THE ECOLOGY OF VERY SMALL-SEEDED SHADE-TOLERANT TREES AND SHRUBS IN LOWLAND RAIN-FOREST IN SINGAPORE

A study was made in Bukit Timah Nature Reserve of niche differentiatio n among 11 woody species that have very small seeds (22-460 mu g) and establish in deep shade, i.e., where the indirect site factor (isf) un der cloudy conditions is less than or equal to 2%, and one taxonomical ly related light-demander (seed mass 33 mu g). Comparative observation s were made on changing light requirement with age. The species varied from shrubs and treelets (Urophyllum) via small to medium-height tree s (Ficus, Pternandra) to tall trees (Gynotroches and Pellacalyx). In g eneral, the shorter species fruited in deep shade while the taller nee ded direct irradiance. All produced ripe fruit at least once a year. I n 1-3% isf seedling mortality over II months was 24% for Urophyllum hi rsutum and 57% for Pternandra echinata (two strongly shade-tolerant sp ecies); survivors produced appreciable new stem and leaf tissue. For t wo more light-demanding species (Ficus grossularioides and F. fistulos a) mean lamina area was much larger in small gaps (10% isf) than in th e understorey (< 3% isf), while for Urophyllum hirsutum it was much sm aller. These three species showed the greatest difference in SLA. Then were few significant intraspecific differences between gaps and under storey in concentrations of N, P or K. In a valley where there was no significant difference in isf between a path and adjacent understorey, seedlings of species with seeds < 1 mg mean dry mass were five times as dense on transects along steep pathside banks as in the understorey 5 m upslope. The mean mid-bank slope was 66 degrees (cf. 20 degrees u pslope). On mid-bank transects litter had a cover of 36% (cf. 95% upsl ope), and was thinner (0.8 vs 3.2 cm). The soil at 0-7 cm depth into t he mid-bank had much lower concentrations of organic matter, total N a nd P (but not total K) and fine roots than that of upslope transects. All species were found to have vesicular-arbuscular mycorrhiza when 1- 2 m tall in deep shade, several having very dense infections. Seedling form and responses are discussed in relation to potential adaptive va lue and possible evolutionary origin.