Functional trait-based frameworks for understanding leaf litter and wood decomposition in tropical dry forests

Background/Question/Methods

Plant functional traits refer to characteristics that affect performance. Recently, there has been much interest in understanding the extent to which functional traits can be used to predict ecosystem processes and services. We examined the extent to which functional traits explain variation in decomposition rates among tropical dry forest trees. We worked in a Costa Rican forest with a mean annual rainfall of 1600mm and a 5-6 month dry season. In our first study, we quantified leaf traits (specific leaf area, toughness, nitrogen and phosphorus resorption) and initial litter chemistry of 26 species, and we related these traits to leaf litter mass loss and nitrogen dynamics in replicate litterbags set out at the beginning of the wet or dry season and harvested over a 650-day period. In our second study, we established a long-term wood decomposition experiment. We selected eight species that vary in wood density and elemental composition. Four replicates of uniform size per species were placed in two different sites, and every six months we cut log slices and measured the densities of bark, heartwood, and sapwood, in addition to evidence of termites.

Results/Conclusions

In our leaf litter study, mass loss was strongly correlated with rainfall; all species lost mass during the wet season but not during the dry season. When pooled across species, there was little nitrogen mineralization or immobilization for bags set out in the wet season until 2 to 5 months, when N was released. In contrast, bags set out to decompose in the dry season immobilized nitrogen rapidly and did not start to release nitrogen until after one year, and this immobilization occurred in the absence of decomposition. Despite large interspecific differences in the traits listed above, percent mass remaining for litterbags set out during the wet season or dry season was best correlated to initial litter chemistry (lignin/N). The percent change in litter N content was best correlated to initial litter N concentrations. 

Patterns of early-stage wood decomposition are complex; after one year, sapwood and hardwood densities increased, decreased, or were unchanged. Initial wood density did not explain these patterns. Rather, changes in sapwood densities were positively correlated with initial zinc concentrations, while changes in heartwood were negatively correlated to initial calcium concentrations. We conclude that precipitation and initial lignin and nutrient concentrations are the best predictors of both leaf and early-stage wood decomposition.