How Patterns of Cloudiness affect Vegetation Water Status in Mountain Regions

Cloud-affected forests (CAFs) are extremely vulnerable to changes in climate and are highly susceptible to drought, as these ecosystems depend on seasonal low cloud and fog cover. CAFs can be found at high elevations in the tropics and at the headwaters of major river basins, such as the Amazon; at middle and low elevations in coastal mountains; and in complex terrain with persistent low-level moisture convergence. The growing season in the CAF microclimate includes long periods of low-level cloud and fog; as much as 70% in the southern Appalachians. Studies have found that when rainfall is inadequate, fog helps to stave off plant water stress, allowing for continued photosynthesis during otherwise dry periods. It is posited that trees in CAFs take advantage of this unique microclimate through an adaptation whereby water vapor is taken up through the leaves, i.e. foliar water uptake (FWU), which may lead to enhanced photosynthesis rates. In this study, we investigate patterns of leaf wetting during periods of fog and cloud in CAFs in Robinson Forest, KY, USA and Monteverde, Costa Rica and link them with changes in plant water status. At Robinson Forest, fog forms within the valley as the result of temperature inversions, while low-level clouds forming along the slopes leads to immersed conditions in Monteverde. We measure changes in stomatal conductance, leaf water potential, leaf wetness, and sap flow under fog and no fog conditions in each of these distinct microclimate regimes. Initial results show that dominant canopy trees in both locations undergo FWU; however differences exist in terms of timing and rates of reverse sap flow. This work sets the foundation for understanding how projected decreases in cloud immersion under future climate scenarios will affect vegetation water status in CAFs.