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Biomass dynamics associated with deforestation, fire, and, conversion to cattle pasture in a Mexican tropical dry forest
Forest Ecology and Management 176 (2003) 1±12
Biomass dynamics associated with deforestation, ®re, and, conversion to cattle pasture in a Mexican tropical dry forest J.B. Kauffmana,*, M.D. Steelea, D.L. Cummingsa, V.J. Jaramillob a
b
Department of Fisheries and Wildlife, Oregon State University, Corvallis, OR 97333, USA Departamento de EcologõÂa de los Recursos Naturales, Instituto de EcologõÂa, Universidad Nacional Autonoma de MeÂxico, A.P. 27-3 Xangari, Morelia, Michoacan, Mexico Received 1 January 2002
Abstract Few studies have described losses of biomass and structure arising from the conversion of tropical dry forest (TDF) to pasture, a common land-use practice in the most widespread forest type in the Neotropics. We sampled total aboveground biomass (TAGB) at ®ve distinct periods during the process of forest conversion to pasture. TAGB was measured after the primary forests were cut, after the initial forest slash ®res, and 1 year after pasture establishment. After 2 years, TAGB was quanti®ed before and after the ®rst pasture ®re conducted on site. To capture the variability in biomass burning, two treatments (Baja and Alta) based upon degree of biomass consumed were established in a randomized block design with three treatment blocks. Mean TAGB of the slashed primary forest was 118 Mg ha 1 in the Baja plots and 135 Mg ha 1 in the Alta plots. The slash ®res resulted in biomass loss of 62% of the TAGB (75 Mg ha 1) in the Baja treatment and 80% (108 Mg ha 1) in the Alta treatment. The greatest treatment differences in consumption occurred in the large wood (>7.6 cm diameter). Fires in the Baja treatment consumed 31% of the large wood, but only 7% in the Alta treatment. Prior to the ®rst pasture ®re, TAGB was 40.3 Mg ha 1 in the Baja treatment, and 20.9 Mg ha 1 in the Alta treatment. The pasture ®res consumed 63% of the TAGB in the Baja sites and 75% of the TAGB in the Alta treatment. Following the pasture ®res, the TAGB was 14.8 Mg ha 1 in the Baja treatment and 7.6 Mg ha 1 in the Alta treatment. In the ®rst 2 years of land cover change from dry forest to pasture there was a dramatic decline in TAGB (and hence aboveground C pools) totaling 87 and 94% of that of pre-disturbance forests. The total biomass lost via ®re and decomposition was 113 and 132 Mg ha 1; ®re accounted for 89±92% of the loss while decomposition/disappearance comprised 2±11% of the total biomass loss. The widespread distribution of tropical dry forests, their high rates of deforestation, and their high rates of biomass consumption during ®res suggest that they are signi®cant anthropogenic sources of atmospheric C. The dramatic loss of biomass and associated high degree of ®re severity may also affect future site productivity and the capacity for these sites to function as C pools in the future. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Deforestation; Fire; Forest biomass; Land conversion; Tropical dry forest; Tropical pasture; Biomass burning; Cattle pastures; Fire ecology
1. Introduction *
Corresponding author. Tel.: 1-541-737-1625; fax: 1-541-737-3590. E-mail address: [email protected] (J.B. Kauffman).
Land cover change involving deforestation and biomass burning of tropical forests is a signi®cant global concern due to emissions of greenhouse gasses,
0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 2 2 7 - X
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negative effects on biodiversity, and losses in site productivity (Murphy and Lugo, 1995; Watson et al., 2000). Rates of land conversion from forest to pasture or agriculture are higher in tropical dry forest than any other Neotropical forest type (Janzen, 1988; Murphy and Lugo, 1995; Masera et al., 1997). Tropical dry forests (TDF) comprise 42% of all tropical and subtropical forests and contain the highest human population densities in Latin America (Murphy and Lugo, 1986). Mooney et al. (1995) described TDF as occurring in the tropical regions where there are several months of severe, even absolute, drought, and are distinguished from savannas by the presence of tree dominated, occasionally closed canopy systems. Tropical dry forest (TDF) is the most abundant tropical forest type in Mexico covering 1:6 105 km2 (Masera et al., 1997). These areas are undergoing rapid rates of land cover change. It has been estimated that 1.4±1.9% of all Mexican tropical deciduous forests were being converted annually to pastures or other uses (Masera et al., 1997, Trejo and Dirzo, 2000). Therefore, it is important to have a better understanding of the effects of land conversion on biomass and nutrient dynamics of TDF in order to predict their contribution as sources and sinks of atmospheric C at country to continental scales. Very little is known of the biomass dynamics (loss and regrowth) associated with ®re and pasture establishment in TDF landscapes. While numerous studies have quanti®ed biomass losses associated with deforestation and biomass burning in moist and wet forests (Fearnside et al., 1993; Guild et al., 1998; Kauffman et al., 1995) few studies have examined it in dry forests (Kauffman et al., 1993). Expansion of cattle ranching has been by far the leading factor encouraging deforestation in the tropical forests of Mexico (Masera et al., 1997) and pasture establishment is the most common fate of converted TDF (Maass, 1995). Similar to other regions of the Neotropics, ®res are a commonly used approach to pasture management in TDF landscapes. Fires are frequently set to maintain the productivity, forage quality, and grass composition of cattle pastures (Uhl and Buschbacher, 1985; Kauffman et al., 1998). Yet we know of no published studies that have quanti®ed ®re behavior and biomass losses of pasture ®res in TDF landscapes. The overall objective of this study was to quantify the biomass dynamics and ®re behavior associated
with the initial phases of conversion of primary TDF to cattle pasture in the Chamela region, Mexico. This was experimentally examined with two separate ®re severity scenarios designed to encompass the variability that occurs in ®res for land conversion in this landscape. Treatments included a low consumption ®re (the ``Baja'' treatment) conducted when fuels were comparatively higher in moisture compared to the other treatment, a high consumption ®re (the ``Alta'' treatment). During the 3-year study period, we also quanti®ed: (1) biomass losses between ®re events; (2) vegetative regrowth for the 2 years following slash ®res; (3) biomass losses from a second ®re in the newly formed pastures and; (4) the cumulative losses of biomass associated with the conversion of TDF to cattle pasture. 2. Study site This study was conducted from 1993 to 1995 on the Ejido San Mateo, Jalisco MeÂxico near the EstacioÂn BiologõÂa Chamela of the Universidad Nacional Autonoma de MeÂxico (198300 N, 105830 W). The landscape is classi®ed as a Mesoamerican (Ceballos, 1995), Subtropical (Holdridge, 1967), seasonally dry deciduous forest (Murphy and Lugo, 1995). The climate is highly seasonal with a pronounced dry season. Precipitation averages 679 mm with the majority falling during the months of June±October (GarcõÂa-Olivia et al., 1995). Mean temperature is 24.9 8C with less than a 5 8C difference between the coolest and warmest months. The landscape is one of steep, low hills (slopes from 40 to 60%). Upland soils are relatively young, shallow, (0.5± 1.0 m in depth), isohyperthermic, Typic Ustorthents. They are poorly structured, sandy loam in texture, derived from rhyolite and have a pH of 6.0±6.5. Plant species richness of the Chamela region consists of 544 genera and 124 families and the principal vegetation of this upland site are deciduous trees in the Fabaceae family (Lott, 1993). Trees have a density > 4325 ha 1, are low in stature, and are relatively small in diameter (3±20 cm dbh; Jaramillo et al., in press). The rate of deforestation for the Chamela region has varied between 0.4 and 2.2% annually in the past 25 years (A. Miranda, personal communication).
J.B. Kauffman et al. / Forest Ecology and Management 176 (2003) 1±12
3. Approaches to land conversion and utilization in the Chamela region Biomass burning optimizes labor energy and capital expenditures for land clearing and the planting of agronomic crops or pasture for livestock (Bye, 1995). Communal landowners (Ejidatarios) in two Ejidos of the Chamela region have stated that slashand-burn is a common practice due to its low cost and high ef®ciency (GonzaÂlez-Flores, 1992). Decisions concerning the location and size of the plots as well as the time for forest conversion are heavily in¯uenced by the availability of economic resources and the uncertainty in land-use regulations. These include the availability of deforestation permits and the time involved for obtaining government loans to establish new pastures or to purchase cattle (GutieÂrrez-AlcalaÂ, 1993). Slashing is performed with chainsaw and machete at the onset of the dry season and slash ®res are set at the end of the dry season ( late April±early May). The precise day for the burn is determined by each landowner according to the morning weather; it requires a clear sky, high temperatures, and strong, constant winds. A stick dipped in kerosene is used to ignite litter and twigs of the felled vegetation along the perimeter of the plot. No ®relines (fuel breaks) are built to prevent the spread of ®re into adjacent forests. Even though litter mass is quite high (Jaramillo et al., in press) and moisture contents are quite low (4±6 months without rain), ®re does not spread into the forest beyond the ®rst few meters. Soon after burning, plots are seeded with pasture grasses, commonly Panicum maximum Jacq. and Cenchrus ciliaris L., and more recently with Andropogon gayanus L. Traditionally, a portion of the plot is also planted with corn, squash, or beans. These crops are harvested after a few months and then cattle are introduced into the pastures. Pastures are burned every 2 or 3 years to maintain productivity and decrease woody invasion. In summary, the generalized approaches to landuse/land conversion in the Chamela region are: (1) slashing existing forest with machete and chainsaw at the onset of the dry season; (2) removal of usable wood (due to the remoteness of our study site little if any wood was removed during this study); (3) allow the forest slash to dry during the dry season; (4) burn the slashed forest just prior to the onset of the rainy
3
season; (5) plant corn or other crops and pasture grasses immediately after burning; (6) harvest crops 3±4 months after planting, and introduce cattle to pasture; and, (7) burn pasture every 2±3 years after establishment. 4. Methods The experimental site was approximately 3.5 ha in size and encompassed an entire area to be converted to pasture by the Ejidatario. Three replicate blocks were established and each was subdivided into treatment sub-blocks consisting of a low (Baja) and high (Alta) consumption/severity burn treatments. Metal posts were used to delineate plot boundaries. The ®re severity treatments were randomly assigned to each subblock. The differences in ®re treatments were accomplished by varying the drying time between slash and burning. In 1993, Alta sites were slashed the last week of January and burned 9 May 1993 (95 days between slashing and burning). Baja sites were slashed the second week of February and burned 9 April 1993 (65 days between slashing and burning). No precipitation occurred during the period between forest cutting and ®re. To determine patterns of biomass decline associated with land conversion, we sampled total aboveground biomass (TAGB) at ®ve distinct time periods: (1) in 1993 before; and (2) immediately after slash burning; (3) towards the end of the 1994 dry season (about 1 year after the slash ®re) during the rastrojo (corn stubble)/early pasture establishment phase; (4) in 1995, prior to; and (5) immediately following the planned pasture ®re for this site. For all ®res, the combustion factor, de®ned as the percent of biomass lost during a ®re, was calculated as the percent difference in biomass before and after burning. Within each sub-block, 15 permanent transects were established (n 45 for each treatment) where total slashed and surface biomass components were measured. At each sampling period, along each transect, biomass of downed wood was quanti®ed non-destructively using the planar intercept technique (Brown and Roussopoulos, 1974; Van Wagner, 1968; Kauffman et al., 1995). Downed wood was partitioned into standard moisture timelag classes based upon diameter (Deeming et al., 1977; Table 1). Length of the sampling
