The segmentation hypothesis, a framework for understanding plant drought adaptive strategy, has long been based on hydraulic resistance and vulnerability. Storage of water and carbohydrate resources is another critical function and shapes plant drought adaption and fitness together with hydraulic efficiency and vulnerability. However, patterns and implications of the interdependency of stored water and carbohydrate resources in the context of the segmentation hypothesis are poorly understood. We measured resource pools (relative water content [RWC] soluble sugar [SS] and starch [S]) and anatomical features of leaves and supporting twigs for 36 trees in a subtropical population during the dry season when the Budyko's aridity index was 0.362. For each tree, we rank-transformed the RWC (RWCrank), SS (SSrank), and S (Srank) and characterised the resource segmentation within organs using Ln(RWCrank/SSrank) and Ln(RWCrank/Srank). We also assessed the resource segmentation between organs using the difference in resource pools between leaves and twigs (RWCleaf-twig, SSleaf-twig, and Sleaf-twig). Resource segmentation was much more effective than the organ-level resource pool alone in predicting intraspecific variation of tree growth rates. Fast-growing individuals were mainly characterised by lower leaf Ln(RWCrank/SSrank), higher twig Ln(RWCrank/SSrank), and lower SSleaf-twig. The resource segmentation strategy of fast-growing individuals was associated with anatomical attributes that facilitate phloem SS loading and unloading and thus water supply upstream. Our results highlight that resource segmentation is an important dimension of plant drought adaptive strategies and enables better prediction of tree growth vigour than resource pool attributes individually.

Both elevated temperature and reduced precipitation have been related to growth declines in Douglas-fir (Pseudotsuga menziesii) in the northwest U.S. However, the impact of high vapor pressure deficit (VPD) on Douglas-fir growth and physiological stress is not fully understood. We investigated how inter- and intra-annual rainfall and VPD correlated to the radial growth and carbon isotope signature (delta C-13) of latewood for similar to 50-year-old Douglas-fir trees in the western Cascade Mountains in Oregon. Latewood delta C-13 reflects variation in stomatal restriction of photosynthetic gas exchange and, therefore, was used as a proxy for the relative degree of physiological water stress. We cored three trees at each of nine sampling sites (n = 27 trees) and used a moving window analysis to test the period of the year in which VPD and rainfall best predicted mean latewood radial growth and delta C-13. Latewood growth, measured as the basal area increment, was more sensitive to daytime VPD than the timing and amounts of rainfall, especially in early summer. In contrast, delta C-13 was equally sensitive to the average daytime VPD and total rainfall during spring and summer. We used the results of the moving window analysis in a linear mixed effects model to test how the effect of VPD and rainfall on yearly latewood growth and delta C-13 differed among our nine sites. We found no evidence for statistical differences in the effects of VPD and rainfall on growth (p = 0.93 and p = 0.91) or delta C-13 (p = 0.31 and p = 0.81) among our nine sites. However, the marginal effects of VPD on latewood growth at each site were weakly related to soil moisture deficits at 100 cm suggesting that site-to-site differences in soil moisture availability may be important in buffering the negative effects of seasonal aridity on growth. In contrast, there was no evidence that soil moisture differences among sites influenced the marginal effects of VPD on latewood delta C-13. We conclude that increases in VPD during summer are likely to reduce latewood growth and increase water stress in Douglas-fir in the Pacific Northwest region. However, more research is needed to better understand the magnitude of this effect across sites with variable subsurface water storage and microclimate.

To examine the mechanisms associated with growth, survival, and mortality under drought conditions during early developmental stages, physiological and structural parameters were measured on Pseudotsuga menziesii (PSME) and Pinus ponderosa (PIPO) first-year seedlings grown under two moisture regimes (drought and control) in a common garden experiment. By the end of the 76-day experiment, PSME mortality in the drought and control treatments had reached 30.4

