Seedling growth and mortality of four shade-tolerant canopy tree species in the rain forest of Puerto Rico following Hurricane Hugo

Author

William H. Petty

Date of Award

5-1993

Degree Type

Thesis

Degree Name

Master of Science

Major

Ecology and Evolutionary Biology

Major Professor

J. Frank McCormick

Committee Members

Dewey Bunting, Clif Amundsen, Sandy McLaughlin

Abstract

Hurricane Hugo struck the rain forest of Puerto Rico on September 18, 1989. Three months after the hurricane, permanent transects were established to measure post-hurricane microenvironments, growth, and survival of seedlings of four shade-tolerant canopy tree species (Dacroydes excelsa, Sloanea berteriana, Manilkara bidentata, and Buchenavia capitata). Transects were placed on slopes in or near the Bisley Watersheds of the Luquillo Experimental Forest. Microenvironmental conditions in the post-hurricane environment were characterized by solar irradiance levels 5 to 47 percent full sunlight. Air (@ 25 cm) and soil (@ -2 cm) temperatures ranged between 19.3 - 33.5 °C and 19.4 - 30.7 °C, respectively. Vapor pressure deficit ranged from 0.07 kPa to 1.1 kPa. Microenvironmental measurements indicate that Hurricane Hugo created canopy openings on slopes equivalent to gaps in the forest canopy ranging from slightly less than 200 m² to greater than 400 m². Initial population sizes of small and large seedlings (0-15 cm and >15-50 cm in height, respectively) and saplings (>50-200 cm in height) varied significantly between species. For large seedlings of all species, positive correlations were found between monthly height growth increment and solar exposure index. Mean growth rates for large seedlings ranged from 0.83 cm mo^-1 for Sloanea to 5.87 cm mo^-1 for Buchenavia. Limited gas exchange measurements suggest that seedling growth rate is influenced more by dark respiration than by photosynthesis. Seedling mortality from 3 to 15 months after the hurricane ranged from 23% for Sloanea to 81% for Dacroydes. Despite high mortality, all species increased sapling densities by at least 100 stems ha^-1 during the first 16 months following Hurricane Hugo. Where data were available for comparison, recruitment rates were greater than those measured under pre-hurricane conditions. Significant differences in growth and mortality were found among species. There was no consistent pattern, however, with regard to growth and mortality. Resource partitioning with respect to light was not evident, based on the fact that all species increased sapling densities under near identical light conditions. Species did, however, demonstrate different survival strategies in the post-hurricane microenvironment. Sloanea exhibited a "conservative" strategy with slow growth rates and low mortality. Dacroydes and Buchenavia showed a "maximal performance" strategy, compensating for high mortality with either fast growth rates (Buchenavia) or high initial seedling densities (Dacroydes). Relatively rapid growth rates were crucial for Buchenavia which had the lowest initial seedling densities of the four species. Manilkara combined intermediate growth and mortality with moderate initial seedling densities resulting in a "moderate" strategy. Recruitment ratios (number of stems recruited into next larger size class per initial number of stems) were inversely proportional to initial population size, suggesting possible density-dependent factors influencing seedling and sapling recruitment. Refoliation of surviving canopy trees, along with rapid growth of pioneer species, reduced the amount of solar radiation reaching the forest floor. Growth of shade-tolerant species, diminished during the study, but, was still two and a half to eight times greater than under pre-hurricane conditions for Manilkara. Over the course of the study, mortality increased for some size classes (Sloanea, Manilkara and Buchenavia small seedlings and Dacroydes and Buchenavia large seedlings) and decreased for others (Sloanea and Manilkara large seedlings and Dacroydes small seedlings). Disturbance caused by Hurricane Hugo created microenvironment conditions which facilitated accelerated regeneration of these four shade-tolerant rain forest species. Measurements of microenvironment and shade-tolerant seedling performance indicate that the impacts of Hurricane Hugo on slopes of the Bisley Watersheds are similar to those of large gaps.

Recommended Citation

Petty, William H., "Seedling growth and mortality of four shade-tolerant canopy tree species in the rain forest of Puerto Rico following Hurricane Hugo. " Master's Thesis, University of Tennessee, 1993.
https://trace.tennessee.edu/utk_gradthes/11986