The role of pseudocyphellae and thallus traits in modulating drying rates of the lichen Crocodia aurata (Ach.) Link ( Peltigeraceae ) across a precipitation gradient in a Costa Rican tropical forest

Abstract Lichens have long been recognized as indicator species of environmental health and microclimate stability because they passively regulate their internal water status. Rather, lichens modulate their environment-to-internal water status relationships through structural adaptations in the thallus. Pseudocyphellae, breaks in the upper and lower cortex that expose the medulla, are present across unrelated lineages of lichens. However, they have rarely been studied from a functional physiological perspective in the tropical ecosystem, an ecologically important yet understudied biome. To understand the role of traditionally measured thallus-related traits (thickness, area, specific mass and water holding capacity) and pseudocyphellae-related traits (size and density) in regulating water status, we investigated the tropical lichen Crocodia aurata ( Peltigeraceae ) at three sites situated along a precipitation gradient in Monteverde, Costa Rica. We hypothesized that drying rate (k) is predicted by differences in water availability along a natural precipitation gradient, and that thallus and pseudocyphellae traits drive variation in k across this gradient. To test this hypothesis, we calculated the drying rate and measured thallus traits in 60 thalli across three sites spanning ~600 m of elevation and differing by 1000–1500 mm in annual precipitation. Consistent with ecophysiological theory, which suggests that lichens in wetter habitats will dry faster to maintain a favourable, intermediate water status, we found that samples of Crocodia aurata from wetter sites dried significantly faster than those from drier sites. All traditionally measured thallus traits predicted drying rate while pseudocyphellae-related traits did not significantly predict drying rate. This suggests that pseudocyphellae may not play a primary role in regulating the drying process in Crocodia aurata , in response to water availability along a tropical precipitation gradient. Instead, traditionally measured thallus traits remain the dominant drivers of drying rates. These findings contribute to our understanding of the acclimation of lichens to different climatic conditions and emphasize the complex interaction between environmental factors and morphological traits. Studying water relations and corresponding functional traits of these poikilohydric organisms is critical if we are to understand how these ecologically important organisms interact with their environments as a function of specific functional traits, and hence understand future impacts of changing climates.