title: Impacts of cloud immersion on microclimate, photosynthesis and water relations of Abies fraseri (Pursh.) Poiret in a temperate mountain cloud forest.
abstract: The red spruce-Fraser fir ecosystem [Picea rubens Sarg.-Abies fraseri (Pursh) Poir.] of the southern Appalachian mountains, USA, is a temperate zone cloud forest immersed in clouds for 30-40% of a typical summer day, and experiencing immersion on about 65% of all days annually. We compared the microclimate, photosynthetic gas exchange, and water relations of Fraser fir trees in open areas during cloud-immersed, low-cloud, or sunny periods. In contrast to sunny periods, cloud immersion reduced instantaneous sunlight irradiance by 10-50%, and midday atmospheric vapor pressure deficit (VPD) was 85% lower. Needle surfaces were wet for up to 16 h per day during cloud-immersed days compared to <1 h for clear days. Shoot-level light-saturated photosynthesis (A (sat)) on both cloud-immersed (16.0 micromol m(-2) s(-1)) and low-cloud (17.9 micromol m(-2) s(-1)) days was greater than A (sat) on sunny days (14.4 micromol m(-2) s(-1)). Daily mean A was lowest on cloud-immersed days due to reduced sunlight levels, while leaf conductance (g) was significantly higher, with a mean value of 0.30 mol m(-2) s(-1). These g values were greater than commonly reported for conifer tree species with needle-like leaves, and declined exponentially with increasing leaf-to-air VPD. Daily mean transpiration (E) on immersed days was 43 and 20% lower compared to sunny and low-cloud days, respectively. As a result, daily mean water use efficiency (A/E) was lowest on cloud-immersed days due to light limitation of A, and high humidity resulted in greater uncoupling of A from g. Thus, substantial differences in photosynthetic CO2 uptake, and corresponding water relations, were strongly associated with cloud conditions that occur over substantial periods of the summer growth season.
authors.affiliation: Department of Biology, Wake Forest University, P.O. Box 7325, Reynolda Station, Winston-Salem, NC 27109-7325, USA. reinhaks@wfu.edu
authors.forename: Keith
authors.initials: K
authors.lastname: Reinhardt
authors.forename: William K
authors.initials: WK
authors.lastname: Smith
mesh_terms: D028202:Abies / Q000502:physiology*; D000704:Analysis of Variance; D001061:Appalachian Region; D002245:Carbon Dioxide / Q000378:metabolism; D008834:Microclimate; D010788:Photosynthesis; D018515:Plant Leaves / Q000502:physiology; D018526:Plant Transpiration; D012044:Regression Analysis; D012621:Seasons; D013472:Sunlight; D014197:Trees / Q000502:physiology; D014867:Water
chemical_list: D014867:Water; D002245:Carbon Dioxide