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Premise of the Study: Both freezing and drought cause cellular dehydration, and they elicit similar increases in protective compounds, which suggests that these stresses could potentially interact. We examined whether the physiological changes that occur in response to freezing in the fall and spring could affect subsequent survival and growth after summer drought., Methods: We froze Poa pratensis tillers in the late fall, early spring, or late spring at 0, -5, or -10°C for 3 d and then subjected them to no drought (-0.025 MPa), moderate drought (-0.140 MPa), or severe drought (-0.250 MPa) for 3 wk in the summer. We quantified survival and total biomass after a 3-wk recovery period, and we determined leaf soluble sugar concentrations before and then 0, 30, and 55 d after freezing., Key Results: For survival and biomass, there were significant interactions between freezing and drought. Spring frozen tillers had the highest biomass and survival following severe drought, whereas fall freezing did not significantly increase the biomass of tillers following the severe drought. Increased drought tolerance after spring freezing did not appear to be associated with increased soluble sugar content, given that the freezing effects on leaf glucose, fructose, and sucrose content were absent 55 d post freezing., Conclusions: Our results demonstrate that multiple stresses that occur over different seasons can interact; this interaction is highly relevant to herbaceous species in northern temperate regions that are experiencing more intense and frequent stress as a result of changes in snow cover and extreme climatic events., (© 2016 Botanical Society of America.)

The evolutionary dynamics of tree species are influenced by their specific climatic environments, and their ability to persist is determined by adaptive strategies such as broad climate tolerance, phenotypic plasticity or genetic differentiation. Biogeographical predictions indicate that populations located at the edges of their ranges are more likely to experience heightened vulnerability to climatic fluctuations due to approaching tolerance limits. However, if local phenotypic acclimation or genetic adaptation has taken place, trees near the edge of their range could demonstrate comparable sensitivity to freeze and drought as the rest of the sampled population. Nevertheless, there remains uncertainty regarding the extent of developmental and evolutionary adjustments in climate sensitivity across the entire ranges of many tree species.Here, we document the biogeography of tree growth sensitivity to freeze and soil water and vapour pressure deficits during 1950–2018 using an extensive multi‐species tree‐ring dataset of 35,784 trees at 4535 sites covering boreal, temperate conifer and temperate deciduous forests of Canada. We quantify the relationships between tree radial growth increment and seasonal climate variables and explore factors driving the observed patterns of annual growth and climate sensitivity such as species, regional climate and local site conditions, and tree age and size.Freeze and drought had widespread impacts on tree growth that were contingent on the presence of focal species in interaction with tree size. An indirect growth thermal limitation towards cold/wet edges, occurring through the site moisture conditions, was observed in seven widespread species (e.g. Picea glauca, Picea mariana and Larix laricina). Moreover, six species had negative drought impacts more strongly expressed towards their warm/dry edges (e.g. Abies balsamea, Betula papyrifera and Pseudotsuga menziesii). However, widespread Picea, Pinus and Populus species showed no indication of increased sensitivity to soil water conditions at these edges.Synthesis. Our findings support the idea of evolutionary or acclimatization adjustments in the development of populations in response to long‐term climate conditions experienced in their respective locations. This underscores the importance of incorporating phenotypic and genomic data into future analyses of climate change impacts, which would enhance our ability to predict potential ecological shifts. [ABSTRACT FROM AUTHOR]

With global warming, forests are facing an increased exposure to compound soil and atmospheric drought (CSAD) events, characterized by low soil water content (SWC) and high vapor pressure deficit (VPD). Such CSAD events trigger responses in both ecosystem and forest floor CO2 fluxes, of which we know little about. In this study, we used multi-year daily and daytime above canopy (18 years; 2005–2022) and daily forest floor (five years; 2018–2022) eddy-covariance CO2 fluxes of a Swiss forest site (montane mixed deciduous forest; CH-Lae). The objectives were (1) to characterize CSAD events at CH-Lae; (2) to quantify the impact of CSAD events on ecosystem and forest floor daily CO2 fluxes; and (3) to identify the major drivers and their temporal contributions to changing ecosystem and forest floor CO2 fluxes during CSAD events and CSAD growing seasons. Our results showed that the growing seasons of 2015, 2018, and 2022, were the top three driest (referred as CSAD years) at CH-Lae since 2005, with similar intensity and duration of the respective CSAD events, but considerably different pre-drought conditions. The CSAD events reduced daily mean net ecosystem productivity (NEP) in all three CSAD years, with highest reduction during 2022 (30 % decrease). This reduction in daily mean NEP was largely due to decreased gross primary productivity (GPP; >15 % decrease) rather than increased ecosystem respiration (Reco) during CSAD events. Furthermore, forest floor respiration (Rff) decreased during the CSAD events in 2018 and 2022 (no measurements in 2015), with a larger reduction in 2022 (>40 %) than in 2018 (<25 %) compared to the long-term mean (2019–2021). Using data-driven machine learning methods, we identified the major drivers of NEP and Rff during CSAD events. While daytime mean NEP during 2015 and 2018 CSAD events was limited by VPD or SWC, respectively, daytime mean NEP during the 2022 CSAD event was strongly limited by both SWC and VPD. Air temperature always had always negative effects, net radiation positive effects on daytime mean NEP during all CSAD events. Daily mean Rff during the 2018 CSAD event was driven by soil temperature and SWC, but severely limited by SWC during the 2022 CSAD event. We found that a multi-layer analysis of CO2 fluxes in forests is necessary to better understand forest responses to CSAD events, particularly if the first signs we saw of acclimation to such CSAD events for our forest are found elsewhere as well. We conclude that such events have multiple drivers with different temporal contributions, making prediction of site-specific CSADs and forest long-term responses to such conditions more challenging. [ABSTRACT FROM AUTHOR]

