cent (‘low’) of the mean weight per unit area found in the squares were applied as plot litter treatments. (The last category was considered appropriate only for eucalyptus, where it still provided an appreciable ground cover.) Replication of low litter for eucalyptus and high litter treatment plots for pine was possible within the research design (Figures 2 and 3). To assess the conservational impact of a post-fire cover of pine needles fallen from the scorched tree canopy, all needles from a burnt pine plot were removed and all
j!-,
I
August 1992
Fire
-
I
November 1992
Pre-treatment phase
June 1993
TREATMENT
Post-treatment phase July 1994 Figure 2 340
Research design: eucalyptus
bounded plots
Soil conservarion and forest wildfires in Portugal: R A Shakesbv et 01.
July 1991 November 1992
Pre-treatment phase
June 1993
TREATMENT
fl
Post-treatment phase
Mature
July 1994
L-J Figure 3
Research design: pine bounded plots
vegetation cut at soil level. The collected needles represented a 38 per cent ground cover. (The impact of eucalyptus leaves was not considered as either they do not fall before logging or they tend to be blown off exposed slopes.) for planting (3) High soil losses resulting from ground preparation by rip-ploughing eucalyptus seedlings have been reported in previous work (Shakesby et ctl., 1994; Coelho et al., 1995a, 1995b). The possible soil conservational benefits of minimum tillage compared with rip-ploughing were investigated by planting on and around a burnt pine plot at the pine site eucalyptus seedlings at a typical density for the region and restricting ground disturbance to c. 50cm diameter around each seedling. A ripploughed plot at the burnt eucalyptus site provided comparative data.
The bounded plots were comparatively small (8 m x 2 m) in response to the restricted downslope distances between forest tracks, the small size of tree stands of a single treatment and the practical limitations of recording soil losses and overland flow accurately for larger areas during high-magnitude events. Each plot was linked to a sediment trap, tipping bucket flow recorder and a series of large collecting tanks (Shakesby et al., 1991; Walsh er ul., 1995). Overland flow was mostly recorded storm by storm. Ideally, soil losses would have been monitored on the same basis, but logistical constraints involved in sediment trap changes and cleaning precluded a higher frequency and resulted in periods of unequal length. Soil losses were monitored over a combination
Soil conservation
and forest wildfires in Portugal: R A Shakesby et al.
of seven long (49-108 days) and two short (9 and 22 days) periods. The pre-treatment phase (November 1992-June 1993) comprised five measurement periods and the posttreatment phase (July 1993-July 1994) four measurement periods. Rainfall was measured using standard raingauges at the plot sites. Daily and hourly rainfall intensities were extrapolated from weekly autographic records at Falgueirinho (altitude 460 m) and Castanheira (200 m) in the study area (Figure I). From previous work, it was known that vegetation recovery and the development of a stone cover are factors affecting soil loss considerably during the first few years after disturbance (Shakesby er ul., 1994). Plot vegetation and stone ( > 0.5 cm) covers were estimated in June 1993 and July 1994 from mosaics of vertical photographs (taken from a 3-m-high movable frame) and from field surveys. Vegetation cover was determined directly from the mosaics but stone cover was estimated using Graftik Optilab image analysis of quartz stones on representative plot areas and total stone cover was calibrated by field surveys (details in Coelho er al., 1995b).
