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Plants rely primarily on rainfall infiltrating their root zones – a supply that is inherently variable, and fluctuations are predicted to increase on most of the Earth's surface. Yet, interrelationships between water availability and plant use on short timescales are difficult to quantify and remain poorly understood. To overcome previous methodological limitations, we coupled high-resolution in situ observations of stable isotopes in soil and transpiration water. We applied the approach along with Bayesian mixing modeling to track the fate of 2H-labeled rain pulses following drought through soil and plants of deciduous tree ecosystems. We resolve how rainwater infiltrates the root zones in a nonequilibrium process and show that tree species differ in their ability to quickly acquire the newly available source. Sessile oak (Quercus petraea) adjusted root uptake to vertical water availability patterns under drought, but readjustment toward the rewetted topsoil was delayed. By contrast, European beech (Fagus sylvatica) readily utilized water from all soil depths independent of water depletion, enabling faster uptake of rainwater. Our results demonstrate that species-specific plasticity and responses to water supply fluctuations on short timescales can now be identified and must be considered to predict vegetation functional dynamics and water cycling under current and future climatic conditions.

Sudden and widespread forest die-back and die-off (e.g., Huang & Anderegg, 2012) and increased mortality rates (e.g., Peng et al., 2011) in many forest ecosystems across the globe have been linked to drought and elevated temperatures (Allen et al., 2010, Fig. 1). Furthermore, these observations have caused a focus on the physiological mechanisms of drought-induced tree mortality (e.g. McDowell et al., 2008) and many studies, both observational and manipulative, have been carried out to explain tree death during drought from a physiological perspective.

1. Amazonian droughts are predicted to become increasingly frequent and intense, and the vulnerability of Amazonian trees has become increasingly documented. However, little is known about the physiological mechanisms and the diversity of drought tolerance of tropical trees due to the lack of quantitative measurements. 2. Leaf water potential at wilting or turgor loss point (pi(tlp)) is a determinant of the tolerance of leaves to drought stress and contributes to plant-level physiological drought tolerance. Recently, it has been demonstrated that leaf osmotic water potential at full hydration (pi(o)) is tightly correlated with pi(tlp). Estimating pi(tlp) from osmometer measurements of pi(o) is much faster than the standard pressure-volume curve approach of pi(tlp) determination. We used this technique to estimate pi(tlp) for 165 trees of 71 species, at three sites within forests in French Guiana. Our data set represents a significant increase in available data for this trait for tropical tree species. 3. Tropical trees showed a wider range of drought tolerance than previously found in the literature, pi(tlp) ranging from -1.4 to -3.2 MPa. This range likely corresponds in part to adaptation and acclimation to occasionally extreme droughts during the dry season. 4. Leaf-level drought tolerance varied across species, in agreement with the available published observations of species variation in drought-induced mortality. On average, species with a more negative pi(tlp) (i.e. with greater leaf-level drought tolerance) occurred less frequently across the region than drought-sensitive species. 5. Across individuals, pi(tlp) correlated positively but weakly with leaf toughness (R-2 = 0.22, P = 0.04) and leaf thickness (R-2 = 0.03, P = 0.03). No correlation was detected with other functional traits (leaf mass per area, leaf area, nitrogen or carbon concentrations, carbon isotope ratio, sapwood density or bark thickness). 6. The variability in pi(tlp) among species indicates a potential for highly diverse species responses to drought within given forest communities. Given the weak correlations between pi(tlp) and traditionally measured plant functional traits, vegetation models seeking to predict forest response to drought should integrate improved quantification of comparative drought tolerance among tree species.

1. The xylem pressure inducing 50% loss of hydraulic conductivity due to embolism (P₅₀) is widely used for comparisons of xylem vulnerability among species and across aridity gradients. However, despite its utility as an index of resistance to catastrophic xylem failure under extreme drought, P₅₀ may have no special physiological relevance in the context of stomatal regulation of daily minimum xylem pressure and avoidance of hydraulic failure under non-extreme conditions. Moreover, few studies of hydraulic architecture have accounted for the buffering influence of tissue hydraulic capacitance on daily fluctuations in xylem pressure in intact plants. 2. We used data from 104 coniferous and angiosperm species representing a range of woody growth forms and habitat types to evaluate trends in three alternative xylem hydraulic safety margins based on features of their stem xylem vulnerability curves and regulation of daily minimum stem water potential (Ψstem min) under non-extreme conditions: (i) Ψstem min - P₅₀, (ii) Ψstem min - Pe, the difference between Ψstem min and the threshold xylem pressure at which loss of conductivity begins to increase rapidly (Pe) and (iii) Pe - P₅₀, an estimate of the steepness of the vulnerability curve between Pe and P₅₀. Additionally, we assessed relationships between xylem capacitance, species-specific set-points for daily minimum stem water potential and hydraulic safety margins in a subset of species for which relevant data were available. 3. The three types of hydraulic safety margin defined increased with decreasing species-specific set-points for Ψstem min, suggesting a diminishing role of stem capacitance in slowing fluctuations in xylem pressure as Ψstem min became more negative. The trends in hydraulic safety were similar among coniferous and angiosperm species native to diverse habitat types. 4. Our results suggest that here is a continuum of relative reliance on different mechanisms that confer hydraulic safety under dynamic conditions. Species with low capacitance and denser wood experience greater daily maximum xylem tension and appear to rely primarily on xylem structural features to avoid embolism, whereas in species with high capacitance and low wood density avoidance of embolism appears to be achieved primarily via reliance on transient release of stored water to constrain transpiration-induced fluctuations in xylem tension.