Background and aims: The root barrier to radial O2 loss is a trait induced during soil flooding restricting oxygen loss from the roots to the anoxic soil. It can also restrict radial water loss, potentially providing tolerance towards drought during conditions of water deficit. Several root traits (aerenchyma and xylem vessels area) respond in a similar way to soil flooding and low soil water potentials. Therefore, we hypothesised that root acclimations to soil flooding prime plants to withstand conditions of water deficit. Methods: We raised plants in hydroponics mimicking contrasting soil water conditions (aerated controls for well-watered soils; stagnant, deoxygenated solutions for flooded soils, and aerated solutions with different PEG6000 concentrations to mimic conditions of water deficit). We used O2 microsensors and gravimetric measurements to characterize the formation of a barrier to radial O2 loss during conditions of water deficit, and measured key anatomical root traits using light microscopy. Results: Several root traits were induced in stagnant conditions as well as in conditions of water deficit, including the barrier to radial O2 loss. The tightness of the barrier to water loss was similar in both stagnant and PEG6000 treatments. Moreover, plants growing in stagnant conditions tolerated a following severe condition of water deficit, whereas those growing in mimicked well-watered conditions did not. Conclusions: We demonstrated that plants growing in stagnant conditions can withstand following severe conditions of water deficit. We propose that key root traits, such as the barrier to radial O2 loss, which are induced in stagnant conditions as well as mild conditions of water deficit, prime the plants for a following severe condition of water deficit. [ABSTRACT FROM AUTHOR]

Water stress can trigger acclimation responses and damage plants. The aim of this study was to evaluate the integrative responses of cotton hydraulic conductance, leaf photosynthesis, and carbon metabolism to short-term drought and subsequent rewatering. A water-controlled pot experiment was conducted in 2020, with soil water drying continuing for one day (D1), two days (D2), and three days (D3) after it reached 40% ± 5% of the soil water holding capacity at the blooming stage of cotton, and the soil was then rewatered to the soil water holding capacity. We investigated how the stem hydraulic conductance, gas exchange, and biochemical traits of cotton were affected by imposed drought stress and subsequent rewatering. The hydraulic characteristics of cotton in the D2 and D3 treatments evolved with damage, complete closure of stomatal conductance, and complete deterioration of photosynthesis, in addition to severe floating changes in the carbon metabolism affected by drought. The leaves' functional characteristics after rewatering cannot be completely recovered to full-irrigation levels, and the recovery extent was strongly linked to the duration. Consequently, it is considered desirable to maintain normal physiological activity during the cotton reproductive period, and the drought episode can be sustained for 1 day in a long-term perspective when the soil water content is depleted to 40% ± 5% of the soil water holding capacity. These results can provide in-depth ideas for better understanding the hydraulic and physiological responses of cotton to drought episodes and rewatering, and they can help drought-affected cotton to cope with future climate change. [ABSTRACT FROM AUTHOR]

• Different temperatures, air humidity, soil moisture content, and light intensity had significant effects on the photosynthesis and chlorophyll fluorescence characteristics of Distylium chinense from heterogeneous habitats. • D. chinense adopted different photosynthetic acclimation patterns and species-specific water use strategies under heterogeneous habitats. • D. chinense adjusted leaf area, specific leaf area, stomatal conductance, photochemical quenching coefficient and non-photochemical quenching to photochemically fix and store more carbon to meet their needs for growth under heterogeneous habitats. Distylium chinense is an evergreen dominant shrub with strong adaptability under heterogeneous habitats, including natural riparian zone (NRZ), anti-seasonal water level fluctuation zone (ASWLFZ), and no water level fluctuation zone (NWLFZ) in the Three Gorges Reservoir Region (TGRR) in China. In order to understand the photosynthetic ecological adaptability and habitat adaptation strategies of D. chinense under heterogeneous habitats, its photosynthesis and chlorophyll fluorescence parameters were investigated. The results showed that photosynthetic characteristics were significantly different among heterogeneous habitats (p <0.05). The PSII potential activity and PSII maximum photochemical efficiency (F v /F m) of D. chinense in the ASWLFZ were the highest among heterogeneous habitats. Non-photochemical quenching coefficient (qN) in the NWLFZ was higher than that in the NRZ and ASWLFZ (p <0.05). Stepwise regression analysis showed that water use efficiency and stomatal conductance were correlated with specific leaf area (SLA), leaf area and leaf dry weight. Regression curve fitting and canonical correspondence analysis revealed that light intensity, soil water content, air humidity and temperature were the main factors affecting photosynthetic characteristics and leaf functional traits. Our results evidenced that D. chinense adapted to the high water content and low light intensity environments, especially in the valley waterfall habitats, by increasing stomatal conductance and net photosynthetic rate, reducing SLA and maintaining moderate qN and F v /F m with adopting a profligate water use pattern to improve photosynthetic productivity. Therefore, D. chinense has a wide photosynthetic acclimation to different environments, which provided baseline information for the conservation and ecological restoration in the newly formed hydro-fluctuation zone and other similar degenerative riparian ecosystem. [ABSTRACT FROM AUTHOR]