1
2 1201
560
100
6 6718
Figure 4
Daily rainfall for the nine monitoring periods at the pine bounded plot site. (Daily rainfall for the eucalyptus site has a similar pattern of rainfall events but with lower totals) 342
Results Rair$dl
and overland flow
Daily rainfall over the monitoring period for the burnt pine plots is given in Figure 4 (rainfall at the eucalyptus site is similar but with smaller totals) and summary rainfall characteristics at both sites are shown in Table 1. Rainfall at the nearest long-term weather station (Campia, altitude 470 m, c.20 km northeast of the study area) in the rainfall years (October-September) 1992/3 and 1993/4 was respectively 22.8 per cent below and 12.6 per cent above the long-term average. The post-treatment phase (July 1993-July 1994) was much wetter than the pre-treatment phase (November 1992-June 1993) with twice as many daily falls >lO mm, 3-5 times as many days with >25 mm and about 2-4 times as many hours with intensities >lO mm (Table I). Overland flow at both burnt-plot sites was higher in the post-treatment than pretreatment phase (Tables 2 and 3) reflecting the rainfall differences between the sites. Application of large amounts of eucalyptus litter may have reduced the amount of overland flow increase in the post-treatment phase, explaining why the high and medium eucalyptus treatment plots have smaller post-treatment/pre-treatment ratios (1.1% 1.66) of overland flow percentages than those for low and untreated (2.06-4.93) plots. For pine. there is no discernible relationship between the treatments and their ratios.
Soil loss from individual plots at the burnt eucalyptus and pine sites in the nine monitoring periods is shown in Figures 5 and 6 and is expressed in terms of soil loss in grammes. unitrain erosion rate (gmm-’ of rainfall), and unit overland flow erosion rate (g mm-’ of overland flow). The latter two measures are helpful in comparing soil loss in periods differing in length and rainfall amount. Eucalyptus litter impact. In the pre-treatment phase, eucalyptus soil losses vary both between periods and between plots. For example, in period 4, with the highest losses and rainfall, loss from untreated plot A (1819g) is over seven times that from untreated plot B (243 g). In the post-treatment phase, low and untreated plot losses for individual periods are higher than in all (for untreated B plot) or all but one (untreated A and low A and B) of the pre-treatment periods (Figure 5).
Table 1 Rainfall at the plot sites during the pre- and post-treatment Falgorosa
Period Pre-treatment. periods 1-S Post-treatment. periods 6-9
(eucalyptus)” Days >25 mm
periods
Lourizela
Daily max. (mm)
Days >lO mm
678.4
39.6
24
5
2
147 I .8
63.5
so
17
9
Total (mm)
monitoring
Hours >lO mm
(pine)h
Daily max. (mm)
Days >lO mm
919.1
70.4
29
6
2027.0
8X.X
60
31
Total (mm)
Days >25 mm
Hours >lO mm IO 23
~’Altitude I IOm; mean annual rainfall 1301 mm “Altitude 320 m: mean annual rainfall 1782 mm
343
Soil conservation and forest wildfires in Portugal: R A Shakesby et al. Table 2 Summary
of overland
tlow in pre- and post-treatment
phases: eucalyptus Overland flow (mm)
Period
Rainfall (mm)
Untreated A
Low A
Untreated B
Low B
Medium
High
(a) Pre-treatment (periods 2-5)b
644.9
55.4 (8.6)
39.9 (6.2)
63.3 (9.8)
26.4 (4.1)
104.7 (16.2)
76.9 (11.9)
(b) Post-treatment (periods 6-9)
1471.8
392.3 (26.7)
188.5 (12.8)
362.8 (24.7)
297.1 (20.2)
280.8 (19.1)
283.8 (19.3)
2.06
2.52
4.93
(b) %/(a) 8
3.10
“Ovedand flow as a percentage of rainfall is given in parentheses bThe calculations relate only to the periods in the pre-treatment and post-treatment Source: after Walsh et al. (1994)
1.18
1.62
phases indicated
Considering unit-rain and unit overland flow erosion rates, apart from period 5, posttreatment values are similar to or higher than pre-treatment ones. The clear exception is the high treatment plot, where all post-treatment values are lower than pre-treatment ones. For both unit erosion rates, period 5 has the highest values, despite having the least rainfall (18.7 mm in 3 days following a 16-day virtually dry warm interlude). Soil losses at the ‘control’ plot in unburnt eucalyptus forest were negligible (totalling only 56.9g, August 1992-May 1994). Pine litter and needle removal impacts. As with eucalyptus plots, soil losses from burnt pine plots (Figure 6) vary considerably between periods and plots. Unlike the eucalyptus plots, post-treatment soil losses and unit erosion rates for the burnt pine plots tend to be similar to or lower than pre-treatment ones for all except the bare treatment plot (soil loss and unit-rain erosion rate only). Similarly to the eucalyptus plots, lowest rainfall in period 5 corresponds to the highest (or second highest) unit-rain and unit overland flow erosion rates. The mature pine plot soil loss was negligible (63.3 g, July 1992-May 1994).
