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Trade-offs between seedling growth, thinning and stand stability in Sitka spruce stands: a modelling analysis
Forest Ecology and Management 187 (2004) 105–115
Trade-offs between seedling growth, thinning and stand stability in Sitka spruce stands: a modelling analysis Sophie E. Halea,*, Peter E. Levyb, Barry A. Gardinera a
b
Forest Research, Northern Research Station, Roslin, Midlothian EH25 9SY, UK Centre for Ecology and Hydrology, Bush Estate, Penicuik, Midlothian EH26 0QB, UK
Received 3 September 2002; received in revised form 16 June 2003; accepted 16 June 2003
Abstract The forest industry is increasingly adopting alternative silvicultural systems, involving regeneration beneath an existing forest canopy, rather than clear-felling and replanting. To apply these silvicultural systems in windy regions such as Britain and Ireland, it is essential that the interactions between thinning intensity, stand stability and seedling growth are properly understood. Here, we present a modelling analysis of the three key relationships between: (i) stand density and the proportion of incident radiation transmitted through a forest canopy as a stand is thinned; (ii) transmitted radiation and seedling growth, and (iii) stand density and stand stability. These relationships were examined using separate models of radiative transfer (MAESTRO), seedling growth, and stand stability/wind risk (ForestGALES). Output from the three models was synthesised to calculate whether a given stand thinned to a pre-defined stability limit would allow sufficient light to penetrate the canopy for seedling growth. A minimum transmittance of 20% was identified as a requirement for seedling growth, which corresponds to removing 45% of stand basal area. A thinning of this intensity left some stands susceptible to unacceptable wind damage, especially in old or previously thinned stands on soils where rooting is impeded. The results emphasised the fact that rooting conditions, thinning history and age of intervention are major constraints on the silvicultural options. In general, older stands are not suitable for conversion to continuous cover forestry (CCF) systems, and the transformation process should begin at pole stage, when heavy thinning does not leave the stand unstable. The analysis approach used here illustrates the potential for combining models to address complex forest management issues. # 2003 Elsevier B.V. All rights reserved. Keywords: Continuous cover forestry; Transformation; Wind risk; Seedling growth
1. Introduction The forest industry in Britain has traditionally relied on the patch clearfelling silvicultural system combined with artificial regeneration (McIntosh, 1995). However, recently there has been growing interest in continuous cover forestry (CCF) management strategies, which do *
Corresponding author. Tel.: þ44-131-445-2176; fax: þ44-131-445-5124. E-mail address: [email protected] (S.E. Hale).
not have the same impacts on the landscape, and may provide ecological benefit from long-term woodland cover. These systems generally involve seedling establishment beneath an existing canopy, thereby retaining continuous forest cover on the site. Whether establishment is achieved by natural regeneration or by planting, conditions beneath the canopy must be suitable for seedling survival and growth. One crucial issue in CCF is the trade-off between success of seedling establishment and windthrow risk, which arises when stand density is reduced by
0378-1127/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0378-1127(03)00313-X
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thinning. Reducing the stand density results in more solar radiation reaching the forest floor, allowing the growth of natural regeneration or planted stock. However, when the canopy is opened up, the risk of windthrow is increased (Gardiner et al., 1997), and it is important to be able to understand and predict these trade-offs if we are to develop CCF on a scientific basis. This risk may increase over the next century if high winds become more frequent as a result of climatic change (Quine and Gardiner, 2002). Sitka spruce (Picea sitchensis (Bong.) Carr.) is the main commercial species in Britain, and comprises 29% of the forest area in Britain (S. Smith, Forest Research, pers. comm.). Previous work in Sitka spruce plantations has quantified the radiation regime beneath the canopy, and how this changes as the stand is thinned (Hale, 2001, 2003). The aim of this study was to quantify the trade-offs between seedling establishment and windthrow risk which result from changing stand density in a Sitka spruce plantation. Specifically, we wished to determine whether substantial growth of seedlings can be expected beneath stands that are stable, and secondly, whether the effects trade-off linearly or an optimum stand density exists. There were three stages in the analysis: (i) a seedling growth model was used to estimate the transmittance required for seedling growth beneath a forest canopy, (ii) field data from a thinning trial and a 3D radiative transfer model were used to predict the proportion of incident radiation transmitted through a forest canopy as the stand was thinned, (iii) ForestGALES (Dunham et al., 2000), a forest stand wind risk model, was used to calculate whether a stand thinned to a pre-defined stability limit (defined as a damaging wind speed with a return period of 10 years) would allow sufficient light to penetrate the canopy for seedling growth.
carbon throughout the seedling is driven by internal concentration gradients. In this study, the model was run for 1-year-old Sitka spruce seedlings, with initial weight of 20 g, using 1996 meteorological data from stations in north-east Scotland (Aboyne: 57850 N, 28500 W), south Scotland (Drumalbin: 558370 N, 38450 W), north Wales (Capel Curig: 53860 N, 38560 W) and south-east England (London: 518300 N, 0870 W). It was rerun a number of times, reducing the incident radiation in the input file from 100 to 0%, to simulate the reduction in radiation received by seedlings beneath the canopy.
2. Methods
L ¼ 3:96d 29:92
2.1. Seedling growth
We chose to simulate the stands with the highest and lowest basal areas in Table 2 (see Section 2.3), and in combination with the two extreme assumptions concerning leaf area, we should thereby encompass the entire range of relationships between t and B found within the stands.