Projections of global climate models suggest that ongoing human-induced climate change will lead to an increase in the frequency of severe droughts in many important agricultural regions of the world. Eco-hydrological models that integrate current understanding of the interacting processes governing soil water balance and plant growth may be useful tools to predict the impacts of climate change on crop production. However, the validation status of these models for making predictions under climate change is still unclear, since few suitable datasets are available for model testing. One promising approach is to test models using data obtained in "space-for-time" substitution experiments, in which samples are transferred among locations with contrasting current climates in order to mimic future climatic conditions. An important advantage of this approach is that the soil type is the same, so that differences in soil properties are not confounded with the influence of climate on water balance and crop growth. In this study, we evaluate the capability of a relatively simple eco-hydrological model to reproduce 6 years (2013–2018) of measurements of soil water contents, water balance components and grass production made in weighing lysimeters located at two sites within the TERENO-SoilCan network in Germany. Three lysimeters are located at an upland site at Rollesbroich with a cool, wet climate, while three others had been moved from Rollesbroich to a warmer and drier climate on the lower Rhine valley floodplain at Selhausen. Four of the most sensitive parameters in the model were treated as uncertain within the framework of the GLUE (generalized likelihood uncertainty estimation) methodology, while the remaining parameters in the model were set according to site measurements or data in the literature. The model satisfactorily reproduced the measurements at both sites, and some significant differences in the posterior ranges of the four uncertain parameters were found. In particular, the results indicated greater stomatal conductance as well an increase in dry-matter allocation below ground and a significantly larger maximum root depth for the three lysimeters that had been moved to Selhausen. As a consequence, the apparent water use efficiency (above-ground harvest divided by evapotranspiration) was significantly smaller at Selhausen than Rollesbroich. Data on species abundance on the lysimeters provide one possible explanation for the differences in the plant traits at the two sites derived from model calibration. These observations showed that the plant community at Selhausen had changed significantly in response to the drier climate, with a significant decrease in the abundance of herbs and an increase in the proportion of grass species. The differences in root depth and leaf conductance may also be a consequence of plasticity or acclimation at the species level. Regardless of the reason, we may conclude that such adaptations introduce significant additional uncertainties into model predictions of water balance and plant growth in response to climate change. [ABSTRACT FROM AUTHOR]

Future climate will be characterized by an increase in frequency and duration of drought and warming that exacerbates atmospheric evaporative demand. How trees acclimate to long-term soil moisture changes and whether these long-term changes alter trees' sensitivity to short-term (day to months) variations of vapor pressure deficit (VPD) and soil moisture is largely unknown. Leaf gas exchange measurements were performed within a long-term (17 years) irrigation experiment in a drought-prone Scots pine-dominated forest in one of Switzerland's driest areas on trees in naturally dry (control), irrigated, and 'irrigation-stop' (after 11 years of irrigation) conditions. Seventeen years of irrigation increased photosynthesis (A) and stomatal conductance (g s) and reduced g s sensitivity to increasing VPD and soil drying. Following irrigation-stop, gas exchange decreased only after 3 years. After 5 years, maximum carboxylation (V cmax) and electron transport (J max) rates in irrigation-stop recovered to similar levels as to before the irrigation-stop. These results suggest that long-term release from soil drought reduces the sensitivity to VPD and that atmospheric constraints may play an increasingly important role in combination with soil drought. Moreover, our study indicates that structural adjustments lead to an attenuation of initially strong leaf-level acclimation to strong multiple-year drought. [ABSTRACT FROM AUTHOR]

