Plots in the Jamaican montane forest were fertilized with nitrogen or with phosphorus to test the hypothesis that growth of trees in this natural forest is limited by the supply of N and P from the soil.
Once a year from 1983 to 1986, urea was added to one plot (at 150 kg N ha−1 y−1) and triple superphosphate was added to another (at 50 kg P ha−1 y−1). In each of these plots and in two control plots, foliage of four common tree species was collected immediately before each fertilizer addition. Trunk growth was measured in 105 individuals.
Foliar N concentrations were not significantly higher in trees fertilized with N compared to control trees. In Dendropanax cf. pendulus and Hedyosmum arborescens fertilization with N resulted in lower P concentrations but only after the third year of fertilization, possibly due to dilution by increased leaf production. Mean trunk diameter growth was significantly higher in the N-fertilized trees than in controls.
Mean foliar P concentrations were higher in Podocarpus urbanii and Clethra occidentalis following fertilization with P, but only after two years of fertilization. Trunk diameter growth was greater in the P fertilized plot.
Thus growth in some Jamaican montane forest trees was limited by the natural supplies of N and of P.