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Table 1 Size classes in which the downed wood was partitioned for biomass calculations, transect lengths (m), and the mean speci®c gravity (g cm3) of wood collected each year of the study Wood diameter (cm)
Transect length (m)
Specific gravity (1993)
Specific gravity (1994)
Specific gravity (1995)
0.0±0.64 0.65±2.54 2.55±7.6 >7.6 (sound) >7.6 (rotten)
1 3 10 15 15
0.74 0.55 0.76 0.78 0.61
0.65 0.76 0.70 0.62 0.23
0.71 0.76 0.70 0.75 0.23
plane varied with the size class of wood material: 1 m for wood < 0:64 cm diameter; 3 m for wood 0.65± 2.54 cm diameter; 10 m for wood 2.54±7.6 cm diameter; and 15 m for wood > 7:6 cm in diameter. Biomass was determined by calculating volume and then multiplying volume by the mean speci®c gravity of wood samples. Each year, at least 50 samples of wood within each size class were randomly collected to estimate the mean speci®c gravity. Speci®c gravity was determined through division of the oven-dry weight of the sample by its volume (weight displaced in water). Residual standing trees were measured within three 225 m2 plots in each sub-block (n 9/treatment). The diameter at breast height (dbh) was measured to calculate standing tree biomass using the equation in MartõÂnez-YrõÂzar et al. (1992). Stumps were measured in three 20 m 1 m permanent plots in each sub-block (n 9/treatment). The diameter and height of all stumps were measured to calculate volume. Each volume measurement was then multiplied by the speci®c gravity of large wood to calculate mass. The biomass of attached foliage (i.e. the mass of leaves that remained attached to the slashed wood) was ascertained through determination of the ratio between its biomass and that of the wood particles < 0:65 cm in diameter (Kauffman et al., 1993; Kauffman et al., 1995). The ratio of attached foliage to twig biomass was calculated by collecting 50 random samples, oven drying, and then weighing the wood and leaf materials, separately. The mass of attached foliage was then estimated by multiplying the biomass of this wood size class by the ratio. In the slashed forest, surface litter composed of leaves, fruits, seeds, and bark fragments was collected in 50 cm 50 cm plots established 1m away from the end of each woody biomass transect (n 45/treatment). Dicot seedlings (<3 cm dbh) were also col-
lected in these plots. In the following years after the sites were converted to pasture, grasses, corn residue, cattle manure, dicot seedlings, and litter were also collected in these microplots, separated, oven dried and weighed at the station laboratory. Following the ®res, the same data were collected from paired microplots placed 2 m away from the end of each wood biomass transect. Ash mass was collected immediately after burning in nine 50 cm 50 cm microplots in the 1993 slash ®res (n 27/treatment) and six 50 cm 50 cm microplots in the 1995 pasture ®res (n 18/treatment). Ash was collected using a vacuum cleaner and a portable generator. At the time of ignition, we measured air temperature, relative humidity, and wind speed. During the ®res we measured the ¯ame length, height, depth, angle, and rate of spread of the ¯ame front (Alexander, 1982). At least two observers visually estimated these ¯ame parameters at ®ve to seven random locations within at each ®re. From ¯ame length measurements, we calculated ®reline intensity (i.e. the rate of energy release per unit length of the ¯ame front) using the equation given in Alexander (1982). The statistical analysis performed was analysis of variance. Statistical analyses included comparisons of aboveground pre-®re versus post-®re biomass, comparisons between treatments at each phase of pasture establishment, percent biomass consumption, biomass loss due to decomposition, and regrowth. 5. Results The TAGB of the slashed forest in sub-blocks ranged from 113 to 137 Mg ha 1 with a mean of 118 Mg ha 1 in the Baja treatment and 135 Mg ha 1 in the Alta treatment (Table 2). There were no sig-
J.B. Kauffman et al. / Forest Ecology and Management 176 (2003) 1±12
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Table 2 Total aboveground biomass (Mg ha 1) and the quantity of biomass consumed (the combustion factor, %) during the slashed forest phase of land conversion of tropical dry forest to pasture BajaÐlow severity Pre-fire Litter Attached foliage Dicots Total fine fuels Wood debris (diameter, 0±0.64 0.65±2.54 2.55±7.62 7.63±20.5 (sound) 7.63±20.5 (rotten) 7.63±20.5 (total) >20.5 (sound) >20.5 (rotten) >20.5 (total) Total wood Standing trees Stumps Total aboveground biomass Ash
AltaÐhigh severity
Post-fire
Combustion factor
Pre-fire
Post-fire
Combustion factor
7.4 0.47 0.17 8.1
0.61 0.03 0.06 0.61
0.0 0.04 0.0 0.04
0.0 0.02 0.0 0.06
100 88.9 100 99.2
0.0 3.8 0.0 0.29
7.4 0.41 0.2 8.0
0.56 0.03 0.04 0.56
0.0 0.0 0.0 0.02
0.0 0.0 0.0 0.02
100 100 100 99.9
0.0 0.13 0.0 0.05
cm) 2.6 16.8 24.7 29.4 4.4 33.8
0.17 1.2 1.9 2.8 0.88 2.9
0.23 2.0 6.1 18.0 3.2 21.2
0.09 0.78 0.84 3.2* 1.1 3.1
88.8 89.1 70.8 31.7 85.1 28.7
3.8 3.6 4.7 11.0 7.2 10.0
2.4 15.5 28.0 31.4 5.0 36.4
0.19 1.2 1.8 3.5 1.2 4.1
0.01 0.46 2.8 5.5 2.0 7.5
0.01 0.160 0.522 1.3 0.93 1.5
99.9 96.8 87.7 78.5 86.5 62.3
0.13 0.97 3.7 5.7# 9.9 14.4
7.7 2.7 0.8 0.8 8.5 2.8
3.1 1.7* 0.0 0.0 3.1 1.7
56.1 22.9 100 0.0 60.9 20.8
86.4 4.2 9.9 4.0 13.9 5.0
32.7 4.0 0.8 0.8 10.1 2.6*
62.4 4.1 78.1 21.9 32.7 19.7
91.3 5.9 14.8 4.6 20.9 5.7
15.2 2.5 1.0 1.0 10.6 3.1
80.4 4.0 95.7 4.3 49.8 19.0
118.2 2.8
43.6 16.4
62.4 15.1
134.9 1.4
26.8 4.1
80.2 2.8
3.9 0.8
7.8 2.8 1.2 0.83 9.0 2.8
4.4 1.9* 0.0 0.0 4.4 1.9
71.6 19.4 100 0.0 77.3 15.7
4.7 0.4
Biomass was signi®cantly different (P < 0:10) before and after ®re within treatments in all components except noted by an asterisk (). A hash symbol (#) indicates a signi®cant difference in the combustion factor between treatments.
ni®cant differences between treatments in any of the pre-®re biomass components. Slashed wood comprised the majority of the TAGB with a mass of 86 and 91 Mg ha 1 in the Baja and Alta treatments. Total ®ne fuels (i.e. litter, dicot, and attached foliage) was 8 Mg ha 1 in both treatments comprising 7 and 6% of the TAGB in the Baja and Alta treatments, respectively. Standing tree biomass was 10 and 15 Mg ha 1 and stump biomass was 14 and 21 Mg ha 1 in the Baja and Alta treatments, respectively. Decisions on the timing of the burning and the manner in which the plots were burned were that of the Ejiditarios. All slash ®res were conducted at similar ambient air temperatures and relative humidity (Table 3). Fires were conducted at midday when temperatures were highest (27±288) and relative humidity was lowest (47±53%). The ®reline intensity of the slash ®res was very high (7076 and 7761 kW m 1) with a mean ¯ame length of 4.7 m.
The mean rate of spread of the ®res was quite different between treatments due to differences in the ignition patterns. The Baja sites were burned by backing and ¯anking ignition patterns (i.e. ignited only on the upslope and sides of the burn units while the Alta sites were burned with head ®res (i.e. ignited at the bottom of the units and allowed to burn up slope). The slash ®res resulted in signi®cant declines in TAGB as well as in most components of aboveground biomass (Table 2). In the Alta treatment, the combustion factor was higher (80%) and there was less variation between blocks (a range from 75 to 80%) compared to the Baja treatment (a mean combustion factor of 60% and a range of 32±78%). The mean post®re TAGB was 44 and 27 Mg ha 1 in the Baja and Alta treatments, respectively. Very high rates of consumption were measured for the ®ne fuels (i.e. litter, attached foliage, and dicots where combustion factors were 89±100%). In the Baja treatment, there was not a
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J.B. Kauffman et al. / Forest Ecology and Management 176 (2003) 1±12
Table 3 Weather conditions at the time of burning and ®re behavior of slash and pasture ®res, 1993 and 1995 Treatment
BajaÐlow severity
AltaÐhigh severity
Pasture fire (treatments combined)
Slash date Fire date Days between slashing and fire Weather at time of fire Relative humidity (%) Air temperature (8C) Wind speed (km h 1)
Mid-February 9±10 April 1993 65 days
Late-January 3±9 May 1993 95 days
15 June 1995
Flame length (m) Flame height (m) Flame depth (m) Flame angle (8) Rate of spread (m min 1) Fireline intensity (kW m 1)
53 27 13.3
47 28 12.8
67 31 12.8
4.8 3.5 3.6
4.6 3.3 4.6
3.5 4.3 18
65 1.2 7761
52 16.2 7076
statistically signi®cant decline in large wood (>7.6 cm in diameter) or in the mass of stumps. These had the lowest combustion factor of all components in the ecosystem. The biomass of all components signi®cantly decreased following ®re in the Alta treatment except for sound wood > 20:5 cm in diameter (P 0:2). The most abundant component of aboveground biomass was downed wood 7.6±20.5 cm in diameter. This component comprised 23±24% of the pre-®re TAGB. Signi®cant differences in combustion between treatments (P 0:09) were detected within this component (i.e. a combustion factor of 29% in the Baja treatment and a combustion factor of 62% in the Alta treatment). Apparently, the differences in time in which the slash was allowed to dry most strongly affected differences in the degrees of consumption of the large wood components. Following ®re, the biomass of all wood components was twice as high in the Baja treatment (33 Mg ha 1) compared to the Alta treatment (15 Mg ha 1). One year after slash ®res, the TAGB of the young pasture/rastrojo phase of establishment was 40 Mg ha 1 in Baja plots and 29 Mg ha 1 in Alta plots (Table 4). This represented a loss in biomass of 3.2 Mg ha 1 in the Baja treatment and a gain in TAGB of 2.4 Mg ha 1 in the Alta treatment. The gains in aboveground biomass from 1993 arose from the growth of pasture grasses and corn. The mass of standing corn stalks was 1.4 Mg ha 1 in the Baja treatment and 1.9 Mg ha 1 in the Alta treatment. Dead grass mass was 0.5±1.1 Mg ha 1. Litter, which was
66 133 3912
largely comprised of corn and grass residue, had a biomass of 3.2 Mg ha 1 in both treatments. Losses in biomass between 1993 and 1994 were largely due to the disappearance of the residual forest biomass (i.e. Table 4 Total aboveground biomass (Mg ha 1) during the early pasture/ rastrojo phase of land conversion from tropical dry forest, approximately 1 year following conversion from tropical dry forest BajaÐlow severity Litter Dicot Corn Dead grass Total fine fuels Wood debris (diameter, cm) 0±0.64 0.65±2.54 2.55±7.62 7.63±20.5 (sound) 7.63±20.5 (rotten) 7.63±20.5 (total) >20.5 (sound) >20.5 (rotten) >20.5 (total) Total wood Standing trees Stumps Total aboveground biomass
AltaÐhigh severity
3.2 0.4 1.4 1.1 6.1
0.4 0.2 0.2 0.4 0.8
3.1 0.3 1.9 0.5 5.8
0.4 0.01 0.02* 0.02 0.1
0.2 1.9 9.1 14.4 0.3 14.7
0.04 0.5 0.9 9.4 0.1 2.4
0.07 0.7 3.1 6.5 0.2 6.7
0.0 0.2 0.6 1.0 0.1 1.1
0.6 0.6 0.0 0.0 0.6 0.6 26.4 1.0 6.9 40.4
3.2 1.0 1.6 11.5
0.0 0.0 0.2 0.2 0.2 0.2 10.7 2.1 10.6 29.2
1.3 1.0 5.6 8.2
A asterisk () indicates that biomass was signi®cantly different (P < 0:10) between treatments.