This review examined one of the effects of climate change that has only recently received attention, i.e., climate change impacts on the distribution and toxicity of chemical contaminants in the environment. As ecosystem engineers, earthworms are potentially threatened by the increasing use of pesticides. Increases in temperature, precipitation regime changes, and related extreme climate events can potentially affect pesticide toxicity. This review of original research articles, reviews, and governmental and intergovernmental reports focused on the interactions between toxicants and environmental parameters. The latter included temperature, moisture, acidification, hypoxia, soil carbon cycle, and soil dynamics, as altered by climate change. Dynamic interactions between climate change and contaminants can be particularly problematic for organisms since organisms have an upper and lower physiological range, resulting in impacts on their acclimatization capacity. Climate change variables such as temperature and soil moisture also have an impact on acidification. An increase in temperature will impact precipitation which might impact soil pH. Also, an increase in precipitation can result in flooding which can reduce the population of earthworms by not giving juvenile earthworms enough time to develop into reproductive adults. As an independent stressor, hypoxia can affect soil organisms, alter bioavailability, and increase the toxicity of chemicals in some cases. Climate change variables, especially temperature and soil moisture, significantly affect the bioavailability of pesticides in the soil and the growth and reproduction of earthworm species. [ABSTRACT FROM AUTHOR]

The adaptability of plants to drought not only includes their ability to resist drought stress, but also their ability to recover after stress is relieved. In this study, a weighting method was used to control the soil water content to produce a soil water stress gradient. The effects of drought and rewatering on the changes in osmotic adjustment substance content, antioxidant enzyme activity, and photosynthetic characteristics of potted Dicranostigma henanensis seedlings were measured on Day 28 after the imposition of watering treatments and Day 7 after rewatering. During the drought stress process, the relative electrical conductivity, thiobarbituric acid, water use efficiency, and proline content displayed a continuously increasing trend. Further, the net photosynthetic rate, stomatal conductance, and transpiration rate constantly decreased, while the chlorophyll content first increased and then decreased. After rehydration, superoxide dismutase (EC 1.15.1.1) activity and photosynthetic parameters quickly recovered to the CK level (soil moisture is 75%-80% of the maximum water holding capacity in the field), indicating that D. henanensis plants have a strong ability to repair the damage caused by drought stress. In particular, the photosynthetic machinery may have sophisticated regulation and repair mechanisms, which may be associated with its stable photosystem. Collectively, our findings demonstrate that the D. henanensis plant has a strong ability to adapt to arid environments, and therefore could be an excellent ornamental flower for landscaping in arid and semiarid areas. [ABSTRACT FROM AUTHOR]

Summary: Understanding the genetic and physiological basis of abiotic stress tolerance under field conditions is key to varietal crop improvement in the face of climate variability. Here, we investigate dynamic physiological responses to water stress in silico and their relationships to genotypic variation in hydraulic traits of cotton (Gossypium hirsutum), an economically important species for renewable textile fiber production.In conjunction with an ecophysiological process‐based model, heterogeneous data (plant hydraulic traits, spatially‐distributed soil texture, soil water content and canopy temperature) were used to examine hydraulic characteristics of cotton, evaluate their consequences on whole plant performance under drought, and explore potential genotype × environment effects.Cotton was found to have R‐shaped hydraulic vulnerability curves (VCs), which were consistent under drought stress initiated at flowering. Stem VCs, expressed as percent loss of conductivity, differed across genotypes, whereas root VCs did not. Simulation results demonstrated how plant physiological stress can depend on the interaction between soil properties and irrigation management, which in turn affect genotypic rankings of transpiration in a time‐dependent manner.Our study shows how a process‐based modeling framework can be used to link genotypic variation in hydraulic traits to differential acclimating behaviors under drought. [ABSTRACT FROM AUTHOR]

Soil carbon losses to the atmosphere through soil respiration are expected to rise with ongoing temperature increases, but available evidence from mesic biomes suggests that such response disappears after a few years of experimental warming. However, there is lack of empirical basis for these temporal dynamics in soil respiration responses, and for the mechanisms underlying them, in drylands, which collectively form the largest biome on Earth and store 32% of the global soil organic carbon pool. We coupled data from a 10 year warming experiment in a biocrust‐dominated dryland ecosystem with laboratory incubations to confront 0–2 years (short‐term hereafter) versus 8–10 years (longer‐term hereafter) soil respiration responses to warming. Our results showed that increased soil respiration rates with short‐term warming observed in areas with high biocrust cover returned to control levels in the longer‐term. Warming‐induced increases in soil temperature were the main drivers of the short‐term soil respiration responses, whereas longer‐term soil respiration responses to warming were primarily driven by thermal acclimation and warming‐induced reductions in biocrust cover. Our results highlight the importance of evaluating short‐ and longer‐term soil respiration responses to warming as a mean to reduce the uncertainty in predicting the soil carbon–climate feedback in drylands. [ABSTRACT FROM AUTHOR]

