Climate models predict widespread increases in both drought intensity and duration in the next decades. Although water deficiency is a significant determinant of plant survival, limited understanding of plant responses to extreme drought impedes forecasts of both forest and crop productivity under increasing aridity. Drought induces a suite of physiological responses; however, we lack an accurate mechanistic description of plant response to lethal drought that would improve predictive understanding of mortality under altered climate conditions. Here, proxies for leaf cellular damage, chlorophyll a fluorescence, and electrolyte leakage were directly associated with failure to recover from drought upon rewatering in Brassica rapa (genotype R500) and thus define the exact timing of drought-induced death. We validated our results using a second genotype (imb211) that differs substantially in life history traits. Our study demonstrates that whereas changes in carbon dynamics and water transport are critical indicators of drought stress, they can be unrelated to visible metrics of mortality, i.e. lack of meristematic activity and regrowth. In contrast, membrane failure at the cellular scale is the most proximate cause of death. This hypothesis was corroborated in two gymnosperms (Picea engelmannii and Pinus contorta) that experienced lethal water stress in the field and in laboratory conditions. We suggest that measurement of chlorophyll a fluorescence can be used to operationally define plant death arising from drought, and improved plant characterization can enhance surface model predictions of drought mortality and its consequences to ecosystem services at a global scale.

Eddy covariance nighttime fluxes are uncertain due to potential measurement biases. Many studies report eddy covariance nighttime flux lower than flux from extrapolated chamber measurements, despite corrections for low turbulence. We compared eddy covariance and chamber estimates of ecosystem respiration at the GLEES Ameriflux site over seven growing seasons under high turbulence [summer night mean friction velocity (u*) = 0.7 m s(-1)], during which bark beetles killed or infested 85% of the aboveground respiring biomass. Chamber-based estimates of ecosystem respiration during the growth season, developed from foliage, wood, and soil CO2 efflux measurements, declined 35% after 85% of the forest basal area had been killed or impaired by bark beetles (from 7.1 ± 0.22 μmol m(-2) s(-1) in 2005 to 4.6 ± 0.16 μmol m(-2) s(-1) in 2011). Soil efflux remained at ~3.3 μmol m(-2) s(-1) throughout the mortality, while the loss of live wood and foliage and their respiration drove the decline of the chamber estimate. Eddy covariance estimates of fluxes at night remained constant over the same period, ~3.0 μmol m(-2) s(-1) for both 2005 (intact forest) and 2011 (85% basal area killed or impaired). Eddy covariance fluxes were lower than chamber estimates of ecosystem respiration (60% lower in 2005, and 32% in 2011), but the mean night estimates from the two techniques were correlated within a year (r(2) from 0.18 to 0.60). The difference between the two techniques was not the result of inadequate turbulence, because the results were robust to a u* filter of >0.7 m s(-1). The decline in the average seasonal difference between the two techniques was strongly correlated with overstory leaf area (r(2) = 0.92). The discrepancy between methods of respiration estimation should be resolved to have confidence in ecosystem carbon flux estimates.

In subalpine watersheds of the intermountain western United States, snowpack melt is the dominant water input to the hydrologic system. The primary focus of this work is to understand the partitioning of water from the snowpack during the snowmelt period and through the remainder of the growing season. We conducted a time‐lapse electrical resistivity tomography (ERT) study in conjunction with a water budget analysis to track water from the snow‐on through snow‐off season (May–August 2015). Seismic velocities provided an estimate of regolith thickness while transpiration measurements from sap flow in conifer trees provided insight into root water uptake. We observed four hydrologic process‐periods and found that deep flow and tree water fluxes are the primary pathways through which water moves off of the hillslope. Overland flow and interflow were negligible. We observed temporal changes in vadose zone water content more than 3.0 m below the surface. Our results show that vertical flow through the thin soil mantle overlaying coarse colluvial regolith was the primary pathway to a local unconfined aquifer.

Drought predisposes conifer forests to bark beetle attacks and mortality. Although plant hydraulic stress mechanistically links to tree mortality, its capacity to predict trees' susceptibility to beetle attacks has not been evaluated. Further, both tree size and water supply could influence plant hydraulic stress, but their relative importance remained unknown. In this study, we modeled plant hydraulic stress of individual trees in a mixed forest of Lodgepole pine (Pinus contorta), Engelmann spruce (Picea engelmannii), and Subalpine fir (Abies lasiocarpa) in southern Wyoming, using an integrated model of plant hydraulics and hydrology, ground surveys of tree size as well as physiological and geophysical measurements. Based on the established link between plant hydraulic stress and tree mortality, we found interspecific differences in the relative importance of water availability and tree size. Pine mortality was best explained by the combination of tree size and water supply, and fir mortality was best explained by variations in water supply. We next compared the prediction of beetle attack by modeled plant hydraulic stress versus tree size and found tree size best explained beetle attack consistently for all three species. Taken together, our results suggested beetle attack was primarily influenced by beetle preference for large trees, potentially as food sources, rather than more hydraulically stressed trees. These findings highlighted the importance of integrated understanding of biotic/abiotic factors and their mechanistic pathways in order to accurately predict the sustainability of forests susceptible to drought and beetle outbreaks.

