In situ, direct observation of seasonal embolism dynamics in Aleppo pine trees growing on the dry edge of their distribution

Tree Growth, Contraction and Recovery: Disentangling Soil and Atmospheric Drought Effects

We investigate how soil and atmospheric droughts jointly impact tree growth and recovery dynamics in a semi-arid pine forest, leveraging high-resolution stem diameter variation data and an irrigation experiment. The irrigated plot, where soil drought was mitigated, served as a benchmark to isolate the effects of atmospheric drought and distinguish them from the compound drought conditions experienced by control trees. Using a suite of tools based only on stem diameter variation, we identified growth modes that vary in accordance with soil water availability. Control trees showed negligible growth during the dry season but rapidly recovered with the onset of the wet season, matching the baseline growth rates of the irrigated trees, suggesting minimal compromise in hydraulic functioning. Our main finding is that heatwaves consistently depress stem-expansion rates, regardless of treatment. However, during the dry season, this negative impact diverges sharply between the treatments. Because irrigated trees benefit from a hydraulic buffer supplied by ample soil water and thus retain a positive growth baseline, the depression merely slows their expansion, whereas control trees already near zero are driven into net contraction. These findings offer new understanding of how trees balance growth, contraction, and recovery under varying drought conditions, revealing the pivotal role of soil water in shaping drought responses across seasons. As climate change intensifies the frequency and severity of drought events, this knowledge is critical for anticipating shifts in tree growth and resilience.

Mixed hydraulic responses to drought in six common woody species from a dry evergreen sclerophyll forest in South Africa

Despite the emergence of a general conceptual framework for woody tree response to drought, few studies link variation in functional traits of coexisting species to drought outcomes in diverse plant communities. We use a natural drought event to test an ecological prediction from the embolism avoidance hypothesis: that co-existing species of a single growth form (woody trees) will converge upon traits that avoid embolism during all but the most severe droughts. We evaluated hydraulic traits and drought responses of six common woody tree species from South Africa's Albany Subtropical Thicket. For each species, we measured laboratory-based xylem vulnerability and Pressure-Volume curves, and in situ minimum water potentials and four metrics of drought canopy damage during a dry period as well as a subsequent wetter period. We also quantified leaf construction and plant architecture traits, including tree height, Huber value and leaf mass per area (LMA). Species varied in the water potential associated with 50% loss of xylem function (P50), and turgor loss point, leading to between-species variation in stomatal and hydraulic safety margins. All species were shown to withstand leaf xylem water potentials more negative than -4.5 MPa before experiencing embolism. Predicted percent embolism during the dry period was associated with whole-plant drought damage but only following recovery. The LMA, modulus of elasticity, Huber value and tree height were also associated with drought damage, albeit less predictably so. Our results provide support for the embolism avoidance hypothesis and demonstrate how knowledge of species' hydraulic traits can predict canopy dieback during drought events. However, our study also reveals mixed functional responses to drought within a single major growth form (i.e., woody trees) within a community that is composed of multiple growth forms, highlighting the complexity of predicting drought outcomes in diverse communities.

Stomatal Plasticity Maintains Water Potential Homeostasis in Pinus radiata Needles

Vapour pressure deficit (VPD) is a primary determinant of stomatal behaviour and water balance in plants. With increasing global temperature, the accompanying rise in VPD is likely to have a significant impact on the performance of plant species in the future. However, the plasticity of stomatal response to VPD remains largely unexplored. This study examines the plasticity of whole plant stomatal conductance (gc) response to VPD in Pinus radiata plants grown under two temperatures and a water-deficient treatment over a period of 3 months. The soil-stem water potential gradient (ΔΨ), gc and soil-stem hydraulic conductance (Ks-s) were evaluated. The different treatment groups showed significant differences in maximum gc relating to differences in Ks-s, however, gc dynamic response to VPD was very similar in all treatments such that ΔΨ was conserved once VPD increased above an average threshold of 0.64 kPa. The ability to robustly quantify water potential regulation in Pinus presents opportunities to explore variation in this globally important tree genus as well as providing a new approach to characterize the regulation of gas exchange in response to VPD.

