"Long-term studies have revealed that the structure and dynamics of many tropical forests are changing, but the causes and consequences of these changes remain debated. To learn more about the forces driving changes within tropical forests, we investigated shifts in tree species composition over the past 25 years within the 50-ha Forest Dynamics Plot on Barro Colorado Island (BCI), Panama, and examined how observed patterns relate to predictions of (1) random population fluctuations, (2) carbon fertilization, (3) succession from past disturbance, (4) recovery from an extreme El Niño drought at the start of the study period, and (5) long-term climate change. We found that there have been consistent and directional changes in the tree species composition. These shifts have led to increased relative representations of drought-tolerant species as determined by the species' occurrence both across a gradient of soil moisture within BCI and across a wider precipitation gradient from a dry forest near the Pacific coast of Panama to a wet forest near its Caribbean coast. These nonrandom changes cannot be explained by stochastic fluctuations or carbon fertilization. They may be the legacy of the El Niño drought, or alternatively, potentially reflect increased aridity due to long-term climate change. By investigating compositional changes, we increased not only our understanding of the ecology of tropical forests and their responses to large-scale disturbances, but also our ability to predict how future global change will impact some of the critical services provided by these important ecosystems.
Tropical forests face many challenges, including both natural climatic events, such as El Niño-associated droughts (Newbery and Lingenfelder 2004, Phillips et al. 2009), and large-scale anthropogenic disturbances, including habitat loss and climate change (Wright 2005, Wright and Muller-Landau 2006, Clark 2007, Bradshaw et al. 2008, Peres et al. 2010). Demographic responses to natural and anthropogenic disturbances will differ among tree species (Körner 2004), potentially resulting in altered inter-specific competitive interactions. These changes, in conjunction with the predicted migrations of species as their distributions shift to remain at equilibrium with changing climate (Beaumont et al. 2007, Feeley and Silman 2010a, b, Feeley et al. 2011), should manifest as directional shifts in the floristic composition of forest stands (e.g., Condit et al. 1996a, c, Condit 1998a, Beckage et al. 2008). Changes in species composition will, in turn, have important consequences for ecosystem-level responses to global change (Körner 2004, Bunker et al. 2005). For example, changes in the relative abundance of heavy- vs. light-wooded tree species may result in augmented or reduced carbon stores, respectively (Bunker et al. 2005). Studies of compositional changes are therefore imperative to understanding the impacts of past disturbances and predicting the effects of future global changes on tropical forests and their contributions to the global carbon budget and other ecosystem services.
In conclusion, the forest of BCI has exhibited remarkably directional and consistent changes in tree species composition since the early 1980s. These changes cannot be explained through stochastic fluctuations, increasing atmospheric CO2 concentrations, and carbon fertilization, or through succession from a past large-scale disturbance event. One explanation supported by the observed patterns is that the floristic changes reflect responses to increased aridity in the plot, possibly due to increasing temperatures and/or a past decrease in rainfall. Alternatively, the observed floristic changes may be the result of the extreme drought that occurred early in the census period, causing high mortality of drought-intolerant species, especially in the unmeasured seedling/sapling size classes. Changes in the seedling/sapling community, due either to past decreases in rainfall or an extreme drought, would in turn lead to differential recruitment and resultant long-term changes in the adult tree community. If this is the case, future censuses may eventually reveal a shift in the floristic community back toward the pre-disturbance composition. The relative importance of extreme events vs. long-term trends in driving compositional changes at BCI and in other ecosystems is clearly a question that deserves further attention. This is especially true given that some models predict that the frequency and magnitude of extreme events such as El Niño droughts will increase (Timmermann et al. 1999), potentially leading to changes in species composition/structure from which forests have insufficient time to recover."
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