RESUMO
Competition among trees is an important driver of community structure and dynamics in tropical forests. Neighboring trees may impact an individual tree's growth rate and probability of mortality, but large-scale geographic and environmental variation in these competitive effects has yet to be evaluated across the tropical forest biome. We quantified effects of competition on tree-level basal area growth and mortality for trees ≥10-cm diameter across 151 ~1-ha plots in mature tropical forests in Amazonia and tropical Africa by developing nonlinear models that accounted for wood density, tree size, and neighborhood crowding. Using these models, we assessed how water availability (i.e., climatic water deficit) and soil fertility influenced the predicted plot-level strength of competition (i.e., the extent to which growth is reduced, or mortality is increased, by competition across all individual trees). On both continents, tree basal area growth decreased with wood density and increased with tree size. Growth decreased with neighborhood crowding, which suggests that competition is important. Tree mortality decreased with wood density and generally increased with tree size, but was apparently unaffected by neighborhood crowding. Across plots, variation in the plot-level strength of competition was most strongly related to plot basal area (i.e., the sum of the basal area of all trees in a plot), with greater reductions in growth occurring in forests with high basal area, but in Amazonia, the strength of competition also varied with plot-level wood density. In Amazonia, the strength of competition increased with water availability because of the greater basal area of wetter forests, but was only weakly related to soil fertility. In Africa, competition was weakly related to soil fertility and invariant across the shorter water availability gradient. Overall, our results suggest that competition influences the structure and dynamics of tropical forests primarily through effects on individual tree growth rather than mortality and that the strength of competition largely depends on environment-mediated variation in basal area.
Assuntos
Florestas , Madeira , África , Brasil , Ecossistema , Clima TropicalRESUMO
BACKGROUND: Wildlife has been traditionally used by forest communities as a source of protein, and the Peruvian Amazon is no exception. The articulation of colonist and indigenous communities to urban centers and markets results in changes in livelihood strategies and impacts on wildlife populations. To address the threat of overhunting and forest conversion, we provide a generalized characterization of colonist and indigenous communities and their hunting activities near Pucallpa, Ucayali, Peru. METHODS: A semi-structured household survey was conducted to characterize hunters and describe their prey collections. The data were analyzed by conducting a Kruskal-Wallis test, a multiple regression analysis, and by estimating the harvest rate (H). RESULTS: Less wealthy households were more actively engaged in hunting for food security and as a livelihood strategy. Additionally, older hunters were associated with higher hunting rates. Although the percentage of hunters was relatively low, estimated hunting rates suggest overharvesting of wildlife. Lowland pacas (Cuniculus paca) were the most frequently hunted prey, followed by red brocket deer (Mazama americana) and primates. While hunting intensity was not significantly different between indigenous and colonist communities, hunting rate disparities suggest there are different types of hunters (specialized vs. opportunistic) and that prey composition differs between communities. CONCLUSION: Close monitoring of wildlife populations and hunting activities is ideal for more accurately determining the impact of hunting on wildlife population and in turn on forest health. In lack of this type of information, this study provides insight of hunting as a shifting livelihood strategy in a rapidly changing environment at the forest/agriculture frontier.