RESPONSES TO NUTRIENT ADDITION AMONG SEEDLINGS OF 8 CLOSELY-RELATED SPECIES OF SHOREA IN SRI-LANKA

1 Responses to the addition of P and Mg are described for eight specie s of Shorea section Doona (Dipterocarpaceae) which vary in their adult distribution across a topographic/soils gradient at Sinharaja Forest Reserve, Sri Lanka. 2 All combinations of the two nutrients resulted i n increased dry mass yield, seedling height and leaf number after 24 m onths for seedlings of these species growing in pots of soil taken fro m a nearby Pinus plantation compared to a control which did not receiv e nutrients. In the presence of P, dry mass yield, leaf number and lat eral root ratio declined in response to increasing Mg. In the presence of Mg, seedling height and stem mass ratio increased in response to i ncreasing P. 3 After 24 months maximum dry mass yield in response to n utrient addition was greater for the four species which occur on more nutrient-rich soils in the held. Maximum percentage increase in dry ma ss in response to nutrient addition was negatively correlated with mea n dry mass of unfertilized seedlings, but the relationship did not dis criminate between species according to adult distribution across the c atena. 4 There was no relation between seedling root mass ratio (RMR) or phenotypic plasticity in seedling RMR and adult distribution in rel ation to nutrient supply. 5 In Shorea section Doona, trade-offs betwee n seedling 'responsiveness' and 'tolerance' to nutrient supply, in ter ms of either growth or dry mass allocation, are not powerful determina nts of differences in adult distribution in relation to nutrient suppl y. Differential dry mass yield in response to nutrient addition was de termined by differences in seed size and seedling relative growth rate s under the experimental conditions, as well as the degree of response to nutrient addition. Response to nutrient addition may be constraine d by seedling characteristics relating to variation in irradiance and water availability.