Untangling a Decline in Tropical Forest Resilience: Constraints on the Sustainability ofShifting Cultivation Across the Globe

Deborah Lawrence1,5, Claudia Radel2, Katherine Tully1, Birgit Schmook3, and Laura Schneider4

1 Department of Environmental Sciences, University of Virginia, Clark Hall, PO Box 400123, Charlottesville, Virginia 22904, U.S.A.

2 Department of Environment & Society, Utah State University, 5215 Old Main Hill, Logan, Utah 84322, U.S.A.

3 El Colegio de la Frontera Sur, Av del Centenario Km 5.5, Chetumal, Quintana Roo, Mexico

4 Department of Geography, Rutgers University, 54 Joyce Kilmer Drive, Piscataway, NJ 08854, U.S.A.

ABSTRACT

Shifting cultivators depend on forest biomass inputs to nourish their crops. For them, forest resilience has an immediate impact: it affects crop productivity. A declinein the rate of recovery following shifting cultivation would ultimately affect local, regional and global carbon budgets, with feedbacks to climate. Yet the long-termimpacts of shifting cultivation have been quantified in only six locations. In this study, we reanalyze data from these locations to determine whether the rate of biomassrecovery is the same from cycle to cycle. Further, using case studies in Southern Yucatan, Mexico and West Kalimantan, Indonesia, we investigate the ecological andsocioeconomic factors that affect forest resilience and thus determine whether or not shifting cultivation is sustainable. The reanalysis links aboveground biomassrecovery following shifting cultivation to site productivity, forest age, fallow length, history of cultivation, and soil texture. Across locations, biomass accumulation ratedeclines by 9.3 percent with each cycle of shifting cultivation. Per cycle change in biomass accumulation rate is significantly more negative in younger forests and foreststhat experience a shorter fallow period. However, more detailed analyses for two case studies suggest that a purely ecological framework is of limited effectiveness inexplaining variability in the effect of repeated shifting cultivation. Rather, socioeconomic factors such as migration, subsidies, roads, and settlement history can alter theoutcome of shifting cultivation by limiting the accumulation and use of local knowledge.

Key words: biomass accumulation; fallow length; forest age; Indonesia; local knowledge; Mexico; migration; secondary forest.

SHIFTING CULTIVATION WAS THE DOMINANT FORM of agriculture in

most tropical areas until the end of the twentieth century, when po-

litical and economic integration at many geographic scales encour-

aged more intensive agriculture. As a major driver of deforestation

for centuries, shifting cultivation has left an ecological legacy around

the world. Global data are sparse, but perhaps 500 million people

(ca 10 percent of global population) practiced shifting cultivation inthe 1980s (Mertz et al. 2009). Where shifting cultivation continues,

the legacy of use determines biomass regeneration in fallows, and

ultimately, the yield of future crops (Lawrence 2005, Zarin et al.2005). Where conservation has intervened, this legacy determines

the potential of newly protected areas to provide carbon benefits in

addition to biodiversity value (Lawrence et al. 2005, Leisz et al.2007, Vester et al. 2007). Where permanent cropping systems have

emerged (Agus & van Noordwijk 2005, Sturgeon 2007), the legacywill continue to influence the productivity of the land.

Land use change, predominantly in the tropics, is responsible

for approximately 15–20 percent of current global carbon emis-

sions (Canadell et al. 2007). Some tropical deforestation occurs in

mature forest, however, much occurs in secondary forests. Substan-

tial, unacknowledged uncertainty exists surrounding the ability of

deforested lands to resequester carbon because of a failure to con-

sider the effects of land use history. Biomass recovery varies withland use type (Pascarella et al. 2000, Holl 2007) and intensity (Uhl

et al. 1988, Hughes et al. 1999). Multiple cycles of use have no

reported effect in some regions and a modest or dramatic negative

effect in others (Table 1).

Although forest age and disturbance type have been incorpo-

rated into some models, global carbon cycle models do not explic-

itly address the issue of repeated use (see Friedlingstein et al. 2006).

Arguably, land use type may be distinguished by remote sensing orsocioeconomic surveys and incorporated into a spatial framework

for simulation models at various scales. Disturbance intensity and

frequency are more difficult to ascertain even for one region. A

closer assessment of regional and global trends in patterns of recov-

ery is essential to determine whether the number of cycles of dis-

turbance should be incorporated into global carbon models and

whether this modification can be done relatively simply. Much of

the tropical forest landscape has been subjected to shifting cultiva-tion in the recent past, even as shifting cultivation is evolving into

other land use systems (Hansen & Mertz 2006, Padoch et al. 2007).

Understanding its legacy, at broad scales, is critical to assessing the

role of tropical forests in the global carbon cycle.