The seedling growth model (Levy et al., 2000) is driven by radiation, temperature and humidity. These determine stomatal conductance and photosynthesis, and thus the carbon uptake of the leaves. Movement of
2.2. Below-canopy radiation environment Two methods were used to estimate the canopy transmittance of radiation in the photosynthetic waveband, t, from stand characteristics. In a thinning experiment in a Scottish Sitka spruce plantation in the stem exclusion phase (sensu Oliver and Larson, 1990), Hale (2003) derived a relationship between t and the residual basal area, B, expressed as a percentage of that in the original, unthinned stand: t ¼ expð0:029 BÞ
(1)
Secondly, a 3D radiative transfer model ‘‘MAESTRO’’ (Wang and Jarvis, 1990) was used to predict t as a function of tree height, leaf area and spacing. The model represents canopies as an array of cones with a specified leaf area density, which may absorb, transmit and scatter both direct and diffuse radiation. Radiation reaching an array of 48 points on the ground was used to calculate t. Two contrasting assumptions concerning leaf area were made: (i) initial leaf area index (LAI) of all stands was 9.0, which represents typical maximum LAI values for Sitka spruce stands (Whitehead et al., 1984; Milne et al., 1985; Jarvis, 1994) or (ii) tree leaf area, L (m2), was proportional to stem diameter, d (cm), according to the equation of McIntosh (1984): (2)
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2.3. ForestGALES ForestGALES is a PC-based model that predicts the wind risk to British forests as a function of stand, site and windiness characteristics (Dunham et al., 2000; Gardiner et al., 2000). The forest stand is specified in terms of species, stocking, top height and mean diameter of the trees. The site is represented by soil type, cultivation and drainage variables, which affect the rooting, and therefore the risk of the trees overturning. The windiness is expressed as detailed aspect method of scoring (DAMS; Bell et al., 1995), which incorporates the prevailing wind climate and the topography to describe the exposure of a site. In Britain, DAMS scores range from 7 (e.g. sheltered valley bottom) to about 29 (e.g. exposed mountain top). ForestGALES calculates separate probabilities for stem breakage and the overturning of the whole tree. For the purposes of this study, wind risk was defined as whichever was higher, and acceptable wind risk was defined as a return period of 10 years or more (Mason and Kerr, 2001).
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Table 1 Summary of the different silvicultural and environmental scenarios used in the stand simulations (initial spacing was 2.0 m in all cases) Variable
Scenario
Yield class DAMS Rooting depth
12, 16, 20 10, 14, 18 Deep (brown earth, notch planting), shallow (gley, deep ploughing) 20, 30, 45, 60 Intermediate thinning, no thinning 100, 90, 80, . . ., 20, 10
Age (years) Previous management Basal area (%)
medium and high yield stands were simulated (yield class: 12, 16 and 20, respectively), based on an initial spacing of 2 m. Each stand was assumed to have conformed to standard management as in the yield tables, with either no thinning or intermediate thinning until a given age throughout a typical rotation period. At this time (age: 20, 30, 45 or 60 years) the stand was subjected to a single heavy thinning. This was simulated by reducing stocking and basal area from 100 to 10% in 10% decrements. Table 2 give details of stand characteristics at the time of thinning.
2.4. Silvicultural factors 2.5. Environmental factors A number of stand scenarios were simulated (Table 1), based on the British forest management yield tables (Edwards and Christie, 1981). Low,
Three windiness scenarios were chosen, representing sheltered, moderately exposed and very exposed
Table 2 Characteristics of the simulated stands, pre-thinning: yield class (m3 ha1 yr1), age (years), top height (m), average tree height (m), average diameter at 1.3 m (cm), stocking (trees per hectare) and basal area (m2 ha1) Yield class
12 12 12 12 16 16 16 16 20 20 20 20
Age
20 30 45 60 20 30 45 60 20 30 45 60
Top height
7.3 12.5 19.2 23.7 10.8 16.8 24.1 28.7 11.7 18.8 27.0 32.2
Average height
5.6 11.2 18.3 23.1 9.3 15.7 23.5 28.4 10.3 17.9 26.6 32.1
No previous thinning
Previous intermediate thinning
Average diameter
Stocking
Basal area
Average diameter
Stocking
Basal area
11 16 22 26 15 20 26 31 15 21 29 34
2309 2123 1547 1209 2232 1734 1203 995 2193 1551 1061 906
24 43 58 65 37 53 66 74 41 56 71 82
11 18 29 36 16 25 37 45 17 28 42 52
2309 1057 571 401 1251 651 368 274 1064 521 286 212
24 28 36 40 24 32 39 43 24 32 39 44
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sites (DAMS 10, 14 and 18, respectively). Soil type, cultivation and drainage affect the stability of a tree with respect to overturning (Fraser and Gardiner, 1967; Nicoll and Ray, 1996; Ray and Nicoll, 1998). All modelled stands were analysed for good rooting conditions (brown earth with notch planting) and poor rooting conditions (gley soil with deep ploughing). Note that the risk of breakage is the same regardless of the rooting depth: it is only the risk of overturning that is affected.
3. Results 3.1. Seedling growth model Fig. 1a shows the output from the seedling growth model. Differences between the four sites were small. It can be seen that below 5% of incident radiation there is no growth, and seedling death is predicted to occur. Above 5–10% transmittance, seedling growth increases non-linearly to almost 60 g in full radiation.
0.08 Modelled Growth (kg y-1)
(a) 0.06
0.04
0.02
0.00 0.0
0.2
0.4
0.6
0.8
1.0
14 WR
(b) Diameter Growth (mm y-1)
12 10
WH
8
SF HS
6 4 LP 2 0 0.0
0.2
0.4 0.6 Canopy transmittance
0.8
1.0
Fig. 1. (a) Modelled growth of 1-year-old Sitka spruce seedlings as a function of proportion of incident radiation transmitted through a canopy, over the year for which the model was run, using meteorological input data from Aboyne (*), Capel Curig (*), Drumalbin (!) and London (5). (b) Observed variation of seedling growth with canopy transmittance for five species in British Colombia. WR: western red cedar [Thuja plicata Donn.]; WH: western hemlock [Tsuga heterophylla (Raf.) Sarg.]; SF: subalpine fir [Abies lasiocarpa (Hook.) Nutt.]; HS: hybrid spruce [Picea glauca (Moench) Voss: P: sitchensis (Bong.) Carr.]; LP: lodgepole pine [Pinus contorta var latifolia] (Coates and Burton, 1999).
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1.0
transmittance
0.8
0.6
0.4
0.2
0.0 0
20
40
60
80
100
Stand basal area (% original) Fig. 2. Relationship between transmittance and percentage of original basal area derived by Hale (2003) (&), and from the MAESTRO simulations for the stands with the highest and lowest basal areas (yield class 20 at age 60 and yield class 12 at age 20, respectively). LAI was assumed to vary with either the basal area relative to that of the unthinned stand (rel), or the absolute basal area (abs). (*) YC12, age 20, rel; (*) YC20, age 60, rel; (!) YC12, age 20, abs; (5) YC20, age 60, abs.
