Responses to simulated drought and elevated nutrient supply among shade-tolerant tree seedlings of lowland tropical forest in Singapore

Seedlings of three tree species of lowland tropical rain forest in Singapore grown in pots of forest soil at low irradiance were subjected to simulated drought and nutrient addition treatments. The gravimetric soil water contents applied during drought simulations were within the range found during an eight-month survey of soil moisture content at 10 understory sites in primary forest in Singapore. Aspects of plant growth and dry mass allocation were influenced strongly by simulated drought for all three species. Morphological responses to the low-frequency watering regime were similar to those described for herbaceous crop plants subjected to water shortage, and suggest chat the tree seedlings used are capable of withstanding substantial periods of low soil water availability. In addition, analysis of covariance showed that height and leaf area growth of seedlings of Antidesma cuspidatum were limited by Mg and height growth of seedlings of Vatica maingayi by one or more macro-nutrients. These results suggest that tree seedlings of tropical lowland rain forests may be limited by the availability of soil resources, as well as irradiance, when growing in deep shade.