Responses of Tropical Plants to Nutrients and Light on a Landslide in Puerto Rico

1 To determine whether availability of mineral nutrients limited growth of pioneer and nonpioneer species differently, we transplanted Phytolacca rivinoides (an annual pioneer species), Cecropia schreberiana (a pioneer tree species), Palicourea riparia (a shrub of tree-fall gaps and understorey) and Manilkara bidentata (a nonpioneer canopy tree) into a landslide in Luquillo Experimental Forest, Puerto Rico. 2 Plots were established in exposed parent material in the open zone and along the edge of a landslide. Within each plot, control and treatments of nitrogen, phosphorus, and N + P were randomly assigned to subplots. Addition of N produced increases in total soil N, ammonium, and nitrate in the open plots, but not in the edge plots. Addition of P produced increases in extractable P in both sets of plots. 3 Biomass of the pioneer species responded significantly to both N and P. Biomass of the nonpioneer species responded only to added N. The effect of location (open vs. edge) on growth was variable. 4 For the pioneer species, the concentration of foliar P was increased by P fertilization, but foliar N did not increase significantly in response to fertilization. For the nonpioneer species, both foliar N and P increased in response to fertilization by N and P, respectively. 5 Light-saturated photosynthetic rate $(A_{\max.})$ of C. schreberiana increased in response to N fertilization in the open plots. In contrast, photosynthesis of P. riparia and M. bidentata in the open plots was unaffected by fertilization. $A_{\max.}$ of P. rivinoides was also unaffected, but sample sizes were very small due to mortality from an outbreak of lepidopteran larvae. 6 There was no effect of fertilization on photoinhibition as assayed by chlorophyll fluorescence. P. riparia may have experienced moderate photoinhibition in the open zone, as shown by reduced growth as well as reduced ratios of variable to maximum fluorescence. 7 Availability of nitrogen appeared to limit growth of pioneer and nonpioneer species on the landslide. Availability of phosphorus appeared to limit growth of pioneer species, perhaps because they were nonmycorrhizal (P. rivinoides) or weakly facultatively mycorrhizal (C. schreberiana). Neither pioneer nor nonpioneer species appeared to be particularly well adapted to colonize exposed parent material, for growth was very slow in the absence of added nutrients.