Stomata play a significant role in the Earth's water and carbon cycles, by regulating gaseous exchanges between the plant and the atmosphere. Under drought conditions, stomatal control of ...transpiration has long been thought to be closely coordinated with the decrease in hydraulic capacity (hydraulic failure due to xylem embolism). We tested this hypothesis by coupling a meta‐analysis of functional traits related to the stomatal response to drought and embolism resistance with simulations from a soil–plant hydraulic model. We report here a previously unreported phenomenon: the existence of an absolute limit by which stomata closure must occur to avoid rapid death in drought conditions. The water potential causing stomatal closure and the xylem pressure at the onset of embolism formation were equal for only a small number of species, and the difference between these two traits (i.e. safety margins) increased continuously with increasing embolism resistance. Our findings demonstrate the need to revise current views about the functional coordination between stomata and hydraulic traits and provide a mechanistic framework for modeling plant mortality under drought conditions.
Xylem cavitation resistance has profound implications for plant physiology and ecology. This process is characterized by a ‘vulnerability curve’ (VC) showing the variation of the percentage of ...cavitation as a function of xylem pressure potential. The shape of this VC varies from ‘sigmoidal’ to ‘exponential’. This review provides a panorama of the techniques that have been used to generate such a curve. The techniques differ by (i) the way cavitation is induced (e.g. bench dehydration, centrifugation, or air injection), and (ii) the way cavitation is measured (e.g. percentage loss of conductivity (PLC) or acoustic emission), and a nomenclature is proposed based on these two methods. A survey of the literature of more than 1200 VCs was used to draw statistics on the usage of these methods and on their reliability and validity. Four methods accounted for more than 96% of all curves produced so far: bench dehydration–PLC, centrifugation–PLC, pressure sleeve-PLC, and Cavitron. How the shape of VCs varies across techniques and species xylem anatomy was also analysed. Strikingly, it was found that the vast majority of curves obtained with the reference bench dehydration-PLC method are ‘sigmoidal’. ‘Exponential’ curves were more typical of the three other methods and were remarkably frequent for species having large xylem conduits (ring-porous), leading to a substantial overestimation of the vulnerability of cavitation for this functional group. We suspect that ‘exponential’ curves may reflect an open-vessel artefact and call for more precautions with the usage of the pressure sleeve and centrifugation techniques.
The knowledge about potential climate change impacts on forests is continuously expanding and some changes in growth, drought induced mortality and species distribution have been observed. However ...despite a significant body of research, a knowledge and communication gap exists between scientists and non-scientists as to how climate change impact scenarios can be interpreted and what they imply for European forests. It is still challenging to advise forest decision makers on how best to plan for climate change as many uncertainties and unknowns remain and it is difficult to communicate these to practitioners and other decision makers while retaining emphasis on the importance of planning for adaptation.
In this paper, recent developments in climate change observations and projections, observed and projected impacts on European forests and the associated uncertainties are reviewed and synthesised with a view to understanding the implications for forest management. Current impact assessments with simulation models contain several simplifications, which explain the discrepancy between results of many simulation studies and the rapidly increasing body of evidence about already observed changes in forest productivity and species distribution. In simulation models uncertainties tend to cascade onto one another; from estimating what future societies will be like and general circulation models (GCMs) at the global level, down to forest models and forest management at the local level.
Individual climate change impact studies should not be uncritically used for decision-making without reflection on possible shortcomings in system understanding, model accuracy and other assumptions made. It is important for decision makers in forest management to realise that they have to take long-lasting management decisions while uncertainty about climate change impacts are still large. We discuss how to communicate about uncertainty – which is imperative for decision making – without diluting the overall message. Considering the range of possible trends and uncertainties in adaptive forest management requires expert knowledge and enhanced efforts for providing science-based decision support.
•Uncertainty is inherent to climate change impact assessments.•Extreme events are only weakly represented in many assessments.•The range of possible impacts has so far been underestimated in most studies.•Some general trends are common to all climate projections.•Guidance is needed to interpret state-of-the-art knowledge and give helpful advice.
