Plant community responses to stand-level nutrient fertilization in a secondary tropical dry forest.

The size of the terrestrial carbon (C) sink is mediated by the availability of nutrients that limit plant growth. However, nutrient controls on primary productivity are poorly understood in the geographically extensive yet understudied tropical dry forest biome. To examine how nutrients influence above- and belowground biomass production in a secondary, seasonally dry tropical forest, we conducted a replicated, fully factorial nitrogen (N) and phosphorus (P) fertilization experiment at the stand scale in Guanacaste, Costa Rica. The production of leaves, wood, and fine roots was monitored through time; root colonization by mycorrhizal fungi and the abundance of N-fixing root nodules were also quantified. In this seasonal forest, interannual variation in rainfall had the largest influence on stand-level productivity, with lower biomass growth under drought. By contrast, aboveground productivity was generally not increased by nutrient addition, although fertilization enhanced growth of individual tree stems in a wet year. However, root growth increased markedly and consistently under P addition, significantly altering patterns of stand-level biomass allocation to above- vs. belowground compartments. Although nutrients did not stimulate total biomass production at the community scale, N-fixing legumes exhibited a twofold increase in woody growth in response to added P, accompanied by a dramatic increase in the abundance of root nodules. These data suggest that the relationship between nutrient availability and primary production in tropical dry forest is contingent on both water availability and plant functional diversity.