J.B. Kauffman et al. / Forest Ecology and Management 176 (2003) 1±12
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Table 5 Total Zaboveground biomass (Mg ha 1) and the quantity of biomass consumed (the combustion factor, %) before and following ®re during the early pasture phase of land conversion from tropical dry forest, 1995 BajaÐlow severity Pre-fire Litter Dicot Live grass Dead grass Manure Total fine fuels
5.0 0.6 0.4 6.8 0.2
Post-fire 0.37 0.2 0.3 2.1 0.2
13.0 2.2
Wood debris (diameter, cm) 0±0.64 0.1 0.03 0.65±2.54 1.2 0.4 2.55±7.62 6.9 0.7 7.63±20.5 (sound) 9.2 1.8 7.63±20.5 (rotten) 1.1 0.36 7.63±20.5 (total) 10.4 2.0 >20.5 (sound) >20.5 (rotten) >20.5 (total)
AltaÐhigh severity
0.6 0.63 0.6 0.4 1.2 0.73
Total wood Standing trees Stumps
19.8 2.4 0.8 0.7 6.7 2.2
Total aboveground biomass Ash
40.3 6.9
0.0 0.6 0.0 0.1 0.0
0.0 0.3* 0.0 0.05 0.0
0.6 0.27 0.03 0.30 3.0 3.9 0.0 4.9
0.02* 0.16* 0.4* 0.91 0.0 1.0
Combustion factor 100 93.6 100 95.5 100
0.0 12.7 0.0 10.3 0.0
Pre-fire 5.5 1.0 0.2 3.3 1.0
Post-fire 0.5 0.6 0.2 0.6 0.4
0.2 0.0 0.0 0.1 0.0
0.2 0.0 0.0 0.1 0.0
Combustion factor 94.3 100 100 94.2 100
5.8 0.0 0.0 12.7 0.0
86.0 7.0
11.0 1.2
0.3 0.2
87.5 8.0
72.2 77.7 50.0 58.4 100 54.4
0.03 0.7 3.1 8.8 0.1 8.9
0.0 0.1 2.1 1.1 0.0 1.4
100 70.2 18.4 81.6 100 72.2
18 8.8 7.0 8.3 0.0 8.8
0.02 0.2 0.5 1.5 0.1 1.5
0.0 0.1 0.5 0.42 0.0 0.5
0.0 15.1 16.2 6.3# 0.0 11.2
0.0 0.0 0.0 0.0 0.0 0.0
100 0.0 100 0.0 100 0.0
0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0
100 0.0 100 0.0 100 0.0
8.3 1.1* 0.4 0.3* 5.5 2.1*
52.3 6.5 59.0 5.4 32.7 19.2
12.6 1.8 0.9 0.5 4.6 1.8
3.8 0.7 0.5 0.5* 3.0 2.0*
67.9 6.0 58.9 5.5 49.2 24.4
63.1 4.5
29.0 2.8
7.6 3.0
75.0 7.9
14.8 2.9 2.1 0.15
2.3 0.2
Data were collected approximately 2 years following conversion from dry forest. Biomass was signi®cantly different (P < 0:10) before and after ®re within treatments in all components except noted by an asterisk (). A hash symbol (#) indicates a signi®cant difference in the combustion factor between treatments.
the mass of the downed wood, standing trees, and stumps combined). In the year following the 1993 post-®re measurements, the residual forest biomass declined from 43.6 to 34.3 Mg ha 1 in the Baja treatment and from 26.8 to 23.4 Mg ha 1 in the Alta treatment. We found that the mean speci®c gravity of large wood declined from 0.78 to 0.62 during this period of time, (Table 1) suggesting losses were due to decomposition. Differences in the loss of wood to decomposition between treatments indicates that less readily decomposable wood remained on site in the Alta treatment compared to the Baja treatment. Two years following slash burning, the TAGB was 40 Mg ha 1 in the Baja treatment and 29 Mg ha 1 in the Alta treatment (Table 5). There was little change in the TAGB from the previous year. The relatively stable
level of TAGB during this time period re¯ected a balance between increases in grass biomass with losses in wood biomass through decomposition. In 1995, pre-®re total ®ne fuels (grasses, litter, and dicots) was 11±13 Mg ha 1 and comprised 32±37% of the TAGB. The residual forest biomass was 27 Mg ha 1 in the Baja treatment and 18 Mg ha 1 in the Alta treatment. Thus, 2 years following the initial slash ®re, residual forest biomass declined by 16.3 Mg ha 1 in the Baja treatment and 8.7 Mg ha 1 in the Alta treatment. This was a signi®cant decrease in wood mass during this time period for the Baja treatment (P 0:04) but not for the Alta treatment (P 0:15). This suggests there was a compensation in biomass losses with greater decomposition occurring on sites with lower levels of combustion due to ®re.
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J.B. Kauffman et al. / Forest Ecology and Management 176 (2003) 1±12
Fig. 1. Biomass dynamics associated with the initial 2 years of land conversion from primary tropical dry forest to pasture at the Ejido San Mateo, Jalisco, Mexico.
J.B. Kauffman et al. / Forest Ecology and Management 176 (2003) 1±12
The entire study area was burned on 15 June 1995. Conditions at the time of burning were quite dry as no signi®cant precipitation had fallen on the site since the previous October. Because of the extremely ¯ammable conditions of the pasture and the proximity of the blocks to one another, it was impossible to burn the treatments separately and maintain any resemblance to actual land-use practices. Pasture ®res were ignited at the bottom of the slope and allowed to burn upslope as a head®re. This resulted in a very rapid rate of spread (a mean of 133 m min 1). By comparison, the rate of spread of head®res through the grasses of the pastures was 8.2 times more rapid than that for head®res in the slash fuels of the Alta treatment. Mean ¯ame length of the ®res was 3.5 m with a ®reline intensity of 3911 kW m 1. It required < 15 min for the entire study area to burn. Pastures ®res resulted in a decline in TAGB from 40 to 15 Mg ha 1 (a combustion factor of 63%) in the Baja treatment and from 29 to 8 Mg ha 1 (a combustion factor of 75%) in the Alta treatment (Table 5). While the combustion factor was higher in the Alta treatment compared to the Baja treatment, it had a lower quantity of biomass consumed by ®re due to a lower pre-®re TAGB (P 0:2). A mean of 25.5 Mg ha 1 of biomass was consumed in the Baja treatment and 21.4 Mg ha 1 was consumed in the Alta treatment. Essentially all of the ®ne fuels, (grasses, litter, and dicots) were consumed by ®re in all plots. The combustion factor of wood 7.6±20.3 cm in diameter was signi®cantly higher (P 0:03) in the Alta treatment (82%) than the Baja treatment (58%). Wood losses by this pasture ®re were 12 Mg ha 1 in the Baja treatment and 9 Mg ha 1 in the Alta treatment. Following the pasture ®res, the dominant pools of aboveground mass were stumps and large wood debris. In the ®rst 2 years of land cover change from TDF to cattle pasture, there was a dramatic decline in aboveground biomass (Fig. 1). TAGB decreased from 118 to 15 Mg ha 1 in the Baja sites and from 135 to 8 Mg ha 1 in the Alta sites. A total of 100±130 Mg ha 1 of biomass was consumed by the slash and pasture ®res combined. The cumulative decomposition/ disappearance losses of the residual forest biomass in the time period between ®res was estimated to be 16.3 Mg ha 1 in the Baja treatment and 8.8 Mg ha 1 in the Alta treatment. The total biomass lost via
9
®re and decomposition was 113 and 132 Mg ha 1; ®re accounted for 89±98% of this loss while decomposition/ disappearance comprised 2±11% of the total biomass loss. 6. Discussion Intensively sampling biomass on ®ve occasions during the early phases of land conversion of forest to pasture allowed us to quantify the patterns and causes of biomass decline associated with this common activity (Fig. 1). We measured large reductions in TAGB and found that the majority of biomass losses occurred as a result of biomass burning, while smaller amounts were lost through decomposition, cattle grazing and other processes (e.g. erosion). The initial ®re resulted in biomass losses of 75 and 108 Mg ha 1 in the Baja and Alta treatments. Between ®re events we estimated that decomposition/disappearance losses were 16.2 Mg ha 1 in the Baja treatment and 8.7 Mg ha 1 in the Alta treatment. Pasture ®res resulted in the decline of an additional 21±26 Mg ha 1 (Fig. 1). These results are quite different than estimates of combustion and decomposition that have been used to estimate C emissions and sequestration in tropical forests at country to continental-level scales. For example, in calculating C emissions arising from the deforestation of Mexican dry forests Masera et al. (1997) used an estimate of 40% as the quantity of the C pool lost via slash and pasture ®res. For Amazonian (Brazil) evergreen forests, Fearnside (1992) suggested the combustion factor of biomass would be 28%, with 35% of the aboveground C released through three reburns. He estimated 62% of the C would be released by decay. In contrast, for the tropical dry forest landscape of this study we found that combustion releases by ®re were 62±80% for slash ®res and 63±75% for pasture ®res. Decomposition/ disappearance losses equaled only 7±12% of the pre®re forest biomass. There are many factors that affect the quantity of biomass consumed by a ®re. These factors include: (1) climateÐnumber of rainless days between slashing and burning, humidity, wind speed, and temperature at the time of burning; (2) the composition and structure of the vegetation, particularly the biomass and size class of the slashed forest fuels; and (3)
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J.B. Kauffman et al. / Forest Ecology and Management 176 (2003) 1±12
few estimates of biomass losses associated with land conversion exist. In a slashed second growth TDF in northeastern Brazil, Kauffman et al. (1995) also found very high rates of biomass combustion (i.e. a combustion factor of 78±95%). However, there were great differences in the structure of the Brazilian slashed forests compared to those measured in this study. The TAGB of the 16-year old secondary Brazilian dry forest was 74 Mg ha 1. Large wood (<7.6 cm) only comprised 3±9% of the TAGB at the Brazil site while large wood comprised 34±36% of the TAGB in the San Mateo sites. This likely re¯ects the structural differences in the secondary forests compared to the primary forest. There are great differences in the biomass and response to ®re associated with land cover change in tropical evergreen forests compared to tropical dry forests. Compared to TDFs, tropical evergreen forests have a greater TAGB (Table 6) and ®res associated