Summary: Recent research has shown that plant acclimation to diverse patterns of light intensity modifies the dynamics of their stomatal response. Therefore, whether plants are grown in controlled conditions or in the field may impact their stomatal dynamics.We analysed the stomatal dynamics of two Populus euramericana and two Populus nigra genotypes grown in the field under contrasting water availability. By comparing their stomatal dynamics with that of the same genotypes grown in a glasshouse, we were able to test whether differences between these growing conditions interacted with genotypic differences in affecting stomatal dynamics and responses to soil water deficit.We found that, despite higher stomatal density and smaller size, in the field stomatal dynamics were much slower than in the glasshouse. Overall, differences among genotypes and their response to soil water deficit were much less pronounced in the field compared with the glasshouse.These results indicate that stomatal dynamics are regulated by both genotype‐specific and environmental factors. Moreover, having slower stomata may be advantageous under some conditions. While stomatal dynamics were linked with whole‐plant transpiration per leaf area in both experiments, the contribution of stomatal morphology varies dependent on the environmental conditions. [ABSTRACT FROM AUTHOR]

The relationship between introduced trees roots and soils in which they grow is the most important factor influencing the adaptation, growth and health of these trees. Therefore, the objective of this study was to identify which physical soil properties enhance or limit the vitality of the studied introduced trees in the Arboretum Mlyňany. Soil properties were studied in seven soil profiles under dense monocultures of Chamaecyparis lawsoniana, Liriodendron tulipifera, Juniperus Chinensis, Thuja orientalis, Thuja plicata, Picea orientalis and Pinus nigra. The results showed that all stagnic horizons had exceeded the limit values of total porosity and bulk density, hence these horizons were compacted. Based on the soil and climatic requirements of the examined trees we conclude that the soil properties of their sites in arboretum are suitable for: Chamaecyparis lawsoniana, Liriodendron tulipifera, Thuja orientalis and Pinus nigra. Nevertheless, physical properties in profiles under Picea orientalis and Juniperus Chinensis do not permit rapid drainage of water, what is unfavourable for healthy development of these two species; while Thuja plicata demanding high moisture supply is grown on soil with high coarse porosity, a prerequisite of fast rainwater drainage. However, since none of the studied introduced trees had suffered from physiological disorders or diseases, they may be declared acclimatized well in the soil-climate conditions described in this study.

Asphodelus aestivus Brot. (Walter) is the dominant geophyte species on degraded areas of eastern Mediterranean Basin. Because it has a stress-tolerant strategy and it is a competitive ruderal. We studied the activities of antioxidant enzymes (ascorbate peroxidase, APX; superoxide dismutase, SOD; catalase, CAT) in the root tubers of A. aestivus from Bursa, Turkey over one year. There was a seasonal variation in antioxidant enzyme activities. The CAT and APX showed similar variation model in which the activity was highest in July (9.3 ± 2.7 units/mg protein and 141.9 ± 52.7 units/mg protein) and lowest in August (1.2 ± 0.6 units/mg protein and 14.5 ± 2.3 units/mg protein). The SOD activity increasing in summer months was reached up the highest level in late summer (297.6 ± 48.1 units/mg protein). The significant negative correlations were also found between enzyme activities and water contents of soils (p<0.05). The increased antioxidant enzyme activities in summer and late summer periods may be a protective and acclimation mechanism against drought stress. These results indicate that drought can induce enzymatic antioxidant defence system in A. aestivus and this system may help to this species to spread on degraded Mediterranean environments. [ABSTRACT FROM AUTHOR]

Climate change will alter global precipitation patterns, making it increasingly important that we understand how ecosystems will be impacted by more frequent and severe droughts. Yet most drought studies examine a single, within-season drought, and we know relatively little about the impacts of multiple droughts that occur within a single growing season. This distinction is important because many plant species are able to acclimate physiologically, such that the effects of multiple droughts on ecosystem function deviate significantly from the effects of cumulative, independent droughts. Unfortunately, we know relatively little about the ability of dominant species to acclimate to drought in drought-sensitive ecosystems like semi-arid grasslands. Here, we tested for physiological acclimation to multiple drought events in two dominant shortgrass steppe species: Bouteloua gracilis (C4) and Elymus elymoides (C3). Neither species exhibited physiological acclimation to drought; leaf water potential, stomatal conductance, and photosynthesis rates were all similarly affected by a single, late period drought and a second, late period drought. Biomass was lowest in plants exposed to two droughts, but this is likely due to the cumulative effects of both an early and late period drought. Our results suggest that late period droughts do exert weaker effects on biomass production of two dominant shortgrass species, but that the weaker effects are due to ontogenetic changes in plant physiology as opposed to physiological acclimation against multiple droughts. As a consequence, current ecosystem models that incorporate grass phenology and seasonal physiology should provide accurate predictions of primary production under future climates. [ABSTRACT FROM AUTHOR]

