Species‐specific stomatal response of trees to drought – a link to vegetation dynamics?

Question: Is stomatal regulation specific for climate and tree species, and does it reveal species‐specific responses to drought? Is there a link to vegetation dynamics? Location: Dry inner alpine valley, Switzerland Methods: Stomatal aperture (θ_E) of Pinus sylvestris, Quercus pubescens, Juniperus communis and Picea abies were continuously estimated by the ratio of measured branch sap flow rates to potential transpiration rates (adapted Penman‐Monteith single leaf approach) at 10‐min intervals over four seasons. Results: θ_E proved to be specific for climate and species and revealed distinctly different drought responses: Pinus stomata close disproportionately more than neighbouring species under dry conditions, but has a higher θ_E than the other species when weather was relatively wet and cool. Quercus keeps stomata more open under drought stress but has a lower θ_E under humid conditions. Juniperus was most drought‐tolerant, whereas Picea stomata close almost completely during summer. Conclusions: The distinct microclimatic preferences of the four tree species in terms of θ_E strongly suggest that climate (change) is altering tree physiological performances and thus species‐specific competitiveness. Picea and Pinus currently live at the physiological limit of their ability to withstand increasing temperature and drought intensities at the sites investigated, whereas Quercus and Juniperus perform distinctly better. This corresponds, at least partially, with regional vegetation dynamics: Pinus has strongly declined, whereas Quercus has significantly increased in abundance in the past 30 years. We conclude that θ_E provides an indication of a species' ability to cope with current and predicted climate.     

Causes of reduced leaf‐level photosynthesis during strong El Niño drought in a Central Amazon forest

Sustained drought and concomitant high temperature may reduce photosynthesis and cause tree mortality. Possible causes of reduced photosynthesis include stomatal closure and biochemical inhibition, but their relative roles are unknown in Amazon trees during strong drought events. We assessed the effects of the recent (2015) strong El Niño drought on leaf‐level photosynthesis of Central Amazon trees via these two mechanisms. Through four seasons of 2015, we measured leaf gas exchange, chlorophyll a fluorescence parameters, chlorophyll concentration, and nutrient content in leaves of 57 upper canopy and understory trees of a lowland terra firme forest on well‐drained infertile oxisol. Photosynthesis decreased 28% in the upper canopy and 17% in understory trees during the extreme dry season of 2015, relative to other 2015 seasons and was also lower than the climatically normal dry season of the following non‐El Niño year. Photosynthesis reduction under extreme drought and high temperature in the 2015 dry season was related only to stomatal closure in both upper canopy and understory trees, and not to chlorophyll a fluorescence parameters, chlorophyll, or leaf nutrient concentration. The distinction is important because stomatal closure is a transient regulatory response that can reverse when water becomes available, whereas the other responses reflect more permanent changes or damage to the photosynthetic apparatus. Photosynthesis decrease due to stomatal closure during the 2015 extreme dry season was followed 2 months later by an increase in photosynthesis as rains returned, indicating a margin of resilience to one‐off extreme climatic events in Amazonian forests.     

Prolonged experimental drought reduces plant hydraulic conductance and transpiration and increases mortality in a piñon–juniper woodland

Plant hydraulic conductance (k_s) is a critical control on whole‐plant water use and carbon uptake and, during drought, influences whether plants survive or die. To assess long‐term physiological and hydraulic responses of mature trees to water availability, we manipulated ecosystem‐scale water availability from 2007 to 2013 in a piñon pine (Pinus edulis) and juniper (Juniperus monosperma) woodland. We examined the relationship between k_s and subsequent mortality using more than 5 years of physiological observations, and the subsequent impact of reduced hydraulic function and mortality on total woody canopy transpiration (E_C) and conductance (G_C). For both species, we observed significant reductions in plant transpiration (E) and k_s under experimentally imposed drought. Conversely, supplemental water additions increased E and k_s in both species. Interestingly, both species exhibited similar declines in k_s under the imposed drought conditions, despite their differing stomatal responses and mortality patterns during drought. Reduced whole‐plant k_s also reduced carbon assimilation in both species, as leaf‐level stomatal conductance (g_s) and net photosynthesis (A_n) declined strongly with decreasing k_s. Finally, we observed that chronically low whole‐plant k_s was associated with greater canopy dieback and mortality for both piñon and juniper and that subsequent reductions in woody canopy biomass due to mortality had a significant impact on both daily and annual canopy E_C and G_C. Our data indicate that significant reductions in k_s precede drought‐related tree mortality events in this system, and the consequence is a significant reduction in canopy gas exchange and carbon fixation. Our results suggest that reductions in productivity and woody plant cover in piñon–juniper woodlands can be expected due to reduced plant hydraulic conductance and increased mortality of both piñon pine and juniper under anticipated future conditions of more frequent and persistent regional drought in the southwestern United States.     

