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Conversely, tree species that survive under arid conditions might provide vital information on successful drought resistance strategies. Although Acacia Vachellia species dominate many of the globe's deserts, little is known about their growth dynamics and water-use in situ. Stem diameter dynamics, leaf phenology, and sap flow were monitored during 3 consecutive years in five Acacia raddiana trees and five Acacia tortilis trees in the Arid Arava Valley, southern Israel annual precipitation mm, restricted to October-May.

We hypothesized that stem growth and other tree activities are synchronized with, and limited to single rainfall or flashflood events. Unexpectedly, cambial growth of both Acacia species was arrested during the wet season, and occurred during most of the dry season, coinciding with maximum daily temperatures as high as 45 degrees C and vapor pressure deficit of up to 9kPa.

Summer growth was correlated with peak sap flow in June, with almost year-round activity and foliage cover. To the best of our knowledge, these are the harshest drought conditions ever documented permitting cambial growth. These findings point to the possibility that summer cambial growth in Acacia under hyper-arid conditions relies on concurrent leaf gas exchange, which is in turn permitted by access to deep soil water. Soil water can support low-density tree populations despite heat and drought, as long as recharge is kept above a minimum threshold. Understanding which species are able to recover from drought, under what conditions, and the mechanistic processes involved, will facilitate predictions of plant mortality in response to global change.

In response to drought, some species die because of embolism-induced hydraulic failure, whilst others are able to avoid mortality and recover, following rehydration. Several tree species have evolved strategies to avoid embolism, whereas others tolerate high embolism rates but can recover their hydraulic functioning upon drought relief.

Here, we focus on structures and processes that might allow some plants to recover from drought stress via embolism reversal. We provide insights into how embolism repair may have evolved, anatomical and physiological features that facilitate this process, and describe possible trade-offs and related costs.

The mechanism is energetically demanding, and the costs to plants include metabolism and transport of soluble sugars, water and inorganic ions. We propose that embolism repair should be considered as a possible component of a hydraulic efficiency-safety' spectrum.


We also advance a framework for vegetation models, describing how vulnerability curves may change in hydrodynamic model formulations for plants that recover from embolism. The combined effects of local adaptation and phenotypic plasticity influencing plant performance are relevant to understand the capacity for genetic responses to climate change. Pima halepensis is a native species of low- to mid-elevation Mediterranean forests with a high ecological value in drought-prone areas.

Thus, it is of utmost importance to determine its adaptive structure for key traits such as growth or survival. Here, we analyse a highly unbalanced dataset collated from different common-garden networks that cover the distribution range of the species. A total of 82 range-wide populations were evaluated in nine Mediterranean trials located in Israel, Italy and Spain.

A climate classification of populations allowed for the definition of six different groups, or ecotypes, which showed contrasting performances for tree height and survival at age The effects of ecotypic differentiation and among-ecotypes genetic variation in plasticity were disentangled by fitting stability models accounting for interaction and heteroscedasticity in genotype-by-environment tables. For growth, a Finlay-Wilkinson model suggested high predictability of ecotypic plastic responses in P. However, differences in mean height of ca. For survival, ecotypic differences were approximately constant across trials, suggesting lack of genotype-by-environment effects.

Sub-humid cool climate populations from the eastern Mediterranean e. Altogether, our results indicate a general adaptive syndrome by which less reactive ecotypes to ameliorated conditions e. The reported ecotypic differentiation constitutes the basis for tailoring infra-specific responses to climate and disentangling the relationship between adaptive variation and resilience towards climatic warming for this exemplary Mediterranean pine.

Tree growth enhancement under elevated [CO2] is much smaller than originally anticipated; yet carbon overabundance can lead to increased wood carbon storage and to stomata] downregulation and hence reduced water use. Notably, all three outcomes increase tree drought resistance. Here we studied growth, water relations, and nonstructural carbohydrates of 60 lemon saplings growing in CO2-controlled rooms at the same greenhouse, under , , and ppm CO2].

A new ecophysiological approach to forest-water relationships in arid climates. By I. Gindel

At each [CO2] level, 10 saplings were exposed to 1-month dry-down after 2 months of standard irrigation, followed by re-watering for another month. The other 10 saplings served as controls. Under drought, tree growth was maintained at elevated, but not ambient, CO2, linked with mild vs. CO2-induced increases in root and shoot starch were transient but significant.


Our results suggest that when predicting tree growth in a warmer and drier future, concomitant atmospheric CO2 concentration must be considered. In young lemon trees, elevated CO2 partially compensated for drought effects on tree growth and water status, and might delay some of the effects of the anthropogenic climate change. Broad-scale forest die-off associated with drought and heat has now been reported from every forested continent, posing a global-scale challenge to forest management.

Climate-driven die-off is frequently compounded with other drivers of tree mortality, such as altered land use, wildfire, and invasive species, making forest management increasingly complex. We assembled a synthesis of 14 forest die-off case studies from around the globe, each with sufficient information to infer impacts on forest dynamics and to inform management options following a forest die-off event. We found that our diverse set of mortality case studies fit into three broad classes of ecosystem trajectories: 1 single-state transition shifts, 2 ecological cascading responses and feedbacks, and 3 complex dynamics where multiple interactions, mortality drivers, and impacts create a range of possible state transition responses.

