The trade-off between photosynthetic rate and thallus moisture-demand explains lichen habitat association with the temperate rainforest

Temperate rainforests are globally rare, covering less than 1% of the Earth's surface, with 15% of their suitable climate space located in Europe. These ecosystems are uniquely defined by diverse cryptogamic species, particularly epiphytic lichens, which play crucial roles in forest biodiversity, trophic interactions, and biogeochemical processes. However, the ecophysiology of temperate rainforest lichens, with the potential to explain their regional to local distribution, their primary productivity, growth rates and biomass accumulation, remains under researched. This study asked whether the coexistence of morphologically different species within Scotland's temperate rainforest is driven by adaptation and subsequent species-sorting into contrasting microclimatic moisture environments. We examined seven species, analysing their biogeographic distributions and physiological responses to controlled moisture and light gradients to understand their degree of association with the temperate rainforest habitat. Our results indicated that species with the strongest temperate rainforest association had higher thallus water requirements (OptWC, MinWC and MaxWC) to achieve maximal photosynthesis (MaxNP) and that these appeared mediated by morphological traits. We found that, when morphological traits relating to water capture and retention, including layers of rhizine and tomentum, are linked to physiological optima, they can begin to explain how species across a spectrum of morphologies are differentially adapted or acclimated and associated to different degrees with the temperate rainforest climate. These findings also underscore the significance of ecophysiological knowledge for predicting the impacts of climate change on temperate rainforest biodiversity, since species' microhabitat responses will be pivotal in understanding broader ecological shifts.