Santiago-García, Ricardo J.

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    Performance of native tree species planted under nurse trees for dry forest restoration in Puerto Rico
    (2010) Santiago-García, Ricardo J.; Van-Bloem, Skip J.; College of Agricultural Sciences; Thaxton, Jarrod M.; Schröder, Eduardo C.; O'Farrill, Hipólito; Department of Crops and Agro-Environmental Sciences; Borges, Sonia
    Tropical dry forests are the most threatened tropical terrestrial ecosystem. Most of the original dry forest has been converted to agriculture or pastureland. The need to restore these forests is critical, but a lack of specific restoration techniques hinders its progress. This project explored the use of nurse trees to restore dry forests in Puerto Rico. Three plots with 400 native saplings in total were planted under 100 Leucaena leucocephala nurse trees at four different planting distances and directions. The planting directions (north, south, east and west) and planting distances (from close to the trunk in the first planting distance to full sun in the fourth distance) were tested for growth, mortality, light environment (assessed as leaf area index) and the variation in leaf morphology of ten native tree species. Leaf area index decreased moving away from the nurse tree, indicating the presence of a light gradient under the nurse trees. However, there was no relationship between leaf area index and growth in either in the planted saplings or the reference plot in a mature forest, indicating that factors other than light are affecting growth. Overall mortality, 37.5%, was low compared to other dry forest and nurse tree restoration projects. Mortality at the Cabo Rojo National Wildlife Refuge was 17.5% greater at full sun in the fourth planting distance, reflecting greater solar radiation and possibly more water soil evaporation, heat loading or photoinhibition. Otherwise, trends in mortality by distance or direction from the nurse tree were species specific. Erythroxylum areolatum had a better performance moving away from the nurse tree with 23% lower mortality in the third and fourth planting distances. Pisonia albida had only one dead individual at the first planting distance, suggesting improved attainment closer to the nurse tree. Stahlia monosperma mortality responded to planting direction, with no dead individuals at the north planting direction. Bourreria succulenta, Stahlia monopserma and Trichilia hirta had no mortality under nurse trees at the Cabo Rojo National Wildlife Refuge. Growth varied by species, with Erythroxylon areolatum, Pisonia albida and Trichilia hirta having 0.13 m or less of growth and Bursera simaruba, Citharexylum fruticosum and Tabebuia heterophylla having an average of 0.28 m of growth or greater. Overall, planting closer to the nurse tree at the first and second planting distances increased growth by 0.10 m. indicating a facilitative effect. However, since leaf area index was not related to growth, then other factors like water availability could be affecting growth. Also, no relationship was found between leaf area index and survivorship with the variation in specific leaf mass or leaf density. For the planted saplings, growth increased as leaf density increased. Greater variation in specific leaf mass and leaf density in dry forest species does not indicate increased growth or survivorship, as has been shown in wetter forest types. The results of this study suggests that using Leucaena leucocephala as a nurse tree can be a promising tool for dry forest restoration since they can lower mortality depending on the species and generally increases growth of native saplings when planted closer to the trunk of the nurse tree.