From these curves, we have arbitrarily delimited a transmittance value of around 20% as necessary to obtain seedling growth beneath a forest canopy. At this level, the seedlings achieve about half of the growth possible in full sunlight. According to Eq. (1), 20% transmittance is equivalent to removing about 45% of the basal area in a neutral uniform thinning (Fig. 2). This is about twice as much as current recommendations, which suggest that 20% is the maximum basal area to be removed if significant wind damage is to be prevented (Mason and Kerr, 2001). 3.2. Below-canopy radiation environment Fig. 2 shows the relationship between t and B derived by Hale (2003), together with MAESTRO simulations of the expected extreme cases. The MAESTRO simulations are not very sensitive to the assumptions concerning tree leaf area, and show that t is influenced more by canopy structure, i.e. whether the trees are large and widely spaced (YC20, age 60) or small and densely packed (YC12, age 20). However, given that these are the extreme cases, where the
initial basal areas range from 22 to 82 m2 ha1 (with initial LAIs ranging from 3.8 to 9.8), the relationship is surprisingly conservative. Furthermore, given that the canopy structure of actual stands shows considerable variation around the values given in the yield tables, we consider the simple equation of Hale (2003) to give a reasonable estimation of t for the purposes of this study. 3.3. ForestGALES runs Fig. 3 shows the output from the seedling growth model together with that from ForestGALES for four stands covering a range of stabilities. The stand in case (i) is stable even with the heaviest thinning, and so wind damage need not be an important consideration in management. In case (ii), the stand is acceptably stable when thinned to approximately 40% of the original basal area, where seedling growth is possible. Seedling growth and the risk of wind damage trade-off linearly against each other where B is 40–60%. In case (iii), stand stablility declines linearly with thinning but no seedling growth is achieved before the stand
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0.06 200
Seedling Growth (kg)
0.05
i
ii
iii 150
0.04
0.03
100
0.02
Wind damage return period (years)
250
0.07
50 0.01 iv 0.00
0 0
20
40
60
80
100
Stand basal area (% original) Fig. 3. The return period for wind damage, calculated using ForestGALES, for four stands at a range of stand densities representing different thinnings (basal area 100% down to 10%). The modelled increase in annual seedling growth as stand density is reduced is also shown (average of the four locations shown in Fig. 1). (i) Yield class (YC)12, no previous thinning, thinned at age 20, DAMS 18; (ii) YC12, no previous thinning, thinned at age 45, DAMS 14; (iii) YC20, previous intermediate thinning, thinned at age 20, DAMS 18; (iv) YC16, previous intermediate thinning, thinned at age 32, DAMS 18.
reaches the acceptable stability limit. Initiating transformation by implementing a heavy thinning in this stand would not be a sensible management strategy. In case (iv), the stand is in danger of wind damage even without thinning, and no intervention would be recommended. Note that Fig. 3 represents conditions immediately after thinning. In subsequent years, as the trees grow and the crowns develop, one would expect the wind risk and the transmitted light will change. The transmittance achieved by thinning each modelled stand to the chosen stability limit (10-year return period) was obtained by relating Eq. (1) with the residual basal area at the stability limit. Figs. 4 and 5 show these transmittance values for different combinations of DAMS, timing of thinning, yield class, management regime and rooting scenario. The bold line in each plot marks a transmittance of 0.2, the minimum required for seedling growth. Colours darker than this indicate lower levels of transmittance, where the wind risk is higher so the stand cannot be so heavily thinned. Lighter colours indicate that the wind risk to the stand is less, permitting a heavier thinning, resulting in a higher
transmittance and greater seedling growth. The following points can be drawn from the graphs: (i) The dark areas are in the upper right portion of the plots, indicating that the risk of wind damage increases with age and with DAMS (i.e. as the trees get taller or the site becomes more exposed). This can be seen in all of the 12 plots, e.g. for a yield class 16 stand with no previous thinning and good rooting, at age 40 it could be heavily thinned on a DAMS 14 site, but not a DAMS 18 site; in contrast, at age 50 heavy thinning is no longer possible on a DAMS 14 site. (ii) For a given management regime and rooting depth, the risk increases with yield class, e.g. a yield class 12 stand with no previous thinning on a moderately exposed site could be heavily thinned up to age 60 years, but at higher yield classes stability would become limiting at age 40–45 years. This is because at low yield classes the trees are shorter at a given age, and are therefore less prone to wind damage.
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Previously unthinned
DAMS
YC12
DAMS
YC16
Previous intermediate thinning
18
18
14
14
10 20
30
40
50
60
10 20
18
18
14
14
10 20
30
40
50
111
60
10 20
18
18
14
14
30
40
50
60
30
40
50
60
30
40
50
60
DAMS
YC20
10 20
30
40
50
Age at thinning (years)
60
10 20
Age at thinning (years)
Fig. 4. Transmittance obtained when simulated stands of different age and DAMS were thinned to the stability limit (return period for damage of 10 years) on freely draining soils with good rooting conditions. The bold line (0.2) indicates the minimum transmittance required for seedling growth. For the sites with no previous thinning, failure at the stability limit was by breakage; for the previously thinned sites the risk of breakage and overturning were similar.
(iii) Previously unthinned stands on the sites with good rooting have a much greater risk of breakage than of overturning. In contrast, on sites with impeded rooting, the risk of overturning is slightly greater than that of breakage, because of the shallower rooting depth. The overall risk of damage is not much different on the two soil types, with a heavy intervention
possible up to age 40 on moderately exposed sites (later for low yield class stands). (iv) For previously thinned stands on sites with good rooting, the risk of overturning and breaking are approximately the same. These stands are the most stable of the simulated stands, with heavy thinning possible for all except the highest DAMS and yield class. In contrast, previously thinned stands on
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Previously unthinned
DAMS
YC12
Previous intermediate thinning
18
18
14
14
10 20
30
40
50
60
10 20
18
18
14
14
30
40
50
60
30
40
50
60
30
40
50
60
DAMS
YC16
10 20
30
40
50
60
10 20
18
18
14
14
DAMS
YC20
10 20
30
40
50
Age at thinning (years)
60
10 20
Age at thinning (years)
Fig. 5. Transmittance obtained when simulated stands of different age and DAMS were thinned to the stability limit (return period for damage of 10 years) on soils with impeded rooting depth. The bold line (0.2) indicates the minimum transmittance required for seedling growth. Failure at the stability limit in all cases was by overturning.
sites with poor rooting are the most unstable of the stands simulated, with a high risk of overturning.