topographic features such as slope, which can affect the rate of ®re spread and continuity of forest fuels (Pyne, 1984). The length of the dry season in this region of Mexico and the small structure of the forest trees resulted in high percentages of biomass consumed. Based upon differences in the two treatments examined in this study, we found the time period between slashing and burning likely affected the amount of moisture left in the slash, particularly in the large woody debris where consumption rates were signi®cantly different among treatments. Upon inspection of the post burn conditions, local Ejidatarios expressed that post-®re conditions in this experiment re¯ected the range of slash consumption present in successful land clearing practices. While the rates of land conversion have been suggested to be higher in TDF than other forest types (Janzen, 1988; Murphy and Lugo, 1995; Masera et al., 1997; Trejo and Dirzo, 2000) surprisingly
Table 6 Total aboveground biomass (Mg ha 1) before and after burning (Mg ha 1) and the combustion factor (%) for ®res in selected Neotropical forests and pastures Pre-fire
Post-fire
Combustion factor
Forests Tropical dry forest, Mexico Low severity High severity
118.2 2.8 134.9 1.4
43.6 16.4 26.8 4.1
63 80
Tropical dry forest, Brazil Low severity Medium High
73.8 2.7 74.0 5.0 73.7 1.9
16.4 3.0 8.3 2.8 4.0 1.1
78 88 95
Tropical evergreen/Brazil Jacunda Maraba Santa barbara Jamari Balteke Sergpipe Pastures Tropical cattle pasture, Mexico from TDF Low severity High severity Tropical cattle pasture, from evergreen forest, Brazil Francisco Durval Joao Jamari
292.4 434.6 290.2 361.2 354.8 398.8
35.8 72.2 20.4 36.8 47.8 44.7
40.3 6.9 29.0 2.8
139.9 207.1 165.1 155.4 187.5 85.2
24.3 53.7 9.2 24.1 5.8 30.0
14.8 2.9 7.6 3.0
52 52 43 57 47 54
63 75
Source This study
Kauffman et al. (1993)
Kauffman et al. (1995)
Guild et al. (1998)
This study
Kauffman et al. (1998) 53.3 119.2 72.7 66.3
4.8 35.0 13.1 13.3
8.8 2.0 94.7 35.2 38.2 9.0 45.6 11.0
83 21 47 31
Guild et al. (1998)
J.B. Kauffman et al. / Forest Ecology and Management 176 (2003) 1±12
with land conversion have a lower combustion factor (Kauffman et al., 1992, Guild et al., 1998). For example, utilizing very similar methods Kauffman et al. (1995) and Guild et al. (1998) reported the TAGB of slashed evergreen forest in the Brazilian Amazon ranged from 290 to 435 Mg ha 1 and the combustion factor ranged from 43 to 54% (Table 6). The absolute biomass consumed was 126± 228 Mg ha 1 in the slashed tropical evergreen forest ®res compared to a biomass consumption of 74± 108 Mg ha 1 in the TDF slash ®res of this study. This suggests that the contribution of CO2 and other greenhouse gasses to the atmosphere arising from land cover change of the Mexican tropical dry forest is not trivial. Masera et al. (1997) calculated that 320,000 ha of Mexican TDF is lost each year via land conversion. Using these data and results from this study, we estimate that ®res associated with dry forest to pasture conversion in Mexico could potentially result in annual losses from 23 to 35 Mg of aboveground biomass. This does not include the losses associated with ®res in pastures which are likely another prominent source of greenhouse gasses. The pasture ®res in this study consumed 21± 26 Mg ha 1 (Fig. 1). We would not expect the quantity of biomass consumed in future pasture ®res to be as high because little residual forest biomass remained after the pasture ®res measured in this study (Table 5). Nevertheless, 11±12 Mg ha 1 of the ®ne fuel biomass that accumulated since pasture establishment (i.e. litter, dicot, live grass, dead grass, and manure) was consumed. In older pastures of the Chamela region, Jaramillo et al. (in press) found these components to be 7±8 Mg ha 1. These ®ne fuels, as well as any residual wood would likely be consumed by future ®res. There is a wide variation in biomass and ®re effects among pastures in tropical forests. In Neotropical evergreen forests, the TAGB of cattle pastures ranges from 7 to 119 Mg ha 1 (Hughes et al., 2000; Kauffman et al., 1998). When pastures are burned in the Amazon, the proportion of the TAGB consumed is variable with combustion factors ranging from 21 to 84% (Table 6). The decline in TAGB due to pasture ®res in the Amazon was 21±45 Mg ha 1 (Guild et al., 1998; Kauffman et al., 1998) which is similar to, or higher than losses in the pasture ®res of this study. The wide range in variation between pasture biomass and
11
quantities of biomass consumed by ®re suggests that site/forest speci®c data are needed to accurately quantify biomass and nutrient dynamics associated with land-use in tropical ecosystems. The losses in aboveground biomass associated with land cover change in the TDF of this study are quite dramatic. With a decline in TAGB of 87 to 94% associated with land conversion we would expect similar declines in the ecosystem pools of C as well as many nutrients. Jaramillo et al. (in press) reported that the TAGB of TDFs in the Chamela region comprised 41% of the total ecosystem C pool. Thus, current patterns of land-use as described in this study will likely affect ecosystem productivity as well as limiting land management options for the future. Alternative land-uses that lessen the dramatic impact on biomass decline, ecosystem structure, and soil exposure, while providing a sustainable means of income are in need of development. Acknowledgements This project was funded by NSF grant DEB 9118854 ``Phosphorus, Carbon, and Nitrogen Dynamics in Perturbed Neotropical Dry Forest''. We are grateful to Mr. Ramiro PenÄa and the Ejido San Mateo for allowing us to work on their lands. Lisa Ellingson, Pedro GonzaÂlez, Daniela Roth, and Georgina GarcõÂa provided invaluable assistance in the ®eld. We thank personnel at the EstacioÂn BiologõÂa Chamela, Universidad Nacional AutonoÂma de MeÂxico for their assistance in making this project possible. We also thank the Instituto de EcologõÂa, Universidad Nacional AutonoÂma de MeÂxico, Campus Morelia, for generously providing of®ce space and facilities to complete the ®nal drafts of this manuscript. References Alexander, M.E., 1982. Calculating and interpreting forest ®re intensities. Can. J. Bot. 60, 349±357. Brown, J.K., Roussopoulos, P.J., 1974. Eliminating biases in the planar intersect method for estimating volumes of small fuels. For. Sci. 20, 350±356. Bye, R., 1995. Ethnobotany of the Mexican tropical dry forests. In: Bullock, S.H., Mooney, H.A., Medina, E. (Eds.), Seasonally Dry Tropical Forests. Cambridge University Press, Cambridge, pp. 423±438.
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Ceballos, G., 1995. Vertebrate diversity, ecology, and conservation in Neotropical dry forests. In: Bullock, S.H., Mooney, H.A., Medina, E. (Eds.), Seasonally Dry Tropical Forests. Cambridge University Press, Cambridge, pp. 195±220. Deeming, J.E., Burgan, R.E., Cohen, J.D., 1977. The national ®re danger rating systemÐ1978. USDA Forest Service General Technical Report INT-39, US Forest Service Intermountian Research Station, Ogden, UT, USA. Fearnside, P.M., 1992. Carbon emissions and sequestration in forests: case studies from seven developing countries, Vol. 2 Brazil. In: Makundi, W., Sathge, J. (Eds.), EPA Climate Change Division Environmental Protection Agency Washington, DC and Environment division Lawrence Berkeley Laboratory, Berkeley, CA, USA. Fearnside, P.M., Leal Jr., N., Fernandes, F.M., 1993. Rainforest burning and the global carbon budget: Biomass, combustion ef®ciency, and charcoal formation in the Brazilian Amazon. J. Geophy. Res. 98, 16733±16743. GarcõÂa-Olivia, F., Maass, J.M., Galicia, L., 1995. Rainstorm analysis and rainfall erosivity of a seasonal tropical region with strong cyclonic in¯uence on the Paci®c coast of Mexico. J. Appl. Meteorol. 34, 2491±2498. GonzaÂlez-Flores, P.C., 1992. El manejo del fuego en el sistema de roza, tumba y quema en la selva baja caducifolia de Chamela, Jalisco. Tesis de Licenciatura en BiologõÂa. Facultad de Ciencias, Universidad Nacional AutoÂnoma de MeÂxico, MeÂxico, D.F. Guild, L.S., Kauffman, J.B., Ellingson, L.J., Cummings, D.L., Castro, E.A., Babbitt, R., Ward, D.E., 1998. Dynamics associated with total aboveground biomass, C, and nutrient pools and biomass burning of primary forest and pasture in Rondonia, Brazil during SCAR-B. J. Geophys. Res. 103, 32091±32100. GutieÂrrez-AlcalaÂ, A.R.M. 1993. La ganaderõÂa extensiva en el troÂpico seco Mexicano: causas, consecuencias y manifestaciones en su medio social. Tesis de Licenciatura en GeografõÂa. Facultad de FilosofõÂa y Letras, Universidad Nacional AutoÂnoma de MeÂxico, MeÂxico, D.F. Holdridge, L.R., 1967. Life Zone Ecology. Tropical Science Center, San Jose, Costa Rica, p. 206. Hughes, R.F., Kauffman, J.B., Jaramillo, V.J., 2000. Ecosystemscale impacts of deforestation and land-use in a humid tropical region of Mexico. Ecol. Appl. 10, 515±527. Janzen, D.H., 1988. Tropical dry forests the most endangered major tropical ecosystem. In: Wilson, E.O. (Ed.), Biodiversity. National Academy Press, Washington, DC, pp. 130±137. Jaramillo, V.J., Kauffman J.B., RenterõÂa-Rodriguez L., Cummings, D.L., Ellingson L.J., 2003. Biomass, C, and N pools in Mexican tropical dry forest landscapes. Ecosystems, in press.
