Drought resistance and recovery of photosystem II activity in a Mediterranean semi-deciduous shrub at the seedling stage

Phlomis fruticosa is one of the main species of post-fire Mediterranean communities, occupying areas with post-fire grazing or altered precipitation pattern and consequently water stress imposed on seedlings of co-occurring species. Seedlings of woody perennials often differ from their mature individuals regarding their resistance of photosynthetic performance to environmental stresses. Such differences have been reported for tree species but there is a lack of information regarding shrub species. In the present study, we tried to detect changes in (PSII) activity imposed by water stress in P. fruticosa seedlings as well as its capability for recovery after rehydration. Maximum PSII photochemical efficiency decreased only under severe water stress exactly as in mature plants in the field. However, leaf chlorophyll (Chl) content was almost stable regardless of leaf relative water content (RWC). We assume that the photoprotective chlorophyll loss process, reported for many mature Mediterranean species (including P. fruticosa), has not yet been developed at the seedling stage. On the other hand, photoprotection through an increase of the relative amount of non-Q_B-reducing centers was found during dehydration. Non-photochemical quenching (NPQ) contributed to protection from photodamage until moderate water stress but was significantly suppressed under severe water stress. Both processes were reversed after rehydration. The above characteristics enabled seedlings not only to survive during aggravating drought but also to maintain a considerable part of their effective quantum yield and perform significant electron transport even at extremely low relative water content (RWC). This was confirmed with measurements in a semi-natural environment (pots) and under real steady state conditions regarding adaptation of the photosynthetic machinery to prevailing light intensities.     

本地和外来草本物种对水分条件时间异质性的可塑响应

极端气候导致的干旱和水淹事件频发，影响了外来植物和本地植物的生长。为了解外来种和本地种植物对干旱和水淹事件发生顺序的响应，探讨草本植物适应水分时间异质性的策略，该文以美国蒙大拿州西部4种本地植物和4种外来植物为研究对象，将所有植物分别进行持续湿润(对照，CK)、水淹-干旱(I-D)和干旱-水淹(D-I)处理，并观测一系列形态和生物量特征的变化。结果表明：(1)与CK相比，D-I和I-D处理均显著降低了外来种的总生物量(P<0.05)。(2)D-I显著降低了本地种早期总生物量、后期地下生物量和根冠比，但显著提高了其后期的相对生长(P<0.05)。(3)D-I处理显著降低了所有植物的地下-地上生物量关系的异速指数，外来种异速指数显著高于本地种(P<0.05)。综上认为，极端事件(水淹和干旱)的发生顺序能改变外来植物和本地植物的生物量分配，早期干旱比后期干旱更容易减少植物生物量的积累，但能促进本地种后期的生长；本地种在环境胁迫下不被降低的总生物量表现说明维持表型稳定的能力较强；D-I处理下本地种和外来种地上和地下生物量关系的分配方式不同。     

Responses of leaf gas exchange, water relations, and water consumption in seedlings of four semiarid tree species to soil drying

To understand the response patterns to soil drying and the water use properties of commonly reforested trees in the semiarid Loess Plateau region of China, a glasshouse experiment was carried out with the seedlings of four species, i.e., Robinia pseudoacacia, Armeniaca sibirica, Syringa oblata, and Quercus liaotungensis. Severe water stress induced by withholding water resulted in permanent wilting of most of the seedlings pot-cultured with sandy soil in 8–12 days. Predawn and midday leaf water potentials and gas exchange characteristics (e.g., stomatal conductance) in the seedlings did not show marked changes until the volumetric soil water content decreased to about 0.05. As the soil water content decreased further, these physiological parameters rapidly declined, approaching their minimal levels at the stage of permanent wilting. The response of each parameter to soil water content changes was fitted with a non-linear saturation curve. Though the results suggested that the general pattern of responses to soil drying was identical among the species, quantitative differences in drought tolerance and water use properties were detected. Leaf stomatal conductance in R. pseudoacacia and A. sibirica showed earlier responses to reduced predawn leaf water potentials. However, water use characteristics and specific leaf area indicated that these two species consumed more water and may not be as drought tolerant as S. oblata and Q. liaotungensis. These results may provide important information to compare the reforestation species with respect to soil drying.     

