Topography in tropical forests enhances growth and survival differences within and among species via water availability and biotic interactions

Abstract

Topography is associated with variation in soil water, biogeochemical properties and climate, which drive diversity by filtering species and promoting niche differences. However, the potential for topography to promote fitness differences and diversity among tree species and populations remains poorly tested in tropical rainforests, especially at small spatial scales in everwet climates. We reciprocally transplanted tree seedlings between ridge and riparian sites and manipulated neighbour abundance and water availability to assess growth and survival differences both among species and between populations within species in response to changes in biotic interactions and soil water gradients associated with topographic heterogeneity. Seedling growth rates were higher on the ridge, but probability of survival was lower on the ridge than the riparian site. Topography also altered growth and survival responses to water availability such that seedlings in the inundated soils in the riparian site had the lowest growth and survival but increased rapidly with moderate soil drying. By contrast, growth and survival on the ridge were generally unresponsive to drying, although severe drought on the ridge reinforced differences among species in growth rates and probability of survival. The patterns of growth and survival within species did not provide evidence of local adaptation between seedlings from lowland and upslope origins. However, within species, topographic seed-origin determined the response of seedling growth and survival to increasing neighbour abundance, indicative of divergent selective pressures between individuals growing in different topographic environments. Combined, these results suggest that topographic heterogeneity promotes tropical forest diversity both at the species level via environmental filtering due to water availability and at the population level via functional responses to the density of neighbouring vegetation.