Plasticity influencing the light compensation point offsets the specialization for light niches across shrub species in a tropical forest understorey
Résumé
Shade tolerance can be defined as the light level at which plants can survive and possibly grow. This light level is referred to as the whole-plant light compensation point (LCP). The LCP depends on multiple leaf and architectural traits. We are still uncertain how often interspecific trait differences allow species to specialize for separate light niches, as observed between shade-tolerant species and light-demanding species. Alternatively, trait plasticity may allow many species to grow in similar light conditions. We measured leaf and architectural traits of up to 1.5-year-old seedlings of 15 sympatric Psychotria shrub species grown at three light levels. We used a 3D plant model to estimate the impacts of leaf traits, architectural traits and plant size on the whole-plant light compensation point (LCPplant). Plant growth rates were estimated from destructive harvests and allometric relationships. At lower light levels, plants of all species achieved a lower leaf light compensation point (LCPleaf). The light interception efficiency (LIE), an index of self-shading, decreased with increasing plant size and was therefore lower in high-light treatments where plants grew more rapidly. When corrected for size, LIE was lower in the low-light treatment, possibly as a result of lower investments in woody support. Species did not show trade-offs in growth under low- and high-light conditions, because species with the greatest plasticity in LCPplant and underlying traits (LCPleaf and LIE) achieved the highest growth rates at lower light levels. Synthesis. The interspecific differences in LCPplant did not result in a growth or survival trade-off between low- and high-light conditions. Instead, these differences were more than offset by the greater plasticity in LCPplant in some species, which was driven by greater plasticity in both leaves and architecture. The most plastic species achieved the fastest growth at different light levels. The results show that plasticity largely neutralizes the separation of light niches amongst species in this forest understorey genus and imply that differential preferences of species for either gaps or forest understorey occur in later life phases or are driven by other stress factors than low light alone.