Detection of tree stress from 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. Based on SAPFLUXNET measurements of sap flow across tropical, temperate and boreal biomes, we demonstrate how variations in the diurnal cycle of 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 seasonal variations in the morning slope are positively associated with top soil moisture (0–30 cm). The area of the diurnal cycle, characterizing the degree of daily hysteresis between sap flow and VPD, increases with sap flow downregulation before peak VPD and is sensitive to temperature and soil moisture variability at seasonal time scales. While in situ sensors can provide continuous sap flow data, we aim to evaluate the potential to estimate 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 propose that sub-daily microwave observations, with at least three sub-daily overpasses could be used to characterize the sub-daily hysteresis and enable improved monitoring of tree hydraulic stress and, consequently, biosphere dynamics.