Constraints on forest regrowth have always affected manage-

ment decisions by local stakeholders. Increasingly, national and in-

ternational stakeholders are evaluating forest regrowth, with the

advent of Reduced Emissions from Deforestation and Degradation(REDD) as a mechanism for conferring carbon credits on good forest

stewards (Singh 2008). Understanding forest regrowth is also critical

for predicting the effects of land use change on the global carbon

cycle, with implications for climate. Finally, from a theoretical

Received 29 September 2008; revision accepted 5 July 2009.5Corresponding author; e-mail: dl3c@virginia.edu

BIOTROPICA 42(1): 21–30 2010 10.1111/j.1744-7429.2009.00599.x

r 2009 The Author(s) 21

Journal compilation r 2009 by The Association for Tropical Biology and Conservation

SPECIAL SECTION

perspective, assessing constraints on forest regrowth represents an

opportunity to move beyond case studies of land use change.The same land use changes occur in many parts of the world,

yet the land change science community has not done enough com-

parative work to understand whether and how the outcomes differ

(but see Tucker et al. 2005, Nagendra 2008). To make theoretical

advances in land change science, we need to derive generalities,

where possible, from multiple case studies (e.g., Turner et al. 2003).

The most effective way to do so is to hold the general type of land

change constant (e.g., shifting cultivation) while examining varia-tion in drivers and consequences. The coupled social–ecological

system is never truly replicated from place to place, but core

elements recur. Through a comparative examination of case stud-

ies in different regions of the tropics, we hope to build a frame-

work for understanding the major constraints on tropical forest

transitions.

In this paper, we determine the long-term effects of shifting

cultivation on forest resilience, defined as the ability of the forest torecover biomass at a similar rate following subsequent disturbances.

First, we use published data from six tropical areas to determine the

mean effect of each additional cycle of shifting cultivation on bio-

mass production. Second, we ask whether effect size varies with soil

texture, inherent productivity, average fallow length, age of the for-

ests being compared, or history of the forests being compared. Fi-

nally, to extend insights derived from this analysis, we closely

examine two case studies in Mexico and Indonesia, for which wehave considerable socioeconomic as well as ecological data.

METHODS

REANALYSIS OF FOREST BIOMASS RECOVERY.—We gathered data from

published studies on the effect of repeated shifting cultivation (Ta-

ble 1). Our objective was to compare secondary forest stands with

different histories, defined by the number of slash–burn–crop–fal-

low cycles. Thus, knowledge of the number of prior cycles at a givenstand was required; only six studies present these data. Previous

work at both the local (Lawrence 2005) and regional scale ( Johnson

et al. 2000, Eaton & Lawrence 2009) demonstrated the importance

of accounting for inherent site fertility (soils, climate) as well as

forest age when comparing forest stands. Therefore, we only com-pared stands from the same location (village or landscape) that were

no more than 1 or 2 years apart in age but had been subject to a

different number of fallow cycles. We included only one age� his-

tory combination (e.g., 10 yr old, two cycles) per location to avoid

over-sampling a given age or history. When two or more stands

shared age and history, we used mean biomass values to

describe such a stand. These criteria yielded 31 comparisons

(among 47 stands) at six locations, predominantly in the neotro-pics (Table 2).

For each stand, we divided live aboveground biomass (Mg/ha)

by forest age (years) to yield an estimate of annual biomass incre-

ment (ABI in units of Mg/ha/yr). As the stands were relatively

young (mean age 8.2 yr), a linear model of biomass increase was

better than a nonlinear model (Steininger 2000). For each pair of

stands, we subtracted the ABI of the stand with fewer cycles from

the ABI of the stand with more cycles, and then divided by thedifference in number of cycles. The result was a per cycle change

in ABI:

Per cycle change in ABI ¼ABIi � ABIj

i � j

where ABIi is the annual biomass increment of a stand age x with i(more) cycles and ABIj is the annual biomass increment of a stand

of the same age with j (fewer) cycles. A negative number indicates

slower regrowth with each subsequent cycle of shifting cultivation; a

positive number indicates faster regrowth. We expressed the differ-ence as a percent of initial ABI (ABIj), referred to as ‘per cycle

difference’ below. This formulation normalized the data, allowing

for similar proportional effects regardless of the inherent produc-

tivity of the site. If all sites show a similar proportional effect, a

global ‘fix’ could be applied to model future carbon sequestration as

a function of disturbance history.

A one-sided t-test of relative per cycle difference determined

whether the overall effect of repeated cultivation differed signifi-cantly from zero (N = 29). Two outliers were excluded; both in-

volved a stand with ABI one to two orders of magnitude lower than

TABLE 1. Reported changes in aboveground biomass increment (ABI, Mg/ha/yr) after multiple cycles of shifting cultivation, according to the original studies.

