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Changes in the Carbon Balance of Tropical Forests: Evidence from Long-Term Plots

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Science  16 Oct 1998:
Vol. 282, Issue 5388, pp. 439-442
DOI: 10.1126/science.282.5388.439

Abstract

The role of the world's forests as a “sink” for atmospheric carbon dioxide is the subject of active debate. Long-term monitoring of plots in mature humid tropical forests concentrated in South America revealed that biomass gain by tree growth exceeded losses from tree death in 38 of 50 Neotropical sites. These forest plots have accumulated 0.71 ton, plus or minus 0.34 ton, of carbon per hectare per year in recent decades. The data suggest that Neotropical forests may be a significant carbon sink, reducing the rate of increase in atmospheric carbon dioxide.

Tropical forests contain as much as 40% of the C stored as terrestrial biomass (1) and account for 30 to 50% of terrestrial productivity (2). Therefore, a small perturbation in this biome could result in a significant change in the global C cycle (3, 4). Recent micrometeorological research suggests that there is a net C sink in mature Amazonian forests (5, 6), but the ability to draw firm conclusions is hampered by the limited spatial and temporal extent of these measurements. Another approach, applying atmospheric transport models to measured global distributions of CO2, O2, and their isotopes (7), has yielded conflicting results. We report a third approach to explore the role of mature tropical forests in the global C cycle, namely, the use of permanent sample plots (PSPs). PSPs, established by foresters and ecologists to monitor tree growth and mortality, have the potential to yield C accumulation estimates that are at once both geographically extensive and of high spatial and temporal resolution.

We compiled data on basal area (cross-sectional area of trees per unit ground area) from mature tropical forest plots (8) that meet appropriate a priori criteria (9). Basal area of trees is a well-substantiated surrogate measure of total biomass in tropical forests (10), so changes due to tree growth and mortality provide an effective measure of changes in biomass. We tested for changes in mature tropical forest biomass in each of four nested regions: the humid tropics (153 plots), the humid Neotropics (120 plots), the humid lowland Neotropics (108 plots), and Amazonia (97 plots) (11). These plots represent more than 600,000 individual tree measurements tropics-wide.

We conducted two analyses with the information available. For each region, we first calculated the mean rate of change in tree basal area across sites, based on the difference between the initial and final census at each geographically distinct site (12). Sites may contain one or more floristically and edaphically similar plots (13). In the second analysis, we estimated basal area change as a function of calendar year and derived an estimate of regional net accumulated biomass through time. Data for this approach were derived for each site by first computing differences between each successive census, then by linear interpolation between successive censuses for years when measurements were not taken, and finally for each year by averaging change across all contributing plots. Measurement errors were corrected by comparing multiple measurements of the same tree over time (14). Basal area values were converted to aboveground biomass estimates by using an allometric model developed for lowland forest in central Amazonia and by using correction factors to account for the biomass of lianas and small trees (15).

Biomass has increased in mature forest sites in the humid Neotropics (1.11 ± 0.54 t ha–1 year–1; mean ± 95% confidence intervals), the humid lowland Neotropics (1.08 ± 0.59 t ha–1 year–1), and in Amazonia (0.97 ± 0.58 t ha–1year–1) (16). The entire pantropical dataset also shows an increase in biomass (0.77 ± 0.44 t ha–1 year–1), but the signal is dominated by the Neotropical pattern, and there has not been a significant change in Paleotropical sites (tropical Africa, Asia, Australia) (–0.18 ± 0.59 t ha–1 year–1) (17). In the Neotropics (tropical Central and South America), the mean value of biomass change has been positive for most years since widespread PSP monitoring began (18). In Amazonia, where most inventories are located, plots have on average gained biomass in most years since at least the late 1970s (Fig. 1). By 1990, mature forest sites in all three nested Neotropical regions had on average accumulated substantial biomass (Fig. 2).

Figure 1

Annual aboveground biomass change in Amazonian forests, 1975–96. Mean (solid circles), 95% confidence intervals (dotted line), and 5-year moving average (solid line) are shown.

Figure 2

Cumulative aboveground net biomass change (tons per hectare per year) in humid forests in: (A) the Tropics since 1958; (B) the Paleotropics (tropical Africa, Asia, Australia) since 1958; (C) the Neotropics (tropical Central and South America) since 1967; (D) the lowland Neotropics since 1971; (E) Amazonia since 1975. Annual mean (solid line) and 95% confidence interval (dotted line) values are based on the cumulative changes in individual sites since the first year and are scaled by a/b, where a = the cumulative time elapsed since the first year and b = the mean monitoring period per site up to each year end.

