Forest Structure and Productivity in Puerto Rico's Luquillo Mountains

Net aboveground primary production (NPP) at 725 m in elevation in the colorado forest (montane rain forest, sensu Beard; lower montane wet forest, sensu Holdridge) of the Luquillo Mountains of Puerto Rico averaged 7.60 t/ha yr on two 0.4-ha permanent plots and was determined by summing aboveground biomass accrual (0.59 t/ha yr), litterfall (6.80 t/ha yr), and herbivory (0.21 t/ha yr). The total leaf area index (trees, epiphytes, and ground vegetation) was 4.95 m2/i2, the aboveground woody volume and biomass, 220 m3/ha and 130 t/ha, respectively, and standing herbivory and the herbivory rate, 5.1 and 4.0 percent/yr, respectively. The mean standing crop of litter was 680 g/m2, with a turnover rate of 0.78 times per year. Of the structural features studied over a 600-m elevational gradient in the Luquillo Mountains, the number of trees per hectare, basal area, and soil organic matter increased with elevation; whereas, the specific leaf area, canopy height, range of tree diameters, forest volume and biomass, leaf area index, and species richness declined. Of the dynamic features studied, ingrowth and mortality of trees, tree growth (diameter, volume, and biomass growth), litterfall, loose litter, standing herbivory and herbivory rates, litter turnover, aboveground woody NPP, and total NPP all declined with an increase in aboveground elevation. The structural and floristic impoverishment of forests and changes in forest dynamics with ascent in the Luquillo Mountains reflect cooler temperatures, fog, and heavier rainfalls that interact to produce saturated soils and retard the mineralization of organic matter.