The impacts of rising vapour pressure deficit in natural and managed ecosystems

An exponential rise in the atmospheric vapour pressure deficit (VPD) is among the most consequential impacts of climate change in terrestrial ecosystems. Rising VPD has negative and cascading effects on nearly all aspects of plant function including photosynthesis, water status, growth and survival. These responses are exacerbated by land-atmosphere interactions that couple VPD to soil water and govern the evolution of drought, affecting a range of ecosystem services including carbon uptake, biodiversity, the provisioning of water resources and crop yields. However, despite the global nature of this phenomenon, research on how to incorporate these impacts into resilient management regimes is largely in its infancy, due in part to the entanglement of VPD trends with those of other co-evolving climate drivers. Here, we review the mechanistic bases of VPD impacts at a range of spatial scales, paying particular attention to the independent and interactive influence of VPD in the context of other environmental changes. We then evaluate the consequences of these impacts within key management contexts, including water resources, croplands, wildfire risk mitigation and management of natural grasslands and forests. We conclude with recommendations describing how management regimes could be altered to mitigate the otherwise highly deleterious consequences of rising VPD.

Vapour pressure deficit was not a primary limiting factor for gas exchange in an irrigated, mature dryland Aleppo pine forest

Climate change is often associated with increasing vapour pressure deficit (VPD) and changes in soil moisture (SM). While atmospheric and soil drying often co-occur, their differential effects on plant functioning and productivity remain uncertain. We investigated the divergent effects and underlying mechanisms of soil and atmospheric drought based on continuous, in situ measurements of branch gas exchange with automated chambers in a mature semiarid Aleppo pine forest. We investigated the response of control trees exposed to combined soil-atmospheric drought (low SM, high VPD) during the rainless Mediterranean summer and that of trees experimentally unconstrained by soil dryness (high SM; using supplementary dry season water supply) but subjected to atmospheric drought (high VPD). During the seasonal dry period, branch conductance (gbr ), transpiration rate (E) and net photosynthesis (Anet ) decreased in low-SM trees but greatly increased in high-SM trees. The response of E and gbr to the massive rise in VPD (to 7 kPa) was negative in low-SM trees and positive in high-SM trees. These observations were consistent with predictions based on a simple plant hydraulic model showing the importance of plant water potential in the gbr and E response to VPD. These results demonstrate that avoiding drought on the supply side (SM) and relying on plant hydraulic regulation constrains the effects of atmospheric drought (VPD) as a stressor on canopy gas exchange in mature pine trees under field conditions.

Rapid leaf xylem acclimation diminishes the chances of embolism in grapevines

Under most conditions tight stomatal regulation in grapevines (Vitis vinifera) avoids xylem embolism. The current study evaluated grapevine responses to challenging scenarios that might lead to leaf embolism and consequential leaf damage. We hypothesized that embolism would occur if the vines experienced low xylem water potential (Ψx) shortly after bud break or later in the season under a combination of extreme drought and heat. We subjected vines to two potentially dangerous environments: (i) withholding irrigation from a vineyard grown in a heatwave-prone environment, and (ii) subjecting potted vines to terminal drought 1 month after bud break. In the field experiment, a heatwave at the beginning of August resulted in leaf temperatures over 45 °C. However, effective stomatal response maintained the xylem water potential (Ψx) well above the embolism threshold, and no leaf desiccation was observed. In the pot experiment, leaves of well-watered vines in May were relatively vulnerable to embolism with 50% embolism (P50) at -1.8 MPa. However, when exposed to drought, these leaves acclimated their leaf P50 by 0.65 MPa in less than a week and before reaching embolism values. When dried to embolizing Ψx, the leaf damage proportion matched (percentage-wise) the leaf embolism level. Our findings indicate that embolism and leaf damage are usually avoided by the grapevines' efficient stomatal regulation and rapid acclimation of their xylem vulnerability.

Relationships between xylem embolism and tree functioning during drought, recovery, and recurring drought in Aleppo pine

Recent findings suggest that trees can survive high levels of drought-induced xylem embolism. In many cases, the embolism is irreversible and, therefore, can potentially affect post-drought recovery and tree function under recurring droughts. We examined the development of embolism in potted Aleppo pines, a common species in hot, dry Mediterranean habitats. We asked (1) how post-drought recovery is affected by different levels of embolism and (2) what consequences this drought-induced damage has under a recurring drought scenario. Young trees were dehydrated to target water potential (Ψx ) values of -3.5, -5.2 and -9.5 MPa (which corresponded to ~6%, ~41% and ~76% embolism), and recovery of the surviving trees was measured over an 8-months period (i.e., embolism, leaf gas-exchange, Ψx ). An additional group of trees was exposed to Ψx of -6.0 MPa, either with or without preceding drought (Ψx of -5.2 MPa) to test the effect of hydraulic damage during repeated drought. Trees that reached -9.5 MPa died, but none from the other groups. Embolism levels in dying trees were on average 76% of conductive xylem and no tree was dying below 62% embolism. Stomatal recovery was negatively proportional to the level of hydraulic damage sustained during drought, for at least a month after drought relief. Trees that experienced drought for the second time took longer to reach fatal Ψx levels than first-time dehydrating trees. Decreased stomatal conductance following drought can be seen as "drought legacy," impeding recovery of tree functioning, but also as a safety mechanism during a consecutive drought.

