Nitorgen and Phosphorus Fertilization Effects on Venezuelan Montane Forest Trunk Growth and Litterfall

In a montane rain forest a °2500 m in Venezuela we fertilized plots with N, with P, or 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 doubled those in control plots (1.4 vs. 0.6 mm/yr). Total litterfall averaged 4.3 Mg°ha — 1 °yr 1 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 that P in this forest.