Observations of tropical forests have revealed large variations in aboveground biomass, plant composition, ecosystem structure, and biogeochemical functioning across plots. A relatively large amount of variability remains even when precipitation and stand age are controlled for. Here, we analyze the extent to which variability in ecosystem processes and characteristics emerge from variation in soils and nutrients. First, we performed simulations of tropical dry forest secondary succession using a mechanistic, numerical model. The model dynamically couples ED2 (vegetation dynamics), MEND (microbial-enzyme decomposition), and N-COM (plant-microbe competition for nitrogen and phosphorus). Second, we compare measurements from a nutrient fertilization experiment to model simulations. The experiment involves 16 forest inventory plots in Guanacaste, Costa Rica, including controls, nitrogen fertilization (+N), phosphorus fertilization (+P), and +NP treatments. The experiment began in 2015. We predicted that (1) our simulations of secondary succession would show large between-plot variability because of differences in soils and nutrient limitation, (2) after 3 years of a nutrient fertilization experiment, fertilization would result in significant changes in belowground processes.
Results/Conclusions
In simulations of secondary succession, the spread in plant biomass was about 30% of the mean. The accumulated biomass was positively correlated with the initial amount of non-occluded soil P. A simulation with artificially large N and P deposition rates had a much smaller spread in accumulated aboveground biomass, confirming a nutrient limitation dynamic. The proportional amounts of different PFTs simulated in the different plots also varied widely and depended on differing degrees and N and P limitation. In the nutrient fertilization experiment, we found that fine root biomass was larger in the +P and +NP plots than in the control or +N plots. This result was also found in the model simulations. In the model, (i) fertilization shifted the vegetation from being more nutrient-limited to being more water-limited, and (ii) without fertilization, plants’ ability to construct fine roots was limited by low nutrient reserves. Overall, our simulations and experiments show that variations in soil and nutrient status can lead to large variations in tropical dry forest functioning, structure, and composition. Prognostic model simulations of tropical dry forest ecosystems should account for variations in the soil template.