Growth, carbon-isotope discrimination, and drought-associated mortality across a Pinus ponderosa elevational transect

Drought‐ and insect‐associated tree mortality at low‐elevation ecotones is a widespread phenomenon but the underlying mechanisms are uncertain. Enhanced growth sensitivity to climate is widely observed among trees that die, indicating that a predisposing physiological mechanism(s) underlies tree mortality. We tested three, linked hypotheses regarding mortality using a ponderosa pine (Pinus ponderosa) elevation transect that experienced low‐elevation mortality following prolonged drought. The hypotheses were: (1) mortality was associated with greater growth sensitivity to climate, (2) mortality was associated with greater sensitivity of gas exchange to climate, and (3) growth and gas exchange were correlated. Support for all three hypotheses would indicate that mortality results at least in part from gas exchange constraints. We assessed growth using basal area increment normalized by tree basal area [basal area increment (BAI)/basal area (BA)] to account for differences in tree size. Whole‐crown gas exchange was indexed via estimates of the CO₂ partial pressure difference between leaf and atmosphere (p_a?p_c) derived from tree ring carbon isotope ratios (δ¹³C), corrected for temporal trends in atmospheric CO₂ and δ¹³C and elevation trends in pressure. Trees that survived the drought exhibited strong correlations among and between BAI, BAI/BA, p_a?p_c, and climate. In contrast, trees that died exhibited greater growth sensitivity to climate than trees that survived, no sensitivity of p_a?p_c to climate, and a steep relationship between p_a?p_c and BAI/BA. The p_a?p_c results are consistent with predictions from a theoretical hydraulic model, suggesting trees that died had a limited buffer between mean water availability during their lifespan and water availability during drought – i.e., chronic water stress. It appears that chronic water stress predisposed low‐elevation trees to mortality during drought via constrained gas exchange. Continued intensification of drought in mid‐latitude regions may drive increased mortality and ecotone shifts in temperate forests and woodlands.     

Long‐term climate and competition explain forest mortality patterns under extreme drought

Rising temperatures are amplifying drought‐induced stress and mortality in forests globally. It remains uncertain, however, whether tree mortality across drought‐stricken landscapes will be concentrated in particular climatic and competitive environments. We investigated the effects of long‐term average climate [i.e. 35‐year mean annual climatic water deficit (CWD)] and competition (i.e. tree basal area) on tree mortality patterns, using extensive aerial mortality surveys conducted throughout the forests of California during a 4‐year statewide extreme drought lasting from 2012 to 2015. During this period, tree mortality increased by an order of magnitude, typically from tens to hundreds of dead trees per km², rising dramatically during the fourth year of drought. Mortality rates increased independently with average CWD and with basal area, and they increased disproportionately in areas that were both dry and dense. These results can assist forest managers and policy‐makers in identifying the most drought‐vulnerable forests across broad geographic areas.     

锐齿栎水力结构和生长对降雨减少的响应

为探究气候变化背景下降雨减少对森林的影响,2013年在宝天曼锐齿栎天然次生林原位建立了3块降水减少(截雨)样地,研究降雨减少对锐齿栎水碳关系和生长的影响。结果表明:降雨减少后锐齿栎枝条水势显著低于对照,最低水势为(-1.36±0.11)MPa,但锐齿栎木质部栓塞88%的水势值为-3.19MPa,叶片气孔关闭时的水势值为-2.5MPa,故降雨减少在这一地区没有对锐齿栎水力结构造成严重的干扰。降雨减少后,锐齿栎的叶片、韧皮部和木质部的总非结构性碳浓度与对照没有显著差异。木质部导管密度和叶片气孔密度变大,而导管直径和气孔长度变小。在天气较为干旱时,降雨减少处理的锐齿栎气孔导度日变化呈"双峰"曲线,而在湿润天气时呈"单峰"曲线且中午峰值显著大于对照。降雨减少处理的锐齿栎木材密度、Huber值、比叶面积和胸径生长与对照没有显著差异。降雨减少后锐齿栎树木没有遭受水力失衡或碳饥饿的危害,生长也没有受到显著影响,但是水力输导系统发生了适应性调节。     