Suitability of Drought-Preconditioning Techniques in Mediterranean Climate

Abstract Arid and semiarid ecosystems in the Mediterranean are under high risk of desertification. Revegetation with native well‐adapted evergreen shrubs is desirable, but techniques for successful establishment of these species are not fully developed. Transplant shock is a key hurdle to plantation success. The application of a drought‐preconditioning treatment during the last months of nursery culture is a potential technique for reducing transplant shock. This technique has been widely applied in boreal habitats and humid temperate areas. Three representative Mediterranean species (Pistacia lentiscus, Quercus coccifera, and Juniperus oxycedrus seedlings) were exposed to classic drought‐preconditioning treatment consisting of reductions in the watering regime. The effects of preconditioning on seedling quality were assessed by cell water relationships (pressure–volume curves), minimal transpiration, leaf capacitance, chlorophyll fluorescence, and gas exchange. Moreover, seedlings were exposed to transplant shock (intense drought period) during which water potential (predawn and midday) and maximal photochemical efficiency were evaluated to establish seedling performance. Results showed that preconditioning did not affect cell water relationships and minimal transpiration in any of the three species. Preconditioned seedlings of P. lentiscus maintained higher water content during desiccating conditions as a consequence of an increase in leaf water content at full turgor. These changes allowed plants to maintain higher net CO₂ assimilation rates and an elevated photosystem II status, facilitating an increase in drought survival. Preconditioning improved the performance of Q. coccifera and J. oxycedrus seedlings, but these two species were much less responsive than P. lentiscus seedlings. Finally, results suggest that sensitivity to drought preconditioning may be related to drought tolerance or avoidance strategy of each species. Drought‐related strategies should be considered to optimize management scale preconditioning.     

Responses of riparian trees to interannual variation in ground water depth in a semi-arid river basin

We investigated the physiological and growth responses of native (Populus fremontii S. Wats. and Salix gooddingii Ball) and exotic (Tamarix chinensis Lour.) riparian trees to ground water availability at the free‐flowing Hassayampa River, Arizona, during dry (1997) and wet (1998) years. In the drier year, all species experienced considerable water stress, as evidenced by low shoot water potentials, low leaf gas exchange rates and large amounts of canopy dieback. These parameters were significantly related to depth of ground water (DGW) in the native species, but not in T. chinensis, in 1997. Canopy dieback was greater in the native species than in T. chinensis when ground water was deep in 1997, and dieback increased rapidly at DGW > 2·5–3·0 m for the native species. Analysis of combined data from wet and dry years for T. chinensis tentatively suggests a similar physiological sensitivity to water availability and a similar DGW threshold for canopy dieback. In 1998, shoot water potential and leaf gas exchange rates were higher and canopy dieback was lower for all species because of increased water availability. However, T. chinensis showed a much larger increase in leaf gas exchange rates in the wet year than the native species. High leaf gas exchange rates, growth when water is abundant, drought tolerance and the maintenance of a viable canopy under dry conditions are characteristics that help explain the ability of T. chinensis to thrive in riparian ecosystems in the south‐western United States.     

Arthropod diversity of exotic vs. native Robinia species in northern Arizona

1 Arthropods were collected on native locust, Robinia neomexicana A. Gray, and exotic Robinia pseudoacacia L. in northern Arizona over a 2‐year period to determine the number of arthropod species and number of individuals present. 2 More arthropod species were found on the native (251) than on the exotic Robinia (174). 3 Greater species diversity was likewise found on the native than the exotic. The five most numerous insects collected each year accounted for 81% to 91% of the total number collected on the exotic and native Robinia in 1997 and 1998. Only 12 species occurred on both the native and exotic Robinia in both years. 4 These findings are discussed in the context of using exotic trees in plantations and ecological theory regarding rates of arthropod species accumulation on exotic hosts.     