The rise in temperatures with climate warming exposes trees and forests on Earth to a triple threat through increased soil drought, enhanced atmospheric drought, and growing heat stress. Understanding which tree species are susceptible to mortality under a more arid future climate is urgent. Here, we review recent progress in our understanding of the drought and heat response of Douglas-fir (Pseudotsuga menziesii, DF) in all relevant fields of research, spanning from stomatal regulation and photosynthetic responses, plant water status dynamics and the vulnerability of the hydraulic system, to adaptive responses of the root system, the climate sensitivity of growth, and climate change-related tree vitality declines and die-off. The species’ high productivity is linked to large leaf areas at maturity, which cause fairly high interception and transpiration rates and often result in effective soil moisture depletion and reduced groundwater recharge. While hydraulic safety is high in DF branch xylem, embolism seems common in the xylem of terminal branchlets and small roots during summer drought. The existing evidence suggests that the photosynthetic apparatus and growth rate of DF are fairly heat-sensitive in comparison to other tree species, with impairment of photosynthesis starting at temperatures of ∼40 °C. A key growth-limiting factor is a high atmospheric saturation deficit, which causes partial stomatal closure and growth decline in summer, explaining high productivity in more humid maritime climates. We explore population, provenance and variety differences in the resistance to drought and heat and the related capacity to adapt, and compare the performance of Douglas-fir to that of other tree species. Across provenances and families, trade-offs between productivity and drought resistance, cold resistance and heat resistance have emerged, and more drought-resistant provenances are often less cold-hardy. Recent hot droughts have caused vitality decline and local stand-level die-off in DF in the drier parts of the U.S. and in some European regions. For the economically important coastal variety (P. m. var. menziesii), the existing evidence suggests considerable vulnerability to a future warmer and drier climate, especially in the warmer lowlands, while the interior variety (P. m. var. glauca) may perform better. Perspectives for the silviculture of Douglas fir in a warmer world are outlined.

Microbial communities are found throughout the biosphere, from human guts to glaciers, from soil to activated sludge. Understanding the statistical properties of such diverse communities can pave the way to elucidate the common mechanisms ...Multiple ecological forces act together to shape the composition of microbial communities. Phyloecology approaches—which combine phylogenetic relationships between species with community ecology—have the potential to disentangle such forces but are often ...

Uncertainties surrounding tree carbon allocation to growth are a major limitation to projections of forest carbon sequestration and response to climate change. The prevalence and extent to which carbon assimilation (source) or cambial activity (sink) mediate wood production are fundamentally important and remain elusive. We quantified source-sink relations across biomes by combining eddy-covariance gross primary production with extensive on-site and regional tree ring observations. We found widespread temporal decoupling between carbon assimilation and tree growth, underpinned by contrasting climatic sensitivities of these two processes. Substantial differences in assimilation-growth decoupling between angiosperms and gymnosperms were determined, as well as stronger decoupling with canopy closure, aridity, and decreasing temperatures. Our results reveal pervasive sink control over tree growth that is likely to be increasingly prominent under global climate change.

Deep soil water utilization allows plants to cope with drought stress. However, little is known about the roles of the understorey layers in driving spatio‐temporal variations of deep soil water in forests and how the patterns of deep soil water use among life‐forms contribute to community assembly processes. We assessed the spatio‐temporal patterns and determinants of deep water utilization of tree, shrub and herb layers in subtropical coniferous plantations and investigated associations between deep water use parameters and dominance and richness of understorey vegetation. We found that the understorey layer had a higher reliance on deep soil water in the dry season, a larger seasonal plasticity of deep soil water uptake, but lower spatial variability in deep soil water utilization than the tree layer. We showed that greater reliance of the tree layer on deep soil water was associated with decreased shrub layer diversity, whereas greater reliance of the shrub layer on deep water was associated with increased herb layer diversity. Synthesis. Our results highlight the roles of understorey layers in driving the temporal dynamics of deep soil water in forests and improve our understanding of how deep soil water use patterns among life‐forms shape community assembly in forests.

Understanding and predicting the relationship between leaf temperature ( T leaf ) and air temperature ( T air ) is essential for projecting responses to a warming climate, as studies suggest that many forests are near thermal thresholds for carbon uptake. Based on leaf measurements, the limited leaf homeothermy hypothesis argues that daytime T leaf is maintained near photosynthetic temperature optima and below damaging temperature thresholds. Specifically, leaves should cool below T air at higher temperatures (i.e., > ∼25–30°C) leading to slopes <1 in T leaf / T air relationships and substantial carbon uptake when leaves are cooler than air. This hypothesis implies that climate warming will be mitigated by a compensatory leaf cooling response. A key uncertainty is understanding whether such thermoregulatory behavior occurs in natural forest canopies. We present an unprecedented set of growing season canopy-level leaf temperature ( T can ) data measured with thermal imaging at multiple well-instrumented forest sites in North and Central America. Our data do not support the limited homeothermy hypothesis: canopy leaves are warmer than air during most of the day and only cool below air in mid to late afternoon, leading to T can / T air slopes >1 and hysteretic behavior. We find that the majority of ecosystem photosynthesis occurs when canopy leaves are warmer than air. Using energy balance and physiological modeling, we show that key leaf traits influence leaf-air coupling and ultimately the T can / T air relationship. Canopy structure also plays an important role in T can dynamics. Future climate warming is likely to lead to even greater T can , with attendant impacts on forest carbon cycling and mortality risk.

