The role of colonising species in the regeneration of dipterocarp rain forest
Regeneration of colonising species following disturbance of primary tropical rain forest in Malaysia was studied in artificial gaps ranging in size up to 30% canopy openness. Effects of gap size on soil temperature and water status were investigated. Spatial variation in density and composition of the soil seed bank beneath undisturbed forest was assessed by germination from soil samples, and seed rain was sampled using seed traps. Seed germination, seedling survival and seedling growth were monitored for two years in gaps and beneath intact canopy. Effects on colonisation of gap size, disturbance to microsite and competition from advance regeneration were studied. Maximum soil temperatures increased with gap size but minimum temperature was unaffected. Soil water content showed no relationship with gap size, but might be higher in gaps than forest after dry periods. Primary forest soil seed bank density was very high (mean 1904 seeds m-2). Climbing shrubs/lianes contributed approximately 75% of seedlings and 50% of taxa. Trees accounted for about 25% composition of taxa, but only 15% of seedlings. Soil seed bank composition differed little among primary forest sites. Similarities in composition between one site and secondary forest possibly reflected recent disturbance. Current seed rain intensity was low, very variable, and not consistently different between gaps and closed forest. Germination in the two years after gap creation was < 5% of the soil seed bank, and appeared greatest in medium-sized gaps. Most germination and accession of new taxa occurred in year one, but both continued at low levels during year two. Ordination analysis suggested that some colonising species regenerated only in large gaps. Both exposure and scarification of the soil increased germination, but had no consistent effects on seedling survival or growth. Competition from advance regeneration was potentially important, increasing mortality and reducing maximum growth rate among colonising seedlings.