Comparison of cloud conditions, microclimatology, photosynthesis, and water relations of Abies fraseri in a temperate mountain cloud forest

Background/Question/Methods

The red spruce-Fraser fir forest (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 on about 65% of all days annually.  We compared the microclimatology, photosynthetic gas exchange, and water relations of Fraser fir during cloud immersed periods versus days with low cloud or sunny conditions. 

Results/Conclusions

Compared to sunny days, cloud immersion reduced sunlight irradiance by 10-50% with an 85% reduction in the vapor pressure deficit (VPD) of the air; this generated lower air and leaf temperatures, resulting in wet needle surfaces for up to 16 hours of a day.  Maximum light-saturated photosynthesis (A_sat) on cloud immersed days was 10% greater than on sunny days, and highest on low cloud days.  However daily carbon gain was lowest on immersed days due to reduced sunlight levels, but not leaf surface wetness.  Leaf conductance (g) was significantly higher on immersed days, with mean values (0.30 mmol m^-2 s^-1) greater than commonly reported for a conifer tree species, and which declined exponentially with increasing leaf-to-air VPD (LAVD).  Transpiration (E) on immersed days was 43% and 20% lower compared to clear and low cloud days, respectively.  The lower A and E on immersed days resulted in lower WUE than on cloudy or sunny days, and a strong exponential relationship between g and LAVD, acted to uncouple A from g during cloud immersion.  Because global climate change is predicted to cause large scale changes in the annual amount and frequency of cloud cover, understanding the influence of cloud interactions with plant water and carbon relations is important for evaluating future impacts on large a number of mountain ecosystems.