Drought-induced cavitation resistance varies considerably between tree species and forest ecosystems (Maherali et al., 2004; Delzon et al., 2010) and is closely linked to survival under severe ...drought in both conifers (Brodribb & Cochard, 2009; Brodribb et al., 2010) and angiosperms (Kursar et al., 2009; Anderegg et al., 2012; Barigah et al., 2013; Urli et al., 2013). Choat et al. (2012) recently reported that most trees operate very close to their threshold of cavitation, leaving them potentially vulnerable to drought-induced mortality in a warmer/drier world (Engelbrecht, 2012). Indeed, species growing in dry environments are more resistant to droughtinduced cavitation (more negative water potential at 50% cavitation, P50) but experience a more negative minimumwater potential (Pmin) than those growing in wet environments. The so-called hydraulic safety margin, the difference between the level of water stress experienced by a species in the field (Pmin) and the level of water stress leading to hydraulic failure, is, therefore, remarkably narrow, whatever the forest species and biome considered (Choat et al., 2012). This pattern provides clues to the global droughtinduced mortality currently observed, even in very wet environments, such as tropical forests (Allen et al., 2010). Klein et al. (2014) play down the functional significance of the hydraulic safety margin in the vulnerability of forests to drought, pointing out the important role played by additional mechanisms, such as the ability of trees to repair embolism. While it is obvious that drought-induced forest dieback is a complex process involving a number of biotic and abiotic factors, we would like to draw the attention of scientists to the state of evidence for embolism repair, thereby guiding research on tree drought resistance into the most relevant and fruitful directions
Hydraulic failure induced by xylem embolism is one of the primary mechanisms of plant dieback during drought. However, many of the methods used to evaluate the vulnerability of different species to ...drought-induced embolism are indirect and invasive, increasing the possibility that measurement artifacts may occur. Here, we utilize x-ray computed microtomography (microCT) to directly visualize embolism formation in the xylem of living, intact plants with contrasting wood anatomy (Quercus robur, Populus tremula × Populus alba, and Pinus pinaster). These observations were compared with widely used centrifuge techniques that require destructive sampling. MicroCT imaging provided detailed spatial information regarding the dimensions and functional status of xylem conduits during dehydration. Vulnerability curves based on microCT observations of intact plants closely matched curves based on the centrifuge technique for species with short vessels (P. tremula × P. alba) or tracheids (P. pinaster). For ring porous Q. robur, the centrifuge technique significantly overestimated vulnerability to embolism, indicating that caution should be used when applying this technique to species with long vessels. These findings confirm that microCT can be used to assess the vulnerability to embolism on intact plants by direct visualization.
Climate warming will increase the drought exposure of many forests world‐wide. It is not well understood how trees adapt their hydraulic architecture to a long‐term decrease in water availability. We ...examined 23 traits characterizing the hydraulic architecture and growth rate of branches and the dependent foliage of mature European beech (Fagus sylvatica) trees along a precipitation gradient (855–594 mm yr⁻¹) on uniform soil. A main goal was to identify traits that are associated with xylem efficiency, safety and growth. Our data demonstrate for the first time a linear increase in embolism resistance with climatic aridity (by 10%) across populations within a species. Simultaneously, vessel diameter declined by 7% and pit membrane thickness (Tₘ) increased by 15%. Although specific conductivity did not change, leaf‐specific conductivity declined by 40% with decreasing precipitation. Of eight plant traits commonly associated with embolism resistance, only vessel density in combination with pathway redundancy and Tₘ were related. We did not confirm the widely assumed trade‐off between xylem safety and efficiency but obtained evidence in support of a positive relationship between hydraulic efficiency and growth. We conclude that the branch hydraulic system of beech has a distinct adaptive potential to respond to a precipitation reduction as a result of the environmental control of embolism resistance.
This study compiles and summarizes the existing knowledge about observed and projected impacts of climate change on forests in Europe. Forests will have to adapt not only to changes in mean climate ...variables but also to increased variability with greater risk of extreme weather events, such as prolonged drought, storms and floods. Sensitivity, potential impacts, adaptive capacity, and vulnerability to climate change are reviewed for European forests. The most important potential impacts of climate change on forest goods and services are summarized for the Boreal, Temperate Oceanic, Temperate Continental, Mediterranean, and mountainous regions. Especially in northern and western Europe the increasing atmospheric CO
2 content and warmer temperatures are expected to result in positive effects on forest growth and wood production, at least in the short–medium term. On the other hand, increasing drought and disturbance risks will cause adverse effects. These negative impacts are very likely to outweigh positive trends in southern and eastern Europe. From west to east, the drought risk increases. In the Mediterranean regions productivity is expected to decline due to strongly increased droughts and fire risks.