Leaf hydraulic safety margin and safety–efficiency trade-off across angiosperm woody species

Leaf hydraulic conductance and the vulnerability to water deficits have profound effects on plant distribution and mortality. In this study, we compiled a leaf hydraulic trait dataset with 311 species-at-site combinations from biomes worldwide. These traits included maximum leaf hydraulic conductance (K_leaf), water potential at 50% loss of K_leaf (P50_leaf), and minimum leaf water potential (Ψ_min). Leaf hydraulic safety margin (HSM_leaf) was calculated as the difference between Ψ_min and P50_leaf. Our results indicated that 70% of the studied species had a narrow HSM_leaf (less than 1 MPa), which was consistent with the global pattern of stem hydraulic safety margin. There was a positive relationship between HSM_leaf and aridity index (the ratio of mean annual precipitation to potential evapotranspiration), as species from humid sites tended to have larger HSM_leaf. We found a significant relationship between K_leaf and P50_leaf across global angiosperm woody species and within each of the different plant groups. This global analysis of leaf hydraulic traits improves our understanding of plant hydraulic response to environmental change.     

Differences in leaf thermoregulation and water use strategies between three co‐occurring Atlantic forest tree species

Given anticipated climate changes, it is crucial to understand controls on leaf temperatures including variation between species in diverse ecosystems. In the first study of leaf energy balance in tropical montane forests, we observed current leaf temperature patterns on 3 tree species in the Atlantic forest, Brazil, over a 10‐day period and assessed whether and why patterns may vary among species. We found large leaf‐to‐air temperature differences (maximum 18.3 °C) and high leaf temperatures (over 35 °C) despite much lower air temperatures (maximum 22 °C). Leaf‐to‐air temperature differences were influenced strongly by radiation, whereas leaf temperatures were also influenced by air temperature. Leaf energy balance modelling informed by our measurements showed that observed differences in leaf temperature between 2 species were due to variation in leaf width and stomatal conductance. The results suggest a trade‐off between water use and leaf thermoregulation; Miconia cabussu has more conservative water use compared with Alchornea triplinervia due to lower transpiration under high vapour pressure deficit, with the consequence of higher leaf temperatures under thermal stress conditions. We highlight the importance of leaf functional traits for leaf thermoregulation and also note that the high radiation levels that occur in montane forests may exacerbate the threat from increasing air temperatures.     

The resprouting response of co‐occurring temperate woody plant and grass species to elevated [CO2]: An insight into woody plant encroachment of grasslands

The phenomenon of woody plant thickening in grasslands has been observed globally and is likely to have widespread ecological consequences. It has been proposed that woody plant thickening is driven in part by rising atmospheric [CO₂] enhancing the resprouting ability of woody plants relative to grasses so they respond more strongly after disturbances such as herbivory and fire. The aim of this study was to examine the CO₂ effect on the resprouting ability of 16 co‐occurring temperate woody plant and grass species (eight species from each growth form). Plants were grown in a controlled glasshouse experiment under ambient (400 ppm) and elevated [CO₂] (600 ppm) for 14 weeks after which their resprouting ability was measured. Root non‐structural carbohydrate (NSC_mass) and nitrogen (N_mass) storage was used as proxies to measure the resprouting ability of woody plants while for the grasses it was measured directly. We found that both the woody plants (22% on average; P = 0.003) and grasses (20% on average; P = 0.003) produced more biomass under elevated [CO₂]. Despite the woody plants not allocating additional carbon to belowground storage under elevated [CO₂], they had significantly greater root NSC_mass (23% on average; P = 0.007) due to increased root biomass production (8% on average; P = 0.007). In contrast, root N_mass of the woody plants did not differ between CO₂ treatments (P = 0.373). Surprisingly, the resprouting ability of the grasses did not significantly differ between the CO₂ treatments (P = 0.067). These results provide evidence that the differing resprouting response of woody plants and grasses under elevated [CO₂] may be contributing to woody plant encroachment of grasslands worldwide.     

