Morphological, photosynthetic and water relations traits underpin the contrasting success of two tropical lichen groups at the interior and edge of forest fragments

Abstract

Forest edges created by fragmentation strongly affect the abiotic and biotic environment. A rarely studied consequence is the resulting impact on non-vascular plants such as poikilohydric lichens, known to be highly sensitive to changes in the microenvironment. We evaluated the impact of forest edge and forest interior on the distribution of two groups of crustose lichens characterized by the presence or absence of a cortex and sought explanations of the outcome in terms of photosynthetic response and water relations. Microclimate, distribution patterns and physiology of cortical and non-cortical lichens were compared at the edge and in the interior of an Atlantic rainforest fragment in Alagoas, Brazil. Ecophysiological aspects of photosynthesis and water relations were studied using chlorophyll a fluorescence analysis, and hydration and rehydration characteristics. Cortical and non-cortical functional groups showed a clear preference for interior and edge habitats, respectively. The cortical lichens retained liquid water more efficiently and tolerated low light. This explains their predominance in the forest interior, where total area cover on host tree trunks reached ca. 40 % (versus ca. 5 % for non-cortical lichens). Although non-cortical lichens exchanged water vapour efficiently, they required high light intensities. Consequently, they were able to exploit well-lit edge conditions where they achieved an area cover of ca. 19 % (versus ca. 7 % for cortical lichens). We provide some of the first data for lichens giving the relative quantity of incident light absorbed by the photosystem (absorptivity). The cortical group achieved higher absorptivity and quantum efficiencies, but at the expense of physiological plasticity; non-cortical lichens showed much decreased values of F_v/F_m and electron transport rates in the forest interior. Morphological and physiological features largely determine the ecophysiological interaction of lichen functional groups with their abiotic environment and, as a consequence, determine their habitat preference across forest habitats. In view of the distinctiveness of their distribution patterns and ecophysiological strategies, the occurrence of cortical versus non-cortical lichens can be a useful indicator of undisturbed forest interiors in tropical forest fragments.