Differential effects of canopy disturbance and litter deposition on litterfall and nutrient dynamics following a simulated hurricane in a tropical forest

Background/Question/Methods

Humid tropical forests have the highest rates of litterfall production globally, fueling rapid nutrient recycling and supporting high net ecosystem production. Climate change is altering the frequency and severity of severe storm events in tropical latitudes, which is likely to feed back on litterfall production and nutrient cycling. Severe storm events affect litterfall and litterfall nutrient dynamics through a combination of canopy disturbance and litter deposition. In this study, we used a large-scale long-term manipulation experiment to explore the separate and combined effects of canopy trimming and litter deposition on litterfall rates and litter nutrient concentrations and content. We tested the hypothesis that litter deposition associated with severe storms partially offsets the negative effects of canopy disturbance, leading to greater litterfall production in sites with litter deposition and canopy opening than with canopy opening alone. We also tested the hypothesis that patterns in litterfall nutrient inputs would be driven primarily by effects on litterfall mass and not by changes litterfall nutrient concentrations, which were predicted to be relatively insensitive to the manipulations. Finally, we explored seasonal dynamics in litterfall and litterfall nutrients in this relatively aseasonal humid tropical forest.

Results/Conclusions

Leaf/twig litter deposition associated with the treatments was roughly equivalent to three times the annual leaf litterfall mass and nutrient content in control plots. Regardless, results showed that canopy trimming was the primary driver of changes in litterfall and associated nutrient cycling. Canopy trimming reduced litterfall mass by 14 Mg ha^-1 over 2.5 years. Nutrient concentrations increased in some litter fractions following trimming, likely due to a combination of changes in species composition and the fractional composition of litterfall, and increased nutrient uptake resulting from lower competition for nutrients. Declines in litterfall mass, however, led to large reductions in litterfall nutrient content with a loss of 143 ± 22 kg N ha^-1 and 7 kg P ha^-1 over the 2.5 years post-trim period. There were no significant effects of litter deposition on litterfall rates or nutrient content. Our results suggest that litterfall production is not strongly limited by nutrients in this forest, and that canopy disturbance has large and lasting effects on carbon and nutrient cycling.