Table 3 Summary
of overland
flow in pre- and post-treatment
phases: pine
Overland flow (mmy
Period
Rainfall (mm)
(a) Pre-treatment (periods 2 + 5)b
351.2
(b) Post-treatmenth (periods 7-9)
1352.5
(b) %/(a) CL
Untreated A
Untreated B
Min. tillage
Medium
High B
Bare
49.0 (14.0)
26.2 (7.5)
22.7 (6.5)
42.4 (12.1)
27.4 (7.8)
22.7 (6.5)
(I 1.7)
396.3 (29.3)
198.1 (14.6)
218.4 (16.1)
258.4 (19.1)
239.4 (17.7)
186.1 (13.8)
306.4 (22.7)
2.27
2.12
2.09
1.95
2.48
1.58
“Overland flow as a percentage of rainfall is given in parentheses “The calculations relate only to the periods in the pre-treatment and post-treatment Source: after Walsh et al. (1994) 344
High A
phases indicated
41.2
I .94
Soil conservation and forest wildfires in Portugal: R A Shake.& et al.
Unit-rain erosion rate (g/mm of rain)
Soil loss (9)
Unit overland flow erosion rate (g/mm of overland flow)
Untreated B
Low A
Medium
High
El -
Pre treatment phase (periods 1-5)
Figure 5
N
Nodata
(gmm-’ of rainfall) and unit overland flow (gmm-’ for the eucalyptus bounded plots
Soil loss (g) and unit-rain
overland flow) erosion rates
Post-treatment phase (periods 6-Q)
of
Minimum tillage and rip-ploughing. Results for the minimum tillage plot (Figure 6) are similar to those for the untreated B pine plot. Soil loss from the rip-ploughed plot in periods 7 and 8 was 4649.4 g, giving a unit-rain erosion rate of 4.8 g mm-‘, which is intermediate between the two untreated eucalyptus plots for these periods. Vegetation and stone cover The average eucalyptus plot vegetation cover changed little during the monitoring period (2.8 per cent, June 1993; 6.0 per cent, July 1994). Stone cover, however, more 345
Soil conservation
and forest wildfhs
in Portugal:
R A Shakesby er al.
Soil loss
Unit-rain erosion fate
Unit overland ftow erosian rate
(g)
(g/mm of rain]
(g/mm of overland Ilow)
-1
Medium
High A
High B
Bare
Minimum tiflage
. cl
S;i Pre-treatmenl phase
(periods
1-5)
Post-treatment phase (periods 6-9)
6 Soil loss (g) and unit-rain 1.gmm-’ of rainfall) and unit overland overland flow) erosion rates for the pine bounded plots
Figure
146
N
Nodata
flow (gmnm’
of
than doubled between these dates, from 17.4 per cent to 42.2 per cent. For pine. vegetation cover rose from 13.0 per cent in June 1993 to 3 1.7 per cent (excluding the bare treatment plot) by July 1994. The higher pine figures reflect the greater time since fire, more conducive local conditions for vegetative growth (northeast-facing for pine and deeper soils compared with southwest-facing and shallower soil for eucalyptus) and the allelopathic tendencies of eucalyptus (chemical inhibition of native plant germination and growth). Pine plot stone cover rose from 18.5 per cent in June 1993 to 44.1 per cent in July 1994.