We document the response of a 2 day wildfire smoke event on a moist temperate coniferous old‐growth forest in the western U.S. Wildfire smoke increased air temperature and suppressed relative humidity and total incoming radiation. Despite these conditions a ∼10% increase in ecosystem photosynthesis was observed. This was explained by a large increase (41%) in ecosystem‐scale stomatal conductance, inferred from measurements of carbonyl sulfide. Increased stomatal conductance contradicts expected stomatal closure at high vapor pressure deficit and was likely caused by an increase in diffuse light and thus shade leaf insolation. Higher ecosystem productivity and transpiration were linked to a greater drawdown of soil moisture. Ecosystem‐scale measurements across a diverse range of ecosystems are needed to better understand the canopy response to wildfire smoke, which is an increasingly common phenomenon across large parts of the world, including the forests of western North America.

The ability of forests to withstand, and recover from, acute drought stress is a critical uncertainty regarding the impacts of climate change on the terrestrial carbon (C) cycle, but it is unclear how drought responses scale from individual trees to whole forests. Here, we assembled a dataset of tree-ring chronologies co-located within the footprint of eddy covariance towers across North America and Europe, with the aim of quantifying the sensitivity of tree radial growth versus gross primary productivity (GPP) during and following drought. We found that drought induced a large decoupling across C cycle processes, whereby GPP was relatively resistant to water stress despite large reductions in tree-ring widths. This decoupling also occurred in the year following drought (i.e., a "drought legacy effect'), and was similar in magnitude in response to both summer and winter droughts. By modeling whole-forest C turnover time, we show that a radial growth-GPP decoupling has important ramifications for the forest C cycle, especially if the C not used to support radial growth is instead allocated towards pools with short residence times. Our results demonstrate that quantifications of drought impacts that rely solely on C uptake are missing this fundamental pathway through which drought alters the forest C cycle and the resulting feedbacks to the climate system.

AbstractUnderstanding long-term insect and pathogen effects on host tree physiology can help forest managers respond to insect and pathogen outbreaks, and understand when insect and pathogen effects on tree physiology will be exacerbated by climate change. Leaf-level physiological processes modify the carbon (C) and oxygen (O) stable isotopic composition of elements taken up from the environment, and these modifications are recorded in tree-rings (see Chaps. 10.1007/978-3-030-92698-4_9, 10.1007/978-3-030-92698-4_10, 10.1007/978-3-030-92698-4_16 and 10.1007/978-3-030-92698-4_17). Therefore, tree-ring stable isotopes are affected by both the tree’s environment and the tree’s physiological responses to the environment, including insects and pathogens. Tree-ring stable isotopes provide unique insights into the long-term effects of insects and pathogens on host tree physiology. However, insect and pathogen impacts on tree-ring stable isotopes are often overlooked, yet can substantially alter interpretations of tree-ring stable isotopes for reconstructions of climate and physiology. In this chapter, we discuss (1) the effects of insects(defoliators, wood-boring, leaf-feeding), pests (parasitic plants), and pathogens(root and foliar fungi) on hostphysiology (growth, hormonal regulation, gas exchange, water relations, and carbon and nutrient use) as they relate to signals possibly recorded by C and O stable isotopes in tree-rings, (2) how tree-ring stable isotopes reveal insect and pathogen impacts and the interacting effects of pathogens and climate on hostphysiology, and (3) the importance of considering insect and pathogen impacts for interpreting tree-ring stable isotopes to reconstruct past climate or physiology.

