Vegetation health monitoring based on sub-daily sap flow variability

Abstract. The terrestrial biosphere plays a critical role in regulating carbon and water fluxes. Rising global temperatures increase atmospheric dryness, which in turn raises atmospheric water demand on vegetation and places. Some plants regulate transpiration losses by closing stomata, at the cost of reduced carbon uptake. Quantifying stomatal regulation and detecting early onset of vegetation stress at large scales remains a challenge. Sap flow in stems responds to water potential gradients between the roots and the atmosphere, and therefore provides a window into transpiration and stomatal regulation. Here, we demonstrate how variations in the diurnal cycle of sub-daily sap flow as a function of vapor pressure deficit (VPD) measurements can elucidate the different levels of plant hydraulic stress. We derive two metrics based on sub-daily responses of sap flow to VPD: the morning sensitivity, given by the slope of the bi-variate relationship, and the area of the diurnal sap flow – VPD curve. We find that the morning slope, is positively associated with top (0-30cm) soil moisture, i.e., soil water availability. The area of the diurnal cycle, characterizing the degree of daily hysteresis between sap flow and VPD, is sensitive to temperature and soil moisture, increasing with sap flow downregulation before peak VPD. While in situ sensors can provide continuous sap flow data, we consider the potential to estimate these descriptors of the diurnal cycle using temporally sparse data. In particular, as sap flow is connected to changes in water storage, which can be estimated using microwave remote sensing, we examine the degree to which the slope and area can be estimated for several acquisition strategies that vary in terms of the numbers of observations and acquisition times. We argue that sub-daily microwave observations, with at least three sub-daily overpasses could be used to characterize the hysteresis and enable improved monitoring of biosphere dynamics and vegetation health.