Responses to Nutrient Addition among Shade-Tolerant Tree Seedlings of Lowland Tropical Rain-Forest in Singapore

1 Two bioassays of growth limitation were carried out for seedlings of four shade-tolerant tree species (Antidesma cuspidatum, Calophyllum tetrapterum, Dipterocarpus kunstleri and Garcinia scortechinii) growing in P-deficient soil taken from lowland dipterocarp forest in Singapore, as a test of the hypothesis that growth would be limited by the availability of phosphorus.

2 Seedlings of only one species, Antidesma cuspidatum, showed increased growth in response to increased nutrient supply and in that case the limiting nutrient was not P. A majority of seedlings of Antidesma, Calophyllum and Garcinia in this experiment possessed VA mycorrhizas.

3 For seedlings of Antidesma, addition of magnesium led to an increase in the concentration of Mg in all fractions and a positive relation between Mg concentrations and dry mass yield. Addition of potassium and calcium resulted in reductions in concentrations of these elements in the leaves of Antidesma.

4 Seedlings of Antidesma, Calophyllum and Dipterocarpus responded to P by altering distribution of dry mass between different plant parts; the pattern of response varied between species. Phosphorus taken up in excess of requirements for vegetative growth was transferred to plant stems rather than leaves.

5 The outcome of pot bioassays may be dependent on factors such as pot size, irradiance and soil moisture conditions; therefore conclusions drawn here need to be tested by field fertilization experiments.