Effects of cloud immersion and elevation on sapling photosynthesis and water status in relic spruce-fir (Picea rubens and Abies fraseri) forest, southern Appalachian Mountains, USA

Background/Question/Methods

The mountaintop spruce-fir communities of the southern Appalachian Mountains, USA are considered endemic, yet characterized as relic and endangered boreal forests that may be persisting due to frequent cloud immersion (>60% of growing season days).  In addition to common cloud immersion, the communities have unique climates of moderate to cool summer temperatures (13.5°C mean maximum air temperature during the growth season) with very high rainfall (>2000 mm year^-1).  Previous studies have proposed a relationship between cloud immersion and the altitudinal distribution pattern of Picea rubens Sarg. (Red Spruce) and Abies fraseri (Pursh) Poir. (Fraser Fir).  The current study examined the microclimate, photosynthesis, and water status of saplings near their upper (1960 m) and lower (1550 m) elevational limits.  Incident sunlight (PPFD), air temperature and humidity were measured every 15 minutes throughout the entire growing season at Mount Mitchell State Park (35°46′13″N 82°15′48″W). Daily xylem water potentials (Ψ) and leaf photosynthesis (A) were measured simultaneously at both sites on completely clear and cloud-immersed days during the entire growing season.  

Results/Conclusions

Cloud immersion occurred more frequently (over 2X the number of days) at the high elevation plot.  On both clear and cloud-immersed days, mean air temperatures were higher at the lower site while PPFD and humidity were greater at the higher site.  On cloud immersed and clear days, A. fraseri photosynthesis was greater (A_max =3.5 versus 2.5 µmol m^-2 s^-1) and had a less negative afternoon water status (xylem water potential Ψ = -0.97 MPa versus -1.37 MPa at the high versus low elevation site, respectively).  Picea rubens followed a similar trend, although the variation between elevations was not as large.  In general, the elevational differences in both photosynthesis and Ψ between elevations were more pronounced later in the season (September) than early (May), especially for A. fraseri at the lower elevation.  With climate change scenarios predicting a higher cloud base and, thus, less immersion at lower elevations, future shifts in the zonal distribution of Picea rubens and Abies fraseri appear probable.