Résumé
A longstanding goal of ecology and conservation biology is to understand
the environmental and biological controls of forest succession. However,
the patterns and mechanisms that guide successional trajectories,
especially within tropical forests, remain unclear. We collected leaf
functional trait and abiotic data across a 110-year chronosequence within
a tropical dry forest in Costa Rica. Focusing on six key leaf functional
traits related to resource acquisition and competition, along with
measures of forest stand structure, we propose a mechanistic framework to
link species composition, community trait distributions, and forest
structure. We quantified the community-weighted trait distributions for
specific leaf area, leaf dry matter concentration, leaf phosphorus
concentration, leaf carbon to nitrogen ratio, and leaf stable isotopic
carbon and nitrogen. We assessed several prominent hypotheses for how
these functional measures shift in response to changing environmental
variables (soil water content, bulk density and pH) across the
chronosequence. Increasingly, older forests differed significantly from
younger forests in species composition, above ground biomass and shifted
trait distributions. Early stages of succession were uniformly
characterized by lower values of community-weighted mean specific leaf
area, leaf stable nitrogen isotope, and leaf phosphorus concentration.
Leaf dry matter concentration and leaf carbon to nitrogen ratio were lower
during earlier stages of succession, and each trait reached an optimum
during intermediate stages of succession. The leaf carbon isotope ratio
was the only trait to decrease linearly with increasing stand age
indicating reduced water use efficiency in older forests. However, in
contrast to expectations, community-weighted trait variances did not
generally change through succession, and when compared to null
expectations were lower than expected. The observed directional shift in
community-weighted mean trait values is consistent with the ‘productivity
filtering’ hypothesis where a directional shift in water and light
availability shifts physiological strategies from ‘slow’ to ‘fast’. In
contrast to expectations arising from niche based ecology, none of the
community trait distributions were over-dispersed. Instead, patterns of
trait dispersion are consistent with the abiotic filtering and/or
competitive hierarchy hypotheses.