X-ray microtomography (microCT) is becoming a valuable noninvasive tool for advancing our understanding of plant–water relations. Laboratory-based microCT systems are becoming more affordable and provide better access than synchrotron facilities. However, some systems come at the cost of comparably lower signal quality and spatial resolution than synchrotron facilities. In this study, we evaluated laboratory-based X-ray microCT imaging as a tool to nondestructively analyse hydraulic vulnerability to drought-induced embolism in a woody plant species. We analysed the vulnerability to drought-induced embolism of benchtop-dehydrated Eucalyptus camaldulensis plants using microCT and hydraulic flow measurements on the same sample material, allowing us to directly compare the two methods. Additionally, we developed a quantitative procedure to improve microCT image analysis at limited resolution and accurately measure vessel lumens. Hydraulic measurements matched closely with microCT imaging of the current-year growth ring, with similar hydraulic conductivity and loss of conductivity due to xylem embolism. Optimized thresholding of vessel lumens during image analysis, based on a physiologically meaningful parameter (theoretical conductivity), allowed us to overcome common potential constraints of some lab-based systems. Our results indicate that estimates of vulnerability to embolism provided by microCT visualization agree well with those obtained from hydraulic measurements on the same sample material.

BackgroundTropical montane cloud forests (TMCFs) are characterized by a unique set of biological and hydroclimatic features, including frequent and/or persistent fog, cool temperatures, and high biodiversity and endemism. These forests are one of the most vulnerable ecosystems to climate change given their small geographic range, high endemism and dependence on a rare microclimatic envelope. The frequency of atmospheric water deficits for some TMCFs is likely to increase in the future, but the consequences for the integrity and distribution of these ecosystems are uncertain. In order to investigate plant and ecosystem responses to climate change, we need to know how TMCF species function in response to current climate, which factors shape function and ecology most and how these will change into the future.ScopeThis review focuses on recent advances in ecophysiological research of TMCF plants to establish a link between TMCF hydrometeorological conditions and vegetation distribution, functioning and survival. The hydraulic characteristics of TMCF trees are discussed, together with the prevalence and ecological consequences of foliar uptake of fog water (FWU) in TMCFs, a key process that allows efficient acquisition of water during cloud immersion periods, minimizing water deficits and favouring survival of species prone to drought-induced hydraulic failure.ConclusionsFog occurrence is the single most important microclimatic feature affecting the distribution and function of TMCF plants. Plants in TMCFs are very vulnerable to drought (possessing a small hydraulic safety margin), and the presence of fog and FWU minimizes the occurrence of tree water deficits and thus favours the survival of TMCF trees where such deficits may occur. Characterizing the interplay between microclimatic dynamics and plant water relations is key to foster more realistic projections about climate change effects on TMCF functioning and distribution.

Nitric oxide (NO) is dynamic molecule implicated in diverse biological functions demonstrating its protective effect against damages provoked by abiotic stresses. The present study investigated that exogenous NO pretreatment (500 µM sodium nitroprusside, 24 h) prevented the adverse effect of drought stress [induced by 10% and 20% polyethylene glycol (PEG), 48 h] on rapeseed seedlings. Drought stress resulted in reduced relative water content with increased proline (Pro) level. Drought stress insisted high H2O2 generation and consequently increased membrane lipid peroxidation which are clear indications of oxidative damage. Drought stress disrupted the glyoxalase system too. Exogenous NO successfully alleviated oxidative damage effects on rapeseed seedlings through improving the levels of nonenzymatic antioxidant pool and upregulating antioxidant enzymes’ activities. Improvement of glyoxalase system (glyoxalase I and glyoxalase II activities) by exogenous NO was significant to improve plants’ tolerance. Nonetheless, regulation of Pro level and improvement of plant–water status were vital to confer drought stress tolerance.

Acoustic emission (AE) analysis allows nondestructive monitoring of embolism formation in plant xylem, but signal interpretation and agreement of acoustically measured hydraulic vulnerability with reference hydraulic techniques remain under debate. We compared the hydraulic vulnerability of 16 species and three crop tree cultivars using hydraulic flow measurements and acoustic emission monitoring, proposing the use of time-dependent AE rates as a novel parameter for AE analysis. There was a linear correlation between the water potential (Ψ) at 50% loss of hydraulic conductivity (P50) and the Ψ at maximum AE activity (Pmaxrate), where species with lower P50 also had lower Pmaxrate (P < 0.001, R 2 = 0.76). Using AE rates instead of cumulative counts for AE analysis allows more efficient estimation of P50, while excluding problematic AE at late stages of dehydration.