4. Discussion 4.1. Implications for stand management The results from this study have implications for whether a particular stand is suitable for conversion to
CCF, or for managing a stand under CCF for maximum stability. Figs. 4 and 5 highlight the fact that there are more silvicultural options for stand management on sites with freely draining soils with good rooting depth than on sites where impeded rooting occurs, with prior thinning making the stand less prone to wind damage during conversion to CCF. The soil type and drainage must therefore be considered as possible constraints to silviculture from an early stage in stand management; this can be facilitated
S.E. Hale et al. / Forest Ecology and Management 187 (2004) 105–115
by assessing the soil moisture regime and rooting depth using ecological site classification (ESC; Pyatt et al., 2001). Also, older stands are more prone to wind damage after a heavy thinning than younger stands. It may therefore not be practical to convert a stand towards the end of its rotation to CCF; rather, manipulation should begin at pole stage, ideally before age 25 years. Although an early thinning may have low economic value, it may improve the long-term stability and timber quality of the stand (Cameron, 2002). In general, previously unthinned stands are at greater risk of wind damage, and therefore less versatile in terms of CCF management, than stands with a history of thinning. The exception to this in the results presented here is on the sites with impeded rooting, where for older stands and those in high yield classes previously thinned stands are more at risk from wind damage (overturning) after a heavy thinning than the stands with no previous thinning. This is because the resistance of a tree to breakage increases with the cube of diameter, whereas the resistance to overturning increases only with the square of diameter (Gardiner et al., 1997). The wind loading on an individual tree increases with spacing. Previously thinned stands have lower stocking and larger diameter trees than the unthinned stands (Table 2). The increase in force on each tree due to the wider spacing is greater than the increased resistance to overturning, therefore, the previously thinned stands are more unstable than the more densely stocked unthinned stands. On deep-rooted sites, the large diameter, widely spaced trees are more stable than the smaller diameter trees in an unthinned stand, because the increase in wind loading due to the wider spacing is more than compensated for by the increased resistance to breakage. On freely draining soils, therefore, a stand should be thinned early to allow the remaining trees to increase in diameter. When the stand is mature enough to produce seed, the trees will be sufficiently stable to tolerate a heavy thinning, so the light environment can be modified to encourage natural regeneration. On a site with impeded rooting, opening up the stand to encourage diameter growth will leave the trees more prone to overturning. Sitka spruce is thought to be intermediate in terms of shade tolerance (Malcolm et al., 2001). Seedlings of more light-demanding species would require more than 20% of incident radiation to be transmitted
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through the canopy, whereas more shade-tolerant species could grow beneath a canopy transmitting less than 20%. The implications for stability of thinning a Sitka spruce overstorey to achieve these light levels can be seen from Figs. 4 and 5, by reading to either side of the bold line. Obviously, if a more shadetolerant species is desired then a lower thinning intensity is required, and can be carried out later, or on a windier site, without compromising the stability of the stand. 4.2. Model limitations The seedling growth model has been validated elsewhere against experimental data (Levy et al., 2000), but no comparable field data exist for Sitka spruce in Britain. However, Fig. 1b shows curves fitted to the observations of Coates and Burton (1999) on seedlings of five species growing in north-central British Columbia (which, due to similarity in latitude and proximity to the coast, has a similar climate and radiation regime to Britain). Although there is interspecific variability, these show response curves of a similar form, with zero growth occurring at 5–15% of incident radiation. In this study, the model was run only for 1 year on 1-year-old seedlings. It may not, therefore, adequately describe the growth of larger seedlings or saplings, as there is evidence that as seedlings grow they require increased light levels (Messier et al., 1999). The limitations of the transmittance vs. basal area model are discussed in Hale (2003). In summary, thinning patterns other than a uniform neutral thinning may produce a different relationship between residual basal area and transmittance. Also, stands at stages later than stem exclusion may have higher transmittance to start with and, therefore, require a less severe thinning to achieve a given light level. The modelled light levels are those obtained immediately after thinning, and do not consider the response of the stand over time, in terms of canopy closure or stability. ForestGALES calculates the wind risk for the ‘mean tree’ in a stand, whereas in reality the larger trees in a stand are more at risk of damage. Also, the old, high yield class stands, and the very widely spaced stands simulated in this study are outhwith the bounds of the data set used to parameterise ForestGALES. As shown in Fig. 3, as a stand is thinned the stability
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decreases. However, in the 45- and 60-year-old stands it was common for the stand to start becoming more stable as the thinning intensity increased (spacing/ height ratios of more than 0.35 and 0.5, respectively). Insufficient work has been done in such widely spaced stands to know whether this increase in stability predicted by the model is realistic.
5. Conclusions This paper has used a radiative transfer model, a seedling growth model, and a wind risk model to address the trade-off between seedling growth and stand stability in the conversion of even-aged Sitka spruce plantations to CCF systems. Results are not meant to be prescriptive, but illustrate the relative importance of different environmental and silvicultural factors, and the nature of the interactions. The results demonstrate the importance of age of intervention and of soil properties and show that, generally, seedling growth and the risk of wind damage trade-off linearly against each other. Models are increasingly used to assist decision-making concerning forestry and land use, and here three models are combined to address a complex forest management issue. As managers are required to achieve wide-ranging and potentially conflicting goals (e.g. natural regeneration, stability, browsing control, sustainability, timber quality, economic return), the use of combined models will be a valuable approach in the future.
