Surviving in the shadows: light responses of co-occurring Rubiaceae species within a tropical forest understory

Abstract The question of how closely related species coexist in the heavily shadowed understory of tropical forests is still not properly clarified. We studied the magnitude of trait variation in twelve morphological, nutritional, and physiological traits related to light response in 85 individuals belonging to nine co-occurring Rubiaceae species in the understory of a tropical forest in Southeast Brazil. Three topographic habitats were chosen from shadowed and more fertile soils in the valley to more open canopies and less fertile soils at slopes and ridge, which were characterized using soil and canopy cover variables. Light curves (A/PAR) were adjusted in five leaves (one leaf per individual plant) for each species and habitat. We compared soil and canopy variables among the topographic habitats, quantified trait dispersion among the nine species, and tested for differences in trait values among habitats and species. Ridge habitat showed the lowest resource availabilities, soil nutrients and water, and a more open canopy. Species distributions in the gradient were best explained by specific leaf area (SLA) and leaf thickness. The Rubiaceae species showed a great investment in leaf nitrogen, regardless of habitat or species. Ridge species maintained low dark respiration rates and light compensation points. Higher SLA values for valley plants allowed greater light-saturated net photosynthetic rates on a mass basis, providing higher positive net carbon balance when compared to the plants in the slope and ridge. Living and survival of these shade plants are possible by very low mean light compensation points. We show that trait differences among topographic habitats in coexisting species of Rubiaceae are important to understanding the organization of tropical forests understory “species swarms”.