The Role of Soil Nutrient Status in Regulating Biomass Fluxes in Lowland Tropical Forests


José A. Medina-Vega1* (, Daniel Zuleta1, Stuart J. Davies1, Jeffrey Chambers2


1Forest Global Earth Observatory, Smithsonian Tropical Research Institute, Washington, DC; 2Lawrence Berkeley National Laboratory, Berkeley, CA



The role of soil nutrient status in regulating the dynamics of tropical forests remains poorly understood. Accurate predictions of tropical forest structure and responses to changing climate, rising carbon dioxide, and disturbance require a much deeper understanding of the interplay between soil nutrient availability and forest production, function, and functional composition. In addition to soil nutrients, tree size is another driver that shapes forest carbon dynamics and determines the interaction between trees and the environment. Small trees have larger contributions to biomass fluxes, i.e., woody productivity (AWP) and woody mortality (AWM), compared to large trees, which dominate biomass stocks. This preliminary study used long-term Barro Colorado Island (BCI) data coupled with a detailed soil survey, including measurements of phosphorus (P), aluminum, iron, magnesium, and manganese (Mn) concentrations, to assess whether soil nutrient availability regulates biomass fluxes as a function of tree size. AWP varied among size classes, being lower at larger size classes, and had a negative association with Mn, suggesting that a high solubility and uptake of toxic metals (e.g., Mn) by trees may constrain productivity. No association of AWP with P was observed, which is expected to limit productivity in tropical forests. This lack of association between P and AWP is consistent with experiments and observations in areas near BCI where it has been suggested that a lack of community-level variation in productivity in relation to P may result from strong species turnover. Moreover, AWM was not related to variation in soil fertility but differed among size classes. However, differences in AWM among size classes were random, suggesting a strong role of stochastic processes in AWM. This analysis will be replicated at the pantropical level by including sites with a larger variability in soil nutrient availability from the Forest Global Earth Observatory network. This work will enable the evaluation and benchmarking of Earth system model nutrient cycle representations by providing pantropical empirical representations of the role of soil nutrient status in regulating the dynamics of tropical forests.