Water flux and canopy conductance of natural versus planted forests in Patagonia, South America

In NW Patagonia, South America, natural shrublands and mixed forests of short Nothofagus antarctica (G. Forst.) Oerst. trees are currently being replaced by plantations with Pseudotsuga menziesii (Mirb) Franco. This land use change is controversial because the region is prone to drought, and replacement of native vegetation by planted forests may increase vegetation water use. The goal of this study was to examine the physiological differences, especially the response of water flux and canopy conductance to microclimate, that lead to greater water use by exotic trees compared to native trees. Meteorological variables and sapflow density of P. menziesii and four native woody species were measured in the growing season 2005–2006. Canopy conductance (gc) was estimated for both the exotic (monoculture) and native (multi-species) systems, including the individual contributions of each species of the native forest. Sapflow density, stand-level transpiration and gc were related to leaf-to-air vapor pressure difference (VPD). All native species had different magnitudes and diurnal patterns of sapflow density compared to P. menziesii, which could be explained by the different gc responses to VPD. Stomatal sensitivity to VPD suggested that all native species have a stronger stomatal control of leaf water potential and transpiration due to hydraulic limitations compared to P. menziesii. In conclusion, differences in water use between a P. menziesii plantation and a contiguous native mixed forest of similar basal area could be explained by different gc responses to VPD between species (higher sensitivity in the native species), in addition to particular characteristics of the native forest structure.     

Comparison of leaf decomposition and macroinvertebrate colonization between exotic and native trees in a freshwater ecosystem

One of the most important sources of energy in aquatic ecosystems is the allochthonous input of detritus. Replacement of native tree species by exotic ones affects the quality of detritus entering freshwater ecosystems. This replacement can alter nutrient cycles and community structure in aquatic ecosystems. The aims of our study were (1) to compare leaf litter decomposition of two widely distributed exotic species (Ailanthus altissima and Robinia pseudoacacia) with the native species they coexist with (Ulmus minor and Fraxinus angustifolia), and (2) to compare macroinvertebrate colonization among litters of the invasive and native species. Litter bags of the four tree species were placed in the water and collected every 2, 25, 39, 71, and 95 days in a lentic ecosystem. Additionally, the macroinvertebrate community on litter bags was monitored after 25, 39, and 95 days. Several leaf chemistry traits were measured at the beginning (% lignin; lignin:N, C:N, LMA) and during the study (leaf total nitrogen). We detected variable rates of decomposition among species (k values of 0.009, 0.008, 0.008, and 0.005 for F. angustifolia, U. minor, A. altissima and R. pseudoacacia, respectively), but we did not detect an effect of litter source (from native/exotic). In spite of its low decay, the highest leaf nitrogen was found in R. pseudoacacia litter. Macroinvertebrate communities colonizing litter bags were similar across species. Most of them were collectors (i.e., they feed on fine particulate organic matter), suggesting that leaf material of either invasive or native trees was used as substrate both for the animals and for the organic matter they feed on. Our results suggest that the replacement of the native Fraxinus by Robinia would imply a reduction in the rate of leaf processing and also a slower release of leaf nitrogen to water.     

Recovery of Populus tremuloides seedlings following severe drought causing total leaf mortality and extreme stem embolism