Current understanding of acclimation processes in desert-shrub species to drought stress in dryland ecosystems is still incomplete. In this study, we measured sap flow in Artemisia ordosica and associated environmental variables throughout the growing seasons of 2013-2014 (May-September period of each year) to better understand the environmental controls on the temporal dynamics of sap flow. We found that the occurrence of drought in the dry year of 2013 during the leaf-expansion and leaf-expanded periods caused sap flow per leaf area (Js) to decline significantly, resulting in a sizable drop in transpiration. Sap flow per leaf area correlated positively with radiation (Rs), air temperature (T), and vapor pressure deficit (VPD), when volumetric soil water content (VWC) was > 0.11 m³ m-3. Diurnal Js was generally ahead of Rs by as much as 6 hours. This lag time, however, decreased with increasing VWC. Relative response of Js to the environmental variables (i.e., Rs, T, and VPD) varied with VWC, Js being more biologically controlled with low decoupling coefficient and thus being less sensitive to the environmental variables during dry periods. According to this study, soil moisture is shown to control sap-flow (and, therefore, plant-transpiration) response in Artemisia ordosica to diurnal variations in biophysical factors. The findings of this study add to the knowledge of acclimation processes in desert-shrub species under drought-associated stress. This knowledge is essential to model desert-shrub-ecosystem functioning under changing climatic conditions. [ABSTRACT FROM AUTHOR]

Black locust (Robinia pseudoacacia) is widely planted throughout semiarid and subhumid regions of the Loess Plateau of China. Determining the changes in transpiration of this species in different climatic areas is important for revealing the acclimation mechanism of black locust and developing suitable forest management practices, particularly in the context of global climate change. Here, sap flow and canopy conductance of black locust plantation trees in semiarid (Yan'an) and subhumid (Yongshou) sites were quantified using Granier-type thermal dissipation probes and concurrent environmental observations from 2012 to 2017. Several physiological parameters were measured throughout the growing season. The results showed that sap flow was correlated with phenological factors across seasons within a year. However, interannual changes in sap flow were affected mainly by the reference evapotranspiration (ET 0) at the Yongshou site, and jointly by precipitation (P), soil water content, and P / ET 0 at the Yan'an site. Sap flow response to meteorological factors showed less discrepancy between pre- and post-rainfall periods at the Yan'an site. Moreover, canopy conductance fluctuated less with a wider range of vapor pressure deficit (VPD) and the slope of canopy resistance as a function of VPD was lower, indicating relatively lower sensitivity of stomatal conductance to environmental factors in Yan'an site. Physiological parameters, except for predawn leaf water potential, were significantly different between the two sites. The results suggested that black locust tended to reduce transpiration, modify leaf morphology, and improve water use efficiency to enhance its adaptability to the dryer site. The species changes stomatal regulation characteristics and general growth rate to acclimatize to distinct water habitats. • Sap flow characteristics of black locust differ between semiarid and subhumid sites. • Dominant environmental factors controlling transpiration varied with habitats. • Adaptability and responses to long-term and short-term drought are different. • Longterm soil water shortage weakened stomatal regulation and growth. • Future intensified drought potentially threatens the plantation stability. [ABSTRACT FROM AUTHOR]

Climate models predict rising temperatures and more frequent and prolonged urban heat islands (UHI) in southern China. The urban vegetations have become a focal point to mitigate the detrimental effects of UHI and to provide cooling through transpiration. However, transpiration in trees such as Ficus concinna in relation to extreme long-term conditions of UHI is scarcely documented. Here, we investigated the transpiration dynamics and its cooling effects in F. concinna induced by changes in site environmental variables in a subtropical megacity, Shenzhen over five consecutive years (2015–2019) based on continuous sap flow measurements. Seasonally, the transpiration (T r) and its cooling effect (i.e., heat energy absorbed (Q) and temperature reduction (ΔT) by T r) were highest in summer, peaking in the month of July with the mean values of 1.98 mm d−1, 4.91 MJ m−2 d−1 and 3.93 °C m−2 d−1, respectively. The highest cooling effect was shown during warmer and wet years. Daily T r had a positive linear relationship with shortwave radiation (R s) in May-June, air temperature (T a) and volumetric soil water content (SWC 30) in July-August and vapor pressure deficit (VPD) in September, respectively. Furthermore, the main regulatory variables of T r within each season were found to be the spring R s , summer T a and SWC 30 , and autumn R s and VPD, which explain 49, 82 and 74% of the variation, respectively. The influence of these variables (i.e., T a , R s and VPD) on T r was modified by the effect of SWC 30. Interannually, T a , SWC 30 and precipitation (PPT) were responsible for most of the observed variation in T r , individually explaining 72, 81, and 66% of the variation. Furthermore, multiple regression model indicated that together T a , SWC 30 and PPT explained 89% of the variation in T r. The diminished sensitivity of T r to environmental variables and enhanced sensitivity to SWC 30 during dry years point to species acclimatization to soil dryness. Our findings clearly indicate the temporal dynamics in T r and its cooling effectiveness for F. concinna over longer timescales. It is suggested that F. concinna could be better suited in response to increasing temperature in subtropical urban areas, as the species may be capable to provide efficient cooling based on its high T r rate and has the potential to mitigate the UHI effect. [ABSTRACT FROM AUTHOR]

