RESPONSES OF TROPICAL PLANTS TO NUTRIENTS AND LIGHT ON A LANDSLIDE INPUERTO-RICO

1 To determine whether availability of mineral nutrients limited growt h of pioneer and nonpioneer species differently, we transplanted Phyto lacca 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) in to a landslide in Luquillo Experimental Forest, Puerto Rico. 2 Plots w ere established in exposed parent material in the open zone and along the edge of a landslide. Within each plot, control and treatments of n itrogen, phosphorus, and N + P were randomly assigned to subplots. Add ition of N produced increases in total soil N, ammonium, and nitrate i n the open plots, but not in the edge plots, Addition of P produced in creases in extractable P in both sets of plots. 3 Biomass of the pione er species responded significantly to both N and P. Biomass of the non pioneer species responded only to added N. The effect of location (ope n vs. edge) on growth was variable. 4 For the pioneer species, the con centration of foliar P was increased by P fertilization, but foliar N did not increase significantly in response to fertilization. For the n onpioneer species, both foliar N and P increased in response to fertil ization by N and P, respectively. 5 Light-saturated photosynthetic rat e (A(max.)) of C. schreberiana increased in response to N fertilizatio n in the open plots. In contrast, photosynthesis of P. riparia and M. bidentata in the open plots was unaffected by fertilization. A(max.) o f 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 chlorophyl l fluorescence. P, riparia may have experienced moderate photoinhibiti on in the open zone, as shown by reduced growth as well as reduced rat ios of variable to maximum fluorescence. 7 Availability of nitrogen ap peared to limit growth of pioneer and nonpioneer species on the landsl ide. Availability of phosphorus appeared to limit growth of pioneer sp ecies, perhaps because they were nonmycorrhizal (P. rivinoides) or wea kly facultatively mycorrhizal (C. schreberiana). Neither pioneer nor n onpioneer species appeared to be particularly well adapted to colonize exposed parent material, for growth was very slow in the absence of a dded nutrients.