Cloud immersion impacts water relations in a relic spruce-fir community in the Southern Appalachian Mountains, USA

Background/Question/Methods

The high elevation spruce-fir communities of the southern Appalachian Mountains are often characterized as relic boreal forests that may be persisting due to frequent cloud immersion.  Previous studies have proposed a relationship between cloud immersion and the altitudinal distribution pattern of these two conifer tree species, Picea rubens (Red Spruce) and Abies fraseri (Fraser Fir), but few have examined the effect of cloud immersion on incident sunlight throughout the leaf canopy and the resulting effects on ecophysiology such as photosynthesis and water relations.  Incident sunlight (photosynthetic photon flux density, PPFD) was measured every ten minutes throughout the growing season of 2010 at four different canopy levels within the spruce-fir forest and in an adjacent open area at Mount Mitchell State Park (35°46′13″N 82°15′48″W).  Daily xylem water potential measurements were taken monthly from May to September on both juvenile and adult trees of each species.  A combination of PPFD, temperature, humidity and live images from Mount Mitchell were used to classify each morning and afternoon as either immersed, cloudy, or sunny.  

Results/Conclusions

The most common day classification was morning immersion with afternoon clouds (29.6%).  In total, 60.8% of the summer days experienced cloud immersion for at least 2 hours of the day.  In comparison to clear days, completely immersed days drastically reduced the maximum (1314 µmol/m²s and 1838 µmol/m²s) and cumulative PPFD (11.09 mol/m² and 38.03 mol/m²).  While average daily temperatures were unaffected by cloud immersion, maximum daily temperatures decreased by 2.1°C and minimum daily temperatures increased by 2.5°C.  For daily and seasonal water availability, there was a greater decrease in afternoon water status (up to 0.5 MPa) on clear days versus immersed days in both juvenile and adults of each species.   Juveniles experienced lower water potentials than adults of both species throughout the summer season.  Cloud immersion also acted to decrease water loss on both a daily and seasonal basis.  With climate change scenarios predicting a higher cloud base (resulting in less immersion) and dryer conditions, examining the effects of immersion on the ecophysiology of Picea rubens and Abies fraseri is important for evaluating the future survival of these relic forests.