• Nighttime sapflow per leaf area J s n exhibited thresholded-response to T a , VPD and G s. • Seasonal variation in T a and VPD controlled J s n in wet and G s and SWC in dry years. • Interannual variation in T a and G s directly controlled J s n , while indirectly by SWC. • J s n responses to biophysical factors underscores the acclimation to dry conditions. • J s n significantly contributed to the daily total sap flow with interannual mean of 11%. The impact of extreme desert conditions on long-term nighttime sap flow dynamics in desert-dwelling shrubs is poorly understood. Based on heat-balance measurements in a semiarid area of northwest China, we investigated the nighttime sap flow (transpiration) dynamics in Salix psammophila as affected by changes in site biophysical variables over a six-year period (2012-2017). Nocturnally, sap flow rate per leaf area (J s n) formed a positive linear relationship with air temperature (T a), water vapor pressure deficit (VPD) and stomatal conductance (G s), when T a < 17 °C, VPD < 1 kPa, and G s < 0.004 mol m−2 s−1, remaining unchanging or forming a negative linear relationship, when T a , VPD and G s were greater than these limits. This relationship was modified by soil water content (SWC) through its effects on G s. The multiple regression model showed that together T a , VPD and G s explained 63% of the variation in J s n. Seasonally, pairwise relationships between T a and VPD and G s and SWC explained 76 and 70% of the variation in J s n during wet and dry years, respectively. Interannually, T a and G s were able to explain a significant portion of observed variation in J s n , individually explaining about 74 and 83% of that variation. Interannual control on J s n also varied with SWC, T a and G s explaining 84% of the variation. The diminished sensitivity of J s n to both T a and VPD and increased sensitivity to SWC during dry years, suggested that the shrubs had a physiological capacity to adapt to dry soil conditions. Daytime sap flow rates (J s d) were significantly greater than those at night (i.e., J s n), with an interannual mean difference of 0.15 ± 0.04 g cm−2 d−1 and coefficient of variation of 27%. In addition, J s n formed a positive linear association with J s d during the growing season. Mean J s n contributed to the daily total sap flow (i.e., J s n /J s 24-h × 100) by 8-14% over the six years, with an interannual mean of 11%. Our findings underscore the importance of nighttime sap flow in S. psammophila and the importance of temperature and SWC in controlling nighttime water losses through transpiration. It is suggested that the proportion of nighttime sap flow could increase during plant acclimatization in response to increasing temperature and water shortages with future climate change. [ABSTRACT FROM AUTHOR]

• Interannual variation in J s was controlled by PPT-induced-VWC and LAI • Long term summer drought reduce J s and stomatal conductance more than spring drought • The reduction highlights physiological adjustment in acclimation to drought • Among three species, HM was least sensitive, more resistant and resilient to drought Extreme seasonal droughts frequently occur in semiarid and arid areas of the world. They can exert a profound influence on the physiological state of desert plants. Short-term rainfall and long-term water shortages in the Mu Us Desert have differentiated the role of individual species with their distinct ability to resist and recover from drought. This study explores how the scale of rainfall events affects interannual variation in sap flow in three dominant desert-shrub species [i.e., Salix psammophila (SP), Artemisia ordosica (AO), and Hedysarum mongolicum (HM)], and whether the associated changes in soil-water dynamics helped to modify sap flow. As a result, interannual variation in sap flow (J s) was mostly driven by precipitation (PPT), Volumetric soil water content (VWC), and leaf area index (LAI), with their individual influences varying as a function of plant phenophases. The species acclimated to long-term summer drought (> 30 days, i.e., no rain over a 30-day period or longer) by reducing stomatal conductance in the three species by 66.5, 59.5, and 43.5%, respectively, suggesting a physiological adjustment in water conservation in response to drought. Average summer drought (June–July) reduced J s in the three species by 55.5 (in SP), 42 (AO), and 28.5% (HM), more than spring drought (May–June) at 28% in SP and 9.5% in AO. Among those species, HM exhibited the least sensitivity and the highest overall resistance and resilience to drought. The relationship between J s and VWC was modulated by the effect of PPT. Our findings provide compelling evidence that HM is seen to be better suited to future climatic warming, as the species may be capable of accessing deep groundwater reserves replenished by large PPT pulses (5–10 mm), in sustaining its physiological activity over longer periods. These results could help formulate a selection process in determining which shrub species to plant in the sustainable management of desert ecosystems. [ABSTRACT FROM AUTHOR]