These results show that (i) there is considerable spatial and temporal variability in rates of biomass change, yet (ii) on average, plots have gained biomass, and (iii) the increase has been especially marked in lowland Neotropical sites. There has been no statistically detectable change in biomass in African and Asian plots, but our coverage of these areas (18 sites) is sparser than in the Neotropics (50 sites), so we concentrate our discussion on the Neotropics. If the difference between Neotropical and Paleotropical forests is genuine, it may reflect differing climatic factors or perhaps greater human disturbance in the more densely populated Paleotropics (19).

Before extrapolating these results to the biomass of Neotropical forests as a whole, it is important to consider whether the PSPs were representative of the broader region. Neotropical forests are heterogeneous (20), and our dataset spans much of the natural variation in Amazonian forests (21). The number of extra-Amazonian lowland and montane samples also corresponds to the approximate coverage of each region (22). Recent debate (23) has centered on two potential problems in monitoring: (i) research activity having a negative impact on tree survivorship and growth and (ii) plots becoming increasingly subject to edge effects as surrounding forest is fragmented (24). These effects would increase mortality relative to growth, thus causing a decline in measured biomass—the opposite of our result. A further possibility is that there could be a bias in the PSPs compared to the surrounding forest, by systematic avoidance or underreporting of forests that underwent natural catastrophic disturbances or smaller scale disturbance due to localized tree death. Although it is difficult to quantify such a bias, there is little evidence for it in our dataset (25), and the increase in biomass is larger than can be accounted for simply by the dynamics of a few large trees (26).

Our results are therefore indicative of a widespread increase in the biomass of surviving Neotropical forests over recent decades. There are a number of mechanisms that may explain this change: (i) a response to continental-scale cyclical climate change; (ii) recovery from widespread disturbance, either natural or anthropogenic; (iii) enhanced forest productivity due to a secular change in climate or increased nutrient availability.

Because Earth's climate fluctuates, forest stocks of C might be responding to past climatic events. The El Niño–Southern Oscillation (ENSO) may be one long-term driver of cyclical changes in forest dynamics (27). In El Niño years, most of Amazonia receives below-normal rainfall (28), but our data show that Amazon forests gained biomass before, during, and after the intense 1982–83 ENSO (Fig. 1). It is possible that regional forest biomass is recovering from earlier greater disturbances, either from drought or from the impacts of indigenous peoples who have experienced steep population declines since the 16th century (29). The biomass increase could also be a response to recent anthropogenic global change. There is some evidence for an increase in temperate and tropical forest productivity (30), and even mature ecosystems may gain biomass if plant productivity is stimulated (4). Candidate factors for nutrient fertilization include increasing atmospheric CO2 (31) and increased N and P deposition from Saharan dust (32) and biomass burning (33).

To estimate regional C sequestration rates, we first converted aboveground biomass into C stocks, using allometric data obtained in central Amazonia (34). The increase in biomass on Amazonian plots is equivalent to a net uptake of 0.62 ± 0.37 t C ha–1 year–1. Multiplying this by the estimated area of humid forest in lowland Amazonia (22) produces a mature forest biomass C sink of 0.44 ± 0.26 Gt C year–1. Similarly, the estimated annual C sink in lowland Neotropical humid forest is 0.52 ± 0.28 Gt C; it is 0.62 ± 0.30 Gt C for all mature humid neotropical forests. Our method suggests a lower C uptake rate than estimates from eddy covariance studies in Rondônia (1.0 t ha–1 year–1) (2) and near Manaus (5.9 t ha–1year–1) (6). The discrepancy may reflect the limited spatial and temporal extent of eddy covariance measurements, or else be indicative of significant increases in the necromass and soil pools (35), which are not accounted for in our analysis.

Our results suggest that mature Neotropical forest biomass may account for ∼40% of the so-called “missing” terrestrial C sink (36). Hence, intact forests may be helping to buffer the rate of increase in atmospheric CO2, thereby reducing the impacts of global climate change. However, the C sink in mature forests appears vulnerable to several factors. There is likely to be an upper limit to the biomass a forest stand can hold. Moreover, deforestation, logging (37), increased fragmentation and edge-effect mortality (23, 24), regional drying and warming (38), and possible intensification of El Niño phenomena (39) may limit and even reverse the sink provided by mature forest. A dedicated large network of permanent biomass plots could provide vital insight into the future role of tropical forests in the global C cycle.

  • * To whom correspondence should be addressed. E-mail: O.Phillips{at}geog.leeds.ac.uk (O.L.P); YMalhi{at}ed.ac.uk(Y.M.)

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