Interpretation
of the results
Interpreting differences in treatment impacts is complicated by the inherent, sometimes large, pre-treatment inter-plot soil loss differences and by progressive changes in plot responses as vegetation recovered and stone cover developed. A better assessment of the treatment impacts on soil losses was made by viewing ratios of post- to pre-treatmentphase soil losses for untreated plots as the ‘control’ or ‘expected’ change in soil losses between the two phases; ratios for treated plots were then compared with the mean ratio for the appropriate (pine or eucalyptus) pair of untreated plots (Tuhks 4 and 5).
Eucul_yptus litter upplicution Post-fire soil losses were reduced substantially in burnt eucalyptus forest by applications of logging litter (Tuhle 4). Even the 8.8 per cent ground cover (10 per cent by weight of available eucalyptus litter) of the low-treatment plots reduced soil losses by about half (46 per cent and 64 per cent) compared with untreated plots. Thus, instead of an increase in soil losses of 3.4 and 7 times, as occurred for untreated plots from pre-treatment to posttreatment phase, losses increased only 1.9 and 2.8 times for the low-treatment plots and the 89.5 per cent cover of the high-treatment plot reduced soil losses to only 5 per cent of the expected amounts indicated by untreated plot losses. The relatively small increase in overland flow following treatment (Table 2) can only partly explain this soil loss decline. most being the result of litter protecting against rainsplash and trapping sediment transported by overland flow.
Table 4 Effects of different post-fire treatments
Plot treatment Untreated A Untreakd B Isw A Low B Medium High Rip-ploughed‘
on soil loss at burnt eucalyptus
Pre-treatment loss (t ha-‘)
Post-treatment loss (t ha-’ )
plots
Impact of treatment in post-treatment phase“
Percentage soil loss change due to treatment”
1.92
6.57
0.46 2.23
3.24 4.13
0.36
+I
1.3I
3.65
0.53
-46
1.17 I.58
I59 0.42 2.73
0.26 0.05
-73 -9s
I .o
(Post-/pre-treatment-phaatt “Impact of treiltmrnt (IT)
0
hoi1 low for treated plot)
= (Post-/pre-treatment-phase
soil losses for untreated plo1\)/2
“Calculakd for treated plot as (IT X 100) - 100 ‘The rip-ploughed plot was monitored only for 21/10/93-4/S/93
(periods 7 and 8)
347
Soil conservation and forest wildfires in Portugal: R A Shakesby et al. ‘&ble 5 Effects of different post-fire treatments
on soil loss at burnt pine plots Impact of treatment in post-treatment phase”
Plot treatment
Pre-treatment loss (t ha-‘)
Post-treatment loss (t ha-‘)
Untreated A Untreated B
1.52 0.86
I .09 0.56
1.0
Minimum Medium High A High B Bare
0.86 4.55 2.24 2.5 1 0.87
0.46 2.01 1.14 1.35 6.79
0.77 0.64 0.73 0.78 11.2
tillage
(Post-/pre-treatment-phase Ympact of treatment
0 -23 -36 -27 -22 + 1020
soil loss for treated plot)
(IT) = (Post-/pre-treatment-phase
hCalculated
Percentage soil loss change due to treatmentb
soil losses for untreated
plots)/2
for treated plot as (IT X 100) - 100
Although fire is conventionally, considered as inducing or intensifying soil hydrophobicity (DeBano, 1991), in the Agueda Basin unburnt and burnt forest soils when dry are similarly hydrophobic (Doerr et al., 1996). However, since the burnt soils dry out more rapidly, drought-enhanced soil hydrophobicity (Burch et al., 1989; Soto et al., 1994) becomes more rapidly established than in mature forest, leading to increased overland flow and erosion in rainstorms following droughts (Terry and Shakesby, 1993; Walsh et al., 1995). This explains the high unit erosion rates in period 5. Pine litter application At the burnt pine site, although perhaps partly masked by rapid vegetation recovery (in the third year after fire; post-treatment phase) not apparent at the eucalyptus plots (in the second year after fire; post-treatment