History gradient Location

Mean annual

rainfall (mm)

Mean annual

temperature (1C)

Per cycle change

in ABIa (%) Reference

2 cycles Rondonia, Brazil 2350 25 None Hughes et al. (2000)

2 cycles Manaus, Brazil 2180 27 None Gehring et al. (2005)

3–4 cycles Santa Cruz, Bolivia 1600 23 None Steininger (2000)

3–4 cycles Southern Yucatan, Mexico 900–1400 25 –26b Eaton & Lawrence (2009)

10 cycles West Kalimantan, Indonesia 3600 28 –2 Lawrence (2005)

10 burnsc Amazonia –12.5 Zarin et al. (2005)

aAnnual aboveground biomass increment = Mg/ha divided by forest age. Derived from data in the original studies.bCalculated from a reported decline of 64 percent from the first cycle to the third or fourth (two to three cycles).cStudy included shifting cultivation and pasture, which does not have the same type of fallow. Rather than number of cycles, the study focused on the number of fires.

22 Lawrence, Radel, Tully, Schmook, and Schneider

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the other 46 stands. Further t-tests examined the effect for a given

location, but sample size (N = 1–8 pairs) severely limited statistical

power. Thus, we relaxed the a value to 0.10 to assess significancewithin a location. Relative per cycle difference was subjected to

analysis of variance (N = 29) by soil texture as reported in the orig-

inal studies. Finally, using linear and logarithmic regression, we de-

termined whether effect size depended on forest age, productivity

(ABI of the stand with fewer cycles), average fallow length, starting

point in history (number of cycles for the stand with fewer cycles),

and the span over which the comparison was made (difference in

number of cycles).Biomass was not assessed uniformly, though most studies pro-

vided data on woody stems and palms 4 5 cm diameter at breast

height (dbh). We used the original data to calculate effect size.

Measurement differences did not influence the effect size because

only sites within locations (usually studied by one set of authors)

were compared, and the effect size is proportional to the initial

measurements. However, one stand in Zona Bragantina, sampled

originally by Moran with a dbh cut-off of 2 cm was compared witha stand sampled by Vieira with a cut-off of 5 cm dbh. To correct for

biases introduced by these differences, we used relationships be-

tween dbh cut-off, proportion of total biomass 4 1 cm dbh, and

the age-dependent proportion of total biomass 4 5 cm dbh to

estimate aboveground biomass 4 5 cm dbh (see Appendix S1,

from data on 36 stands in Read & Lawrence 2003). In addition,

for analyzing the effect of inherent site productivity, we derivedABI4 5 cm dbh in four cases (Hughes et al. 2000: 2.54 cm

cut-off; Gehring et al. 2005: 0.2 cm cut-off). For the same analysis,

we derived biomass4 5 cm dbh from raw data for the Yucatan

(D. Lawrence, unpubl. data). We emphasize that the published data

were used to calculate relative per cycle loss (percent), the depen-

dent variable in all analyses.

REASSESSING TWO CASE STUDIES.—The case studies of Indonesia and

Mexico represent two situations described by the reanalysis: noeffect and negative effect of repeated shifting cultivation. Although

we would have liked to fully explore constraints on resilience in all

six regions, we had access to both ecological and socioeconomic

data for the same time period only for West Kalimantan and the

southern Yucatan. We examined ecological parameters (climate,

soil nutrients, soil organic matter, soil pH, invasive species) and

ecological aspects of management (burn preparation, field size, fal-

low length, cropping intensity, crop species) to determine whetherpredicted differences in effects on ABI were borne out by our an-

alyses. Then, we interpreted the results through a consideration of

socioeconomic factors that may affect the ecological outcome of

TABLE 2. Data included in the re-analysis of secondary forest regrowth following shifting cultivation.

Location

# of

pairs

Age

(range, yr)

# of cycles

(range)

Fallow length

(yr)a Soil textureb Vegetation sampled

ABIc (range,

Mg/ha/yr)

Americas

Santa Cruz, Boliviad 4 5–15 1–4 4–7 Nonsandy Trees1stemmed palms4 5 cm 3.8–9.6

Manaus, Brazile 2 3–5 1–2 1–2 Nonsandy Lianas, trees and arborescent palms4 0.2 cm 20.0–39.2

(3.1–6.0)

Rondonia, Brazilf 1 4 1–2 4 Nonsandy Foliage, residual sound and successional

wood4 2.54 cm

6.5–6.9

(2.6–2.8)

Zona Bragantina, Brazilg 8 3–10 1–10 3–5 Sandy Trees4 5 cm dbh 0.4–4.4

S. Yucatan, Mexicoh 8 2–18 1–4 6–9 Nonsandy All stems4 1 cm dbh 2.3–11.0

(0–3.0)

Asia

W. Kalimantan,

Indonesiai

8 9–12 1–10 20 Sandy,

Nonsandy

All stems4 5 cm dbh 3.8–6.4

aFallow length drawn from the original study, or from the literature.bSoil texture as described by the authors.cAnnual biomass increment (aboveground live biomass divided by forest age) from the original data. Diameter cut-off varies from study to study. If only one set of

values is shown, data are from stems 4 5 cm dbh. If two sets are shown, a different cut-off was used in the original study; see Table 2. In parentheses, ABI for

stems4 5 cm dbh; see methods for details.dSteininger (2000).eGehring et al. (2005).fHughes et al. (2000).gVieira et al. (2003) and Moran et al. (2000) data extracted from Zarin et al. (2005).hEaton & Lawrence (2009).iLawrence (2005).jABI for biomass4 5 cm dbh, see methods for calculations.

