Good days, bad days: Weather patterns driving bryophyte carbon‐exchange dynamics in tropical rainforest canopies

Abstract The physiology of tropical mosses, liverworts and lichens may be challenged by the daily weather patterns in lowland rainforests. Since these poikilohydric organisms can only photosynthesize and respire when hydrated, unfavourable timing of light and water availability (i.e. desiccation during light hours and hydration during dark hours) may lead to small or negative diel (24‐h) carbon budget, especially in the tree crowns, where desiccation frequently occurs. Here, we tested this ‘unfavourable timing’ hypothesis that explains the low abundance of bryophytes and lichens in tropical lowland forests with a high frequency of weather patterns that limit carbon gain and increase carbon losses. By example of the forest surrounding the Yasuní Scientific Station (YSS) in the Ecuadorian Amazon, our approach consisted of three steps. We used the carbon‐exchange model for poikilohydric organisms PoiCarb (Version 1.1) to simulate diel carbon budgets at a 10‐min temporal resolution for 290 days based on canopy climate data from inside a tree crown and species‐specific physiological parameters. Our simulations explicitly included parameter uncertainty; hence, we were able to discriminate simulations based on their certainty. We implemented a classification tree analysis of the modelled diel carbon budgets for Neckeropsis undulata , the most abundant bryophyte species in the studied canopies. This analysis indicated that limiting light intensities, night‐time hydration and daytime desiccation caused net carbon losses on more than two‐thirds of the modelled days, while net carbon gains occurred only on bright days with rain. We validated these classification rules for an independent subset of days for the same species and for two companion species ( Plagiochila raddiana and Symbiezidium transversale ). Our validation indicates >70% accuracy in classifying diel carbon budgets. Our findings show that tropical lowlands are a physiologically marginal environment for bryophytes, mainly due to their frequent low‐light cloudy days and, secondarily, due to sunny days with no moisture availability, limiting carbon gain in the studied canopy bryophytes. Our threefold approach constitutes a tool for predicting bryophyte physiological performance under current and changing weather and climate patterns. Read the free Plain Language Summary for this article on the Journal blog.