The influence of soil temperature and water content on belowground hydraulic conductance and leaf gas exchange in mature trees of three boreal species

Understanding stomatal regulation is fundamental to predicting the impact of changing environmental conditions on vegetation. However, the influence of soil temperature (ST) and soil water content (SWC) on canopy conductance (g_s) through changes in belowground hydraulic conductance (k_bg) remains poorly understood, because k_bg has seldom been measured in field conditions. Our aim was to (a) examine the dependence of k_bg on ST and SWC, (b) examine the dependence of g_s on k_bg and (c) test a recent stomatal optimization model according to which g_s and soil-to-leaf hydraulic conductance are strongly coupled. We estimated k_bg from continuous sap flow and xylem diameter measurements in three boreal species. k_bg increased strongly with increasing ST when ST was below +8°C, and typically increased with increasing SWC when ST was not limiting. g_s was correlated with k_bg in all three species, and modelled and measured g_s were well correlated in Pinus sylvestris (a model comparison was only possible for this species). These results imply an important role for k_bg in mediating linkages between the soil environment and leaf gas exchange. In particular, our finding that ST strongly influences k_bg in mature trees may help us to better understand tree behaviour in cold environments.     

Last‐century changes of alpine grassland water‐use efficiency: a reconstruction through carbon isotope analysis of a time‐series of Capra ibex horns

The ecophysiological response of an alpine grassland to recent climate change and increasing atmospheric CO₂ concentration was investigated with a new strategy to go back in time: using a time‐series of Capra ibex horns as archives of the alpine grasslands' carbon isotope discrimination (¹³Δ). From the collection of the Natural History Museum of Bern, horns of 24 males from the population of the Augstmatthorn–Brienzer Rothorn mountains, Switzerland, were sampled covering the period from 1938 to 2006. Samples were taken from the beginning of each year‐ring of the horns, representing the beginning of the horn growth period, the spring. The horns' carbon ¹³C content (Δ¹³C) declined together with that of atmospheric CO₂ over the 69‐year period, but ¹³Δ increased slightly (+0.4‰), though significantly (P<0.05), over the observation period. Estimated intercellular CO₂ concentration increased (+56 μmol mol^?1) less than the atmospheric CO₂ concentration (+81 μmol mol^?1), so that intrinsic water‐use efficiency increased by 17.8% during the 69‐year period. However, the atmospheric evaporative demand at the site increased by approximately 0.1 kPa between 1955 and 2006, thus counteracting the improvement of intrinsic water‐use efficiency. As a result, instantaneous water‐use efficiency did not change. The observed changes in intrinsic water‐use efficiency were in the same range as those of trees (as reported by others), indicating that leaf‐level control of water‐use efficiency of grassland and forests followed the same principles. This is the first reconstruction of the water‐use efficiency response of a natural grassland ecosystem to last century CO₂ and climatic changes. The results indicate that the alpine grassland community has responded to climate change by improving the physiological control of carbon gain to water loss, following the increases in atmospheric CO₂ and evaporative demand. But, effective leaf‐level water‐use efficiency has remained unchanged.     

Gas exchange and water‐use efficiency in plant canopies

In this review, I first address the basics of gas exchange, water‐use efficiency and carbon isotope discrimination in C₃ plant canopies. I then present a case study of water‐use efficiency in northern Australian tree species. In general, C₃ plants face a trade‐off whereby increasing stomatal conductance for a given set of conditions will result in a higher CO₂ assimilation rate, but a lower photosynthetic water‐use efficiency. A common garden experiment suggested that tree species which are able to establish and grow in drier parts of northern Australia have a capacity to use water rapidly when it is available through high stomatal conductance, but that they do so at the expense of low water‐use efficiency. This may explain why community‐level carbon isotope discrimination does not decrease as steeply with decreasing rainfall on the North Australian Tropical Transect as has been observed on some other precipitation gradients. Next, I discuss changes in water‐use efficiency that take place during leaf expansion in C₃ plant leaves. Leaf phenology has recently been recognised as a significant driver of canopy gas exchange in evergreen forest canopies, and leaf expansion involves changes in both photosynthetic capacity and water‐use efficiency. Following this, I discuss the role of woody tissue respiration in canopy gas exchange and how photosynthetic refixation of respired CO₂ can increase whole‐plant water‐use efficiency. Finally, I discuss the role of water‐use efficiency in driving terrestrial plant responses to global change, especially the rising concentration of atmospheric CO₂. In coming decades, increases in plant water‐use efficiency caused by rising CO₂ are likely to partially mitigate impacts on plants of drought stress caused by global warming.     

