Gap Dynamic Modeling of a Subtropical Dry Forest: Effects of Hurricanes on Forest Change and Productivity in Climate Change Scenarios

Because of human pressures, the need to understand and predict the long-term dynamics and development of subtropical dry forests is urgent. The new gap model ZELIG-TROP was developed, parameterized, and successfully modeled the forest dynamics of the Puerto Rico subtropical dry forest. Along with new forest parameterization, additions to ZELIG-TROP increased the level of detail in gap models by including a new natural disturbance routine and by tracking litterfall, leaf production, coarse woody debris, and carbon components at the individual tree level. This research developed a valuable tool to understand the process of rehabilitating dry tropical forests, understand the unique aspects and predict the future direction of subtropical dry forests. In addition to validating a new model, simulating the succession of secondary forests on abandoned fields was achieved, showing that secondary forests have a delayed response to recover and reach a mature forest status (100+ years). Realistic hurricane simulations were implemented within the modeled Puerto Rican subtropical dry forest from ZELIG-TROP, for the first time. A major gap in the literature is to compare storm effects on vegetation from storms varying in intensity, duration, and frequency that are forecasted in climate change scenarios. Increasing hurricane intensity of severe storms did not create a large shift in the average aboveground biomass, net primary productivity (NPP), annual litterfall, leaf production, coarse woody debris, or annual autotrophic live carbon accumulation (AALCA) from that of historical hurricane regime (control). During both control and increased hurricane ii intensity scenarios, AALCA was negative indicating a carbon transfer from the live forest to the surrounding ecosystem. Increasing the frequency of hurricanes had a much stronger effect on forest dynamics. In comparison to the control scenario, increased hurricane frequency decreased the aboveground biomass by 5% , increased NPP by 32% , increased leaf production, decreased litterfall and coarse woody debris, and AALCA became positive indicating a carbon sink into the forest over 600 years. Overall community composition does not substantially change from the control treatment when there is an increase in hurricane intensity or frequency, but species diversity was the highest during all treatments which increased the frequency of hurricanes.

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