phase), the absence of a clear impact for the medium and high pine litter treatments (Table 5) is attributed to their lower application densities (0.9 kg rnw2 and 1.8 kgm-’ for pine compared with 2.3 kg mm2 and 4.6 kg mm2 for eucalyptus), the associated sparse cover (4.8 and 8.3 per cent) and the rigidity of pine twigs and branches, which makes them less effective at damming sediment. That the percentage soil loss change differs little between minimum tillage (where treatment involved ground disturbance and no added litter) and treated plots supports the view that adding pine litter had little impact on soil losses. Pine needle cover The importance of the pine needle ‘carpet’, as suggested by others (see, for example, Connaughton, 1935; Grigal andMcCol1, 1975; Megahan and Molitor, 1975; White and Wells, 1982), is confirmed for Agueda Basin pine forests by the elevenfold increase in post-treatment soil loss at the bare treatment plot. This increase may understate bare soil losses immediately after fire due to (1) the binding effect of needles incorporated in the soil and erosion of many needles prior to treatment; (2) the stabilizing effect of roots of cut plants; and (3) the growth of moss. Minimum
tillage versus rip-ploughing
Minimum tillage has been shown to be a very advantageous post-fire practice for limiting soil losses when planting eucalyptus seedlings because (1) the post- to pre-treatment ratio 348
Soil conservarion and fores
wildfiresin Porrugal:R A Shake& et al.
for the minimum tillage plot differed little from those of the other pine litter treatments, suggesting no marked soil loss increase, and (2) this treatment produced an order of magnitude less sediment than the rip-ploughed plot at Falgorosa in the post-treatment phase and two orders of magnitude less than losses recorded for plots installed immediately following rip-ploughing (see, for example, Shakesby et al., 1994; Coelho et al., 1995a). Whether this treatment is acceptable from a silvicultural viewpoint, however, remains to be assessed. Post-fire recovery at untreated plots That untreated eucalyptus unit-rain erosion rates did not decline in the second year after burning is significant in two respects. First, this pattern is contrary to that usually reported, of highest soil losses within a year of fire with a rapid decline thereafter (Swanson, 1981; Diaz-Fierros et al., 1987; Vega and Diaz-Fierros, 1987; Lavee et al., 1991). In this study, rainstorm timing and magnitude-frequency may have been partly responsible. Thus the higher frequency of intense daily and hourly falls (Table I; Figure 4) may explain the much higher untreated and low-treatment eucalyptus soil losses of the post-treatment phase (Table 4). Secondly, it suggests that even for thin soils, preferential loss of fines (Thomas, 1996) and resulting surface stone lags do not necessarily lead rapidly to post-fire ‘soil exhaustion’ (see also Sala et al., 1994). By the third year after fire, however, previous work (Shakesby et al., 1994) indicates that untreated eucalyptus unit-rain erosion rates decline sharply towards mature forest levels. Percentage overland flow, however, can remain high (still 5 per cent six years after fire) (Walsh et al., 1995), suggesting that by this stage eucalyptus soil loss attenuation results more from reduced sediment supply, mainly through surface stabilization by vegetation (particularly moss, but also grasses and herbs), rather than from reduced overland flow. The comparatively high rates of erosion in the pre-treatment phase for the burnt pine (second year after fire) suggest that the post-fire erosion peak lasts two years, as is the case for eucalyptus. Pine overland flow percentages decline rapidly to mature forest levels (5 per cent by three years and 1 per cent by six years; see Walsh et al., 1995).