Responses of two Acacia species to drought suggest different water-use strategies, reflecting their topographic distribution

Introduction

Soil water availability is a key factor in the growth of trees. In arid deserts, tree growth is limited by very dry soil and atmosphere conditions. Acacia tree species are distributed in the most arid deserts of the globe, therefore they are well adapted to heat and long droughts. Understanding why some plants do better than others in some environments is a key question in plant science.

Methods

Here we conducted a greenhouse experiment to continuously and simultaneously track the whole-plant water-balance of two desert Acacia species, in order to unravel their physiological responses to low water availability.

Results

We found that even under volumetric water content (VWC) of 5-9% in the soil, both species maintained 25% of the control plants, with a peak of canopy activity at noon. Moreover, plants exposed to the low water availability treatment continued growing in this period. A. tortilis applied a more opportunistic strategy than A. raddiana, and showed stomatal responses at a lower VWC (9.8% vs. 13.1%, t₄= -4.23, p = 0.006), 2.2-fold higher growth, and faster recovery from drought stress.

Discussion

Although the experiment was done in milder VPD (~3 kPa) compared to the natural conditions in the field (~5 kPa), the different physiological responses to drought between the two species might explain their different topographic distributions. A. tortilis is more abundant in elevated locations with larger fluctuations in water availability while A. raddiana is more abundant in the main channels with higher and less fluctuating water availability. This work shows a unique and non-trivial water-spending strategy in two Acacia species adapted to hyper-arid conditions.

A montane species treeline is defined by both temperature and drought effects on growth season length

Montane treelines are defined by a threshold low temperature. However, what are the dynamics when the snow-free summer growth season coincides with a 6-month seasonal drought? We tested this fundamental question by measuring tree growth and leaf activity across elevations in Mt Hermon (2814 m; in Israel and Syria), where oak trees (Quercus look and Quercus boissieri) form an observed treeline at 1900 m. While in theory, individuals can be established at higher elevations (minimum daily temperature >6.5 °C for >4 months even at the summit), soil drying and vapor pressure deficit in summer enforces growth cessation in August, leaving only 2-3 months for tree growth. At lower elevations, Q. look Kotschy is replaced by Quercus cerris L. (1300 m) and Quercus calliprinos Webb (1000 m) in accompanying Q. boissieri Reut., and growth season length (GSL) is longer due to an earlier start in April. Leaf gas exchange continues during autumn, but assimilates are no longer utilized in growth. Interestingly, the growth and activity of Q. boissieri were equivalent to that of each of the other three species across the ~1 km elevation gradient. A planting experiment at 2100 m showed that seedlings of the four oak species survived the cold winter and showed budding of leaves in summer, but wilted in August. Our unique mountain site in the Eastern Mediterranean introduces a new factor to the formation of treelines, involving a drought limitation on GSL. This site presents the elevation edge for each species and the southern distribution edge for both the endemic Q. look and the broad-range Q. cerris. With ongoing warming, Q. look and Q. boissieri are slowly expanding to higher elevations, while Q. cerris is at risk of future extirpation.

Seeing is believing: what visualising bubbles in the xylem has revealed about plant hydraulic function

Maintaining water transport in the xylem is critical for vascular plants to grow and survive. The drought-induced accumulation of embolism, when gas enters xylem conduits, causes declines in hydraulic conductance (K ) and is ultimately lethal. Several methods can be used to estimate the degree of embolism in xylem, from measuring K in tissues to directly visualising embolism in conduits. One method allowing a direct quantification of embolised xylem area is the optical vulnerability (OV) technique. This method has been used across different organs and has a high spatial and temporal resolution. Here, we review studies using the OV technique, discuss the main advantages and disadvantages of this method, and summarise key advances arising from its use. Vulnerability curves generated by the OV method are regularly comparable to other methods, including the centrifuge and X-ray microtomography. A major advantage of the OV technique over other methods is that it can be simultaneously used to determine in situ embolism formation in leaves, stems and roots, in species spanning the phylogeny of land plants. The OV method has been used to experimentally investigate the spreading of embolism through xylem networks, associate embolism with downstream tissue death, and observe embolism formation in the field.