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The southern Appalachian spruce–fir ( Picea rubens  Sarg. and  Abies fraseri  (Pursh) Poir.) forest is found only on high altitude mountain tops that receive copious precipitation (&gt; 2000 mm year −1 ) and experience frequent cloud immersion. These high-elevation, temperate rain forests are immersed in clouds on ∼65% of the total growth season days and for 30–40% of a typical summer day, and cloud deposition accounts for up to 50% of their annual water budget. We investigated environmental influences on understory leaf gas exchange and water relations at two sites: Mt. Mitchell, NC (MM; 35°45′53″ N, 82°15′53″ W, 2028 m elevation) and Whitetop Mtn., VA (WT; 36°38′19″ N, 81°36′19″ W, 1685 m elevation). We hypothesized that the cool, moist and cloudy conditions at these sites exert a strong influence on leaf gas exchange. Maximum photosynthesis ( A  max ) varied between 1.6 and 4.0 μmol CO 2  m −2  s −1  for both spruce and fir and saturated at irradiances between ∼200 and 400 μmol m −2  s −1  at both sites. Leaf conductance ( g ) ranged between 0.05 and 0.25 mol m −2  s −1  at MM and between 0.15 and 0.40 mol m −2  s −1  at WT and was strongly associated with leaf-to-air vapor pressure difference (LAVD). At both sites,  g  decreased exponentially as LAVD increased, with an 80–90% reduction in  g  between 0 and 0.5 kPa. Predawn leaf water potentials remained between −0.25 and −0.5 MPa for the entire summer, whereas late afternoon values declined to between −1.25 and −1.75 MPa by late summer. Thus, leaf gas exchange appeared tightly coupled to the response of  g  to LAVD, which maintained high water status, even at the relatively low LAVD of these cloud forests. Moreover, the cloudy, humid environment of these refugial forests appears to exert a strong influence on tree leaf gas exchange and water relations. Because global climate change is predicted to increase regional cloud ceiling levels, more research on cloud impacts on carbon gain and water relations is needed to predict future impacts on these relict forests.

Leaf gas exchange of understory spruce–fir saplings in relict cloud forests, southern Appalachian Mountains, USA