Wood density (WD) is often used as a proxy for hydraulic traits such as vulnerability to drought-induced xylem cavitation and maximum water transport capacity, with dense-wooded species generally being more resistant to drought-induced xylem cavitation, having lower rates of maximum water transport and lower sapwood capacitance than light-wooded species. However, relationships between WD and the hydraulic traits that they aim to predict have not been well established in tropical forests, where modeling is necessary to predict drought responses for a high diversity of unmeasured species. We evaluated WD and relationships with stem xylem vulnerability by measuring cavitation curves, sapwood water release curves and minimum seasonal water potential (Psi(min)) on upper canopy branches of six tree species and three liana species from a single wet tropical forest site in Panama. The objective was to better understand coordination and trade-offs among hydraulic traits and the potential utility of these relationships for modeling purposes. We found that parameters from sapwood water release curves such as capacitance, saturated water content and sapwood turgor loss point (Psi(tlp,x)) were related to WD, whereas stem vulnerability curve parameters were not. However, the water potential corresponding to 50% loss of hydraulic conductivity (P-50) was related to Psi(tlp,x) and sapwood osmotic potential at full turgor (pi(o,x)). Furthermore, species with lower Psi(min) showed lower P-50, Psi(tlp,x) and pi(o,x) suggesting greater drought resistance. Our results indicate that WD is a good easy-to-measure proxy for some traits related to drought resistance, but not others. The ability of hydraulic traits such as P-50 and Psi(tlp,x) to predict mortality must be carefully examined if WD values are to be used to predict drought responses in species without detailed physiological measurements.

This article comments on: Seeking the “point of no return” in the sequence of events leading to mortality of mature trees

Hydraulic capacitance and carbohydrate storage are two drought adaptation strategies of woody angiosperms. However, we currently lack information on their associations and how they are associated with species' degree of isohydry. We measured total stem xylem non-structural carbohydrate (NSC) concentration in the dry and wet seasons, xylem hydraulic capacitance, native leaf water potentials, pressure-volume curve parameters, and photosynthetic performance in 24 woody understory species differing in their degree of isohydry. We found a trade-off between xylem water and carbohydrate storage both in storage capacitance and along a spectrum of isohydry. Species with higher hydraulic capacitance had lower native NSC storage. The less isohydric species tended to show greater NSC depletion in the dry season and have more drought-tolerant leaves. In contrast, the more isohydric species had higher hydraulic capacitance, which may enhance their drought avoidance capacity. In these species, leaf flushing in the wet season and higher photosynthetic rates in the dry season resulted in accumulation rather than depletion of NSC in the dry season. Our results provide new insights into the mechanisms through which xylem storage functions determine co-occurring species' drought adaptation strategies and improve our capacity to predict community assembly processes under drought.

Summary Strategies for deep soil water acquisition ( WA deep ) are critical to a species’ adaptation to drought. However, it is unknown how WA deep determines the abundance and resource economy strategies of understorey shrub species. With data from 13 understorey shrub species in subtropical coniferous plantations, we investigated associations between the magnitude of WA deep , the seasonal plasticity of WA deep , midday leaf water potential (Ψ md ), species abundance and resource economic traits across organs. Higher capacity for WA deep was associated with higher intrinsic water use efficiency, but was not necessary for maintaining higher Ψ md in the dry season nor was it an ubiquitous trait possessed by the most common shrub species. Species with higher seasonal plasticity of WA deep had lower wood density, indicating that fast species had higher plasticity in deep soil resource acquisition. However, the magnitude and plasticity of WA deep were not related to shallow fine root economy traits, suggesting independent dimensions of soil resource acquisition between deep and shallow soil. Our results provide new insights into the mechanisms through which the magnitude and plasticity of WA deep interact with shallow soil and aboveground resource acquisition traits to integrate the whole‐plant economic spectrum and, thus, community assembly processes.

Summary The ratio of leaf internal (ci) to ambient (ca) partial pressure of CO2, defined here as χ, is an index of adjustments in both leaf stomatal conductance and photosynthetic rate to environmental conditions. Measurements and proxies of this ratio can be used to constrain vegetation model uncertainties for predicting terrestrial carbon uptake and water use. We test a theory based on the least‐cost optimality hypothesis for modelling historical changes in χ over the 1951–2014 period, across different tree species and environmental conditions, as reconstructed from stable carbon isotopic measurements across a global network of 103 absolutely dated tree‐ring chronologies. The theory predicts optimal χ as a function of air temperature, vapour pressure deficit, ca and atmospheric pressure. The theoretical model predicts 39% of the variance in χ values across sites and years, but underestimates the intersite variability in the reconstructed χ trends, resulting in only 8% of the variance in χ trends across years explained by the model. Overall, our results support theoretical predictions that variations in χ are tightly regulated by the four environmental drivers. They also suggest that explicitly accounting for the effects of plant‐available soil water and other site‐specific characteristics might improve the predictions.

