Ecological trends in lichen photosynthesis.

Drawing on data for 42 fruticose and foliose lichen species, four aspects of photosynthetic response have been ex&mined in relation to latitude and climatic variables: 1) the maximal rate of net photosynthesis (Pmax), 2) the tissue temperature at which Pmax occurs (Topt), 3) the photon flux density at which Pmax occurs (PhARsat), and 4) the tissue water status at which Pmax occurs. Tissue water status was measured as either relative water content (RWCopt) or percentage dry weight (DWopt). The Topt decreased significantly with increasing latitude; Topt also increased significantly with increasing July solar radiation, July air temperature, daily hours of bright sunshine, daily solar radiation, annual precipitation, and annual evaporation. Both RWCopt and DWopt increased significantly with annual precipitation. The Topt and RWCopt for a lichen could both be more accurately predicted from multiple regressions on macroclimatic variables describing the lichen's habitat. Both the PhARsat and the Pmax, despite their lack of simple univariate relationships to either latitude or climatic variables, could also be predicted by multiple regression. The limitations on the generality and precision of these ecological trends in lichen photosynthesis are discussed in relation to microclimatic considerations, morphological adaptations, and poikilohydric metabolism.