Tropical dry forests (TDF) are complex, functionally diverse ecosystems that exist across a wide range of environmental gradients. Additionally, tropical dry forests are threatened by extensive land-use change and often exist as recovering secondary forests. This combination of environmental stressors imposed by climate and disturbance results in a high potential for multiple resource constraints on forest growth, including water, nitrogen, and phosphorus limitation. Here, we ask: (1) whether we understand TDFs enough to accurately model their biomass and nutrient dynamics? (2) Is there evidence that these forests can become nutrient limited? (3) If nutrient limitation arises, can N2-fixers (a functionally dominant group in TDFs) alleviate N limitation? We use a new version of the Ecosystem Demography (ED2) model that is parameterized for TDFs and incorporates plant-mycorrhizae interactions and multiple resource constraints (carbon, nitrogen, phosphorus, and water). We test the model’s performance against a nutrient gradient of field sites in Costa Rica. We then use the model to quantify the sensitivity of forest growth to nutrient limitation. Ongoing field measurements will further efforts to mechanistically represent belowground processes in the model as we expand to test the model’s performance against additional TDFs sites in Mexico.
Results/Conclusions
We found that the model accurately projects short-term biomass and nutrient dynamics over 5 years when compared to observations. However, the model skill decreases after a decade due to substantial litter accumulation and subsequent forest nutrient limitation from nitrogen sequestration in the litter pool. Interestingly, forest litter accumulation is negatively correlated with site fertility across the nutrient gradient sampled in Costa Rican forests. To explore which factors contribute most to forests becoming nutrient limited, we did a series of sensitivity simulations isolating various factors. Our sensitivity analyses indicate that N2-fixer abundance, decomposition rates, and mycorrhizal nutrient cycling are key factors in overcoming this limitation. These insights improve our understanding of how TDFs function and are especially relevant to recovering secondary TDFs.