Differences in seed germination responses may promote coexistence of four sympatric Piper species

1. Species-specific responses to the range of microsites resulting from canopy gap formation may contribute to coexistence in tropical forests. We investigated the effects of four factors affected by canopy gap formation (red : far-red light, soil nitrate concentrations, soil temperature fluctuations and soil water potential) on the germination response of four pioneer Piper species (P. dilatatum, P. hispidum, P. marginatum and P peltatum) that are typically found in canopy gaps and clearings. 2. All four Piper species required light for maximum germination. However, the ratio of red : far-red light (R : FR) resulting in maximum germination varied between species. Piper peltatum will germinate in simulated understorey light conditions; P. dilatatum and P. hispidum require conditions typically found in small to medium gaps; while P. marginatum requires the conditions appropriate to large gaps. 3. Only P. marginatum was affected by nitrate concentration: elevated concentrations increased the germination rate. This suggests that this species could detect canopy gaps using a combination of high R : FR and elevated soil nitrate concentrations. 4. The germination rate of P. marginatum was least sensitive to low water potentials and high daytime temperatures characteristic of large gaps. Piper peltatum was most sensitive to these treatments, while P. dilatatum and P. hispidum were intermediate in response. 5. A principal components analysis of the ratios of germination in understorey to large gap conditions, for four variables, generated a significant axis that explained 88.5% of the variance in germination response between species. Differential species distribution along this axis, based on species-specific responses, may allow germination to occur in the most suitable microsite for onward growth of the seedling and contribute to species coexistence by reducing interspecific competition.