The effect of soil drought on water-use efficiency in a contrasting Great Basin desert and Sierran montane species

Abstract. The effect of soil drought on water-use efficiency (WUE) and water relations was examined for potted Artemisia tridentata Nutt. and Pinus ponderosa Laws., a dominant Great Basin desert shrub and a Sierran montane tree, respectively. Before the onset of drought, A. tridentata had slightly higher photosynthetic rates than P. ponderosa and A. tridentata maintained positive photosynthetic rates at substantially lower water potentials (Ψ). Complete stomatal closure and cessation of photosynthesis occurred at a Ψ of ca. −2.5 MPa for P. ponderosa and less than −5.0 MPa for A. tridentata. Repeated drought cycles caused a small increase in bulk modulus of elasticity for A. tridentata and neither species exhibited significant osmotic adjustment. WUE was similar at Ψ≥−1.0 MPa but as Ψ decreased P. ponderosa consistently maintained higher WUE than A. tridentata. The primary factor contributing to higher WUE for P. ponderosa was the rapid decrease in stomatal conductance with decreasing Ψ. Comparatively low WUE for A. tridentata , a drought tolerant species, suggests that efficient use of water is a conservative ecophysiological 'strategy' that can be detrimental in a competitive water-limited environment. The combination of profligate use of water and a high degree of drought tolerance may be a more successful combination of physiological characteristics in certain dry habitats.     

Fire deficits have increased drought sensitivity in dry conifer forests: Fire frequency and tree‐ring carbon isotope evidence from Central Oregon

A century of fire suppression across the Western United States has led to more crowded forests and increased competition for resources. Studies of forest thinning or stand conditions after mortality events have provided indirect evidence for how competition can promote drought stress and predispose forests to severe fire and/or bark beetle outbreaks. Here, we demonstrate linkages between fire deficits and increasing drought stress through analyses of annually resolved tree‐ring growth, fire scars, and carbon isotope discrimination (Δ¹³C) across a dry mixed‐conifer forest landscape. Fire deficits across the study area have increased the sensitivity of leaf gas exchange to drought stress over the past >100 years. Since 1910, stand basal area in these forests has more than doubled and fire‐return intervals have increased from 25 to 140 years. Meanwhile, the portion of interannual variation in tree‐ring Δ¹³C explained by the Palmer Drought Severity Index has more than doubled in ca. 300–500‐year‐old Pinus ponderosa as well as in fire‐intolerant, ca. 90–190‐year‐old Abies grandis. Drought stress has increased in stands with a basal area of ≥25 m²/ha in 1910, as indicated by negative temporal Δ¹³C trends, whereas stands with basal area ≤25 m²/ha in 1910, due to frequent or intense wildfire activity in decades beforehand, were initially buffered from increased drought stress and have benefited more from rising ambient carbon dioxide concentrations, [CO₂], as demonstrated by positive temporal Δ¹³C trends. Furthermore, the average Δ¹³C response across all P. ponderosa since 1830 indicates that photosynthetic assimilation rates and stomatal conductance have been reduced by ~10% and ~20%, respectively, compared to expected trends due to increasing [CO₂]. Although disturbance legacies contribute to local‐scale intensity of drought stress, fire deficits have reduced drought resistance of mixed‐conifer forests and made them more susceptible to challenges by pests and pathogens and other disturbances.     

