High-elevation species in some Southern Appalachian communities have been shown to be dependent on frequent cloud immersion. However, little is known about the influence of clouds on rare and endemic species native to high-elevation rock outcrops. These outcrops are characterized by shallow soil and high insolation, exposing species to frequent water stress, which might be partially ameliorated by cloud immersion. We compared leaf gas-exchange characteristics of two rock outcrop specialists (Hydatica petiolaris and Sibbaldiopsis tridentata) and two forest-edge herb generalists (Potentilla simplex and Solidago flexicaulis) growing at a high-elevation outcrop in the Craggy Mountains in North Carolina. At least 1 hr of cloud immersion occurred on 67% of days, mostly in the morning hours, whereas most (54%) daylight hours were cloudy. Temperature was lower and humidity higher on cloudy days. Light response curves did not reveal differences among species. Diurnal gas-exchange curves revealed that leaf-to-air vapor-pressure deficits were lower on cloudy days, and on those days, outcrop specialists had higher stomatal conductance and internal CO2 concentration, but there was no effect on net photosynthetic rates, suggesting little stomatal limitation to photosynthesis on clear days. Water-use efficiency varied among species but was not consistently higher for outcrop specialists. More research is needed to further elucidate the influence of clouds and cloud immersion on rock outcrop communities.
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Vol. 81 • No. 4