Summary 0The reproductive output of Shorea siamensis\ a widespread dipterocarp tree\ was assessed in relation to disturbance and tree density at three sites in western Thailand during the 0885 and 0886~owering seasons[ The locations were similar except in disturbance history\ which was re~ected in decreasing tree density from undisturbed via moderately disturbed to disturbed sites[ 1 Hand pollination experiments showed S[ siamensis to be partially self!incompatible due to di}erential pollen tube growth and abortion of inbred fruit[ Although more than 89) of~owers from trees at all sites were pollinated\ pollen tubes developed in only a small proportion of these~owers[ Both pollen tube development and initial fruit production were highest at the undisturbed site[ Many fruit\ presumably selfed\ were aborted during development at all sites\ but signi_cantly more fruit were aborted at the disturbed site\ resulting in lowered production of mature fruit[ 2 Shorea siamensis was pollinated by small Tri`ona bees\ which exhibited signi_cant declines in intertree movements with increasing distance between~owering trees[ As resource availability did not di}er between sites\ di}erences in mature fruit set were considered to be mediated by changes in pollinator foraging behaviour at di}ering tree densities[ Variation in seed set was found to be negatively correlated with distance to nearest conspeci_c both within and between sites[ 3 At the two least disturbed sites observed seed set values corresponded with those expected by calculation[ However\ seed set at the disturbed site was signi_cantly lower than expected[ 4 Our results suggest that high reproductive success of S[ siamensis is dependent upon cross!pollination which\ through pollinator behaviour\ is a function of tree isolation[ This species may thus be subject to the Allee e}ect\ where population viability is reduced disproportionately with a decline in population size or\ as in this study\ population density[ The implications of these results for population recovery and genetic structure following disturbance are discussed[ Keywords] Allee e}ect\ disturbance\ forest regeneration\ gene~ow\ outcrossing\ pol! lination\ population viability Journal of Ecology "0887# 75\ 351Ð362