Shade acclimation of rainforest leaves to colonization by lichens

Abstract

1. Lichens that live epiphytically on leaves (foliicolous lichens) are one of the most abundant groups of epiphytes in tropical rainforests, with lichen cover on individual leaves often exceeding 50%. In this study we quantified the shading of host leaves by foliicolous lichens, and investigated the capacity of palm leaves growing under contrasting light regimes to photo-acclimate to lichen colonization. 2. Spectral analyses of 11 lichen species with differing thallus colouring and morphology demonstrated that the lichens exhibited absorptance spectra similar to their host leaves. The lichens showed mean light interceptions of 50%, with some species reducing the photosynthetically usable light by up to 70%. 3. Despite such shading, our observations indicated that leaves photo-acclimate to the cover of lichens. The rate of photosynthesis of leaf patches covered by lichens was saturated at lower irradiances, and these patches had higher chlorophyll concentrations than uncolonized leaf areas. Monte Carlo estimates of total daily carbon gain, based on assimilation-irradiance curves and continuous light records, indicated that leaf areas covered by lichens compensated fully for the shading by lichens. 4. These findings do not support the widely held opinion that shading by foliicolous lichens and other epiphylls generally reduces the rate of photosynthesis by their host leaves.