Adaptive capacity consists of the inherent adaptive capacity of trees and forest ecosystems and of socio-economic factors determining the capability to implement planned adaptation. The adaptive capacity in the forest sector is relatively large in the Boreal and the Temperate Oceanic regions, more constrained by socio-economic factors in the Temperate Continental, and most limited in the Mediterranean region where large forest areas are only extensively managed or unmanaged.
Potential impacts and risks are best studied and understood with respect to wood production. It is clear that all other goods and services provided by European forests will also be impacted by climate change, but much less knowledge is available to quantify these impacts. Understanding of adaptive capacity and regional vulnerability to climate change in European forests is not well developed and requires more focussed research efforts. An interdisciplinary research agenda integrated with monitoring networks and projection models is needed to provide information at all levels of decision making, from policy development to the management unit.
Resistance to water‐stress induced cavitation is an important indicator of drought tolerance in woody species and is known to be intimately linked to the anatomy of the xylem. However, the actual ...mechanical properties of the pit membrane are not well known and the exact mode of air‐seeding by which cavitation occurs is still uncertain. We examined the relationship between cavitation resistance and bordered pit structure and function in 40 coniferous species. Xylem pressure inducing 50% loss of hydraulic conductance (P₅₀, a proxy for cavitation resistance) varied widely among species, from −2.9 to −11.3 MPa. The valve effect of the pit membrane, measured as a function of margo flexibility and torus overlap, explained more variation in cavitation‐resistance than simple anatomical traits such as pit membrane, pit aperture or torus size. Highly cavitation resistant species exhibited both a high flexibility of the margo and a large overlap between the torus and the pit aperture, allowing the torus to tightly seal the pit aperture. Our results support the hypothesis of seal capillary‐seeding as the most likely mode of air‐seeding, and suggest that the adhesion of the torus to the pit border may be the main determinant of cavitation resistance in conifers.
► Provided a temperature range of about 7
°C, the altitudinal gradient used here is particularly relevant to calibrate phenological models. ► Most of the phenological models used were able to explain ...and predict accurately the leaf unfolding date for all the tree species considered, whereas they failed to predict senescence date for two out of four deciduous species. ► Overall, dates of leaf unfolding are expected to be advanced in the coming decades and dates of senescence to be delayed. ► The results suggest that chilling temperature could be insufficient for some species at low elevation with winter temperatures rising in the next decades. ► The simulations showed species differences in lengthening of canopy duration and consequently suggested changes in the competitive balance between species over the current century.
Modelling phenology is crucial to assess the impact of climate change on the length of the canopy duration and the productivity of terrestrial ecosystems. Focusing on six dominant European tree species, the aims of this study were (i) to examine the accuracy of different leaf phenology models to simulate the onset and ending of the leafy season, with particular emphasis on the putative role of chilling to release winter bud dormancy and (ii) to predict seasonal shifts for the 21st century in response to climate warming.
Models testing and validation were done for each species considering 2 or 3 years of phenological observations acquired over a large elevational gradient (1500
m range, 57 populations). Flushing models were either based solely on forcing temperatures (1-phase models) or both on chilling and forcing temperatures (2-phases models). Leaf senescence models were based on both temperature and photoperiod.
We show that most flushing models are able to predict accurately the observed flushing dates. The 1-phase models are as efficient as 2-phases models for most species suggesting that chilling temperatures are currently sufficient to fully release bud dormancy. However, our predictions for the 21st century highlight that chilling temperature could be insufficient for some species at low elevation. Overall, flushing is expected to advance in the next decades but this trend substantially differed between species (from 0 to 2.4 days per decade). The prediction of leaf senescence appears more challenging, as the proposed models work properly for only two out of four deciduous species, for which senescence is expected to be delayed in the future (from 1.4 to 2.3 days per decade). These trends to earlier spring leafing and later autumn senescence are likely to affect the competitive balance between species. For instance, simulations over the 21st century predict a stronger lengthening of the canopy duration for
Quercus petraea than for
Fagus sylvatica, suggesting that shifts in the elevational distributions of these species might occur.
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