Responses to nutrient addition among seedlings of eight closely related species of Shorea in Sri Lanka

1 Responses to the addition of P and Mg are described for eight species 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 in increased dry mass yield, seedling height and leaf number after 24 months for seedlings of these species growing in pots of soil taken from a nearby Pinus plantation compared to a control which did not receive nutrients. In the presence of P, dry mass yield, leaf number and lateral root ratio declined in response to increasing Mg. In the presence of Mg, seedling height and stem mass ratio increased in response to increasing P.

3 After 24 months maximum dry mass yield in response to nutrient addition was greater for the four species which occur on more nutrient-rich soils in the held. Maximum percentage increase in dry mass in response to nutrient addition was negatively correlated with mean dry mass of unfertilized seedlings, but the relationship did not discriminate between species according to adult distribution across the catena.

4 There was no relation between seedling root mass ratio (RMR) or phenotypic plasticity in seedling RMR and adult distribution in relation to nutrient supply.

5 In Shorea section Doona, trade-offs between seedling 'responsiveness' and 'tolerance' to nutrient supply, in terms of either growth or dry mass allocation, are not powerful determinants of differences in adult distribution in relation to nutrient supply. Differential dry mass yield in response to nutrient addition was determined by differences in seed size and seedling relative growth rates under the experimental conditions, as well as the degree of response to nutrient addition. Response to nutrient addition may be constrained by seedling characteristics relating to variation in irradiance and water availability.