Demographic diversity in tropical forests: Functional trade-offs and consequences for species distributions
The large numbers of tree species in tropical forests represent a diversity of demographic strategies that are presumably due largely to natural selection. Tree species’ demographic strategies, here defined for a single species as the combination of its mean growth and mortality rates, are often arrayed along an interspecific trade-off axis in which species with faster growth have higher mortality rates and vice-versa. We used data from CTFS-ForestGEO inventory plots to evaluate whether this axis is more strongly shaped by selection to minimize an individual tree’s mortality at low resources or to maximize a tree’s responsiveness to increasing resource availability. If demographic strategies are selected by these rules, then a species’ strategy should predict the shape of its intraspecific growth-mortality relationship. Within plots, species’ demographic strategies were estimated as the first principle component of the interspecific growth-mortality axis. The intraspecific growth-mortality relationship fitted for each species defined individual mortality as a function of prior growth. Assuming a tree’s growth rate is a measure of total resource-availability to it, the intercept and slope thus represent low-resource mortality and the change in mortality with greater resources. Whether distributions of intercept-slope combinations were constrained to limited parameter space was judged using null models.
Interspecific growth-mortality relationships in most forests examined were consistent with widely observed patterns in which faster mean growth is significantly associated with higher mean mortality, although there was variation in the strength of this association. In most cases, species’ demographic strategies were correlated with the intercepts of the intraspecific growth-mortality function, supporting the hypothesis that selection for reducing low-resource mortality is important in tropical forests. Moreover, the shape of this relationship suggested that there may be a minimum low-resource mortality rate achievable, given a species’ demographic strategy. Conversely, the slopes of the within-species growth-mortality function were more weakly correlated with demographic strategies, and species occupying faster-growth/higher-mortality strategies did not consistently have steep, negative slopes. We conclude that natural selection producing the demographic diversity seen in closed-canopy tropical forests may operate more strongly on minimizing mortality rate in resource-poor environments than on maximizing responsiveness to resources. Disturbance-prone forests may be an exception, where fast growth to maturity is important. As anthropogenic change is likely to increase non-equilibrium dynamics in forests across the globe, we expect species with greater responsiveness to favorable growth environments may become more widely distributed.