Testing the effects of aluminum-hyperaccumulating trees and nitrogen-fixing trees on successional processes in Costa Rica

The current state of global deforestation is alarming, with an estimated 13 million hectares of forest cleared each year. Current limitations to forest recovery include a lack of seed availability, soil aluminum (Al) toxicity and low levels of soil phosphorus (P) and nitrogen (N). In this dissertation I examine methods of stimulating natural succession on degraded lands by addressing these specific limitations to recovery.;My first chapter is the first study to quantify aluminum hyperaccumulation by Vochysia guatemalensis (Vochysiaceae) grown across a spectrum of manipulated levels of soil Al. I demonstrate that V. guatemalensis seedlings accumulate as much as 38,000 ppm Al in their leaves, and that, in this study, Al accumulation is not related to soil Al levels.;My second chapter reports the effects of V. guatemalensis on five species of varying successional stages. The first experiment tests the effect of planted V. guatemalensis on the growth of Calophyllum brasiliense (Clusiaceae), a native mid-successional tree species. After three years of growth, V. guatemalensis had a null effect on the growth of C. brasiliense, which supports the tolerance model of succession. In a parallel experiment, I tested the effects of Al-rich V. guatemalensis mulch on the growth of five species of trees, relative to a mulch low in Al. Vochysia guatemalensis mulch had a null effect on the growth of the early successional (V. guatemalensis and Inga spectabilis) and mid successional species (Calophyllum brasiliense), but had a significant positive effect on the growth of the two late successional species (Astronium graveolens and Tabebuia chrysantha ), relative to the control mulch.;Given the positive effect of V. guatemalensis mulch on the growth of A. graveolens, my third chapter tests whether a planted Al-hyperaccumulator (V. ferruginea) has positive, facilitative effects on planted A. graveolens seedlings in the field, or whether the interaction shifts towards competition. This experiment simultaneously examines whether a planted N-fixing legume (Inga spectabilis ) has facilitative or competitive effects on A. graveolens. After four years of growth, there is a null effect of both V. ferruginea and I. spectabilis on A. graveolens, supporting the tolerance model of succession.