Plant transpiration is the largest evaporative flux from most terrestrial ecosystems, playing a dominant role in energy balance, hydrological cycling, ecosystem services and water security (Schlesinger & Jasechko, 2014). Consequently, understanding the mechanisms of plant transpiration and how they relate to plant traits is essential for enhancing agricultural productivity, optimizing land management planning, ecological studies and improving climate modelling. Transpiration rates vary over time and space, and can be measured on a variety of scales (Allen, Pereira, Howell, & Jensen, 2011). Handheld devices can measure leaf-level responses, but are highly labour intensive and prone to scaling errors (Asbjornsen et al., 2011; Mackay, Ewers, Loranty, & Kruger, 2010). On the other hand, tower and watershed-based methods observe total evapotranspiration rates for an entire ecosystem (up to 1 km 2), but these aggregated measurements fail to capture individual physiological responses and water use strategies (Asbjornsen et al., 2011).

Mistletoes are important co-contributors to tree mortality globally, particularly during droughts. In Australia, mistletoe distributions are expanding in temperate woodlands, while their hosts experienced unprecedented heat and drought stress in recent years. We investigated whether the excessive water use of mistletoes increased the probability of xylem emboli in a mature woodland during the recent record drought that was compounded by multiple heatwaves. We continuously recorded transpiration ($T_{SLA}$) of infected and uninfected branches from two eucalypt species over two summers, monitored stem and leaf water potentials ($\Psi $), and used hydraulic vulnerability curves to estimate percent loss in conductivity (PLC) for each species. Variations in weather (vapour pressure deficit, photosynthetic active radiation, soil water content), host species and % mistletoe foliage explained 78% of hourly $T_{SLA}$. While mistletoe acted as an uncontrollable sink for water in the host even during typical summer days, daily $T_{SLA}$ increased up to 4-fold in infected branches on hot days, highlighting the previously overlooked importance of temperature stress in amplifying water loss in mistletoes. The increased water use of mistletoes resulted in significantly decreased host $\Psi _{leaf}$ and $\Psi _{trunk}$. It further translated to an estimated increase of up to 11% PLC for infected hosts, confirming greater hydraulic dysfunction of infected trees that place them at higher risk of hydraulic failure. However, uninfected branches of Eucalyptus fibrosa had much tighter controls on water loss than uninfected branches of Eucalyptus moluccana, which shifted the risk of hydraulic failure towards an increased risk of carbon starvation for E. fibrosa. The contrasting mechanistic responses to heat and drought stress between both co-occurring species demonstrates the complexity of host–parasite interactions and highlights the challenge in predicting species-specific responses to biotic agents in a warmer and drier climate.

The total solar eclipse of August 21, 2017 created a path of totality ~115 km in width across the United States. While eclipse observations have shown distinct responses in animal behavior often emulating nocturnal behavior, the influence of eclipses on plant physiology are less understood. We investigated physiological perturbations due to rapid changes of sunlight and air temperature in big sagebrush ( Artemisia tridentata ssp. vaseyana ), a desert shrub common within the path of eclipse totality. Leaf gas exchange, water potential, and chlorophyll a fluorescence were monitored during the eclipse and compared to responses obtained the day before in absence of the eclipse. On the day of the eclipse, air temperature decreased by 6.4 °C, coupled with a 1.0 kPa drop in vapor pressure deficit having a 9-minute lag following totality. Using chlorophyll a fluorescence measurements, we found photosynthetic efficiency of photosystem II ( Fv’/Fm’ ) recovered to near dark acclimated state (i.e., 87%), but the short duration of darkness did not allow for complete recovery. Gas exchange data and a simple light response model were used to estimate a 14% reduction in carbon assimilation for one day over sagebrush dominated areas within the path of totality for the Western United States.

The prediction of snowmelt in mountainous forests strongly depends on the accurate description of sensible and latent heat turbulent fluxes. Uncertainty about the withincanopy wind conditions especially poses a challenge, with relatively few studies examining both above-and below-canopy turbulent fluxes. In this study, turbulent flux predictions from a state-of-the-art watershed model GEOtop were verified against eddy covariance data from one above-canopy tower and two below-canopy towers in a snow-dominated coniferous forest in south-eastern Wyoming. The model was applied in one-dimensional vertical mode using field-observed vegetation parameters and laboratory-measured soil water retention data. The model was calibrated by identifying optimum values for the canopy fraction and the within-canopy eddy decay coefficient using the brute-force method. Above-canopy sensible heat flux at the Glacier Lakes Ecosystem Experiments Site was predicted reasonably well (r 2 = .851). The prediction of above-canopy latent heat flux was weaker (r 2 = .426). For latent heat flux, errors in 30-min values offset each other when fluxes were aggregated over time, resulting in realistic mean diurnal trends. Below-canopy turbulent flux at two sites in the Libby Creek Experimental Watershed were predicted with variable success with r 2 = .031-.146 for sensible heat flux and r 2 = .445-.581 for latent heat flux. Modelled below-canopy sensible heat flux was too low due to the underestimation of daytime ground surface temperature, because of not enough solar radiation reaching the soil surface. This study suggests that future work on GEOtop and related models should include better parameterizations of the ground surface energy balance to more reliably predict snowmelt and streamflow from mountainous forests. KEYWORDS forest, modelling, snow cover, surface energy balance 1 | INTRODUCTION Streamflow from snow-dominated mountainous ecosystems is an important source of water in many parts of the world, including the Western United States (Bales et al., 2006). The timing of snowmelt is strongly influenced by the canopy and ground surface energy balances. These energy balances show high spatio-temporal variability with differences in elevation, slope, aspect, and vegetation cover, and diurnal and seasonal fluctuations in solar radiation, temperature, and precipitation, all playing a role. As a result, watershed computer