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Ray, D., Nicoll, B.C., 1998. The effect of soil water-table depth on root plate development and stability of Sitka spruce. Forestry 71, 169–182. Wang, Y.P., Jarvis, P.G., 1990. Description and validation of an array model—MAESTRO. Agric. For. Meteorol. 51, 257–280. Whitehead, D., Edwards, W.R.N., Jarvis, P.G., 1984. Conducting sapwood area, foliage area, and permeability in mature trees of Picea sitchensis and Pinus contorta. Can. J. For. Res. 14, 940–947.
Trade-offs between seedling growth, thinning and stand stability in Sitka spruce stands: a modelling analysis Sophie E. Halea,*, Peter E. Levyb, Barry A. Gardinera a
b
Forest Research, Northern Research Station, Roslin, Midlothian EH25 9SY, UK Centre for Ecology and Hydrology, Bush Estate, Penicuik, Midlothian EH26 0QB, UK
Received 3 September 2002; received in revised form 16 June 2003; accepted 16 June 2003
Abstract The forest industry is increasingly adopting alternative silvicultural systems, involving regeneration beneath an existing forest canopy, rather than clear-felling and replanting. To apply these silvicultural systems in windy regions such as Britain and Ireland, it is essential that the interactions between thinning intensity, stand stability and seedling growth are properly understood. Here, we present a modelling analysis of the three key relationships between: (i) stand density and the proportion of incident radiation transmitted through a forest canopy as a stand is thinned; (ii) transmitted radiation and seedling growth, and (iii) stand density and stand stability. These relationships were examined using separate models of radiative transfer (MAESTRO), seedling growth, and stand stability/wind risk (ForestGALES). Output from the three models was synthesised to calculate whether a given stand thinned to a pre-defined stability limit would allow sufficient light to penetrate the canopy for seedling growth. A minimum transmittance of 20% was identified as a requirement for seedling growth, which corresponds to removing 45% of stand basal area. A thinning of this intensity left some stands susceptible to unacceptable wind damage, especially in old or previously thinned stands on soils where rooting is impeded. The results emphasised the fact that rooting conditions, thinning history and age of intervention are major constraints on the silvicultural options. In general, older stands are not suitable for conversion to continuous cover forestry (CCF) systems, and the transformation process should begin at pole stage, when heavy thinning does not leave the stand unstable. The analysis approach used here illustrates the potential for combining models to address complex forest management issues. # 2003 Elsevier B.V. All rights reserved. Keywords: Continuous cover forestry; Transformation; Wind risk; Seedling growth
1. Introduction The forest industry in Britain has traditionally relied on the patch clearfelling silvicultural system combined with artificial regeneration (McIntosh, 1995). However, recently there has been growing interest in continuous cover forestry (CCF) management strategies, which do *
Corresponding author. Tel.: þ44-131-445-2176; fax: þ44-131-445-5124. E-mail address: [email protected] (S.E. Hale).
not have the same impacts on the landscape, and may provide ecological benefit from long-term woodland cover. These systems generally involve seedling establishment beneath an existing canopy, thereby retaining continuous forest cover on the site. Whether establishment is achieved by natural regeneration or by planting, conditions beneath the canopy must be suitable for seedling survival and growth. One crucial issue in CCF is the trade-off between success of seedling establishment and windthrow risk, which arises when stand density is reduced by
0378-1127/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0378-1127(03)00313-X
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thinning. Reducing the stand density results in more solar radiation reaching the forest floor, allowing the growth of natural regeneration or planted stock. However, when the canopy is opened up, the risk of windthrow is increased (Gardiner et al., 1997), and it is important to be able to understand and predict these trade-offs if we are to develop CCF on a scientific basis. This risk may increase over the next century if high winds become more frequent as a result of climatic change (Quine and Gardiner, 2002). Sitka spruce (Picea sitchensis (Bong.) Carr.) is the main commercial species in Britain, and comprises 29% of the forest area in Britain (S. Smith, Forest Research, pers. comm.). Previous work in Sitka spruce plantations has quantified the radiation regime beneath the canopy, and how this changes as the stand is thinned (Hale, 2001, 2003). The aim of this study was to quantify the trade-offs between seedling establishment and windthrow risk which result from changing stand density in a Sitka spruce plantation. Specifically, we wished to determine whether substantial growth of seedlings can be expected beneath stands that are stable, and secondly, whether the effects trade-off linearly or an optimum stand density exists. There were three stages in the analysis: (i) a seedling growth model was used to estimate the transmittance required for seedling growth beneath a forest canopy, (ii) field data from a thinning trial and a 3D radiative transfer model were used to predict the proportion of incident radiation transmitted through a forest canopy as the stand was thinned, (iii) ForestGALES (Dunham et al., 2000), a forest stand wind risk model, was used to calculate whether a stand thinned to a pre-defined stability limit (defined as a damaging wind speed with a return period of 10 years) would allow sufficient light to penetrate the canopy for seedling growth.
carbon throughout the seedling is driven by internal concentration gradients. In this study, the model was run for 1-year-old Sitka spruce seedlings, with initial weight of 20 g, using 1996 meteorological data from stations in north-east Scotland (Aboyne: 57850 N, 28500 W), south Scotland (Drumalbin: 558370 N, 38450 W), north Wales (Capel Curig: 53860 N, 38560 W) and south-east England (London: 518300 N, 0870 W). It was rerun a number of times, reducing the incident radiation in the input file from 100 to 0%, to simulate the reduction in radiation received by seedlings beneath the canopy.
2. Methods
L ¼ 3:96d 29:92
2.1. Seedling growth
We chose to simulate the stands with the highest and lowest basal areas in Table 2 (see Section 2.3), and in combination with the two extreme assumptions concerning leaf area, we should thereby encompass the entire range of relationships between t and B found within the stands.
