RESPONSES TO NUTRIENT ADDITION AMONG SHADE-TOLERANT TREE SEEDLINGS OFLOWLAND 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 t etrapterum, Dipterocarpus kunstleri and Garcinia scortechinii) growing in P-deficient soil taken from lowland dipterocarp forest in Singapor e, as a test of the hypothesis that growth would be limited by the ava ilability of phosphorus. 2 Seedlings of only one species, Antidesma cu spidatum, showed increased growth in response to increased nutrient su pply and in that case the limiting nutrient was not P. A majority of s eedlings of Antidesma, Calophyllum and Garcinia in this experiment pos sessed VA mycorrhizas. 3 For seedlings of Antidesma, addition of magne sium led to an increase in the concentration of Mg in all fractions an d a positive relation between Mg concentrations and dry mass yield. Ad dition of potassium and calcium resulted in reductions in concentratio ns of these elements in the leaves of Antidesma. 4 Seedlings of Antide sma, Calophyllum and Dipterocarpus responded to P by altering distribu tion of dry mass between different plant parts; the pattern of respons e varied between species. Phosphorus taken up in excess of requirement s for vegetative growth was transferred to plant stems rather than lea ves. 5 The outcome of pot bioassays may be dependent on factors such a s pot size, irradiance and soil moisture conditions; therefore conclus ions drawn here need to be tested by field fertilization experiments.