Background/Question/Methods: In higher plants, desiccation stress is expected to result in a trade-off between desiccation tolerance and growth, however these trade-offs have rarely been examined in bryophytes. Using a unique geothermal bryophyte system in Lassen Volcanic National Park we examined distribution drivers, desiccation physiology, and relative growth rates among five bryophyte species, where geothermal vents allow desiccation and thermal stress to be decoupled. In this study, we use field and lab data to address the following questions: 1) How do abiotic factors such as temperature and moisture influence species distributions in geothermal bryophyte systems? 2) Is there evidence that interspecific competition influences species distributions along these gradients? 3) Can physiological response to abiotic stress distinguish among moss species distributions in this extreme environment? To answer these questions, we used chlorophyll fluorescence as an estimate of photosynthetic status on plants in the field and collected data on temperature, humidity, and light levels for each plant. We combined these field collections with lab experiments in which we measured plant response to changes in water content. \Results/Conclusions: We found that midday relative humidity significantly influenced the distribution of A. palustre, the most common species in this system, and relative humidity influenced PSII efficiency (Fv/Fm) in four of the five bryophyte species measured, suggesting that water availability is a key factor in determining overall community structure. Individuals of Campylopus introflexus, a species that occurs in the most extreme sites with respect to water availability and temperature, and A. palustre, showed differential abilities to compensate for desiccation stress. C. introflexus plants from the field were able to maintain high PSII efficiencies for 48 hours at 50% relative water loss, while ramets of A. palustre showed extreme signs of water stress and reduced PSII efficiencies within 12 hours. Further, C. introflexus plants were found to have significantly lower relative growth rates than A. palustre plants (ramets/day), suggesting that there is a trade-off between stress tolerance and growth in this bryophyte system. Results from these studies suggest that the distribution of bryophyte species in the Lassen geothermal system may be explained by both species specific stress tolerance and interspecific competition among species.