Decline in Tropical Forest Biomass Accumulation 23

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multiple cycles of shifting cultivation, including local ecological

knowledge, government subsidies, household integration into the

market economy and remittances from migratory labor. For the

Mexico case, we drew on 2003 socioeconomic data from a survey of203 households in 14 communities (Schmook & Radel 2008); the

ecological data were collected in 1999. Data for West Kalimantan

come from a 1991 survey of 81 households in four villages and fol-

low-up surveys in one of the villages in 1993 (N = 68) and 1995

(N = 40), during which time ecological data were also collected

(Lawrence et al. 1998).

RESULTS

FOREST BIOMASS RECOVERY.—Across all locations, mean above-

ground annual biomass increment (ABI) declined by 9.3 percent

with each cycle of shifting cultivation (one-tailed t-test, P = 0.042).

Four of six locations showed a negative effect, with relative per cycle

changes in ABI between � 6 and � 27 percent (Fig. 1). Santa Cruz

showed a per cycle decline of 6 percent (N = 4, P4 0.10), and ABI

declined by 13 percent in Zona Bragantina (N = 6, Po 0.10). Percycle decline in the Yucatan was 18 percent (N = 8, Po 0.10),

somewhat less than the per cycle decline derived by a different

method with a larger multiage multilocation sample (ca 26 percent

per cycle; Eaton & Lawrence 2009). The greatest decline was evi-

dent in Manaus, where the average decline was 27 percent (N = 2,

P4 0.10). The effect in West Kalimantan was not significantly

different from zero (13%, N = 8, P4 0.30), despite a significant

but modest decline reported originally (� 2% per cycle after thefourth cycle; Lawrence 2005). The matched pair in Rondonia

showed a 6 percent increase from the first to the second cycle.

The relative per cycle effect was more negative in younger

stands (R2 = 0.17, P = 0.028; Fig. 2). The mean per cycle loss in ABI

was 20 percent in 5-yr-old stands vs. 6 percent in 10-yr-old stands.

The per cycle effect did not vary with stand productivity (Fig. S1)

irrespective of whether we tested all comparisons (N = 29) or only

those with a site in its first cycle, indicating the greatest possible

biomass increment (N = 17). Per cycle decline tended to be greater

in stands with a shorter average fallow period, however the trend

was only marginally significant, and only for comparisons spanning

more than two crop-fallow cycles (N = 21, R2 = 0.17, P = 0.06; Fig.3). Effect size was not related to the historical starting point for

comparisons (Fig. S2A). Effect size also did not depend on the span

over which the comparison was made (the difference in number of

cycles between stands; Fig. S2B). Forest resilience did not differ on

sandy vs. nonsandy soils (Fig. S3).

MEXICAN AND INDONESIAN CASE STUDIES: ECOLOGICAL FACTORS.—

Both the original studies and this reanalysis showed a substantialloss in ABI with repeated shifting cultivation in the Yucatan but

little to no decline in West Kalimantan. As measured in the stands

sampled, soil organic matter, pH, and erosion potential predict the

opposite outcome (Table 3). In the Yucatan, relatively large organic

matter pools in a high pH soil should promote nutrient supply.

Mild, rolling topography with little overland flow results in low

water-based erosion and nutrient loss. Effective nutrient supply and

retention processes are critical to continued recovery followingrepeated use. Yet, ABI declines with repeated use in the Yucatan,

where these conditions are apparently met.

Climate variables, on the other hand, favor higher productivity

in West Kalimantan, with concomitant potential for rapid recovery

FIGURE 1. Change in annual biomass increment (ABI, in Mg/ha/yr) with each

cycle of shifting cultivation relative to initial ABI (t-test, N = 29, P = 0.04).

Dashed line indicates the mean decline over all sites (� 9.3% per cycle) in

Rondonia, Brazil (Rond), West Kalimantan, Indonesia (WKal), Santa Cruz,

Bolivia (SCruz), Zona Bragantina, Brazil (ZBrag) and Manaus, Brazil (Man).

Mean� SE for each location. Shaded area represents SE of the grand mean.

FIGURE 2. Per cycle change in annual biomass increment (ABI) relative to

initial ABI as a logarithmic function of the age of the stands compared.

24 Lawrence, Radel, Tully, Schmook, and Schneider

SPECIAL SECTION

of ecosystem processes following shifting cultivation (Table 3).

