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The Physiological Ecology of Fraser FIr in Southern Appalachian Cloud Forests

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Fraser fir (Abies fraseri (Pursh.) Poiret is a tree species endemic to the southern Appalachian Mountains, USA. This species is found in only six mountain-top populations in that span between southwestern Virginia and the Great Smoky Mountains of western North Carolina and eastern Tennessee. Usually associated with red spruce, Picea rubens Sarg., these forests are receive greater than 1500 mm yr-1 precipitation, with as much as 50% of their annual precipitation due to cloud water deposition. Furthermore, these forests are immersed in clouds 30-40% of the time during the summer months. A sharp ecotone between spruce-fir cloud forest and northern hardwood forest types occurs at the same elevation of the cloud immersion altitude (~1500-1700 m). It was hypothesized that clouds, and especially cloud immersion, exert a strong influence on Fraser fir physiological ecology and microclimate that ultimately determine Fraser fir distribution. One goal of the research presented here was to characterize the microclimate of these forests during cloud immersed conditions, compared to cloudy and sunny (clear sky) conditions. During cloud immersed periods, sunlight levels were low and often below the photosynthetic compensation point of Fraser fir. Leaf and air temperatures were substantially lower in cloud immersed periods than during cloudy and sunny conditions. Leaf surfaces due to cloud water deposition were observed for up to 16 hours of the day. The directional nature of light was mostly diffuse under both cloud immersed and low-cloud periods, and altered the relative contribution of both blue and red light wavelengths compared to other light wavelengths in the visible spectrum. The other main goal was to quantify photosynthetic gas exchange (especially carbon and water fluxes) during different types of sky conditions, and use these data to possibly explain the sharp lower elevation boundary of spruce-fir forests that coincides with the average bottom altitude of cloud immersion. It was hypothesized that, much like previous studies at the canopy scale under cloudy skies, leaf level photosynthesis would be greater and more efficient during cloud immersion compared to sunny conditions. However, it was found that while photosynthetic capacity of Fraser fir was slightly greater (<15%) during cloud immersion, average photosynthesis during daytime hours was ~35% lower than on sunny days. Low-cloud days had the greatest photosynthesis and photosynthetic capacity of all types of days. The lower photosynthesis on cloud immersed days was due to low sunlight levels and not the frequent leaf wetness experienced during immersed days. Water use efficiency (photosynthesis/transpiration), which was expected to be very high during cloud immersed conditions due to the cool, moist air conditions (~95-100% relative humidity and leaf to air vapor pressure deficit <0.5 kPa), was lower in immersed conditions than for both low-cloud and sunny conditions. This was because substantial transpiration still occurred during immersed conditions, and with low photosynthesis. However, determination of leaf level conductance and transpiration values were problematic due to frequently wet leaf surfaces, which needs to be addressed in future research projects. Finally, in the studies on the ecophysiology of regenerating size classes, no differences in gas exchange or water relations were found among the youngest age/size classes (germinant seedlings, seedlings, saplings). Furthermore, it was determined in greenhouse studies that gains in shoot level photosynthesis in conifers in diffuse light conditions, as found in and under clouds, and in the understory most of the time except in direct sunflecks, are mostly due to conifer shoot structure and not leaf biochemistry. Thus, benefits of diffuse light to plant ecophysiology seems to be an emergent property, arising certainly at the shoot or branch level, and evident mostly at the organism scale and above.
Reinhardt, Keith (author)
Smith, William (committee chair)
Browne, Robert (committee member)
Carter, Gregory (committee member)
Dimock, Ronald Jr (committee member)
Silman, Miles (committee member)
2009-05-07T14:30:14Z (accessioned)
2010-06-18T18:57:44Z (accessioned)
2009-05-07T14:30:14Z (available)
2010-06-18T18:57:44Z (available)
2009-05-07T14:30:14Z (issued)
Biology (discipline)
http://hdl.handle.net/10339/14710 (uri)
en_US (iso)
Wake Forest University
Release the entire work for access only to the Wake Forest University system for one year from the date below. After one year, release the entire work for access worldwide. (accessRights)
The Physiological Ecology of Fraser FIr in Southern Appalachian Cloud Forests

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