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Our results highlight that although forest die-off events across the globe encompass many different underlying drivers and pathways of ecosystem change, there are commonalities in opportunities for successful management intervention. Green roofs provide many ecosystem services such as regulation of building temperatures, reducing urban heat-island effects and draining rainwater. In addition, they are expected to reduce the high levels of CO2 concentrations in big cities. Previous CO2 fixation studies on green roofs were done by taking long-timeperiod samples using expensive equipment and with limited replication.

To plan green roofs for optimal CO2 reduction, a simple method to quantify CO2 fixation rate in relation to plant species-composition is required. The method we tested is direct measurement of CO2 concentrations with a portable air-quality meter, which allows a large number of samples.

Here we focus on differences in the photosynthetic effect between plots containing the local Mediterranean succulent, Sedum sediforme and plots containing various annuals. In a factorial design presence or absence of Sedum crossed with presence or absence of annuals , we tested the effect of sedum and annual treatments on CO2 concentrations. To compare our results with a commonly used method, and to evaluate the role of the different species, we examined the photosynthetic activity at the single plant level under these treatments by using a portable gas-exchange measuring system.

We found that our method can detect the effect of different green roof plots and can be used as a simple and reliable tool for green-roof planers. We found that annuals reduced CO2 concentrations, but only in the absence of Sedum. Sedum alone had no effect on CO2 concentrations. This emphasizes the importance of integrating plots with annual plants in Sedum-based green roofs. C Elsevier GmbH.

All rights reserved. Green roofs are expected to absorb and store carbon in plants and soils and thereby reduce the high CO2 concentration levels in big cities. Sedum species, which are succulent perennials, are commonly used in extensive green roofs due to their shallow root system and ability to withstand long water deficiencies. Here we examined CO2 fixation and emission rates for Mediterranean Sedum sediforme on green-roof experimental plots.

During late winter to early spring, we monitored CO2 concentrations inside transparent tents placed over 1 m 2 plots and followed gas exchange at the leaf level using a portable gas-exchange system. We found high rates of CO2 emission at daytime, which is when CO2 concentration in the city is the highest Both plot- and leaf-scale measurements showed that these CO2 emissions were not fully compensated by the nighttime uptake.

We conclude that although carbon sequestration may only be a secondary benefit of green roofs, for improving this ecosystem service, other plant species than Sedum should also be considered for use in green roofs, especially in Mediterranean and other semi-arid climates. C Elsevier B.

Dr. Nadine Rühr

Climate change is expected to exacerbate drought for many plants, making drought tolerance a key driver of species and ecosystem responses. Plant drought tolerance is determined by multiple traits, but the relationships among traits, either within individual plants or across species, have not been evaluated for general patterns across plant diversity. We synthesized the published data for stomatal closure, wilting, declines in hydraulic conductivity in the leaves, stems, and roots, and plant mortality for woody angiosperm and 48 gymnosperm species.

We evaluated the correlations among the drought tolerance traits across species, and the general sequence of water potential thresholds for these traits within individual plants. The trait correlations across species provide a framework for predicting plant responses to a wide range of water stress from one or two sampled traits, increasing the ability to rapidly characterize drought tolerance across diverse species.

Analyzing these correlations also identified correlations among the leaf and stem hydraulic traits and the wilting point, or turgor loss point, beyond those expected from shared ancestry or independent associations with water stress alone. Further, on average, the angiosperm species generally exhibited a sequence of drought tolerance traits that is expected to limit severe tissue damage during drought, such as wilting and substantial stem embolism.

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  • This synthesis of the relationships among the drought tolerance traits provides crucial, empirically supported insight into representing variation in multiple traits in models of plant and ecosystem responses to drought. Are mature forests carbon limited? To explore this question, we exposed ca. The site receives substantial soluble nitrogen N via atmospheric deposition, and thus, trees are unlikely N-limited. We used a construction crane to operate the free-air CO2 release system and for canopy access. Here, we summarize the major results for growth and carbon C fluxes.

    Tissue C signals confirmed the effectiveness of the CO2 enrichment system and permitted tracing the continuous flow of new C in trees. Tree responses were individually standardized by pre-treatment signals. However, we did not detect an effect on stem radial growth, branch apical growth and needle litter production. Neither stem nor soil CO2 efflux was stimulated under elevated CO2. The rate at which fine roots filled soil ingrowth cores did not significantly differ between A- and E-trees.

    Since trees showed no stomatal responses to elevated CO2, sap flow remained unresponsive, both in the long run as well as during short-term CO2 on-off experiments. As a consequence, soil moisture remained unaffected. We trapped significantly more nitrate in the root sphere of E-trees suggesting a CO2-stimulated breakdown of soil organic matter, presumably induced by extra carbohydrate exudation priming'.

    A New Ecophysiological Approach to Forest-Water Relationships in Arid Climates.

    The lack of a single enhanced C sink to match the increased C uptake meant a missing C sink. Increased C transport to below-ground sinks was indicated by C transfer to ectomycorrhiza and on to neighbouring trees and by increased C export to soil. We conclude that these tall Picea abies trees are not C limited at current CO2 concentrations and further atmospheric CO2 enrichment will have at most subtle effects on growth, despite. The temporal dynamics of water transport and storage in plants have major implications for plant functioning and survival.