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• 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]

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]

Abstract: New soil moisture sensor systems (SMSs) for irrigation control have been commercialized in recent years. However, limited research has been carried out to evaluate their precision to measure the volumetric soil water content (θ). The objectives of this research were to: (a) determine the relationship between θ and the θ sensed by four commercially available SMSs, (b) quantify the proportion of scheduled irrigation cycles (SICs) that the SMSs bypassed, and (c) determine the θ at which SICs were allowed or bypassed. Sensors from brands Acclima, Rain Bird, Irrometer, and Water Watcher were buried at 7–10cm depth, on plots with common bermudagrass [Cynodon dactylon (L.) Pers.]. A calibrated ECH2O probe was also installed in every plot, at the same depth, to monitor θ continuously. When comparing the ECH2O readings with θ sensed by the SMSs, significant correlations were found for the three Acclima RS500 (AC) systems tested, and for two of the three systems of Irrometer Watermark 200SS/WEM (IM) and Rain Bird MS-100 (RB). Most of the SMS-based treatments bypassed the majority of the SICs during rainy periods, and allowed irrigation during the dry periods. On average, 71% of the SICs were bypassed by the SMS treatments, without detriment to the turfgrass quality. However, most of the SMSs were not found to be precision instruments, because sometimes they bypassed SICs and sometimes they did not, even when reading the same or a lower θ. Considering the average θ range of over which the different SMS treatments always allowed or always bypassed irrigation, brand AC resulted in the significantly narrowest range (1.4%) followed by RB (3.2%), suggesting that they were more consistent and precise in measuring θ than Water Watcher DPS-100 (WW) and IM (7.4 and 7.8%, respectively). These results are consistent with the reported water savings achieved by these SMSs in related studies. [Copyright &y& Elsevier]

Models used to estimate net ecosystem productivity (NEP) during climate change should first be shown to simulate the effects on NEP of interannual variability in current climates. Energy and CO2 fluxes simulated by the ecosystem model ecosys were compared with those measured by eddy covariance over a semi-arid ungrazed grassland near Lethbridge, Alberta to improve confidence in model projections of grassland NEP. Differences between simulated and measured fluxes of CO2 and energy were within the range of uncertainty in measured fluxes during 3 years with declining precipitation (466, 363, and 276mm in 1998, 1999, and 2000, respectively). Standard differences between modelled versus measured CO2 and LE over 3 years were 1.0μmolm-2s-1 and 22Wm-2 which were comparable to standard errors estimated from eddy covariance measurements. However, further research is needed to reconcile differences in modelled vs. measured ecosystem respiration. Annual modelled NEP of the semi-arid ungrazed grassland was +59g Cm-2 in 1998, +5g Cm-2 in 1999, and -33g Cm-2 in 2000. Long-term modelled rate of C accumulation in this grassland under current climate was 26g Cm-2y-1 which is consistent with C accumulation in semiarid grasslands estimated from measurements and models elsewhere. Under climate change projected by the Canadian Regional Climate Model II from the IS92a emissions scenario, increases in transpiration caused by rising temperatures were fully offset by decreases in transpiration caused by rising CO2, thereby alleviating water deficits and lengthening growing seasons. The consequent rise in grassland net primary productivity was largely offset by a rise in heterotrophic respiration, so that C sequestration by the semi-arid ungrazed grassland rose by less than 2gCm-2y-1 under climate change. [Copyright &y& Elsevier]

• 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]

Abstract: In a mesocosm experiment, with bare peat soils exposed to different water levels (WL), we examined whether heterotrophic respiration (Rh) acclimated to a 3 °C temperature increase. Across all WLs, Rh at 15 °C was never lower in the heated treatment than in the unheated treatment, indicating that Rh did not acclimate to the warmer conditions. We hypothesize that this lack of thermal acclimation is due to the unlimited substrate availability in these organic soils. These results imply that peat soils may exhibit a sustained positive feedback to global warming. [Copyright &y& Elsevier]

• 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]