RESPONSE TO PHOSPHORUS SUPPLY OF TROPICAL TREE SEEDLINGS - A COMPARISON BETWEEN A PIONEER SPECIES TAPIRIRA-OBTUSA AND A CLIMAX SPECIES LECYTHIS-CORRUGATA

The highly-weathered acid sandy soils in Guyana, South America, are ve ry low in nutrients, especially in phosphorus (P). Earlier experiments demonstrated that P was growth-limiting for some tree seedlings on th ese soils, but other species failed to increase their growth in respon se to greater P-availability. To investigate this, we measured growth and distribution of biomass and P of tree seedlings, of a pioneer tree species, Tapirira obtusa (Benth.) Mitchell, and of a climax tree spec ies, Lecythis corrugata Poit., at 10 levels of P-supply under controll ed conditions in a glasshouse. At intervals of 3 wk, dry weights of pl ant parts and their phosphorus concentrations were measured. The pione er and the climax species took up similar amounts of P when grown at h igh P-supply. The pioneer tree T. obtusa maintained a low P concentrat ion (0.25 mg g(-1)) independent of P-supply, and used the P taken up t o increase growth., At high P-supply it invested little biomass in roo ts, and reached a relative growth rate (RGR) of 40 mg g(-1) d(-1). The climax tree species, L. corrugata, maintained a low RGR of 10 mg g(-1 ) d(-1) and a constant distribution of biomass at all P-supply rates. It stored the extra P in a structure between the stem and root derived from the former hypocotyl, which persisted for over 6 months after ge rmination. The differences in growth and distribution of biomass and P in response to P-supply of the two species are likely to contribute t o the establishment of their seedlings in the field. If L. corrugata i s capable of re-translocating P from the hypocotyl, this storage of P has ecological advantages for long-term survival, which might be impor tant under low light conditions. Together with a low RGR, it enables a seedling to maintain P-reserves until a gap occurs. In a newly create d gap, P-availability per seedling increases, and pioneers, with their higher P uptake and growth potential, can benefit from these relative ly higher levels of P-availability. This is an important advantage in high-light gap environments where the tallest tree seedling is general ly the most competitive one.