NITROGEN AND PHOSPHORUS FERTILIZATION EFFECTS ON VENEZUELAN MONTANE FOREST TRUNK GROWTH AND LITTERFALL

In a montane rain forest at almost-equal-to 2500 m in Venezuela we fertilized plots with N, with P, or with N and P for 4.5 yr. We measured trunk growth in circumference in control plots and in those fertilized with N and P, and measured litterfall in all four treatments. There were five 12 x 12 m plots per treatment. N was applied at 225 kg.ha-1.yr-1 and P at 75 kg.ha-1.yr-1 for the first 2 yr, and subsequently N was applied at 150 kg.ha-1.yr-1 and P at 50 kg.ha-1.yr-1. Rates of trunk growth in N and P fertilized plots were roughly double those in control plots (1.4 vs. 0.6 mm/yr). Total litterfall averaged 4.3 Mg.ha-1.yr-1 in the control plots. In the 4th yr after fertilization litterfall was significantly higher in plots fertilized with N + P (1.50 g.m-2.d-1) than in control plots (1.00 g.m-2.d-1). There were no significant differences in rates of litterfall between treatments during the first 3 yr after the first fertilization. The changes of rates of litterfall with time were significantly different between treatments. Litterfall in control plots fell by 9% per year, that in P-fertilized plots fell by 5% per year, and litterfall in N- and in N + P-fertilized plots remained approximately constant. Nitrogen concentrations in litterfall were unaffected by fertilization with N or with P. Phosphorus concentrations were significantly increased by P fertilization. We conclude that N is more limiting than P in this forest.