Hydrologically-Influenced Feedbacks Between Phosphorus and Vegetation in Dry Tropical Forests

This research investigates the potential for hydrologically-influenced feedbacks between phosphorus and vegetation to induce ecosystem state changes in dry tropical forests. Since many of these forests are phosphorus (P) limited, any disturbance that influences available P may directly impact vegetation (V). I hypothesized that vegetation also influences P-cycling, and that P-V feedbacks lead to reduced resiliency and state changes in dry tropical forests, especially when disturbed. I designed a model to explore the interactions between P and V dynamics in tropical forests. Using published data to define the relationships between P and V, I determined that feedbacks in these systems (i.e., the role of vegetation in "trapping" P deposition) could be sufficient to induce state shifts. Based on this result, I investigated three regions of P-cycling where I expected to find hydrologically-influenced P-V feedbacks: deposition, leaching, and soil P bioavailability. Associated studies were primarily undertaken in the Mexican dry forest of the Southern Yucatan Peninsular Region. I found that: 1) throughfall [P] was significantly higher in old forests than in young forests after a dry period, supporting the idea that greater plant area in old forests makes them more efficient at "trapping" deposition. However, collection time and storm size also affected throughfall [P]. 2) Deep leachate [P] decreased with increasing forest age, suggesting that old forests lose less P via leachate. There was also a significant negative relationship between forest age and the difference between deep and shallow leachate [P); leachate [P] may decrease down the soil profile in old forests, implying that these forests are highly efficient at conserving P. 3) Soil P bioavailability increased with ii moisture and was strongly pulsed after re-wetting dried soils. Wet-dry pulses may become more common with reduced canopy cover (e.g., less shading) or climate change; plants may be inefficient at conserving strongly pulsed P, increasing the risk for P losses under these conditions.

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