• Substitutable resources fill the same metabolic or stoichiometric need • Acquisition of interdependent resources are tightly linked • Optimizing uptake of both resource types can be based on marginal yield • Uptake assets are allocated to the resource with highest return per unit cost • The algorithm should be broadly applicable for resource acquisition models We develop a hierarchical approach to modeling organism acclimation to changing availability of and requirements for substitutable and interdependent resources. Substitutable resources are resources that fill the same metabolic or stoichiometric need of the organism. Interdependent resources are resources whose acquisition or expenditure are tightly linked (e.g., light, CO 2 , and water in photosynthesis and associated transpiration). We illustrate the approach by simulating the development of vegetation with four substitutable sources of N that differ only in the cost of their uptake and assimilation. As the vegetation develops, it uses the least expensive N source first then uses progressively more expensive N sources as the less expensive sources are depleted. Transition among N sources is based on the marginal yield of N per unit effort expended, including effort expended to acquire C to cover the progressively higher uptake costs. We illustrate the approach to interdependent resources by simulating the expenditure of effort to acquire light energy, CO 2 , and water to drive photosynthesis in vegetation acclimated to different conditions of soil water, atmospheric vapor pressure deficit, CO 2 concentration, and light levels. The approach is an improvement on the resource optimization used in the earlier Multiple Element Limitation (MEL) model. [ABSTRACT FROM AUTHOR]

• Sustained drought, but not repeated, promoted physiological acclimation. • More variable physiological responses occurred in repeated than sustained drought. • Sustained drought led to conservative responses and minimal growth. • Physiology during repeated drought was opportunistic, linked to water availability. Climate change is predicted to result in more variable and extreme precipitation regimes, with potentially longer periods between precipitation events. However, the effects of multiple, sequential drought events on ecosystems and potential acclimation, particularly in grasslands, is poorly understood. In this study, we tested if two simulated drought regimes, (i) sustained drought (soil moisture content reduced from 75 to 25 % field capacity over 14 days and maintained for 29 days) and (ii) repeated drought (two cycles of a reduction to 25 % field capacity, reached after 22 days and 15 days, separated by a rewatering event to 75 % field capacity for 5 days), differentially influence fourteen key physiological responses in the model grass species (Brachypodium distachyon). In addition, we aimed to determine whether plants may acclimate differently to sustained and/or repeated drought events. B. distachyon exhibited plasticity in response to drought, exhibiting two distinct drought resistance responses to the two watering regimes. Plants exposed to sustained drought exhibited early reductions in photosynthesis, stomatal conductance and water potential (43 %, 42 % and 37 %, respectively). However, plants showed physiological acclimation and recovery in these parameters after an additional 3–21 days, but had a ∼50 % lower shoot biomass than well-watered plants. These data demonstrate a conservative, albeit plastic, response to soil water deficit. In contrast, plants exposed to repeated drought responded opportunistically whereby greater fluctuations in photosynthesis, stomatal conductance and water potential (reductions of up to 62 %, 71 % and 90 %, respectively) were closely related to drought and rewatering events. Since plants exposed to repeated drought had a greater reduction in water potential and faster reductions in relative photosynthesis in the second drought event compared to the initial drought event, this may indicate a lack of acclimation under this drought regime. Our results provide evidence for differential responses of B. distachyon to drought and rewatering events, which are dependent on the drought regime (sustained or repeated) and must be carefully considered when interpreting experimental data. [ABSTRACT FROM AUTHOR]

Background and Objectives: Drought occurs more frequently in Northern China with the advent of climate change, which might increase the mortality of tree seedlings after afforestation due to hydraulic failure. Therefore, investigating water relations helps us understand the drought tolerance of tree seedlings. Electrical impedance spectroscopy (EIS) is widely used to assess the responses of plant tissues to stress factors and may potentially reveal the water relations of cells. The aim of this study is to reveal the relationships between EIS and water related parameters, produced by pressure–volume (PV) curves in lacebark pine (Pinus bungeanaZucc.) seedlings reacting to drought stress. Materials and Methods: Four-year-old pot seedlings were divided into three parts (0, 5, and 10 days of drought) before planting, the treated seedlings were then replanted, and finally exposed to post-planting drought treatments with the following soil relative water contents: (i) adequate irrigation (75%–80%), (ii) light drought (55%–60%), (iii) moderate drought (35%–40%), and (iv), severe drought (15%–20%). During the post-planting growth phase, the EIS parameters of needles and shoots, and the parameters of PV curves, were measured coincidently; thus, the correlations between them could be obtained. Results: The extracellular resistance (re) of needles and shoots were substantially reduced after four weeks of severe post-planting drought stress. Meanwhile, the osmotic potential at the turgor-loss point (ψtlp) and the saturation water osmotic potential (ψsat) of shoots decreased after drought stress, indicating an osmotic adjustment in acclimating to drought. The highest correlations were found between the intracellular resistance (ri) of the shoots and ψtlp and ψsat. Conclusions: EIS parameters can be used as a measure of drought tolerance. The change in intracellular resistance is related to the osmotic potential of the cell and cell wall elasticity. Extracellular resistance is a parameter that shows cell membrane damage in response to drought stress in lacebark pine seedlings. [ABSTRACT FROM AUTHOR]