Waiting for the snow to melt…
Transpiration is the upwards flow of water from soil to atmosphere by vegetation. This flux of water is a critical component of the photosynthetic process and an important control on the energy balance of leaves. The spatial and temporal patterns in transpiration have a dominant control of the hydrological balance of watersheds because the use of this water by plants effects how much water delivered by snow or rain remains available downstream for streamflow or groundwater recharge. While transpiration has been an obvious source of interest to ecologists and hydrologists for decades, continuous and distributed measurements of transpiration in alpine systems remain limited. The lack of observed information on transpiration has made it difficult to develop models to predict how and why transpiration varies between species, across hillslopes or in response to climate drivers such as drought. We will use a technique called sap flow to make continuous measurements of transpiration for lodgepole pine, Engelmann spruce, subalpine fir and quaking aspen across topographic gradients in the East River watershed. We will use this data to test how differences in factors such as slope, relief, lithology and upslope drainage influences the water demands between species. We will also test how these abiotic factors interact with biotic factors such as the effective rooting depth and conductivity of xylem tissues to modulate transpiration. We will complement the measurements of transpiration with periodic measurements of the stable isotope ratios of xylem and soil water to help understand the sources of water (such as snow melt vs. summer rain) as well as the soil depth trees are drawing water from. The detailed measurements of transpiration from sap flow will be complemented by a more experimental approach based on continuous measurements of the diameter of trees ? i.e. dendrometry. Changes in the daily cycle of tree circumference is related to storage of water in trees and can therefore be related to total water use. This technique has the advantage of using lower cost and more robust sensors but is less reliable. The work collectively will shed light on how trees compete for available water, particularly at the interface between aspen and conifer stands. This data will be collected synergistically with measurements of canopy temperature from drone flights which can provide spatial information on water use that complements the tree-level temporally continuous data from sap flow. In addition, this data will be used for model validation of a recently developed land surface model. The information gathered on transpiration will also be broadly applicable to understanding the resilience of typical fir, pine, spruce and aspen ecosystems in the western US to climate change.