Wildfires were a frequent source of disturbance in forests of the Western United States prior to Euro-American settlement. Following a series of catastrophic wildfires in the Northern Rockies in 1910, the U.S. Forest Service adopted a broad wildfire suppression policy that has resulted in forests thick with small trees. These crowded trees compete for nutrients and water and experience increased drought stress in summer.

The degree of plant iso/anisohydry is a popular framework for characterising species-specific drought responses. However, we know little about associations between below-ground and above-ground hydraulic traits as well as the broader ecological implications of this framework. For 24 understory shrub species in seasonally dry subtropical coniferous plantations, we investigated contributions of the degree of isohydry to species' resource economy strategies, abundance, and importance value, and quantified the hydraulic conductance (K-h) of above-ground and below-ground organs, magnitude of deep water acquisition (WA(deep)), shallow absorptive root traits (diameter, specific root length, tissue density), and resource-use efficiencies (A(max), maximum photosynthesis rate; PNUE, photosynthetic nitrogen-use efficiency). The extreme isohydric understory species had lower wood density (a proxy for higher growth rates) because their higher WA(deep) and whole-plant K-h allowed higher A(max) and PNUE, and thus did not necessarily show lower abundance and importance values. Although species' K-h was coordinated with their water foraging capacity in shallow soil, the more acquisitive deep roots were more crucial than shallow roots in shaping species' extreme isohydric behaviour. Our results provide new insights into the mechanisms through which below-ground hydraulic traits, especially those of deep roots, determine species' degree of isohydry and economic strategies.

Climate change is a world‐wide threat to biodiversity and ecosystem structure, functioning and services. To understand the underlying drivers and mechanisms, and to predict the consequences for nature and people, we urgently need better understanding of the direction and magnitude of climate change impacts across the soil–plant–atmosphere continuum. An increasing number of climate change studies are creating new opportunities for meaningful and high‐quality generalizations and improved process understanding. However, significant challenges exist related to data availability and/or compatibility across studies, compromising opportunities for data re‐use, synthesis and upscaling. Many of these challenges relate to a lack of an established ‘best practice’ for measuring key impacts and responses. This restrains our current understanding of complex processes and mechanisms in terrestrial ecosystems related to climate change. To overcome these challenges, we collected best‐practice methods emerging from major ecological research networks and experiments, as synthesized by 115 experts from across a wide range of scientific disciplines. Our handbook contains guidance on the selection of response variables for different purposes, protocols for standardized measurements of 66 such response variables and advice on data management. Specifically, we recommend a minimum subset of variables that should be collected in all climate change studies to allow data re‐use and synthesis, and give guidance on additional variables critical for different types of synthesis and upscaling. The protocols are also available online on the ClimEx handbook webpage (https://climexhandbook.w.uib.no/) and we encourage scientists from the climate change research community to get involved, give us feedback and make suggestions for updates to specific protocols. We hope that this is a way to amend the protocols and extend the shelf life of the ClimEx Handbook. The goal of this community effort is to facilitate awareness of the importance and broader application of standardized methods to promote data re‐use, availability, compatibility and transparency. We envision improved research practices that will increase returns on investments in individual research projects, facilitate second‐order research outputs and create opportunities for collaboration across scientific communities. Ultimately, this should significantly improve the quality and impact of the science, which is required to fulfil society's needs in a changing world.