Greater annual rainfall and fewer dry months should promote more

rapid regrowth, and thus greater resilience, in West Kalimantan

than in the Yucatan. The data agree, showing only a 0–2 percent

per cycle decline in ABI for West Kalimantan. Another major eco-

logical factor driving the different trajectories of forest recovery in

West Kalimantan and the Yucatan is climatic variability. Variabilityat the low end of the tropical rainfall gradient may more often lead

to severe drought.

In addition, crop and fallow management in West Kalimantan

seem to put less pressure on the ecosystem. In the area studied, a

long fallow (12–20 yr) usually follows one season of crops (Law-

rence et al. 1998). In the Yucatan, two crops are occasionally taken

from the same field in a given year, and the field is often used over

two subsequent years (Schmook 2008). The average fallow lengthacross the region is 6–7 yr, resulting in a much more intensive sys-

tem. In the Yucatan, special trees (Manilkara zapote and Ceibapentandra) are left standing, especially during the first cycle of

burning. In subsequent cycles, however, all trees are usually felled.

In West Kalimantan, only rarely do trees remain after the burn

(more often during the first cycle). The modest difference in relict

trees is in opposition to the observed effect on forest recovery. Al-

though average field size is similar in both places (a little more than1 ha), field size can range up to 3 ha in the Yucatan. Larger fields

there might inhibit recovery by limiting dispersal into the patch,

though many of the species regenerate through resprouting as well.

Crop species may also play a role in determining the outcome of

repeated cultivation. Maize is a notoriously nitrogen (N) demand-

ing crop. Repeated harvests of an N-rich crop combined with more

frequent burning in the Yucatan may induce N-limitation over

many cycles of shifting cultivation.Finally, both regions experience species invasions that inhibit

succession, thus slowing forest regrowth and reducing future inputs

–80

–60

–40

–20

0

20

40

60

0 10 15 20

Rel

ativ

e pe

r cy

cle

effe

ct o

n A

BI (

%)

Average fallow period (years)

1-2 cycles 3 or more cycles

R2=0.17

P=0.06

5

FIGURE 3. Per cycle change in annual biomass increment (ABI) relative to

initial ABI as a function of the average fallow period of the location in which

the stands were compared.

TABLE 3. Ecological predictors of the inherent ability of the land to sustain shifting cultivation in Southern Yucatan, Mexico and West Kalimantan, Indonesia. Range shown

for plots included in the original studies.

Southern Yucatan Peninsula, Mexicoa West Kalimantan, Indonesiab

Ecological factors predicting greater sustainability in Mexico

Soil pH in water (15 cm) High (7.4–7.7) Low (4.7–5.1)

Soil organic matter (15 cm) High (74–99 Mg/ha SOC) Low (35–64 Mg/ha SOC)

Potential for erosion Low (modest slopes, rapid infiltration, low rainfall) High (steeper slopes, slower infiltration, high rainfall)

Pre-burn treatment of large trees Select species remain in some fields, but most fields are clear cut Trees remain after burning only occasionally

Ecological factors predicting greater sustainability in Indonesia

Rainfall Low (900–1400 mm/yr) High (3600 mm/yr)

Dry season Long (4–7 moo 100 mm) Short (2–3 moo 200 mm)

Climatic variability High interannual rainfall variability Lower interannual rainfall variability

Fallow period Short (6–9 yr) Long (12–20 yr)

Cropping frequency High (1–3 yr per 7–12 yr cycle) Low (1–2 yr per 13–22 yr cycle)

Main crop species Maize Upland dry rice

Ecological factors with no clear prediction

Total soil phosphorus (15 cm) 170–728 kg P/ha (mature forest 190–488 kg/ha)c 116–1435 kg P/ha (mature forest 116–231 kg/ha)

Invasive species Bracken fern (Pteridium aquilinum) Alang-alang grass (Imperata cylindrica)

Field size 1–3 ha 1.1 ha

aData from Lawrence and Foster (2002), Eaton and Lawrence (2009) and Schmook (2008).bData from Lawrence et al. (1998), Lawrence et al. (2005), Lawrence et al. (2007).cPlots in El Refugio, from Diekmann (2004); many overlapped with Lawrence and Foster (2002).

Decline in Tropical Forest Biomass Accumulation 25

SPECIAL SECTION

from stored aboveground biomass. In the Yucatan, bracken fern

(Pteridium aquilinum) experienced a fourfold increase from 1987

to 2001. More than half the communities have at most 2.5 percent

of their land area under bracken. However, in at least one third ofthe communities, 3–14 percent of the land has been invaded by

bracken (Schneider 2006). By intent, the study of forest regrowth

in the Yucatan did not include large bracken fern patches. Bracken

was present, but not dominant, in several stands, and large patches

were nearby in most cases. It may have inhibited recovery in current

or past cycles, but clearly attributing the decline in ABI to species

invasion is difficult. Similarly, in West Kalimantan, alang-alang(Imperata cylindrica) was observed in the understory of some stands,but never dominated a patch. Large invaded patches have been ob-

served in neighboring communities and across the island, much as

seen in the Yucatan, but they were not present in the focal village.