Implications Erosion limitation and fire hazard properties
of logging litter
To limit soil erosion after fire, the results indicate that logging should be carried out differently for pine and eucalyptus. Pine logging litter is less important in reducing soil losses than needles. Thus delaying timber clearance until after needlefall (12-15 weeks after fire) would be a simple but very effective conservational management practice. Since in reality clearance tends not to precede needlefall, this recommendation would not involve any change to current practice. For eucalyptus, however, the logging litter is important in soil loss reduction. To maximize its soil conservational benefit, post-fire logging should be completed as soon as feasible, although this may not be easily implemented elsewhere in Portugal, where eucalyptus stands represent a long-term family asset rather than a commercial crop (Coelho et al., 1995b). Logging litter, however, is also flammable. After wildfire, logging litter, together with leaf litter and ground cover vegetation, rapidly reach critical amounts for renewed wildfire (Vega et al., 1994). Recent research has advocated the reintroduction of traditional labourintensive fuel load reduction practices for Mediterranean forests, such as prescribed burning, grazing and coppicing (Vega et al., 1987; Diaz-Fierros et al., 1990; Naveh, 1990; Rego et al., 1990; Botelho et al., 1994), which have declined with depopulation and other socioeconomic changes. The response to recent increased fire frequency in the Agueda
Soil conservation and forest wildfires in Portugal: R A Shakesby et al.
Basin, however, has been to focus on different following:
fire prevention
measures,
including
the
(1) a publicity campaign stressing the damaging aspects of fire; (2) forest road improvement to encourage people to remain in remote forest areas, providing early warning of fire; (3) establishing a fire watchtower network; (4) clearing undergrowth from 30-m strips either side of selected forest roads; (5) national legislation stipulating that all logging litter should be removed within 15 days of clear-felling (in part to reduce the risk of pest infestation), with a maximum penalty of 6 million Escudos (c. &26000). This last measure is of most relevance to this investigation. For pine, there is no conflict as it is the needle ‘carpet’ (not included in the legislation) that is important for soil conservation rather than the logging litter (which can encourage pest infestation). For eucalyptus, however, there is a direct conflict between the fire hazard and soil conservational aspects of its litter. Two points help to resolve this problem. First, only 10 per cent by weight (0.46 kg mm2) of available litter can halve soil losses, and this need not present a significant fire hazard or increase pest infestation risk appreciably. Secondly, once the bark (a significant proportion of the litter) becomes moulded to the soil surface it presents no major fire hazard, as it remains damp for much of any dry period. Leaving the bark is also advantageous as its nutrient content is high (Hopmans er al., 1993). Post-fire management,
soil longevity und soil quality
An important question arising from this work is the significance of the soil losses for soil longevity and soil quality. Data are available from this and earlier work (Shakesby et al., 1993; Coelho et al., 1995a) to determine soil losses for different post-fire management scenarios, but extrapolating to the longer term (50-100 years) is difficult, not only because of time since fire and treatment type, but also because rainstorm timing and magnitudefrequency strongly influence soil losses. A simple approach incorporating the effect of variation of rainfall amount on soil loss is presented in Table 6. Soil losses for years 1 and 2 (eucalyptus) and 2 and 3 (pine) after fire have been calculated from unit-rain erosion rates for three post-fire rainfall scenarios: the mean, maximum (‘wettest’) and minimum (‘driest’) annual rainfall totals for the site (as predicted from Campia records, 193 l-94). Soil losses for eucalyptus for the wettest rainfall scenario are calculated as three times those in the driest years, reaching 6.73 t ha-’ and 9.47 t ha-’ in years 1 and 2 respectively for the most erosive plot. For pine, corresponding estimates reach 14.37 t ha-’ in year 2 and 2.88 t ha-’ in year 3. Thus, using the figures in Table 6, two consecutive exceptionally wet post-fire years would result in an estimated highest eucalyptus soil loss of 18.94 t ha-‘. The calculated maximum soil loss for pine in year 2 of 14.37 t ha-’ implies that for two exceptionally wet years immediately after fire, the highest soil losses would be greater than for the eucalyptus site. With two exceptionally dry years after fire, eucalyptus