Kauffman, J.B., Cummings, D.L., Ward, D.E., Babbitt, R., 1995. Fire in the Brazilian Amazon: biomass, nutrient pools and losses in slashed primary forests. Oecologia 104, 397±408. Kauffman, J.B., Cummings, D.L., Ward, D.E., 1998. Fire in the Brazilian Amazon 2: biomass, nutrient pools, and losses in cattle pastures. Oecologia 113, 415±427. Kauffman, J.B., Sanford, R.L., Cummings, D.L., Salcedo, I.H., Sampaio, E.V.S.B., 1993. Biomass and nutrient dynamics associated with slash ®res in Neotropical dry forests. Ecology 74, 140±151. Lott, E.J., 1993. Annotated checklist of the vascular ¯ora of the Chamela bay region, Jalisco, Mexico. Occasional Papers California Acad. Sci. 148, 1±60. Maass, J.M. 1995. Conversion of tropical dry forest to pasture and agriculture. In: Bullock, S.H., Mooney, H.A., Medina, E. (Eds.), Seasonally Dry Tropical Forests. Cambridge University Press, Cambridge, pp. 399±422. MartõÂnez-YrõÂzar, A., SarukhaÂn, J., PeÂrez-JimeÂnez, A., RincoÂn, E., Maass, M., SolõÂs-Magallanes, A., Cervantes, L., 1992. Aboveground phytomass of a tropical deciduous forest on the coast of Jalisco Mexico. J. Tropical Ecol. 8, 87±96. Masera, O.R., Ordonez, M.J., Dirzo, R., 1997. Carbon emissions from Mexican forests: current situation and long-term scenarios. Climatic Change 35, 265±295. Mooney, H.A., Bullock, S.H., Medina, E., 1995. Introduction to: seasonally dry tropical forests. In: Bullock, S.H., Mooney, H.A., Medina, E. (Eds.), Seasonally Dry Tropical Forests. Cambridge University Press, Cambridge, pp. 1±9. Murphy, P.G., Lugo, A.E., 1986. Ecology of tropical dry forest. Ann. Rev. Ecol. Syst. 17, 67±88. Murphy, P.G., Lugo, A.E. 1995. Dry Forests of Central America and the Caribbean. In: Bullock, S.H., Mooney, H.A., Medina, E. (Eds.), Seasonally Dry Tropical Forests. Cambridge University Press, Cambridge, pp. 9±34. Pyne, S.J. 1984. Introduction to Wildland Fire B Fire Management in the United States. Wiley, New York. Trejo, I., Dirzo, R., 2000. Deforestation of seasonally dry tropical forest: a national and local analysis in Mexico. Biol. Conservation 94, 133±142. Uhl, C., Buschbacher, R., 1985. A disturbing synergism between cattle ranch burning practices and selective tree harvesting in the eastern Amazon. Biotropica 17, 265±268. Van Wagner, C.E., 1968. The line intersect method in forest fuel sampling. For. Sci. 14, 20±26. Watson, R.J., Noble, I.R., Bolin, B., Ravindranath, N.H., Verado, D.J., Dokken, D.J., 2000. IPCC Special Report On Land-Use, Land Change And Forestry. Cambridge University Press, Cambridge.
Biomass dynamics associated with deforestation, ®re, and, conversion to cattle pasture in a Mexican tropical dry forest J.B. Kauffmana,*, M.D. Steelea, D.L. Cummingsa, V.J. Jaramillob a
b
Department of Fisheries and Wildlife, Oregon State University, Corvallis, OR 97333, USA Departamento de EcologõÂa de los Recursos Naturales, Instituto de EcologõÂa, Universidad Nacional Autonoma de MeÂxico, A.P. 27-3 Xangari, Morelia, Michoacan, Mexico Received 1 January 2002
Abstract Few studies have described losses of biomass and structure arising from the conversion of tropical dry forest (TDF) to pasture, a common land-use practice in the most widespread forest type in the Neotropics. We sampled total aboveground biomass (TAGB) at ®ve distinct periods during the process of forest conversion to pasture. TAGB was measured after the primary forests were cut, after the initial forest slash ®res, and 1 year after pasture establishment. After 2 years, TAGB was quanti®ed before and after the ®rst pasture ®re conducted on site. To capture the variability in biomass burning, two treatments (Baja and Alta) based upon degree of biomass consumed were established in a randomized block design with three treatment blocks. Mean TAGB of the slashed primary forest was 118 Mg ha 1 in the Baja plots and 135 Mg ha 1 in the Alta plots. The slash ®res resulted in biomass loss of 62% of the TAGB (75 Mg ha 1) in the Baja treatment and 80% (108 Mg ha 1) in the Alta treatment. The greatest treatment differences in consumption occurred in the large wood (>7.6 cm diameter). Fires in the Baja treatment consumed 31% of the large wood, but only 7% in the Alta treatment. Prior to the ®rst pasture ®re, TAGB was 40.3 Mg ha 1 in the Baja treatment, and 20.9 Mg ha 1 in the Alta treatment. The pasture ®res consumed 63% of the TAGB in the Baja sites and 75% of the TAGB in the Alta treatment. Following the pasture ®res, the TAGB was 14.8 Mg ha 1 in the Baja treatment and 7.6 Mg ha 1 in the Alta treatment. In the ®rst 2 years of land cover change from dry forest to pasture there was a dramatic decline in TAGB (and hence aboveground C pools) totaling 87 and 94% of that of pre-disturbance forests. The total biomass lost via ®re and decomposition was 113 and 132 Mg ha 1; ®re accounted for 89±92% of the loss while decomposition/disappearance comprised 2±11% of the total biomass loss. The widespread distribution of tropical dry forests, their high rates of deforestation, and their high rates of biomass consumption during ®res suggest that they are signi®cant anthropogenic sources of atmospheric C. The dramatic loss of biomass and associated high degree of ®re severity may also affect future site productivity and the capacity for these sites to function as C pools in the future. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Deforestation; Fire; Forest biomass; Land conversion; Tropical dry forest; Tropical pasture; Biomass burning; Cattle pastures; Fire ecology
1. Introduction *
Corresponding author. Tel.: 1-541-737-1625; fax: 1-541-737-3590. E-mail address: [email protected] (J.B. Kauffman).
Land cover change involving deforestation and biomass burning of tropical forests is a signi®cant global concern due to emissions of greenhouse gasses,
0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 2 2 7 - X
2
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negative effects on biodiversity, and losses in site productivity (Murphy and Lugo, 1995; Watson et al., 2000). Rates of land conversion from forest to pasture or agriculture are higher in tropical dry forest than any other Neotropical forest type (Janzen, 1988; Murphy and Lugo, 1995; Masera et al., 1997). Tropical dry forests (TDF) comprise 42% of all tropical and subtropical forests and contain the highest human population densities in Latin America (Murphy and Lugo, 1986). Mooney et al. (1995) described TDF as occurring in the tropical regions where there are several months of severe, even absolute, drought, and are distinguished from savannas by the presence of tree dominated, occasionally closed canopy systems. Tropical dry forest (TDF) is the most abundant tropical forest type in Mexico covering 1:6 105 km2 (Masera et al., 1997). These areas are undergoing rapid rates of land cover change. It has been estimated that 1.4±1.9% of all Mexican tropical deciduous forests were being converted annually to pastures or other uses (Masera et al., 1997, Trejo and Dirzo, 2000). Therefore, it is important to have a better understanding of the effects of land conversion on biomass and nutrient dynamics of TDF in order to predict their contribution as sources and sinks of atmospheric C at country to continental scales. Very little is known of the biomass dynamics (loss and regrowth) associated with ®re and pasture establishment in TDF landscapes. While numerous studies have quanti®ed biomass losses associated with deforestation and biomass burning in moist and wet forests (Fearnside et al., 1993; Guild et al., 1998; Kauffman et al., 1995) few studies have examined it in dry forests (Kauffman et al., 1993). Expansion of cattle ranching has been by far the leading factor encouraging deforestation in the tropical forests of Mexico (Masera et al., 1997) and pasture establishment is the most common fate of converted TDF (Maass, 1995). Similar to other regions of the Neotropics, ®res are a commonly used approach to pasture management in TDF landscapes. Fires are frequently set to maintain the productivity, forage quality, and grass composition of cattle pastures (Uhl and Buschbacher, 1985; Kauffman et al., 1998). Yet we know of no published studies that have quanti®ed ®re behavior and biomass losses of pasture ®res in TDF landscapes. The overall objective of this study was to quantify the biomass dynamics and ®re behavior associated
with the initial phases of conversion of primary TDF to cattle pasture in the Chamela region, Mexico. This was experimentally examined with two separate ®re severity scenarios designed to encompass the variability that occurs in ®res for land conversion in this landscape. Treatments included a low consumption ®re (the ``Baja'' treatment) conducted when fuels were comparatively higher in moisture compared to the other treatment, a high consumption ®re (the ``Alta'' treatment). During the 3-year study period, we also quanti®ed: (1) biomass losses between ®re events; (2) vegetative regrowth for the 2 years following slash ®res; (3) biomass losses from a second ®re in the newly formed pastures and; (4) the cumulative losses of biomass associated with the conversion of TDF to cattle pasture. 2. Study site This study was conducted from 1993 to 1995 on the Ejido San Mateo, Jalisco MeÂxico near the EstacioÂn BiologõÂa Chamela of the Universidad Nacional Autonoma de MeÂxico (198300 N, 105830 W). The landscape is classi®ed as a Mesoamerican (Ceballos, 1995), Subtropical (Holdridge, 1967), seasonally dry deciduous forest (Murphy and Lugo, 1995). The climate is highly seasonal with a pronounced dry season. Precipitation averages 679 mm with the majority falling during the months of June±October (GarcõÂa-Olivia et al., 1995). Mean temperature is 24.9 8C with less than a 5 8C difference between the coolest and warmest months. The landscape is one of steep, low hills (slopes from 40 to 60%). Upland soils are relatively young, shallow, (0.5± 1.0 m in depth), isohyperthermic, Typic Ustorthents. They are poorly structured, sandy loam in texture, derived from rhyolite and have a pH of 6.0±6.5. Plant species richness of the Chamela region consists of 544 genera and 124 families and the principal vegetation of this upland site are deciduous trees in the Fabaceae family (Lott, 1993). Trees have a density > 4325 ha 1, are low in stature, and are relatively small in diameter (3±20 cm dbh; Jaramillo et al., in press). The rate of deforestation for the Chamela region has varied between 0.4 and 2.2% annually in the past 25 years (A. Miranda, personal communication).