The absence of widespread invasives in the area studied may con-

tribute to the observed resilience in West Kalimantan, but we can-

not confirm it.

DISCUSSION

EVIDENCE FOR BROAD-BASED DECLINE OF SHIFTING CULTIVATION.—

Among the 47 stands studied, in wet and dry forest, on richer and

poorer soils, in the neotropics and the paleotropics, the rate of forest

recovery was reduced 9.3 percent with each cycle of shifting culti-

vation (Fig. 1). By implication, carbon sequestration in secondary

forest fallows is similarly reduced, as is the productivity of the crop

once land is recleared for cultivation. Land under shifting cultiva-tion should be almost half as productive after five cycles and close

to exhausted after 10 cycles (50–200 yr). Measurements made after

clearing of mature forest (common, especially in the early literature)

significantly overestimate the rate of future biomass recovery. Al-

though shifting cultivation is still transforming land at the frontier

of mature tropical forest, most areas are well beyond the first cycle

of use (see Table 1).

Understanding where a region falls in the trajectory of tropicaldeforestation (e.g., the ‘forest transition’; J. Southworth, H. Nagen-

dra & L. Cassidy unpubl. data) is not critical to assessing the impact

of shifting cultivation. The rate of decline does not accelerate with

increasing numbers of cycles. The data are limited, and future data

may provide more nuanced results. Currently, our best prediction is

that the rate of regrowth will decline 9.3 percent per cycle, regard-

less of whether the area in question is a frontier or an area of long

human habitation (Fig. S2). Differentiating by soil texture is notnecessary, a result confirmed by the failure of inherent productivity

to predict the rate of per cycle decline (Figs. S1 and S3). Soil texture

does affect biomass production (as shown by Johnson et al. 2000),

but it does not affect forest resilience in the face of repeated use.

The significant effect of forest age on per cycle decline in ABI

may be linked to the trend with fallow length (Figs. 2 and 3). Shorter

fallows would tend to create younger forests, which here showed

more dramatic negative effects of repeated shifting cultivation. Thus,considering average forest age (and/or average fallow length) would

be useful when incorporating the effects of shifting cultivation, and

potentially other cyclical forest uses, into global carbon models.

Our analysis suggests that predicting forest recovery and future

carbon sequestration may require relatively simple parameterization

to achieve robust results at the global scale, but this is somewhat

unsatisfying. Lack of replication within locations precluded a test ofdifferences among regions, and yet differences are apparent. In

West Kalimantan and Rondonia, shifting cultivation seems to be

more sustainable. Other parts of Brazil, Bolivia and the southern

Yucatan show modest to severe degradation with every cycle (Fig.

1). As we found in stand-level comparisons, increasing fallow

length may enhance forest resilience across regions but inherent

productivity does not (Fig. 4). Lower rainfall seems correlated with

more negative per cycle effects (lower resilience). A simple predic-tion based on rainfall and fallow period is tempting. However, both

rainfall and fallow period are similar between Rondonia and Zona

Bragantina, and yet, the former shows a positive response and the

latter shows a negative response to shifting cultivation. Clearly,

other factors must be in play. Examination of critical ecological

factors in West Kalimantan and the southern Yucatan suggested

that climatic rather than soil factors more strongly determine resil-

ience. Another possibility is that nonecological factors interact withecological factors to affect forest resilience.

SUSTAINABILITY IN WEST KALIMANTAN VS. DECLINE IN SOUTHERN

YUCATAN.—Two major socioeconomic factors differ between the

Yucatan and West Kalimantan: local processes of population

migration and government spending on subsidies and rural infra-

structure. These factors limit the accumulation of local ecological

knowledge and affect the cultivation decisions of local farmers.Thus, ultimately, they also constrain the resilience of the forest.

In West Kalimantan, the trial-and-error learning on which

sound land management is based has occurred in the same village

for over two centuries. Before village establishment, the settlers had

most likely been practicing shifting cultivation in a similar land-

scape not far away (Freeman 1955, Dove 1985). The system has

been well-tested, both in the village itself and under similar soil and

climatic conditions nearby. According to our analysis, it worksquite well.

In contrast, farmers in the Southern Yucatan arrived 30–60 yr

ago following government incentives to reopen the southern fron-

tier of Mexico (Klepeis & Turner 2001). They originate from

other, land-poor parts of Mexico, where both climate and soil con-

ditions differ markedly from the Southern Yucatan 400–1000 km

to the east. Seventy-five percent are from Veracruz, Tabasco and

Chiapas, the remainder from as far away as Michoacan (Schmook2008). For a rotational system, a few decades represent a brief pe-

riod of time. Most parcels have experienced one or two cycles of

shifting cultivation, some up to three or four (Schmook 2008). This

short history, reflecting internal migration processes, limits learning

and adaptive management.