Research on the hydraulics of woody plants is well represented in Plant, Cell & Environment. In the new Virtual Special Issue [https://onlinelibrary.wiley.com/doi/toc/10.1111/(ISSN)1365-3040.hydraulics-of-woody-plants], we have compiled articles in this research field published by PC&E since 2017. They address important questions in plant hydraulics such as: What are the structural and physiological determinants of water transport capacity? Which role does the environment play? How are hydraulics interlinked with stomatal behaviour, transpiration, and carbon assimilation? The efficiency of water transport from roots to leaves depends to a large extent on the properties of the xylem conduits including vessels and tracheids. The role of xylem anatomy in determining properties of the hydraulic pathway is addressed in several studies in this Virtual Special Issue. For example, Ohtsuka, Sack, and Taneda (2018) found that the degree of leaf vein bundle sheath lignification was a strong predictor of leaf hydraulic conductance (Figure 1). Scientists in this area of research often present xylem vulnerability curves, which plot negative pressure in the xylem versus loss of hydraulic conductivity due to drought-induced embolism. The shape of these curves is affected by xylem anatomical characteristics at the pit membrane and vessel scale, along with xylem network topology (Mrad, Domec, Huang, Lens, & Katul, 2018). Pagliarani et al. (2019) studied the role of coordinated molecular and biochemical signals at the interface between xylem parenchyma and vessels in recovery from drought-induced embolism. Plant water transport capacity is threatened not only by droughts but also by a combination of water and heat stress. Grossiord et al. (2017) subjected pine and juniper trees to precipitation reduction, warming, and a combination of both factors for a duration of 5 years and found that there was no active acclimation of hydraulic traits to drying. However, stomatal conductance acclimated to warming, which minimized additive effects of warming and drying, but did not prevent them entirely. In order to predict tree performance in a future drier climate, researchers use mathematical models that include many liquid and vapour phase water transport parameters such as leaf water potential and stomatal conductance. One study using the Mediterranean oak Quercus ilex as an example showed that such models can be made more accurate if they also take the parameter of stem hydraulic capacitance into account (Salomón, Limousin, Ourcival, Rodríguez-Calcerrada, & Steppe, 2017). Hydraulic traits such as stem hydraulic conductivity are also affected by the plant's environment. Plantation-grown pine trees showed greater branch dieback and hydraulic constraints on growth than trees growing in natural forest (Liu et al., 2018). Surprisingly though, hydraulic traits of 12 woody species were more strongly related to rainfall and aridity patterns in their native distribution range than their actual growth environment (Li et al., 2018). Another central question in woody plant hydraulics is the role of the roots in determining whole-plant water transport capacity. Hydraulic failure below ground can determine a species' vulnerability to drought (Johnson et al., 2018), and one of the studies in this issue reports a role for root aquaporins in regulating whole-plant water transport, stomatal conductance, and leaf water status (Rodríguez-Gamir et al., 2019). Hydraulics are also interlinked with plant carbon economy. How gas exchange parameters including the photosynthetic rate were correlated with leaf hydraulic conductance in ferns, gymnosperms, and angiosperms was investigated by Xiong, Douthe, and Flexas (2018). The authors found that these parameters were well correlated under light-saturated conditions but how they responded to varying light intensities was highly species dependent. In the study of Bucci et al. (2019), the diurnal dynamics of leaf gas exchange were associated with leaf hydraulic vulnerability. Species with more vulnerable leaves restricted water loss and carbon assimilation earlier than less vulnerable species. Carbon assimilation is necessary for tree growth, and it has long been known that fast-growing trees have higher mortality rates compared to slow-growing trees. The physiology underlying this phenomenon was investigated in nine Brazilian tropical cloud forest tree species (Eller et al., 2018). The authors measured tree growth using point dendrometers (Figure 2) and showed that higher mortality in fast-growing trees was associated with smaller xylem hydraulic safety margins. The hydraulic safety margin is the difference between the observed minimum water potential in the field and the critical water potential leading to hydraulic failure. Different strategies for dealing with water shortage are often conceptualized as isohydric versus anisohydric behaviours. Generally, isohydric species are thought to decrease stomatal conductance and gas exchange in response to drought, this maintaining a relatively constant minimum leaf water potential. By contrast, anisohydric species allow the water potential to vary while maintaining relatively stable gas exchange rates even during water shortage. Manrique-Alba et al. (2018) compared stem radial growth between an isohydric and an anisohydric conifer species and found that the different strategies translate into alternative growth and water storage strategies under drought and heat. In another field study, Pivovaroff, Cook, and Santiago (2018) showed that the degree of isohydry/anisohydry was correlated with the size of hydraulic safety margins across species in a Mediteranean-type ecosystem in California. More isohydric species had larger hydraulic safety margins, implying that maintenance of a more conservative hydraulic strategy occurs at the expense of carbon gain. Because the utility of the isohydricity/anisohydricity concept has been questioned in the past for its focus on drought, masking the impact of other environmental factors on the relationship between stomatal conductance and leaf water potential, Novick, Konings, and Gentine (2019) have sought to broaden the concept. In their article, they investigate the influence of variations in vapour pressure deficit and leaf area and make recommendations how to incorporate these factors into the isohydry/anisohydry concept. The papers in this Virtual Special Issue address coordination of hydraulic traits with other physiological traits and behaviours within and across plant organs, including their integration into whole-plant hydraulic strategies such as where species operate along a continuum of isohydry to anisohydry. PC&E will continue to add newly accepted papers to the online collection to provide readers with an up-to-date resource. Improved understanding of the functional trade-offs implied in species' hydraulic strategies is necessary to anticipate how plants will perform under extreme climatic events such as droughts and heat waves.