J.B. Kauffman et al. / Forest Ecology and Management 176 (2003) 1±12
3. Approaches to land conversion and utilization in the Chamela region Biomass burning optimizes labor energy and capital expenditures for land clearing and the planting of agronomic crops or pasture for livestock (Bye, 1995). Communal landowners (Ejidatarios) in two Ejidos of the Chamela region have stated that slashand-burn is a common practice due to its low cost and high ef®ciency (GonzaÂlez-Flores, 1992). Decisions concerning the location and size of the plots as well as the time for forest conversion are heavily in¯uenced by the availability of economic resources and the uncertainty in land-use regulations. These include the availability of deforestation permits and the time involved for obtaining government loans to establish new pastures or to purchase cattle (GutieÂrrez-AlcalaÂ, 1993). Slashing is performed with chainsaw and machete at the onset of the dry season and slash ®res are set at the end of the dry season ( late April±early May). The precise day for the burn is determined by each landowner according to the morning weather; it requires a clear sky, high temperatures, and strong, constant winds. A stick dipped in kerosene is used to ignite litter and twigs of the felled vegetation along the perimeter of the plot. No ®relines (fuel breaks) are built to prevent the spread of ®re into adjacent forests. Even though litter mass is quite high (Jaramillo et al., in press) and moisture contents are quite low (4±6 months without rain), ®re does not spread into the forest beyond the ®rst few meters. Soon after burning, plots are seeded with pasture grasses, commonly Panicum maximum Jacq. and Cenchrus ciliaris L., and more recently with Andropogon gayanus L. Traditionally, a portion of the plot is also planted with corn, squash, or beans. These crops are harvested after a few months and then cattle are introduced into the pastures. Pastures are burned every 2 or 3 years to maintain productivity and decrease woody invasion. In summary, the generalized approaches to landuse/land conversion in the Chamela region are: (1) slashing existing forest with machete and chainsaw at the onset of the dry season; (2) removal of usable wood (due to the remoteness of our study site little if any wood was removed during this study); (3) allow the forest slash to dry during the dry season; (4) burn the slashed forest just prior to the onset of the rainy
3
season; (5) plant corn or other crops and pasture grasses immediately after burning; (6) harvest crops 3±4 months after planting, and introduce cattle to pasture; and, (7) burn pasture every 2±3 years after establishment. 4. Methods The experimental site was approximately 3.5 ha in size and encompassed an entire area to be converted to pasture by the Ejidatario. Three replicate blocks were established and each was subdivided into treatment sub-blocks consisting of a low (Baja) and high (Alta) consumption/severity burn treatments. Metal posts were used to delineate plot boundaries. The ®re severity treatments were randomly assigned to each subblock. The differences in ®re treatments were accomplished by varying the drying time between slash and burning. In 1993, Alta sites were slashed the last week of January and burned 9 May 1993 (95 days between slashing and burning). Baja sites were slashed the second week of February and burned 9 April 1993 (65 days between slashing and burning). No precipitation occurred during the period between forest cutting and ®re. To determine patterns of biomass decline associated with land conversion, we sampled total aboveground biomass (TAGB) at ®ve distinct time periods: (1) in 1993 before; and (2) immediately after slash burning; (3) towards the end of the 1994 dry season (about 1 year after the slash ®re) during the rastrojo (corn stubble)/early pasture establishment phase; (4) in 1995, prior to; and (5) immediately following the planned pasture ®re for this site. For all ®res, the combustion factor, de®ned as the percent of biomass lost during a ®re, was calculated as the percent difference in biomass before and after burning. Within each sub-block, 15 permanent transects were established (n 45 for each treatment) where total slashed and surface biomass components were measured. At each sampling period, along each transect, biomass of downed wood was quanti®ed non-destructively using the planar intercept technique (Brown and Roussopoulos, 1974; Van Wagner, 1968; Kauffman et al., 1995). Downed wood was partitioned into standard moisture timelag classes based upon diameter (Deeming et al., 1977; Table 1). Length of the sampling
4
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Table 1 Size classes in which the downed wood was partitioned for biomass calculations, transect lengths (m), and the mean speci®c gravity (g cm3) of wood collected each year of the study Wood diameter (cm)
Transect length (m)
Specific gravity (1993)
Specific gravity (1994)
Specific gravity (1995)
0.0±0.64 0.65±2.54 2.55±7.6 >7.6 (sound) >7.6 (rotten)
1 3 10 15 15
0.74 0.55 0.76 0.78 0.61
0.65 0.76 0.70 0.62 0.23
0.71 0.76 0.70 0.75 0.23
plane varied with the size class of wood material: 1 m for wood < 0:64 cm diameter; 3 m for wood 0.65± 2.54 cm diameter; 10 m for wood 2.54±7.6 cm diameter; and 15 m for wood > 7:6 cm in diameter. Biomass was determined by calculating volume and then multiplying volume by the mean speci®c gravity of wood samples. Each year, at least 50 samples of wood within each size class were randomly collected to estimate the mean speci®c gravity. Speci®c gravity was determined through division of the oven-dry weight of the sample by its volume (weight displaced in water). Residual standing trees were measured within three 225 m2 plots in each sub-block (n 9/treatment). The diameter at breast height (dbh) was measured to calculate standing tree biomass using the equation in MartõÂnez-YrõÂzar et al. (1992). Stumps were measured in three 20 m 1 m permanent plots in each sub-block (n 9/treatment). The diameter and height of all stumps were measured to calculate volume. Each volume measurement was then multiplied by the speci®c gravity of large wood to calculate mass. The biomass of attached foliage (i.e. the mass of leaves that remained attached to the slashed wood) was ascertained through determination of the ratio between its biomass and that of the wood particles < 0:65 cm in diameter (Kauffman et al., 1993; Kauffman et al., 1995). The ratio of attached foliage to twig biomass was calculated by collecting 50 random samples, oven drying, and then weighing the wood and leaf materials, separately. The mass of attached foliage was then estimated by multiplying the biomass of this wood size class by the ratio. In the slashed forest, surface litter composed of leaves, fruits, seeds, and bark fragments was collected in 50 cm 50 cm plots established 1m away from the end of each woody biomass transect (n 45/treatment). Dicot seedlings (<3 cm dbh) were also col-
lected in these plots. In the following years after the sites were converted to pasture, grasses, corn residue, cattle manure, dicot seedlings, and litter were also collected in these microplots, separated, oven dried and weighed at the station laboratory. Following the ®res, the same data were collected from paired microplots placed 2 m away from the end of each wood biomass transect. Ash mass was collected immediately after burning in nine 50 cm 50 cm microplots in the 1993 slash ®res (n 27/treatment) and six 50 cm 50 cm microplots in the 1995 pasture ®res (n 18/treatment). Ash was collected using a vacuum cleaner and a portable generator. At the time of ignition, we measured air temperature, relative humidity, and wind speed. During the ®res we measured the ¯ame length, height, depth, angle, and rate of spread of the ¯ame front (Alexander, 1982). At least two observers visually estimated these ¯ame parameters at ®ve to seven random locations within at each ®re. From ¯ame length measurements, we calculated ®reline intensity (i.e. the rate of energy release per unit length of the ¯ame front) using the equation given in Alexander (1982). The statistical analysis performed was analysis of variance. Statistical analyses included comparisons of aboveground pre-®re versus post-®re biomass, comparisons between treatments at each phase of pasture establishment, percent biomass consumption, biomass loss due to decomposition, and regrowth. 5. Results The TAGB of the slashed forest in sub-blocks ranged from 113 to 137 Mg ha 1 with a mean of 118 Mg ha 1 in the Baja treatment and 135 Mg ha 1 in the Alta treatment (Table 2). There were no sig-
J.B. Kauffman et al. / Forest Ecology and Management 176 (2003) 1±12
5
Table 2 Total aboveground biomass (Mg ha 1) and the quantity of biomass consumed (the combustion factor, %) during the slashed forest phase of land conversion of tropical dry forest to pasture BajaÐlow severity Pre-fire Litter Attached foliage Dicots Total fine fuels Wood debris (diameter, 0±0.64 0.65±2.54 2.55±7.62 7.63±20.5 (sound) 7.63±20.5 (rotten) 7.63±20.5 (total) >20.5 (sound) >20.5 (rotten) >20.5 (total) Total wood Standing trees Stumps Total aboveground biomass Ash
AltaÐhigh severity
Post-fire
Combustion factor
Pre-fire
Post-fire
Combustion factor
7.4 0.47 0.17 8.1
0.61 0.03 0.06 0.61
0.0 0.04 0.0 0.04
0.0 0.02 0.0 0.06
100 88.9 100 99.2
0.0 3.8 0.0 0.29
7.4 0.41 0.2 8.0
0.56 0.03 0.04 0.56
0.0 0.0 0.0 0.02
0.0 0.0 0.0 0.02
100 100 100 99.9
0.0 0.13 0.0 0.05
cm) 2.6 16.8 24.7 29.4 4.4 33.8
0.17 1.2 1.9 2.8 0.88 2.9
0.23 2.0 6.1 18.0 3.2 21.2
0.09 0.78 0.84 3.2* 1.1 3.1
88.8 89.1 70.8 31.7 85.1 28.7
3.8 3.6 4.7 11.0 7.2 10.0
2.4 15.5 28.0 31.4 5.0 36.4
0.19 1.2 1.8 3.5 1.2 4.1
0.01 0.46 2.8 5.5 2.0 7.5
0.01 0.160 0.522 1.3 0.93 1.5
99.9 96.8 87.7 78.5 86.5 62.3
0.13 0.97 3.7 5.7# 9.9 14.4
7.7 2.7 0.8 0.8 8.5 2.8
3.1 1.7* 0.0 0.0 3.1 1.7
56.1 22.9 100 0.0 60.9 20.8
86.4 4.2 9.9 4.0 13.9 5.0
32.7 4.0 0.8 0.8 10.1 2.6*
62.4 4.1 78.1 21.9 32.7 19.7
91.3 5.9 14.8 4.6 20.9 5.7
15.2 2.5 1.0 1.0 10.6 3.1
80.4 4.0 95.7 4.3 49.8 19.0
118.2 2.8
43.6 16.4
62.4 15.1
134.9 1.4
26.8 4.1
80.2 2.8
3.9 0.8
7.8 2.8 1.2 0.83 9.0 2.8
4.4 1.9* 0.0 0.0 4.4 1.9
71.6 19.4 100 0.0 77.3 15.7
4.7 0.4
Biomass was signi®cantly different (P < 0:10) before and after ®re within treatments in all components except noted by an asterisk (). A hash symbol (#) indicates a signi®cant difference in the combustion factor between treatments.
ni®cant differences between treatments in any of the pre-®re biomass components. Slashed wood comprised the majority of the TAGB with a mass of 86 and 91 Mg ha 1 in the Baja and Alta treatments. Total ®ne fuels (i.e. litter, dicot, and attached foliage) was 8 Mg ha 1 in both treatments comprising 7 and 6% of the TAGB in the Baja and Alta treatments, respectively. Standing tree biomass was 10 and 15 Mg ha 1 and stump biomass was 14 and 21 Mg ha 1 in the Baja and Alta treatments, respectively. Decisions on the timing of the burning and the manner in which the plots were burned were that of the Ejiditarios. All slash ®res were conducted at similar ambient air temperatures and relative humidity (Table 3). Fires were conducted at midday when temperatures were highest (27±288) and relative humidity was lowest (47±53%). The ®reline intensity of the slash ®res was very high (7076 and 7761 kW m 1) with a mean ¯ame length of 4.7 m.