cumulative soil losses as low as 0.88-5.90 t ha-’ are implied. These estimates are, however, crude because unit-rain erosion rates derived from the two monitored phases are assumed to be typical, though as Figures 5 and 6 show, they vary considerably from period to period. If the rain in years 1 and 2 of the monitoring programme had been transposed, the relative unit-rain erosion rates are likely to have been rather different. Other points also render the assessment of soil longevity difficult. First, bounded plots represent only ‘top-of-slope’ locations. In middle and footslope locations, upslope overland flow and sediment input could either lead to higher erosion rates (because of greater generated overland flow depths) or lower erosion rates (because sediment from upslope would partly replace eroded sediment) than those recorded at the plots. The 350
Table 6 Post-fire soil losses based on plot data estimated for different rainfall scenarios Year 1 Rainfall scenario Eucalyptus Actual Average wettest Driest
Year 2
Rainfall (mm)
Soil loss range (t ha-‘)
Rainfall (mm)
Soil loss range (t ha-’ )
678.4 I30 I .o
0.46-2.23 0.87-4.13
1472.0
2 120.0 660.0
1.42-6.73 0.44-2. IO
1301.0 2120.0 660.0
3.24-6.57 2.86-5.81 4.66-9.47 1.4.5-2.9.5
919.2 1782.2 2900.0 860.0
0.86-4.55 I .67-8.83 2.72- 14.37 0.82-4.26
Year 3 Rainfall
(mm)
Soil loss range (t ha-‘)
2027.3 1782.2 2900.0 860.0
0.56-2.0 I o.so- 1.77 0.8 I-2.88 0.24-0.85
Pine
Actual Average Wettest Driest
Now: Rainfall estimates based on established relationship of Castanheira and Falgueirinho raingauges in the study area with Campia long-term rainfall records. Wettest and driest annual rainfall estimates have been rounded to the nearest IOmm. Soil loss ranges are based on all plots using unit-rain erosion rates for the pretreatment phase but for the post-treatment phase the ranges exclude values calculated for pine bare treatment and all eucalyptus litter treatment plots
former view is supported by an average 4.5 mm of ground lowering measured in a midslope newly burnt eucalyptus stand with 1193 mm of rainfall (Shakesby et rrl., 1993). Assuming 1.5t ha-’ represents 1 mm soil depth (Morgan 1987), this gives a loss of c. 67 t ha-‘. This is an order of magnitude higher than the plot results in the present study and suggests that the erosional effect of uninterrupted overland flow could be larger than is suggested by them. Secondly, erosion caused directly by logging and rip-ploughing operations is not represented in the plot losses but could well far outweigh them. Although manual logging of the type that occurs in the Agueda Basin causes little on-site erosion (Lal, 1981), excavating and using forest roads can increase soil losses considerably (Reid and Dunne, 1984; Trimble, 1988). More importantly, the rip-ploughing process itself causes considerable downslope soil transfer. Thirdly, little is known about commercial forest soil renewal rates. Estimates for natural conditions range from, for example,
Soil conservation and forest wildfires in Portugal: R A Shakesby et al
(FAO, 1979; Kardell et al., 1986), so that ground preparation involving uprooting the old stumps and planting of seedlings would be needed at least every 30-40 years. If this involved downslope rip-ploughing, then losses would be likely to be an order of magnitude higher than fire-induced losses in the first year (Coelho et al., 1995a). Even with negligible soil renewal rates, these fire-induced soil losses represent net ground lowering after fire of little more than l-2 mm (assuming 15 t ha-’ represents 1 mm of lowering). (Downslope rip-ploughing every 3-4 clear-felling cycles would add about another 3-4 mm of lowering over a 30-40-year period.) These amounts do not appear to present a medium-term threat of substantial soil depth reduction. However, such losses may be highly detrimental to soil quality for the following reasons. First, the sediment removed is not only rich in fines but also in organic matter and nutrients. Whilst losses of nitrogen and potassium can be expected to be replenished relatively quickly from the air and bedrock respectively, the same is not true of phosphorus. Thomas (1996) estimates that, for a 3-cm-thick soil at the burnt eucalyptus site, 55 per cent of available phosphorus was removed in just two years after the August 1992 fire. Even if soil depth were 10 cm, calculations show that 19 per cent of available phosphorus would be removed. Since weathering of schist yields little or no phosphorus, successive fires and accompanying soil losses on thin schist soils in the Agueda Basin would severely deplete them of this vital nutrient for plant growth. Secondly, preferential loss of the water-retaining fines and organic matter (Coelho et al., 1995a; Thomas, 1996) would make establishing eucalyptus seedlings difficult.