The seedling growth model (Levy et al., 2000) is driven by radiation, temperature and humidity. These determine stomatal conductance and photosynthesis, and thus the carbon uptake of the leaves. Movement of
2.2. Below-canopy radiation environment Two methods were used to estimate the canopy transmittance of radiation in the photosynthetic waveband, t, from stand characteristics. In a thinning experiment in a Scottish Sitka spruce plantation in the stem exclusion phase (sensu Oliver and Larson, 1990), Hale (2003) derived a relationship between t and the residual basal area, B, expressed as a percentage of that in the original, unthinned stand: t ¼ expð0:029 BÞ
(1)
Secondly, a 3D radiative transfer model ‘‘MAESTRO’’ (Wang and Jarvis, 1990) was used to predict t as a function of tree height, leaf area and spacing. The model represents canopies as an array of cones with a specified leaf area density, which may absorb, transmit and scatter both direct and diffuse radiation. Radiation reaching an array of 48 points on the ground was used to calculate t. Two contrasting assumptions concerning leaf area were made: (i) initial leaf area index (LAI) of all stands was 9.0, which represents typical maximum LAI values for Sitka spruce stands (Whitehead et al., 1984; Milne et al., 1985; Jarvis, 1994) or (ii) tree leaf area, L (m2), was proportional to stem diameter, d (cm), according to the equation of McIntosh (1984): (2)
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2.3. ForestGALES ForestGALES is a PC-based model that predicts the wind risk to British forests as a function of stand, site and windiness characteristics (Dunham et al., 2000; Gardiner et al., 2000). The forest stand is specified in terms of species, stocking, top height and mean diameter of the trees. The site is represented by soil type, cultivation and drainage variables, which affect the rooting, and therefore the risk of the trees overturning. The windiness is expressed as detailed aspect method of scoring (DAMS; Bell et al., 1995), which incorporates the prevailing wind climate and the topography to describe the exposure of a site. In Britain, DAMS scores range from 7 (e.g. sheltered valley bottom) to about 29 (e.g. exposed mountain top). ForestGALES calculates separate probabilities for stem breakage and the overturning of the whole tree. For the purposes of this study, wind risk was defined as whichever was higher, and acceptable wind risk was defined as a return period of 10 years or more (Mason and Kerr, 2001).
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Table 1 Summary of the different silvicultural and environmental scenarios used in the stand simulations (initial spacing was 2.0 m in all cases) Variable
Scenario
Yield class DAMS Rooting depth
12, 16, 20 10, 14, 18 Deep (brown earth, notch planting), shallow (gley, deep ploughing) 20, 30, 45, 60 Intermediate thinning, no thinning 100, 90, 80, . . ., 20, 10
Age (years) Previous management Basal area (%)
medium and high yield stands were simulated (yield class: 12, 16 and 20, respectively), based on an initial spacing of 2 m. Each stand was assumed to have conformed to standard management as in the yield tables, with either no thinning or intermediate thinning until a given age throughout a typical rotation period. At this time (age: 20, 30, 45 or 60 years) the stand was subjected to a single heavy thinning. This was simulated by reducing stocking and basal area from 100 to 10% in 10% decrements. Table 2 give details of stand characteristics at the time of thinning.
2.4. Silvicultural factors 2.5. Environmental factors A number of stand scenarios were simulated (Table 1), based on the British forest management yield tables (Edwards and Christie, 1981). Low,
Three windiness scenarios were chosen, representing sheltered, moderately exposed and very exposed
Table 2 Characteristics of the simulated stands, pre-thinning: yield class (m3 ha1 yr1), age (years), top height (m), average tree height (m), average diameter at 1.3 m (cm), stocking (trees per hectare) and basal area (m2 ha1) Yield class
12 12 12 12 16 16 16 16 20 20 20 20
Age
20 30 45 60 20 30 45 60 20 30 45 60
Top height
7.3 12.5 19.2 23.7 10.8 16.8 24.1 28.7 11.7 18.8 27.0 32.2
Average height
5.6 11.2 18.3 23.1 9.3 15.7 23.5 28.4 10.3 17.9 26.6 32.1
No previous thinning
Previous intermediate thinning
Average diameter
Stocking
Basal area
Average diameter
Stocking
Basal area
11 16 22 26 15 20 26 31 15 21 29 34
2309 2123 1547 1209 2232 1734 1203 995 2193 1551 1061 906
24 43 58 65 37 53 66 74 41 56 71 82
11 18 29 36 16 25 37 45 17 28 42 52
2309 1057 571 401 1251 651 368 274 1064 521 286 212
24 28 36 40 24 32 39 43 24 32 39 44
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sites (DAMS 10, 14 and 18, respectively). Soil type, cultivation and drainage affect the stability of a tree with respect to overturning (Fraser and Gardiner, 1967; Nicoll and Ray, 1996; Ray and Nicoll, 1998). All modelled stands were analysed for good rooting conditions (brown earth with notch planting) and poor rooting conditions (gley soil with deep ploughing). Note that the risk of breakage is the same regardless of the rooting depth: it is only the risk of overturning that is affected.
3. Results 3.1. Seedling growth model Fig. 1a shows the output from the seedling growth model. Differences between the four sites were small. It can be seen that below 5% of incident radiation there is no growth, and seedling death is predicted to occur. Above 5–10% transmittance, seedling growth increases non-linearly to almost 60 g in full radiation.
0.08 Modelled Growth (kg y-1)
(a) 0.06
0.04
0.02
0.00 0.0
0.2
0.4
0.6
0.8
1.0
14 WR
(b) Diameter Growth (mm y-1)
12 10
WH
8
SF HS
6 4 LP 2 0 0.0
0.2
0.4 0.6 Canopy transmittance
0.8
1.0
Fig. 1. (a) Modelled growth of 1-year-old Sitka spruce seedlings as a function of proportion of incident radiation transmitted through a canopy, over the year for which the model was run, using meteorological input data from Aboyne (*), Capel Curig (*), Drumalbin (!) and London (5). (b) Observed variation of seedling growth with canopy transmittance for five species in British Colombia. WR: western red cedar [Thuja plicata Donn.]; WH: western hemlock [Tsuga heterophylla (Raf.) Sarg.]; SF: subalpine fir [Abies lasiocarpa (Hook.) Nutt.]; HS: hybrid spruce [Picea glauca (Moench) Voss: P: sitchensis (Bong.) Carr.]; LP: lodgepole pine [Pinus contorta var latifolia] (Coates and Burton, 1999).
