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Cloud immersion experienced by high-elevation rock outcrop plants reduces the leaf-to-air vapor pressure deficit (VPD), decreasing transpirational water loss. Frequent cloud immersion might ameliorate water stress in shallow-soil outcrop communities, increasing water use efficiency and growth. Current climate pattern predictions propose that Southern Appalachian cloud immersion frequency will decrease, potentially increasing water stress in rock outcrop plant populations. In this experiment, outcrop specialists Hydatica petiolaris (cliff saxifrage) and Solidago simulans (granite dome goldenrod) were grown in microcosms simulating current, reduced, and absent cloud immersion. Maximum photosynthetic rates, light saturation point and water use efficiency (WUE) increased, and transpiration decreased with decreasing immersion duration. Root mass, root-to-shoot ratio, and specific leaf mass were greatest in the reduced immersion treatment. Simulating nonimmersed abiotic conditions while measuring gas exchange in a leaf cuvette resulted in higher VPD, photosynthetic rate, transpiration, and lower WUE across treatments. Results indicate acclimation in response to immersion duration for some physiological parameters, suggesting the ability of these plants to adapt to changing climatic conditions. Further experimentation is needed to determine the effects of reduced cloud immersion on plant fitness.

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