The mean rate of spread of the ®res was quite different between treatments due to differences in the ignition patterns. The Baja sites were burned by backing and ¯anking ignition patterns (i.e. ignited only on the upslope and sides of the burn units while the Alta sites were burned with head ®res (i.e. ignited at the bottom of the units and allowed to burn up slope). The slash ®res resulted in signi®cant declines in TAGB as well as in most components of aboveground biomass (Table 2). In the Alta treatment, the combustion factor was higher (80%) and there was less variation between blocks (a range from 75 to 80%) compared to the Baja treatment (a mean combustion factor of 60% and a range of 32±78%). The mean post®re TAGB was 44 and 27 Mg ha 1 in the Baja and Alta treatments, respectively. Very high rates of consumption were measured for the ®ne fuels (i.e. litter, attached foliage, and dicots where combustion factors were 89±100%). In the Baja treatment, there was not a
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J.B. Kauffman et al. / Forest Ecology and Management 176 (2003) 1±12
Table 3 Weather conditions at the time of burning and ®re behavior of slash and pasture ®res, 1993 and 1995 Treatment
BajaÐlow severity
AltaÐhigh severity
Pasture fire (treatments combined)
Slash date Fire date Days between slashing and fire Weather at time of fire Relative humidity (%) Air temperature (8C) Wind speed (km h 1)
Mid-February 9±10 April 1993 65 days
Late-January 3±9 May 1993 95 days
15 June 1995
Flame length (m) Flame height (m) Flame depth (m) Flame angle (8) Rate of spread (m min 1) Fireline intensity (kW m 1)
53 27 13.3
47 28 12.8
67 31 12.8
4.8 3.5 3.6
4.6 3.3 4.6
3.5 4.3 18
65 1.2 7761
52 16.2 7076
statistically signi®cant decline in large wood (>7.6 cm in diameter) or in the mass of stumps. These had the lowest combustion factor of all components in the ecosystem. The biomass of all components signi®cantly decreased following ®re in the Alta treatment except for sound wood > 20:5 cm in diameter (P 0:2). The most abundant component of aboveground biomass was downed wood 7.6±20.5 cm in diameter. This component comprised 23±24% of the pre-®re TAGB. Signi®cant differences in combustion between treatments (P 0:09) were detected within this component (i.e. a combustion factor of 29% in the Baja treatment and a combustion factor of 62% in the Alta treatment). Apparently, the differences in time in which the slash was allowed to dry most strongly affected differences in the degrees of consumption of the large wood components. Following ®re, the biomass of all wood components was twice as high in the Baja treatment (33 Mg ha 1) compared to the Alta treatment (15 Mg ha 1). One year after slash ®res, the TAGB of the young pasture/rastrojo phase of establishment was 40 Mg ha 1 in Baja plots and 29 Mg ha 1 in Alta plots (Table 4). This represented a loss in biomass of 3.2 Mg ha 1 in the Baja treatment and a gain in TAGB of 2.4 Mg ha 1 in the Alta treatment. The gains in aboveground biomass from 1993 arose from the growth of pasture grasses and corn. The mass of standing corn stalks was 1.4 Mg ha 1 in the Baja treatment and 1.9 Mg ha 1 in the Alta treatment. Dead grass mass was 0.5±1.1 Mg ha 1. Litter, which was
66 133 3912
largely comprised of corn and grass residue, had a biomass of 3.2 Mg ha 1 in both treatments. Losses in biomass between 1993 and 1994 were largely due to the disappearance of the residual forest biomass (i.e. Table 4 Total aboveground biomass (Mg ha 1) during the early pasture/ rastrojo phase of land conversion from tropical dry forest, approximately 1 year following conversion from tropical dry forest BajaÐlow severity Litter Dicot Corn Dead grass Total fine fuels Wood debris (diameter, cm) 0±0.64 0.65±2.54 2.55±7.62 7.63±20.5 (sound) 7.63±20.5 (rotten) 7.63±20.5 (total) >20.5 (sound) >20.5 (rotten) >20.5 (total) Total wood Standing trees Stumps Total aboveground biomass
AltaÐhigh severity
3.2 0.4 1.4 1.1 6.1
0.4 0.2 0.2 0.4 0.8
3.1 0.3 1.9 0.5 5.8
0.4 0.01 0.02* 0.02 0.1
0.2 1.9 9.1 14.4 0.3 14.7
0.04 0.5 0.9 9.4 0.1 2.4
0.07 0.7 3.1 6.5 0.2 6.7
0.0 0.2 0.6 1.0 0.1 1.1
0.6 0.6 0.0 0.0 0.6 0.6 26.4 1.0 6.9 40.4
3.2 1.0 1.6 11.5
0.0 0.0 0.2 0.2 0.2 0.2 10.7 2.1 10.6 29.2
1.3 1.0 5.6 8.2
A asterisk () indicates that biomass was signi®cantly different (P < 0:10) between treatments.
J.B. Kauffman et al. / Forest Ecology and Management 176 (2003) 1±12
7
Table 5 Total Zaboveground biomass (Mg ha 1) and the quantity of biomass consumed (the combustion factor, %) before and following ®re during the early pasture phase of land conversion from tropical dry forest, 1995 BajaÐlow severity Pre-fire Litter Dicot Live grass Dead grass Manure Total fine fuels
5.0 0.6 0.4 6.8 0.2
Post-fire 0.37 0.2 0.3 2.1 0.2
13.0 2.2
Wood debris (diameter, cm) 0±0.64 0.1 0.03 0.65±2.54 1.2 0.4 2.55±7.62 6.9 0.7 7.63±20.5 (sound) 9.2 1.8 7.63±20.5 (rotten) 1.1 0.36 7.63±20.5 (total) 10.4 2.0 >20.5 (sound) >20.5 (rotten) >20.5 (total)
AltaÐhigh severity
0.6 0.63 0.6 0.4 1.2 0.73
Total wood Standing trees Stumps
19.8 2.4 0.8 0.7 6.7 2.2
Total aboveground biomass Ash
40.3 6.9
0.0 0.6 0.0 0.1 0.0
0.0 0.3* 0.0 0.05 0.0
0.6 0.27 0.03 0.30 3.0 3.9 0.0 4.9
0.02* 0.16* 0.4* 0.91 0.0 1.0
Combustion factor 100 93.6 100 95.5 100
0.0 12.7 0.0 10.3 0.0
Pre-fire 5.5 1.0 0.2 3.3 1.0
Post-fire 0.5 0.6 0.2 0.6 0.4
0.2 0.0 0.0 0.1 0.0
0.2 0.0 0.0 0.1 0.0
Combustion factor 94.3 100 100 94.2 100
5.8 0.0 0.0 12.7 0.0
86.0 7.0
11.0 1.2
0.3 0.2
87.5 8.0
72.2 77.7 50.0 58.4 100 54.4
0.03 0.7 3.1 8.8 0.1 8.9
0.0 0.1 2.1 1.1 0.0 1.4
100 70.2 18.4 81.6 100 72.2
18 8.8 7.0 8.3 0.0 8.8
0.02 0.2 0.5 1.5 0.1 1.5
0.0 0.1 0.5 0.42 0.0 0.5
0.0 15.1 16.2 6.3# 0.0 11.2
0.0 0.0 0.0 0.0 0.0 0.0
100 0.0 100 0.0 100 0.0
0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0
100 0.0 100 0.0 100 0.0
8.3 1.1* 0.4 0.3* 5.5 2.1*
52.3 6.5 59.0 5.4 32.7 19.2
12.6 1.8 0.9 0.5 4.6 1.8
3.8 0.7 0.5 0.5* 3.0 2.0*
67.9 6.0 58.9 5.5 49.2 24.4
63.1 4.5
29.0 2.8
7.6 3.0
75.0 7.9
14.8 2.9 2.1 0.15
2.3 0.2
Data were collected approximately 2 years following conversion from dry forest. Biomass was signi®cantly different (P < 0:10) before and after ®re within treatments in all components except noted by an asterisk (). A hash symbol (#) indicates a signi®cant difference in the combustion factor between treatments.
the mass of the downed wood, standing trees, and stumps combined). In the year following the 1993 post-®re measurements, the residual forest biomass declined from 43.6 to 34.3 Mg ha 1 in the Baja treatment and from 26.8 to 23.4 Mg ha 1 in the Alta treatment. We found that the mean speci®c gravity of large wood declined from 0.78 to 0.62 during this period of time, (Table 1) suggesting losses were due to decomposition. Differences in the loss of wood to decomposition between treatments indicates that less readily decomposable wood remained on site in the Alta treatment compared to the Baja treatment. Two years following slash burning, the TAGB was 40 Mg ha 1 in the Baja treatment and 29 Mg ha 1 in the Alta treatment (Table 5). There was little change in the TAGB from the previous year. The relatively stable
level of TAGB during this time period re¯ected a balance between increases in grass biomass with losses in wood biomass through decomposition. In 1995, pre-®re total ®ne fuels (grasses, litter, and dicots) was 11±13 Mg ha 1 and comprised 32±37% of the TAGB. The residual forest biomass was 27 Mg ha 1 in the Baja treatment and 18 Mg ha 1 in the Alta treatment. Thus, 2 years following the initial slash ®re, residual forest biomass declined by 16.3 Mg ha 1 in the Baja treatment and 8.7 Mg ha 1 in the Alta treatment. This was a signi®cant decrease in wood mass during this time period for the Baja treatment (P 0:04) but not for the Alta treatment (P 0:15). This suggests there was a compensation in biomass losses with greater decomposition occurring on sites with lower levels of combustion due to ®re.
8
J.B. Kauffman et al. / Forest Ecology and Management 176 (2003) 1±12
Fig. 1. Biomass dynamics associated with the initial 2 years of land conversion from primary tropical dry forest to pasture at the Ejido San Mateo, Jalisco, Mexico.