In contrast with other native Populus species in North America, Populus tremuloides (aspen) can successfully establish itself in drought‐prone areas, yet no comprehensive analysis has been performed on the ability of seedlings to withstand and recover from a severe drought resulting in complete leaf mortality. Here, we subjected 4‐month‐old aspen seedlings grown in two contrasting soil media to a progressive drought until total leaf mortality, followed by a rewatering cycle. Stomatal conductance (g_s), photosynthesis and transpiration followed a sigmoid decline with declining fraction of extractable soil water values. Cessation of leaf expansion occurred close to the end of the linear‐decrease phase, when g_s was reduced by 95%. Leaf mortality started after g_s reached the lowest values, which corresponded to a stem–xylem pressure potential (Ψ_xp) of ?2.0 MPa and a percent loss of stem hydraulic conductivity (PLC) of 50%. In plants with 50% leaf mortality, PLC values remained around 50%. Complete leaf mortality occurred at an average stem PLC of 90%, but all seedlings were able to resprout after 6–10 days of being rewatered. Plants decapitated at soil level before rewatering developed root suckers, while those left with a 4‐cm stump or with their stems intact resprouted exclusively from axillary buds. Resprouting was accompanied by recovery of stem hydraulic conductivity, with PLC values around 30%. The percentage of resprouted buds was negatively correlated with the stem %PLC. Thus, the recovery of stem hydraulic conductivity appears as an important factor in the resprouting capacity of aspen seedlings following a severe drought.     

Drought enhances symbiotic dinitrogen fixation and competitive ability of a temperate forest tree

General circulation models project more intense and frequent droughts over the next century, but many questions remain about how terrestrial ecosystems will respond. Of particular importance, is to understand how drought will alter the species composition of regenerating temperate forests wherein symbiotic dinitrogen (N₂)-fixing plants play a critical role. In experimental mesocosms we manipulated soil moisture to study the effect of drought on the physiology, growth and competitive interactions of four co-occurring North American tree species, one of which (Robinia pseudoacacia) is a symbiotic N₂-fixer. We hypothesized that drought would reduce growth by decreasing stomatal conductance, hydraulic conductance and increasing the water use efficiency of species with larger diameter xylem vessel elements (Quercus rubra, R. pseudoacacia) relative to those with smaller elements (Acer rubrum and Liriodendron tulipifera). We further hypothesized that N₂ fixation by R. pseudoacacia would decline with drought, reducing its competitive ability. Under drought, growth declined across all species; but, growth and physiological responses did not correspond to species’ hydraulic architecture. Drought triggered an 80 % increase in nodule biomass and N accrual for R. pseudoacacia, improving its growth relative to other species. These results suggest that drought intensified soil N deficiency and that R. pseudoacacia’s ability to fix N₂ facilitated competition with non-fixing species when both water and N were limiting. Under scenarios of moderate drought, N₂ fixation may alleviate the N constraints resulting from low soil moisture and improve competitive ability of N₂-fixing species, and as a result, supply more new N to the ecosystem.     

Seedling drought stress susceptibility in two deciduous Nothofagus species of NW Patagonia

The physiological capacities of seedlings to cope with drought may be subject to strong selective pressure in the context of future climate scenarios, threatening the regeneration and sustainability of forests. Characterization of the responses and the variability between species is of interest to breeding and domestication programs. In this study, our main goal was to describe some of the physiological mechanisms involved in the drought response of Nothofagus nervosa and N. obliqua, two forest species of ecological and commercial importance (high wood quality) in NW Patagonia. We tested for differences in water status, gas exchange and survival in response to a gradually imposed severe drought. Based on cavitation vulnerability curves and hydraulic conductivity measurements, we can conclude that N. obliqua stems have higher specific hydraulic conductivity and somewhat lower vulnerability to cavitation than N. nervosa stems, leading it to sustain higher stomatal conductance under non-severe drought conditions. N. obliqua had higher photosynthetic capacity than N. nervosa, due both to characteristics of its hydraulic architecture and to its higher metabolic capacity. Our results indicate that both species present characteristics of plants susceptible to water stress. Also, both species showed behavior resembling an anisohydric response. This behavior results from a lack of stomatal control over transpiration while the soil dehydrates, probably accompanied by very high vulnerability to cavitation. In contrast, both species had similar high stomatal sensitivity to vapor pressure deficit when soil water was limiting.     

