COS 33-7
The importance of the timing of fog to foliar uptake and physiological functioning in southern Appalachian cloud forests
In cloud forests, frequent fog can aid in alleviating plant water stress and facilitate carbon gain. Research has suggested that direct foliar uptake of water (FU) is a common strategy for providing a moisture subsidy to leaves and alleviating periods of high water stress. Additionally, the timing of fog during key periods of high water and carbon use can alleviate stresses such as extreme temperatures and sunlight. We examined the prevalence of foliar uptake and the effects of variation in fog timing on plant water status and carbon gain in saplings from the two dominant conifers in high-elevation southern Appalachian cloud forests. These mountain-top communitiesare considered relic, boreal forests that may have persisted because of frequent cloud immersion.Using stable isotopes to identify plant water sources, we measured deuterium values before and after a 24 h period of experimental cloud immersion in Abies fraseri and Picea rubens. Changes in needle water content and xylem water potentials were also measured. Because over 65% of fog events in the southern Appalachian mountains occurs during morning hours, we conducted an additional experiment exposing saplings to morning, afternoon, or evening fog regimes and again measured FU, water potentials, and leaf level gas exchange.
Results/Conclusions
Both species exhibited FU following immersion, leading to strong improvements in water potentials (up to 0.33 MPa). In addition, total needle water content improved 3.7-6.4% following experimental submersion confirming leaf absorption of surface water. In experiment two we examined how the timing of fog would affect foliar uptake following one day of fog and eight days of fog. Foliar uptake in saplings was greatest on the first day of fog contributing up to 50% in morning fog, 23% in afternoon fog, while no FU occurred in evening fog. On day nine, morning fog regimes still resulted in greater FU although the overall quantity was not as strong. Moreover, morning and afternoon fog lead to the greatest improvements in daily water balance and carbon gain, respectively.In southern Appalachian conifers, the timing of fog events and associated benefit to physiological functioning seems to affect the direct utilization of fog water through foliar uptake. With climate change scenarios predicting changes to cloud patterns and frequency, our next steps must examine how changes to this moisture subsidy will affect health, mortality, and distribution of these threatened cloud forests.