Differences in hydraulic architecture account for near-isohydric and anisohydric behaviour of two field-grown Vitis vinifera L. cultivars during drought

A comparative study on stomatal control under water deficit was conducted on grapevines of the cultivars Grenache, of Mediterranean origin, and Syrah of mesic origin, grown near Montpellier, France and Geisenheim, Germany. Syrah maintained similar maximum stomatal conductance (g_max) and maximum leaf photosynthesis (A_max) values than Grenache at lower predawn leaf water potentials, Ψ_leaf, throughout the season. The Ψ_leaf of Syrah decreased strongly during the day and was lower in stressed than in watered plants, showing anisohydric stomatal behaviour. In contrast, Grenache showed isohydric stomatal behaviour in which Ψ_leaf did not drop significantly below the minimum Ψ_leaf of watered plants. When g was plotted versus leaf specific hydraulic conductance, K_l, incorporating leaf transpiration rate and whole‐plant water potential gradients, previous differences between varieties disappeared both on a seasonal and diurnal scale. This suggested that isohydric and anisohydric behaviour could be regulated by hydraulic conductance. Pressure‐flow measurements on excised organs from plants not previously stressed revealed that Grenache had a two‐ to three‐fold larger hydraulic conductance per unit path length (K_h) and a four‐ to six‐fold larger leaf area specific conductivity (LSC) in leaf petioles than Syrah. Differences between internodes were only apparent for LSC and were much smaller. Cavitation detected as ultrasound acoustic emissions on air‐dried shoots showed higher rates for Grenache than Syrah during the early phases of the dry‐down. It is hypothesized that the differences in water‐conducting capacity of stems and especially petioles may be at the origin of the near‐isohydric and anisohydric behaviour of g.     

A dynamic leaf gas‐exchange strategy is conserved in woody plants under changing ambient CO2: evidence from carbon isotope discrimination in paleo and CO2 enrichment studies

Rising atmospheric [CO₂], c_a, is expected to affect stomatal regulation of leaf gas‐exchange of woody plants, thus influencing energy fluxes as well as carbon (C), water, and nutrient cycling of forests. Researchers have proposed various strategies for stomatal regulation of leaf gas‐exchange that include maintaining a constant leaf internal [CO₂], c_i, a constant drawdown in CO₂ (c_a ? c_i), and a constant c_i/c_a. These strategies can result in drastically different consequences for leaf gas‐exchange. The accuracy of Earth systems models depends in part on assumptions about generalizable patterns in leaf gas‐exchange responses to varying c_a. The concept of optimal stomatal behavior, exemplified by woody plants shifting along a continuum of these strategies, provides a unifying framework for understanding leaf gas‐exchange responses to c_a. To assess leaf gas‐exchange regulation strategies, we analyzed patterns in c_i inferred from studies reporting C stable isotope ratios (δ¹³C) or photosynthetic discrimination (?) in woody angiosperms and gymnosperms that grew across a range of c_a spanning at least 100 ppm. Our results suggest that much of the c_a‐induced changes in c_i/c_a occurred across c_a spanning 200 to 400 ppm. These patterns imply that c_a ? c_i will eventually approach a constant level at high c_a because assimilation rates will reach a maximum and stomatal conductance of each species should be constrained to some minimum level. These analyses are not consistent with canalization toward any single strategy, particularly maintaining a constant c_i. Rather, the results are consistent with the existence of a broadly conserved pattern of stomatal optimization in woody angiosperms and gymnosperms. This results in trees being profligate water users at low c_a, when additional water loss is small for each unit of C gain, and increasingly water‐conservative at high c_a, when photosystems are saturated and water loss is large for each unit C gain.     

The combined effects of a long‐term experimental drought and an extreme drought on the use of plant‐water sources in a Mediterranean forest

Vegetation in water‐limited ecosystems relies strongly on access to deep water reserves to withstand dry periods. Most of these ecosystems have shallow soils over deep groundwater reserves. Understanding the functioning and functional plasticity of species‐specific root systems and the patterns of or differences in the use of water sources under more frequent or intense droughts is therefore necessary to properly predict the responses of seasonally dry ecosystems to future climate. We used stable isotopes to investigate the seasonal patterns of water uptake by a sclerophyll forest on sloped terrain with shallow soils. We assessed the effect of a long‐term experimental drought (12 years) and the added impact of an extreme natural drought that produced widespread tree mortality and crown defoliation. The dominant species, Quercus ilex, Arbutus unedo and Phillyrea latifolia, all have dimorphic root systems enabling them to access different water sources in space and time. The plants extracted water mainly from the soil in the cold and wet seasons but increased their use of groundwater during the summer drought. Interestingly, the plants subjected to the long‐term experimental drought shifted water uptake toward deeper (10–35 cm) soil layers during the wet season and reduced groundwater uptake in summer, indicating plasticity in the functional distribution of fine roots that dampened the effect of our experimental drought over the long term. An extreme drought in 2011, however, further reduced the contribution of deep soil layers and groundwater to transpiration, which resulted in greater crown defoliation in the drought‐affected plants. This study suggests that extreme droughts aggravate moderate but persistent drier conditions (simulated by our manipulation) and may lead to the depletion of water from groundwater reservoirs and weathered bedrock, threatening the preservation of these Mediterranean ecosystems in their current structures and compositions.     