J.B. Kauffman et al. / Forest Ecology and Management 176 (2003) 1±12
The entire study area was burned on 15 June 1995. Conditions at the time of burning were quite dry as no signi®cant precipitation had fallen on the site since the previous October. Because of the extremely ¯ammable conditions of the pasture and the proximity of the blocks to one another, it was impossible to burn the treatments separately and maintain any resemblance to actual land-use practices. Pasture ®res were ignited at the bottom of the slope and allowed to burn upslope as a head®re. This resulted in a very rapid rate of spread (a mean of 133 m min 1). By comparison, the rate of spread of head®res through the grasses of the pastures was 8.2 times more rapid than that for head®res in the slash fuels of the Alta treatment. Mean ¯ame length of the ®res was 3.5 m with a ®reline intensity of 3911 kW m 1. It required < 15 min for the entire study area to burn. Pastures ®res resulted in a decline in TAGB from 40 to 15 Mg ha 1 (a combustion factor of 63%) in the Baja treatment and from 29 to 8 Mg ha 1 (a combustion factor of 75%) in the Alta treatment (Table 5). While the combustion factor was higher in the Alta treatment compared to the Baja treatment, it had a lower quantity of biomass consumed by ®re due to a lower pre-®re TAGB (P 0:2). A mean of 25.5 Mg ha 1 of biomass was consumed in the Baja treatment and 21.4 Mg ha 1 was consumed in the Alta treatment. Essentially all of the ®ne fuels, (grasses, litter, and dicots) were consumed by ®re in all plots. The combustion factor of wood 7.6±20.3 cm in diameter was signi®cantly higher (P 0:03) in the Alta treatment (82%) than the Baja treatment (58%). Wood losses by this pasture ®re were 12 Mg ha 1 in the Baja treatment and 9 Mg ha 1 in the Alta treatment. Following the pasture ®res, the dominant pools of aboveground mass were stumps and large wood debris. In the ®rst 2 years of land cover change from TDF to cattle pasture, there was a dramatic decline in aboveground biomass (Fig. 1). TAGB decreased from 118 to 15 Mg ha 1 in the Baja sites and from 135 to 8 Mg ha 1 in the Alta sites. A total of 100±130 Mg ha 1 of biomass was consumed by the slash and pasture ®res combined. The cumulative decomposition/ disappearance losses of the residual forest biomass in the time period between ®res was estimated to be 16.3 Mg ha 1 in the Baja treatment and 8.8 Mg ha 1 in the Alta treatment. The total biomass lost via
9
®re and decomposition was 113 and 132 Mg ha 1; ®re accounted for 89±98% of this loss while decomposition/ disappearance comprised 2±11% of the total biomass loss. 6. Discussion Intensively sampling biomass on ®ve occasions during the early phases of land conversion of forest to pasture allowed us to quantify the patterns and causes of biomass decline associated with this common activity (Fig. 1). We measured large reductions in TAGB and found that the majority of biomass losses occurred as a result of biomass burning, while smaller amounts were lost through decomposition, cattle grazing and other processes (e.g. erosion). The initial ®re resulted in biomass losses of 75 and 108 Mg ha 1 in the Baja and Alta treatments. Between ®re events we estimated that decomposition/disappearance losses were 16.2 Mg ha 1 in the Baja treatment and 8.7 Mg ha 1 in the Alta treatment. Pasture ®res resulted in the decline of an additional 21±26 Mg ha 1 (Fig. 1). These results are quite different than estimates of combustion and decomposition that have been used to estimate C emissions and sequestration in tropical forests at country to continental-level scales. For example, in calculating C emissions arising from the deforestation of Mexican dry forests Masera et al. (1997) used an estimate of 40% as the quantity of the C pool lost via slash and pasture ®res. For Amazonian (Brazil) evergreen forests, Fearnside (1992) suggested the combustion factor of biomass would be 28%, with 35% of the aboveground C released through three reburns. He estimated 62% of the C would be released by decay. In contrast, for the tropical dry forest landscape of this study we found that combustion releases by ®re were 62±80% for slash ®res and 63±75% for pasture ®res. Decomposition/ disappearance losses equaled only 7±12% of the pre®re forest biomass. There are many factors that affect the quantity of biomass consumed by a ®re. These factors include: (1) climateÐnumber of rainless days between slashing and burning, humidity, wind speed, and temperature at the time of burning; (2) the composition and structure of the vegetation, particularly the biomass and size class of the slashed forest fuels; and (3)
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J.B. Kauffman et al. / Forest Ecology and Management 176 (2003) 1±12
few estimates of biomass losses associated with land conversion exist. In a slashed second growth TDF in northeastern Brazil, Kauffman et al. (1995) also found very high rates of biomass combustion (i.e. a combustion factor of 78±95%). However, there were great differences in the structure of the Brazilian slashed forests compared to those measured in this study. The TAGB of the 16-year old secondary Brazilian dry forest was 74 Mg ha 1. Large wood (<7.6 cm) only comprised 3±9% of the TAGB at the Brazil site while large wood comprised 34±36% of the TAGB in the San Mateo sites. This likely re¯ects the structural differences in the secondary forests compared to the primary forest. There are great differences in the biomass and response to ®re associated with land cover change in tropical evergreen forests compared to tropical dry forests. Compared to TDFs, tropical evergreen forests have a greater TAGB (Table 6) and ®res associated
topographic features such as slope, which can affect the rate of ®re spread and continuity of forest fuels (Pyne, 1984). The length of the dry season in this region of Mexico and the small structure of the forest trees resulted in high percentages of biomass consumed. Based upon differences in the two treatments examined in this study, we found the time period between slashing and burning likely affected the amount of moisture left in the slash, particularly in the large woody debris where consumption rates were signi®cantly different among treatments. Upon inspection of the post burn conditions, local Ejidatarios expressed that post-®re conditions in this experiment re¯ected the range of slash consumption present in successful land clearing practices. While the rates of land conversion have been suggested to be higher in TDF than other forest types (Janzen, 1988; Murphy and Lugo, 1995; Masera et al., 1997; Trejo and Dirzo, 2000) surprisingly
Table 6 Total aboveground biomass (Mg ha 1) before and after burning (Mg ha 1) and the combustion factor (%) for ®res in selected Neotropical forests and pastures Pre-fire
Post-fire
Combustion factor
Forests Tropical dry forest, Mexico Low severity High severity
118.2 2.8 134.9 1.4
43.6 16.4 26.8 4.1
63 80
Tropical dry forest, Brazil Low severity Medium High
73.8 2.7 74.0 5.0 73.7 1.9
16.4 3.0 8.3 2.8 4.0 1.1
78 88 95
Tropical evergreen/Brazil Jacunda Maraba Santa barbara Jamari Balteke Sergpipe Pastures Tropical cattle pasture, Mexico from TDF Low severity High severity Tropical cattle pasture, from evergreen forest, Brazil Francisco Durval Joao Jamari
292.4 434.6 290.2 361.2 354.8 398.8
35.8 72.2 20.4 36.8 47.8 44.7
40.3 6.9 29.0 2.8
139.9 207.1 165.1 155.4 187.5 85.2
24.3 53.7 9.2 24.1 5.8 30.0
14.8 2.9 7.6 3.0
52 52 43 57 47 54
63 75
Source This study
Kauffman et al. (1993)
Kauffman et al. (1995)
Guild et al. (1998)
This study
Kauffman et al. (1998) 53.3 119.2 72.7 66.3
4.8 35.0 13.1 13.3
8.8 2.0 94.7 35.2 38.2 9.0 45.6 11.0
83 21 47 31
Guild et al. (1998)
J.B. Kauffman et al. / Forest Ecology and Management 176 (2003) 1±12
with land conversion have a lower combustion factor (Kauffman et al., 1992, Guild et al., 1998). For example, utilizing very similar methods Kauffman et al. (1995) and Guild et al. (1998) reported the TAGB of slashed evergreen forest in the Brazilian Amazon ranged from 290 to 435 Mg ha 1 and the combustion factor ranged from 43 to 54% (Table 6). The absolute biomass consumed was 126± 228 Mg ha 1 in the slashed tropical evergreen forest ®res compared to a biomass consumption of 74± 108 Mg ha 1 in the TDF slash ®res of this study. This suggests that the contribution of CO2 and other greenhouse gasses to the atmosphere arising from land cover change of the Mexican tropical dry forest is not trivial. Masera et al. (1997) calculated that 320,000 ha of Mexican TDF is lost each year via land conversion. Using these data and results from this study, we estimate that ®res associated with dry forest to pasture conversion in Mexico could potentially result in annual losses from 23 to 35 Mg of aboveground biomass. This does not include the losses associated with ®res in pastures which are likely another prominent source of greenhouse gasses. The pasture ®res in this study consumed 21± 26 Mg ha 1 (Fig. 1). We would not expect the quantity of biomass consumed in future pasture ®res to be as high because little residual forest biomass remained after the pasture ®res measured in this study (Table 5). Nevertheless, 11±12 Mg ha 1 of the ®ne fuel biomass that accumulated since pasture establishment (i.e. litter, dicot, live grass, dead grass, and manure) was consumed. In older pastures of the Chamela region, Jaramillo et al. (in press) found these components to be 7±8 Mg ha 1. These ®ne fuels, as well as any residual wood would likely be consumed by future ®res. There is a wide variation in biomass and ®re effects among pastures in tropical forests. In Neotropical evergreen forests, the TAGB of cattle pastures ranges from 7 to 119 Mg ha 1 (Hughes et al., 2000; Kauffman et al., 1998). When pastures are burned in the Amazon, the proportion of the TAGB consumed is variable with combustion factors ranging from 21 to 84% (Table 6). The decline in TAGB due to pasture ®res in the Amazon was 21±45 Mg ha 1 (Guild et al., 1998; Kauffman et al., 1998) which is similar to, or higher than losses in the pasture ®res of this study. The wide range in variation between pasture biomass and
11
quantities of biomass consumed by ®re suggests that site/forest speci®c data are needed to accurately quantify biomass and nutrient dynamics associated with land-use in tropical ecosystems. The losses in aboveground biomass associated with land cover change in the TDF of this study are quite dramatic. With a decline in TAGB of 87 to 94% associated with land conversion we would expect similar declines in the ecosystem pools of C as well as many nutrients. Jaramillo et al. (in press) reported that the TAGB of TDFs in the Chamela region comprised 41% of the total ecosystem C pool. Thus, current patterns of land-use as described in this study will likely affect ecosystem productivity as well as limiting land management options for the future. Alternative land-uses that lessen the dramatic impact on biomass decline, ecosystem structure, and soil exposure, while providing a sustainable means of income are in need of development. Acknowledgements This project was funded by NSF grant DEB 9118854 ``Phosphorus, Carbon, and Nitrogen Dynamics in Perturbed Neotropical Dry Forest''. We are grateful to Mr. Ramiro PenÄa and the Ejido San Mateo for allowing us to work on their lands. Lisa Ellingson, Pedro GonzaÂlez, Daniela Roth, and Georgina GarcõÂa provided invaluable assistance in the ®eld. We thank personnel at the EstacioÂn BiologõÂa Chamela, Universidad Nacional AutonoÂma de MeÂxico for their assistance in making this project possible. We also thank the Instituto de EcologõÂa, Universidad Nacional AutonoÂma de MeÂxico, Campus Morelia, for generously providing of®ce space and facilities to complete the ®nal drafts of this manuscript. References Alexander, M.E., 1982. Calculating and interpreting forest ®re intensities. Can. J. Bot. 60, 349±357. Brown, J.K., Roussopoulos, P.J., 1974. Eliminating biases in the planar intersect method for estimating volumes of small fuels. For. Sci. 20, 350±356. Bye, R., 1995. Ethnobotany of the Mexican tropical dry forests. In: Bullock, S.H., Mooney, H.A., Medina, E. (Eds.), Seasonally Dry Tropical Forests. Cambridge University Press, Cambridge, pp. 423±438.
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