Furthermore, high variability in the timing and amount of rain

in the Yucatan may limit the ability of farmers to ‘read’ the results

of their management. If a drought occurs, or the rain is delayed, ora hurricane intervenes, the ecological feedback necessary to evaluate

decisions about site selection, fallow period, or cropping intensity

does not occur. Instead, yield is somewhat stochastic. Adaptive

26 Lawrence, Radel, Tully, Schmook, and Schneider

SPECIAL SECTION

management is difficult when yield is not consistently tied to land

use decisions but heavily influenced by climatic quirks.

In addition to constraints on the accumulation of local eco-

logical knowledge, government subsidies and infrastructure devel-

opment differ fundamentally between the Yucatan and West

Kalimantan. Farmers in the Yucatan are paid to clear and plant,

through the Program of Direct Payments to the Countryside(PROCAMPO), a response to the North American Free Trade

Agreement (NAFTA). Payments are not tied to the success, or even

the existence, of a crop (Klepeis & Vance 2003), reducing the in-

centive to manage land effectively and sustainably. NAFTA has also

resulted in large price fluctuations for staple crops such as maize

(Ackerman et al. 2003, Turner et al. 2003) that may further reduce

farmers’ incentive to properly manage land in shifting cultivation.National policy, and the international agreement that spawned it,

have direct and indirect effects on farmer management and ulti-

mately, forest resilience.

In West Kalimantan, no such subsidies exist for indigenous

farmers who practice shifting cultivation, although government-

sponsored transmigrants elsewhere have received subsidies for per-

manent cultivation. Although farmers in the study area are inte-

grated into markets for some cash crops like fruit or rubber, uplandcultivation of rice has been primarily for subsistence. Because their

livelihood depends substantially on rice production, the economic

incentives for good management are greater in West Kalimantan.

The proximity of the United States and widespread household

participation in male out-migration significantly affect land use

decisions in the Yucatan. Although young men and women do

migrate out of the region, the newest wave of migration is of male

household heads—the individuals primarily responsible for landuse decisions, and the repository of accumulated agricultural

knowledge (Radel 2005, Radel & Schmook 2008). When a family

member obtains work in the United States, remittances may obvi-

ate the need for sustainable agricultural management. Income and

material consumption increase for households employing migra-

tion, accompanied by lower reliance on subsistence crop produc-

tion (Schmook & Radel 2008). In fact, with reliable remittances,

farming households may do best by clearing young (less productive)fallows simply to qualify for the PROCAMPO subsidy because less

labor is required. In other parts of Latin America out-migration has

been linked to a forest transition (Jokisch & Lair 2002, Rudel et al.2002, Klooster 2003). In El Salvador, forest recovery was positively

correlated with remittance receipts (Hecht et al. 2006). In fact, the

association among globalization, out-migration, and forest recovery

has been posited for all Latin America (Aide & Grau 2004). We

assert that migration and remittances also affect the resilience ofland that remains in agriculture.

Although farmers in West Kalimantan do cross the border to

Malaysia in search of employment (Amster 2005), this phenome-

non was not observed in the village studied. Quantitative studies of

this migration have not been published, but anecdotal evidence

suggests that the flow of people is not comparable to that from

Mexico to the United States (M. H. Amster, pers. comm.). Sub-

stantial engagement in off-farm labor occurs in other parts of Kali-mantan, resulting in local migration (Wadley 1997). However, in

the area we studied, the remittance economy is not as well devel-

oped as it is in Mexico, other parts of Kalimantan or Malaysia

(Amster 2005).

Finally, although farmers in both the southern Yucatan and

West Kalimantan are integrated into markets for cash crops, the

road network is far superior in Mexico. There, road access allowed

middle men to encourage chili pepper cultivation (Keys 2005). Al-though expansion of chili has not caused an obvious shortening of

the fallow in response to land pressure, cash from chili is used to

purchase staple foods (Gurri & Vallejo Nieto 2007). The shift from

–50

–40

–30

–20

–10

0

10

20

500 1500 2500 3500

Mean annual rainfall (mm)

–50

–40

–30

–20

–10

0

10

20

0 10 15 20 25 30

Rel

ativ

e pe

r cy

cle

effe

ct o

n A

BI (

%)

Mean fallow period (years)

–50

–40

–30

–20

–10

0

10

20

0 6

Inherent productivity (ABI in first cycle)

ZB

ZB

ZB

Y

Y

Y

SC

SC

SC

M

M

M

R

R

R

WK

WK

WK

(A)

(B)

(C)

82 4

5

FIGURE 4. Variation among regions in per cycle change in annual biomass in-

crement (ABI) relative to initial ABI as a function of: (A) mean annual rainfall;

(B) mean fallow period (range shown in horizontal bars); and (C) inherent pro-

ductivity as indicated by mean ABI4 5 cm dbh for those sites in the first cycle

(SE of the mean shown in horizontal bars); mean� SE. Y = Yucatan, Mexico;

SC = Santa Cruz, Bolivia; ZB = Zona Bragantina, Brazil; M = Manaus, Brazil;

R = Rondonia, Brazil; WK = West Kalimantan, Indonesia.

