期刊
OECOLOGIA
卷 158, 期 2, 页码 229-238出版社
SPRINGER
DOI: 10.1007/s00442-008-1128-5
关键词
Clouds; Ecophysiology; Fraser fir; Southern Appalachian mountains; Leaf wetness
类别
资金
- William and Flora Hewlett Foundation
- Pisgah National Forest (Grandfather Ranger District)
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 (Asat) on both cloud-immersed (16.0 mu mol m(-2) s(-1)) and low-cloud (17.9 mu mol m(-2) s(-1)) days was greater than Asat on sunny days (14.4 mu mol 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.
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