The Effect Of Soil Nutrients And Light On Tree Growth And Interactions During Tropical Forest Succession: Experiments In Costa Rica.

Natural regeneration in tropical rainforests occurs primarily in treefall gaps where growing saplings encounter a wide range of physical conditions. This dissertation investigates the effect of two important factors, sunlight and soil nutrients, on the growth and interactions of rainforest trees. Addition of fertilizer and reduction of light both resulted in increased relative biomass production by the fastest growing species and suppression of the remaining species due to heavy shading. The experiments were carried out in the Atlantic lowlands of Costa Rica, near the La Selva field station. Six tree species (Carapa guianensis, Cordia alliodora, Dipteryz panamensis, Hampea appendiculata, Pentaclethra macroloba and Stryphnodendron excelsum) representing a range of successional types from pioneer to climax were planted as seeds in a regular pattern in 10 x 10m plots in a cleared field near undisturbed rainforest. In a 2 x 2 factorial design, 5 of 10 plots were fertilized with 10-30-6 NPK, and a 15x50x3m shade house was constructed over 2 fertilized and 2 unfertilized plots which reduced the light levels to 30% of full sunlight. Small monospecific plots of each of the species were established under each of the four conditions. All trees were measured 3 times yearly for height, diameter at 50 and 150cm, and crown diameter. Four plots were sacrificed at the end of one year for detailed measurement and biomass determination. Survival of all species was higher in the shade than in full sunlight, where most mortality resulted from shading by herbaceous plants. Tree growth was not reduced at the lower light level. All species showed increased growth in response to fertilizer, and the increase was greatest in Hampea, which had the highest growth rate of the six species. After two years of growth the largest Hampea were over 8m tall and completely dominated the fertilized plots. As a consequence of the reduced dominance of Hampea in the unfertilized plots, the other species attained their maximum growth in the unfertilized plots. This pattern of increased dominance (and reduced species diversity) in succession ocurring under favorable nutrient conditions is similar to patterns found in a wide variety of ecosystems.Ph.D.Biological SciencesEcologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/127613/2/8224971.pd