不同干旱处理刺槐、侧柏光合特性与内源脱落酸含量的相关关系

为揭示干旱介导植物光合同化能力的内源脱落酸(ABA)调控机制,以北京地区典型树种刺槐和侧柏为研究对象,探讨土壤快速干旱-复水(T1)和慢速干旱-复水(T2)处理下,树木碳同化与ABA器官分配模式间相关关系。结果表明:(1)刺槐与侧柏在T1和T2干旱处理阶段的净光合速率(P_n)、气孔导度(G_s)、蒸腾速率(T_r)均随土壤体积含水量(SWC)下降显著降低(P<0.05)。T1两树种P_n降幅(99.18%和57.30%)大于T2(91.97%和36.69%),且T1、T2刺槐P_n、G_s降幅均大于侧柏。T1和T2旱后复水阶段,T1两树种P_n、G_s恢复程度大于T2,刺槐光合能力恢复至胁迫处理前水平,而侧柏仅部分恢复。(2)T1和T2两树种叶片、小枝、粗根、细根ABA均随SWC下降显著上升(P<0.05),且T2两树种全株ABA上升幅度(21.21%和26.01%)均大于T1(13.71%和8.91%)。T1和T2两树种粗根在旱后复水阶段ABA显著低于干旱阶段,其它器官ABA均维持干旱阶段水平。(3)探究两树种光合特性与ABA相关关系随土壤水分变化的规律发现,T1和T2处理下两树种光合参数与ABA含量呈显著负相关(P<0.05)。SWC下降速度越慢,两树种光合参数与器官ABA协同变化程度越高,且刺槐光合、ABA两者相关系数高于侧柏。综上所述,光合参数、器官ABA含量变幅及其二者相互关系程度均显著受到土壤干旱节律的影响作用,且存在种间特异性。研究旨在揭示植物应对不同干旱胁迫的碳同化与内源激素调控响应机制,为区域城市园林耐旱林木的选培与管理提供理论参考。     

Effectiveness of repeated autumn and spring fires for understorey restoration in weed‐invaded temperate eucalypt woodlands

Question. Can strategic burning, targeting differing ecological characteristics of native and exotic species, facilitate restoration of native understorey in weed‐invaded temperate grassy eucalypt woodlands? Location. Gippsland Plains, eastern Victoria, Australia. Methods. In a replicated, 5‐year experimental trial, the effects of repeated spring or autumn burning were evaluated for native and exotic plants in a representative, degraded Eucalyptus tereticornis grassy woodland. Treatments aimed to reduce seed banks and modify establishment conditions of exotic annual grasses, and to exhaust vegetative reserves of exotic perennial grasses. Treatments were applied to three grassland patch types, dominated by the native grass Austrodanthonia caespitosa, ubiquitous exotic annuals, or the common exotic perennial grass Paspalum dilatatum. Results. The dominant native grass Austrodanthonia caespitosa and native forbs were resilient to repeated fires, and target exotic annuals and perennials were suppressed differentially by autumn and spring fires. Exotic annuals were also suppressed by drought, reducing the overall treatment effects but indicating important opportunities for restoration. The initially sparse exotic geophyte Romulea rosea increased in cover with fire and the impact of this species on native forbs requires further investigation. There was minimal increase in diversity of subsidiary natives with fire, probably owing to lack of propagules. Conclusions. While fire is often considered to increase ecosystem invasibility, our study showed that strategic use of fire, informed by the relative responses of available native and exotic taxa, is potentially an effective step towards restoration of weed‐invaded temperate eucalypt woodlands.     

Elevated [CO2] does not ameliorate the negative effects of elevated temperature on drought‐induced mortality in Eucalyptus radiata seedlings

It has been reported that elevated temperature accelerates the time‐to‐mortality in plants exposed to prolonged drought, while elevated [CO₂] acts as a mitigating factor because it can reduce stomatal conductance and thereby reduce water loss. We examined the interactive effects of elevated [CO₂] and temperature on the inter‐dependent carbon and hydraulic characteristics associated with drought‐induced mortality in Eucalyptus radiata seedlings grown in two [CO₂] (400 and 640 μL L^?1) and two temperature (ambient and ambient +4 °C) treatments. Seedlings were exposed to two controlled drying and rewatering cycles, and then water was withheld until plants died. The extent of xylem cavitation was assessed as loss of stem hydraulic conductivity. Elevated temperature triggered more rapid mortality than ambient temperature through hydraulic failure, and was associated with larger water use, increased drought sensitivities of gas exchange traits and earlier occurrence of xylem cavitation. Elevated [CO₂] had a negligible effect on seedling response to drought, and did not ameliorate the negative effects of elevated temperature on drought. Our findings suggest that elevated temperature and consequent higher vapour pressure deficit, but not elevated [CO₂], may be the primary contributors to drought‐induced seedling mortality under future climates.     