Conclusions Simple post-fire management practices can limit soil losses substantially in the wet pine and eucalyptus forests of north-central Portugal, but attaining worthwhile soil loss reduction with minimal disruption to established post-fire forest practices relies on an appreciation of the quantities, soil conservational qualities and timing of litter availability of different tree species. Thus for eucalyptus, clear-felling after fire is best carried out as soon as possible and litter (particularly bark) should be left on-site. Conversely, for pine, where litter quantities are typically much less and the rigid twigs and branches are comparatively ineffective at reducing soil losses (and harbour pests), the best feasible post-fire management is to delay logging until needlefall from the scorched tree canopy has occurred. The resulting needle ‘carpet’ is highly effective at reducing soil loss by some 90 per cent. Whilst leaving large amounts of eucalyptus litter on site can provide good soil protection, it conflicts directly with fire prevention and pest control regulations. As a compromise, since the bark is the most effective part of the litter at limiting erosional losses and is comparatively rich in nutrients, existing regulations should be modified from removing all litter (bark and timber) to removing just the timber component. Minimum tillage is a far less erosive ground preparation method for planting eucalyptus seedlings than rip-ploughing, but its applicability is questionable: it could only be used to plant seedlings in pine areas (as pine does not regrow from sawn stumps), unless eucalyptus stumps can be killed without having to uproot them by rip-ploughing. Despite near-homogeneity as regards slope angle, soil texture and soil depth at each site, soil loss and overland flow at the plot scale varied as much because of inherent plot variations as treatment impacts. Sufficient plot replication to eliminate this effect was impractical. The solution adopted here, which was to view the response of untreated plots during the post-treatment compared with pre-treatment phase as the expected behaviour of plots for the rainfall patterns experienced, and to compare treated plot changes with these responses, was largely successful. It worked particularly well for eucalyptus plots, where
Soil conservation
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treatment impact was set against a rising trend in soil losses at the untreated plots, but was less successful for pine plots where, because of more advanced post-fire vegetation regrowth, soil losses declined sharply in the post-treatment phase, despite higher rainfall. Scaling up rates of soil loss from the plot data, even for worst-case scenarios, does not suggest imminent, serious soil degradation, but this may be misleading for three reasons. First, these data do not tally with substantial ground lowering rates measured previously in middle and footslope locations, where overland flow was unimpeded by plot boundaries. Secondly, the pine plot results relate to thicker and potentially, therefore, more easily eroded soils than at the eucalyptus site but do not include the critical early post-fire months when losses might have been very large. Thirdly, erosion selectively removes organic-rich fines containing important nutrients, notably phosphorus, which is not easily replaced by weathering.
Acknowledgements We thank particularly Antonio J D Ferreira, Stefan Doerr and Andrew D Thomas for invaluable assistance with fieldwork, data collation and discussion of results. We thank Guy Lewis and Nicola Jones for drawing the diagrams. Financial support was provided by an EU research contract (EV5V-0041) and we thank Dr P Balabanis and Mr D Peter of the European Commission for their interest and support. We also thank two anonymous referees for their very helpful comments.
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