Stomatal responses of Douglas-fir seedlings to elevated carbon dioxide and temperature during the third and fourth years of exposure

Two major components of climate change, increasing atmospheric [CO₂] and increasing temperature, may substantially alter the effects of water availability to plants through effects on the rate of water loss from leaves. We examined the interactive effects of elevated [CO₂] and temperature on seasonal patterns of stomatal conductance (g_s), transpiration (E) and instantaneous transpiration efficiency (ITE) in Douglas‐fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings. Seedlings were grown in sunlit chambers at either ambient CO₂ (AC) or ambient + 180 µmol mol^?1 CO₂ (EC), and at ambient temperature (AT) or ambient + 3·5 °C (ET) in a full‐factorial design. Needle gas exchange at the target growth conditions was measured approximately monthly over 21 months. Across the study period and across temperature treatments, growth in elevated [CO₂] decreased E by an average of 12% and increased ITE by an average of 46%. The absolute reduction of E associated with elevated [CO₂] significantly increased with seasonal increases in the needle‐to‐air vapour pressure deficit (D). Across CO₂ treatments, growth in elevated temperature increased E an average of 37%, and did not affect ITE. Combined, growth in elevated [CO₂] and elevated temperature increased E an average of 19% compared with the ACAT treatment. The CO₂ supply and growth temperature did not significantly affect stomatal sensitivity to D or the relationship between g_s and net photosynthetic rates. This study suggests that elevated [CO₂] may not completely ameliorate the effect of elevated temperature on E, and that climate change may substantially alter needle‐level water loss and water use efficiency of Douglas‐fir seedlings.     

Gas exchange and water relations of spring wheat under full‐season infrared warming

Gas exchange and water relations were evaluated under full‐season in situ infrared (IR) warming for hard red spring wheat (Triticum aestivum L. cv. Yecora Rojo) grown in an open field in a semiarid desert region of the southwest USA. A temperature free‐air controlled enhancement (T‐FACE) apparatus utilizing IR heaters maintained canopy air temperature above 3.0 m Heated plots of wheat by 1.3 and 2.7 °C (0.2 and 0.3 °C below the targeted set‐points of Reference plots with dummy heaters) during daytime and nighttime, respectively. Control plots had no apparatus. Every 6 weeks during 2007–2009 wheat was sown under the three warming treatments (i.e., Control, Heated, Reference) in three replicates in a 3 × 3 Latin square (LSQ) design on six plantings during 4 months (i.e., January, March, September, December), or in a natural temperature variation treatment (i.e., Control) in three replicates in a randomized complete block (RCB) design on nine plantings during 7 months (i.e., January, February, April, June, July, August, October). Soil temperature (T_s) and volumetric soil‐water content (θ_s) were 1.3 °C warmer and 14% lower in Heated compared with Reference plots, respectively. Other than a 1% shading effect, no artifacts on gas exchange or water relations were associated with the IR warming apparatus. IR warming increased carbon gain characteristic of an increase in metabolic rates to higher temperature that may have been attributed to the well‐watered wheat crop and the supplemental irrigation that minimized plant‐to‐air water vapor pressure differences between IR‐warmed and nonwarmed plots. Nevertheless, seasonal oscillations in the IR warming response on carbon gain occurred. IR warming decreased leaf water status and provided thermal protection during freeze events. IR warming is an effective experimental methodology to investigate the impact of global climate change on agronomic cropping and natural ecosystems to a wide range of natural and artificially imposed air temperatures.