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To understand and predict the effects of climate change on organisms requires the disentangling of the effects of temperature and humidity (which can change with temperature). By carefully controlling air moisture as well as temperature we show that Antarctic mosses are not affected by a wide range of temperatures but instead respond strongly to humidity. This finding is crucial to making accurate prediction of how polar vegetation (in which mosses often dominate) will respond to current and future climate changes. Abstract:Predicting impacts of climate change requires an understanding of the sensitivity of species to temperature, including conflated changes in humidity. Physiological responses to temperature and clump-to-air vapour pressure difference (VPD) were compared in two Antarctic moss species, Ceratodon purpureus and Schistidium antarctici. Temperatures from 8 to 24ºC had no significant effects on photosynthesis or recovery from drying, while high VPD accelerated drying. In Schistidium, which lacks internal conduction structures, shoots dried more slowly than the clump, and photosynthesis ceased at high shoot relative water content (RWC), behavior consistent with a strategy of drought avoidance although desiccation tolerant. In contrast, shoots of Ceratodon have a central vascular core, but dried more rapidly than the clump.These results imply that cavitation of the hydroid strand enables hydraulic isolation of extremities during rapid drying, effectively slowing water loss from the clump. Ceratodon maintained photosynthetic activity during drying to lower shoot RWC than Shistidium, consistent with a strategy of drought tolerance. These ecophysiological characteristics provide a functional explanation for the differential distribution of Shistidium and Ceratodon along moisture gradients in Antarctica.Thus, predicting responses of non-vascular vegetation to climate change at high latitudes requires greater focus on VPD and hydraulics than temperature.We hypothesized that: (1) the cosmopolitan species (C. purpureus) would be less sensitive to temperature than the Antarctic endemic (S. antarctici) due to its much wider geographic distribution; however (2) clump-to-air vapor pressure difference (VPD) would have a greater impact on photosynthetic activity than temperature alone, and(3) high VPD would more negatively impact the species lacking specialized hydraulic structures (S. antarctici) by dehydrating shoots faster than they can be replenished. Materials and Methods:Samples from the two species Schistidium antarctici (Cardot) L.I. Savicz and Smirnova and Ceratodon purpureus (Hedw.) Brid. were collected at Casey Station on Bailey Peninsula, Vincennes Bay (66°16'54''S 110°31'28''E), in February 2012 at the end of the summer season. Schistidium antarctici grows in melt ponds and streams while C. purpureus is commonly found in dry locations at this station. Due to strict sampling regulations on very slow growing species from continental Antarctica, only 10cm 2 of each species could be collected. Such a sam...

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