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1.0
transmittance
0.8
0.6
0.4
0.2
0.0 0
20
40
60
80
100
Stand basal area (% original) Fig. 2. Relationship between transmittance and percentage of original basal area derived by Hale (2003) (&), and from the MAESTRO simulations for the stands with the highest and lowest basal areas (yield class 20 at age 60 and yield class 12 at age 20, respectively). LAI was assumed to vary with either the basal area relative to that of the unthinned stand (rel), or the absolute basal area (abs). (*) YC12, age 20, rel; (*) YC20, age 60, rel; (!) YC12, age 20, abs; (5) YC20, age 60, abs.
From these curves, we have arbitrarily delimited a transmittance value of around 20% as necessary to obtain seedling growth beneath a forest canopy. At this level, the seedlings achieve about half of the growth possible in full sunlight. According to Eq. (1), 20% transmittance is equivalent to removing about 45% of the basal area in a neutral uniform thinning (Fig. 2). This is about twice as much as current recommendations, which suggest that 20% is the maximum basal area to be removed if significant wind damage is to be prevented (Mason and Kerr, 2001). 3.2. Below-canopy radiation environment Fig. 2 shows the relationship between t and B derived by Hale (2003), together with MAESTRO simulations of the expected extreme cases. The MAESTRO simulations are not very sensitive to the assumptions concerning tree leaf area, and show that t is influenced more by canopy structure, i.e. whether the trees are large and widely spaced (YC20, age 60) or small and densely packed (YC12, age 20). However, given that these are the extreme cases, where the
initial basal areas range from 22 to 82 m2 ha1 (with initial LAIs ranging from 3.8 to 9.8), the relationship is surprisingly conservative. Furthermore, given that the canopy structure of actual stands shows considerable variation around the values given in the yield tables, we consider the simple equation of Hale (2003) to give a reasonable estimation of t for the purposes of this study. 3.3. ForestGALES runs Fig. 3 shows the output from the seedling growth model together with that from ForestGALES for four stands covering a range of stabilities. The stand in case (i) is stable even with the heaviest thinning, and so wind damage need not be an important consideration in management. In case (ii), the stand is acceptably stable when thinned to approximately 40% of the original basal area, where seedling growth is possible. Seedling growth and the risk of wind damage trade-off linearly against each other where B is 40–60%. In case (iii), stand stablility declines linearly with thinning but no seedling growth is achieved before the stand
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0.06 200
Seedling Growth (kg)
0.05
i
ii
iii 150
0.04
0.03
100
0.02
Wind damage return period (years)
250
0.07
50 0.01 iv 0.00
0 0
20
40
60
80
100
Stand basal area (% original) Fig. 3. The return period for wind damage, calculated using ForestGALES, for four stands at a range of stand densities representing different thinnings (basal area 100% down to 10%). The modelled increase in annual seedling growth as stand density is reduced is also shown (average of the four locations shown in Fig. 1). (i) Yield class (YC)12, no previous thinning, thinned at age 20, DAMS 18; (ii) YC12, no previous thinning, thinned at age 45, DAMS 14; (iii) YC20, previous intermediate thinning, thinned at age 20, DAMS 18; (iv) YC16, previous intermediate thinning, thinned at age 32, DAMS 18.
reaches the acceptable stability limit. Initiating transformation by implementing a heavy thinning in this stand would not be a sensible management strategy. In case (iv), the stand is in danger of wind damage even without thinning, and no intervention would be recommended. Note that Fig. 3 represents conditions immediately after thinning. In subsequent years, as the trees grow and the crowns develop, one would expect the wind risk and the transmitted light will change. The transmittance achieved by thinning each modelled stand to the chosen stability limit (10-year return period) was obtained by relating Eq. (1) with the residual basal area at the stability limit. Figs. 4 and 5 show these transmittance values for different combinations of DAMS, timing of thinning, yield class, management regime and rooting scenario. The bold line in each plot marks a transmittance of 0.2, the minimum required for seedling growth. Colours darker than this indicate lower levels of transmittance, where the wind risk is higher so the stand cannot be so heavily thinned. Lighter colours indicate that the wind risk to the stand is less, permitting a heavier thinning, resulting in a higher
transmittance and greater seedling growth. The following points can be drawn from the graphs: (i) The dark areas are in the upper right portion of the plots, indicating that the risk of wind damage increases with age and with DAMS (i.e. as the trees get taller or the site becomes more exposed). This can be seen in all of the 12 plots, e.g. for a yield class 16 stand with no previous thinning and good rooting, at age 40 it could be heavily thinned on a DAMS 14 site, but not a DAMS 18 site; in contrast, at age 50 heavy thinning is no longer possible on a DAMS 14 site. (ii) For a given management regime and rooting depth, the risk increases with yield class, e.g. a yield class 12 stand with no previous thinning on a moderately exposed site could be heavily thinned up to age 60 years, but at higher yield classes stability would become limiting at age 40–45 years. This is because at low yield classes the trees are shorter at a given age, and are therefore less prone to wind damage.
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Previously unthinned
DAMS
YC12
DAMS
YC16
Previous intermediate thinning
18
18
14
14
10 20
30
40
50
60
10 20
18
18
14
14
10 20
30
40
50
111
60
10 20
18
18
14
14
30
40
50
60
30
40
50
60
30
40
50
60
DAMS
YC20
10 20
30
40
50
Age at thinning (years)
60
10 20
Age at thinning (years)
Fig. 4. Transmittance obtained when simulated stands of different age and DAMS were thinned to the stability limit (return period for damage of 10 years) on freely draining soils with good rooting conditions. The bold line (0.2) indicates the minimum transmittance required for seedling growth. For the sites with no previous thinning, failure at the stability limit was by breakage; for the previously thinned sites the risk of breakage and overturning were similar.
