Facilitating succession on abandoned pasture in Costa Rica using an aluminum-hyperaccumulating tree, Vochysia guatemalensis (Vochysiaceae)

In Costa Rica, more than half of the natural forests have been converted to pasture and agricultural lands. Where plant communities are not established, high levels of rainfall cause excessive erosion and leaching of soil nutrients. As a result, poor nutrient levels, soil acidity, and concomitant soil aluminum (Al) toxicity are primary factors limiting ecosystem recovery. Native pioneer tree species adapted to poor conditions may facilitate the growth of later stage species by improving soil nutrient conditions and by moderating microclimatic factors. On our study site in Costa Rica, Vochysia guatemalensis (Vochysiaceae) seedlings sequester up to 35,000ppm Al in their leaves. Furthermore, the organic acids that chelate Al in the rhizosphere may also free phosphate in the soil. I am interested in the efficacy of V. guatemalensis as a potential facilitator of neighboring tropical tree seedlings. Using a randomized blocked design, I planted a matrix of V. guatemalensis seedlings in Sept, '05 and allowed them to grow for one year. In Sept, '06 I planted mid-successional Calophyllum brasiliense (Clusiaceae; “target”) seedlings inside the matrix of V. guatemalensis trees. Increased growth of target seedlings when grown as neighbors to V. guatemalensis as compared to when grown alone is evidence of facilitation. To tease apart the potential mechanisms of facilitation, I have four additional controls: [target + artificial shade], [target + shade + P fertilizer], [target + shade – Al toxicity], [target + density control]. Initial soil chemistry was not significantly different between treatments or blocks. Height of target trees did not differ significantly between treatments after three months of growth. This is a long term experiment in which soil chemistry, microclimatic factors, and tree growth will be monitored. I predict increased P availability, decreased soil Al toxicity, and moderated microclimate will have a significant effect on target tree growth.