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, art very low in nutrients, especially in phosphorus (P) Earlier experiments demonstrated that P was growth-limiting for some tree seedlings on these soils, but other species failed to increase their growth in response to greater P-availability. To investigate this, we measured growth and distribution (if biomass and P of tree seedlings, of a pioneer tree species, Tapirira obtusa (Benth.) Mitchell, and of a climax tree species, Lecythis corrugata Poit., at 10 levels of P-supply under controlled conditions in a glasshouse. At intervals of 3 wk. dry weights of plant parts and their phosphorus concentrations were measured. The pioneer and the climax species took up similar amounts of P when grown at high P-supply. The pioneer tree T. obtusa maintained a low P concentration (0.25 mg g-1 ) independent of P-supply, and used the P taken up Hi increase growth., At high P-supply it invested little biomass in roots, and reached a relative growth rate (RGR) of 40 mg g-1 d-1 . The climax, tree species, L, corrugate, 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 germination. The differences m growth and distribution of biomass and P in response to P-supply of the two species are likely to contribute to the establishment of their seedlings in the field. If L. corrugata is capable of re-translocating P from the hypocotyl, this storage of P has ecological advantages for long-term survival, which might be important under low light conditions. Together with a low RGR, it enables a seedling to maintain P-reserves until a gap occurs. In a newly created gap, P-availability per seedling increases, and pioneers, with their higher P uptake and growth potential, can benefit from these relatively higher levels of P-availability. This is an important advantage in high-light gap environments where the tallest tree seedling is generally the most competitive one.