(iii) Previously unthinned stands on the sites with good rooting have a much greater risk of breakage than of overturning. In contrast, on sites with impeded rooting, the risk of overturning is slightly greater than that of breakage, because of the shallower rooting depth. The overall risk of damage is not much different on the two soil types, with a heavy intervention
possible up to age 40 on moderately exposed sites (later for low yield class stands). (iv) For previously thinned stands on sites with good rooting, the risk of overturning and breaking are approximately the same. These stands are the most stable of the simulated stands, with heavy thinning possible for all except the highest DAMS and yield class. In contrast, previously thinned stands on
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Previously unthinned
DAMS
YC12
Previous intermediate thinning
18
18
14
14
10 20
30
40
50
60
10 20
18
18
14
14
30
40
50
60
30
40
50
60
30
40
50
60
DAMS
YC16
10 20
30
40
50
60
10 20
18
18
14
14
DAMS
YC20
10 20
30
40
50
Age at thinning (years)
60
10 20
Age at thinning (years)
Fig. 5. Transmittance obtained when simulated stands of different age and DAMS were thinned to the stability limit (return period for damage of 10 years) on soils with impeded rooting depth. The bold line (0.2) indicates the minimum transmittance required for seedling growth. Failure at the stability limit in all cases was by overturning.
sites with poor rooting are the most unstable of the stands simulated, with a high risk of overturning.
4. Discussion 4.1. Implications for stand management The results from this study have implications for whether a particular stand is suitable for conversion to
CCF, or for managing a stand under CCF for maximum stability. Figs. 4 and 5 highlight the fact that there are more silvicultural options for stand management on sites with freely draining soils with good rooting depth than on sites where impeded rooting occurs, with prior thinning making the stand less prone to wind damage during conversion to CCF. The soil type and drainage must therefore be considered as possible constraints to silviculture from an early stage in stand management; this can be facilitated
S.E. Hale et al. / Forest Ecology and Management 187 (2004) 105–115
by assessing the soil moisture regime and rooting depth using ecological site classification (ESC; Pyatt et al., 2001). Also, older stands are more prone to wind damage after a heavy thinning than younger stands. It may therefore not be practical to convert a stand towards the end of its rotation to CCF; rather, manipulation should begin at pole stage, ideally before age 25 years. Although an early thinning may have low economic value, it may improve the long-term stability and timber quality of the stand (Cameron, 2002). In general, previously unthinned stands are at greater risk of wind damage, and therefore less versatile in terms of CCF management, than stands with a history of thinning. The exception to this in the results presented here is on the sites with impeded rooting, where for older stands and those in high yield classes previously thinned stands are more at risk from wind damage (overturning) after a heavy thinning than the stands with no previous thinning. This is because the resistance of a tree to breakage increases with the cube of diameter, whereas the resistance to overturning increases only with the square of diameter (Gardiner et al., 1997). The wind loading on an individual tree increases with spacing. Previously thinned stands have lower stocking and larger diameter trees than the unthinned stands (Table 2). The increase in force on each tree due to the wider spacing is greater than the increased resistance to overturning, therefore, the previously thinned stands are more unstable than the more densely stocked unthinned stands. On deep-rooted sites, the large diameter, widely spaced trees are more stable than the smaller diameter trees in an unthinned stand, because the increase in wind loading due to the wider spacing is more than compensated for by the increased resistance to breakage. On freely draining soils, therefore, a stand should be thinned early to allow the remaining trees to increase in diameter. When the stand is mature enough to produce seed, the trees will be sufficiently stable to tolerate a heavy thinning, so the light environment can be modified to encourage natural regeneration. On a site with impeded rooting, opening up the stand to encourage diameter growth will leave the trees more prone to overturning. Sitka spruce is thought to be intermediate in terms of shade tolerance (Malcolm et al., 2001). Seedlings of more light-demanding species would require more than 20% of incident radiation to be transmitted
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through the canopy, whereas more shade-tolerant species could grow beneath a canopy transmitting less than 20%. The implications for stability of thinning a Sitka spruce overstorey to achieve these light levels can be seen from Figs. 4 and 5, by reading to either side of the bold line. Obviously, if a more shadetolerant species is desired then a lower thinning intensity is required, and can be carried out later, or on a windier site, without compromising the stability of the stand. 4.2. Model limitations The seedling growth model has been validated elsewhere against experimental data (Levy et al., 2000), but no comparable field data exist for Sitka spruce in Britain. However, Fig. 1b shows curves fitted to the observations of Coates and Burton (1999) on seedlings of five species growing in north-central British Columbia (which, due to similarity in latitude and proximity to the coast, has a similar climate and radiation regime to Britain). Although there is interspecific variability, these show response curves of a similar form, with zero growth occurring at 5–15% of incident radiation. In this study, the model was run only for 1 year on 1-year-old seedlings. It may not, therefore, adequately describe the growth of larger seedlings or saplings, as there is evidence that as seedlings grow they require increased light levels (Messier et al., 1999). The limitations of the transmittance vs. basal area model are discussed in Hale (2003). In summary, thinning patterns other than a uniform neutral thinning may produce a different relationship between residual basal area and transmittance. Also, stands at stages later than stem exclusion may have higher transmittance to start with and, therefore, require a less severe thinning to achieve a given light level. The modelled light levels are those obtained immediately after thinning, and do not consider the response of the stand over time, in terms of canopy closure or stability. ForestGALES calculates the wind risk for the ‘mean tree’ in a stand, whereas in reality the larger trees in a stand are more at risk of damage. Also, the old, high yield class stands, and the very widely spaced stands simulated in this study are outhwith the bounds of the data set used to parameterise ForestGALES. As shown in Fig. 3, as a stand is thinned the stability
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decreases. However, in the 45- and 60-year-old stands it was common for the stand to start becoming more stable as the thinning intensity increased (spacing/ height ratios of more than 0.35 and 0.5, respectively). Insufficient work has been done in such widely spaced stands to know whether this increase in stability predicted by the model is realistic.
5. Conclusions This paper has used a radiative transfer model, a seedling growth model, and a wind risk model to address the trade-off between seedling growth and stand stability in the conversion of even-aged Sitka spruce plantations to CCF systems. Results are not meant to be prescriptive, but illustrate the relative importance of different environmental and silvicultural factors, and the nature of the interactions. The results demonstrate the importance of age of intervention and of soil properties and show that, generally, seedling growth and the risk of wind damage trade-off linearly against each other. Models are increasingly used to assist decision-making concerning forestry and land use, and here three models are combined to address a complex forest management issue. As managers are required to achieve wide-ranging and potentially conflicting goals (e.g. natural regeneration, stability, browsing control, sustainability, timber quality, economic return), the use of combined models will be a valuable approach in the future.
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