Decline in Tropical Forest Biomass Accumulation 27

SPECIAL SECTION

producer to purchaser of staple food crops may further erode the

link between household wellbeing and sustainable management of

land that remains in shifting cultivation.

In the West Kalimantan site, substitution of cash crops for ricehas been occurring for decades, also with little effect on fallow

length (Lawrence et al. 1998; but see Mary & Michon 1987, Mertz

et al. 1999, Hansen & Mertz 2006). However, at the time of study,

lack of road access limited cash crops to nonperishable rubber and

illipe nut and may also have hindered market engagement (Law-

rence et al. 1995). Ultimately, a cash economy could simply leave

fewer families engaged in shifting cultivation (as seen in Malaysia;

Amster 2005), rather than causing land degradation. Nevertheless,a focus on subsistence seems to have favored sustainable manage-

ment in the past.

THE INTERACTION OF SOCIOECONOMIC AND ECOLOGICAL FACTORS.—

We propose that given the degree of climatic variability and therelatively recent arrival of farmers to the region, farmers in the

southern Yucatan have not had adequate time to adapt their orig-

inal agricultural methods to appropriate management schemes for

the new environment. We argue that various forces of globalization

hinder the accumulation of local ecological knowledge and inhibit

the effectiveness of that knowledge by providing contradictory eco-

nomic incentives. As a result, the forests of the Yucatan are less

resilient, and the system itself, unsustainable.In comparing ecological constraints on sustainable land use in

West Kalimantan and the Yucatan, and among the six regions, cli-

mate seems to constrain the outcome of repeated shifting cultiva-

tion more than inherent soil characteristics (Figs. S3 and S4).

However, farmers can, with experience, adapt to local climate as

well as soil conditions. Ultimately, crop management may be a

stronger determinant of forest resilience. Crop management repre-

sents the nexus between ecological and socioeconomic constraintson land use. Management is fundamentally a product of local

knowledge. The existence of fire-adapted invasive species in both

the sustainable and the unsustainable case study points to the po-

tential for management failure as well as bad luck. Determining

whether invasion is a result of chance or a predictable outcome of

farmers’ decisions is an important avenue for future research as it

further illuminates the link between ecological and socioeconomic

constraints on sustainable land use (e.g., Schneider & Fernando2009).

Are similar factors at play in parts of Brazil and Bolivia, where

shifting cultivation is also in decline? Is ecological knowledge suffi-

cient to explain the apparent sustainability of shifting cultivation in

Rondonia, despite the fact that it is, like the Yucatan, a frontier of

deforestation? Interdisciplinary research is needed to address these

questions, with a special focus on the role of migration, subsidies,

market engagement and history of occupation.

CONCLUSION

The data currently available suggest that biomass recovery is de-

clining substantially with each cycle of shifting cultivation in a wide

range of tropical forests. Our case studies highlight the potential

indirect effects of socioeconomic factors on forest resilience under

shifting cultivation. Many of these factors link globalization to de-

cision making by farmers: global agricultural commodity markets,an increasingly global labor market and the cash produced by both.

Our analysis suggests that ecological differences may not be suffi-

cient to explain variation in forest resilience under shifting cultiva-

tion. By viewing differences in the outcome of shifting cultivation

through a socioeconomic as well as an ecological lens, it is possible

to understand that the ‘ecologically’ irrational decision to engage in

short-fallow, long-cropping agriculture may in fact be rational.

ACKNOWLEDGMENTS

This research was funded by NSF (Lawrence GFRP and DIG,

LTER-Harvard Forest and Carnegie Mellon University’s Center

for Integrated Studies on Global Change), the A. W. Mellon Foun-

dation, the University of Virginia, and NASA’s Land-cover and

Land-use Change program (NAG 56406). El Colegio de la Front-era Sur provided essential logistical support. We also thank L. Read,

J. Eaton, M. Uriarte, two anonymous reviewers and the Magrann

conferees for their contributions to this manuscript.

SUPPORTING INFORMATION

Additional Supporting Information may be found in the onlineversion of this article:

FIGURE S1. Per cycle change in annual biomass increment (ABI)

relative to initial ABI as a function of the productivity (ABI) of the

stand with fewer cycles.

FIGURE S2. Per cycle change in annual biomass increment (ABI)

relative to initial ABI as a function of cultivation history at both sites.

FIGURE S3. Per cycle change in annual biomass increment (ABI)relative to initial ABI as a function of soil texture, as characterized

by the original authors.

APPENDIX S1. Supporting information on methods used for

biomass assessment.

Please note: Wiley-Blackwell are not responsible for the content or

functionality of any supporting materials supplied by the authors.

Any queries (other than missing material) should be directed to thecorresponding author for the article.

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