Assessing Evidence for a Pervasive Alteration in Tropical Tree Communities

Abstract

In Amazonian tropical forests, recent studies have reported increases in aboveground biomass and in primary productivity, as well as shifts in plant species composition favouring fast-growing species over slow-growing ones. This pervasive alteration of mature tropical forests was attributed to global environmental change, such as an increase in atmospheric CO2 concentration, nutrient deposition, temperature, drought frequency, and/or irradiance. We used standardized, repeated measurements of over 2 million trees in ten large (16–52 ha each) forest plots on three continents to evaluate the generality of these findings across tropical forests. Aboveground biomass increased at seven of our ten plots, significantly so at four plots, and showed a large decrease at a single plot. Carbon accumulation pooled across sites was significant (+0.24 MgC ha−1 y−1, 95% confidence intervals [0.07, 0.39] MgC ha−1 y−1), but lower than reported previously for Amazonia. At three sites for which we had data for multiple census intervals, we found no concerted increase in biomass gain, in conflict with the increased productivity hypothesis. Over all ten plots, the fastest-growing quartile of species gained biomass (+0.33 [0.09, 0.55] % y−1) compared with the tree community as a whole (+0.15 % y−1); however, this significant trend was due to a single plot. Biomass of slow-growing species increased significantly when calculated over all plots (+0.21 [0.02, 0.37] % y−1), and in half of our plots when calculated individually. Our results do not support the hypothesis that fast-growing species are consistently increasing in dominance in tropical tree communities. Instead, they suggest that our plots may be simultaneously recovering from past disturbances and affected by changes in resource availability. More long-term studies are necessary to clarify the contribution of global change to the functioning of tropical forests.

Detecting and projecting changes in forest biomass from plot data

“…Violation of this could lead to artificially low confidence intervals. On BCI, as Chave et al (2008) previously reported for this and other large plots, the confidence intervals are insensitive to the subplot scale for subplot sizes between 0.025 and 1 ha (Figure 14.2B), consistent with the lack of spatial structure at these scales (Figure 14.2A). The sampling uncertainty for the full 50-ha plot that is estimated by bootstrapping over subplots is exactly consistent with the spatial scaling of sampling uncertainty for smaller areas (Figure 14.1A).…”

“…Since the biomass change on the plot is known more exactly, this bootstrapping procedure is technically estimating sampling uncertainty associated with the stochastic process operating within this plot (and that could potentially have produced other outcomes) (e.g. Chave et al 2008). When dividing a plot into subplots for the purposes of within-plot bootstrapping, it is essential that the subplot scale be greater than the integral length scale, the scale at which the spatial semivariogram plateaus, i.e.…”

“…Past approaches to error identification most typically involved absolute cutoffs for diameter growth rates, with more extreme values considered unrealistic and thus necessarily erroneous. For example, diameter increases of more than 35 mm yr −1 , 40 mm yr −1 or 45 mm yr −1 , or decreases of more than 5 mm yr −1 , have been flagged for correction (Chave et al 2003(Chave et al , 2008Lewis et al 2009b;Phillips et al 2009). Where multiple census data are available, these presumed errors are corrected by interpolation or extrapolation where possible.…”

“…Where multiple census data are available, these presumed errors are corrected by interpolation or extrapolation where possible. In other cases, they are set to the mean (Chave et al 2008;Phillips et al 2009) or median (Lewis et al 2009b) diameter growth rates for trees in the same diameter class. Despite the ubiquity of such procedures, we know of no study that has systematically evaluated their potential influence on estimates of forest biomass change.…”

“…Many studies have evaluated changes in biomass stocks in tropical forests using recensuses of field plots Chave et al 2008; Lewis et al 2009b;Phillips et al 1998Phillips et al , 2008, and some have also examined changes in coarse woody productivity Phillips, 1996). Field measurements of tropical forest biomass stocks have focused largely on aboveground woody biomass of trees.…”