Dispersal and recruitment limitation in native versus exotic tree species: life‐history strategies and Janzen‐Connell effects

Life‐history traits of invasive exotic plants are typically considered to be exceptional vis‐à‐vis native species. In particular, hyper‐fecundity and long range dispersal are regarded as invasive traits, but direct comparisons with native species are needed to identify the life‐history stages behind invasiveness. Until recently, this task was particularly problematic in forests as tree fecundity and dispersal were difficult to characterize in closed stands. We used inverse modelling to parameterize fecundity, seed dispersal and seedling dispersion functions for two exotic and eight native tree species in closed‐canopy forests in Connecticut, USA. Interannual variation in seed production was dramatic for all species, with complete seed crop failures in at least one year for six native species. However, the average per capita seed production of the exotic Ailanthus altissima was extraordinary: > 40 times higher than the next highest species. Seed production of the shade tolerant exotic Acer platanoides was average, but much higher than the native shade tolerant species, and the density of its established seedlings (≥ 3 years) was higher than any other species. Overall, the data supported a model in which adults of native and exotic species must reach a minimum size before seed production occurred. Once reached, the relationship between tree diameter and seed production was fairly flat for seven species, including both exotics. Seed dispersal was highly localized and usually showed a steep decline with increasing distance from parent trees: only Ailanthus altissima and Fraxinus americana had mean dispersal distances > 10 m. Janzen‐Connell patterns were clearly evident for both native and exotic species, as the mode and mean dispersion distance of seedlings were further from potential parent trees than seeds. The comparable intensity of Janzen‐Connell effects between native and exotic species suggests that the enemy escape hypothesis alone cannot explain the invasiveness of these exotics. Our study confirms the general importance of colonization processes in invasions, yet demonstrates how invasiveness can occur via divergent colonization strategies. Dispersal limitation of Acer platanoides and recruitment limitation of Ailanthus altissima will likely constitute some limit on their invasiveness in closed‐canopy forests.     

Co‐occurring woody species have diverse hydraulic strategies and mortality rates during an extreme drought

From 2011 to 2013, Texas experienced its worst drought in recorded history. This event provided a unique natural experiment to assess species‐specific responses to extreme drought and mortality of four co‐occurring woody species: Quercus fusiformis, Diospyros texana, Prosopis glandulosa, and Juniperus ashei. We examined hypothesized mechanisms that could promote these species' diverse mortality patterns using postdrought measurements on surviving trees coupled to retrospective process modelling. The species exhibited a wide range of gas exchange responses, hydraulic strategies, and mortality rates. Multiple proposed indices of mortality mechanisms were inconsistent with the observed mortality patterns across species, including measures of the degree of iso/anisohydry, photosynthesis, carbohydrate depletion, and hydraulic safety margins. Large losses of spring and summer whole‐tree conductance (driven by belowground losses of conductance) and shallower rooting depths were associated with species that exhibited greater mortality. Based on this retrospective analysis, we suggest that species more vulnerable to drought were more likely to have succumbed to hydraulic failure belowground.     

黄土高原羊圈沟小流域人工物种和自然物种径向生长对气候变化的响应差异

为揭示黄土高原人工和自然物种径向生长对气候变化的响应差异,在延安羊圈沟小流域分别获取人工和自然物种的树木年轮材料并构建标准年表,其中人工物种为刺槐(Robinia pseudoacacia)和柠条(Caragana korshinskii),自然物种为山杏(Armeniaca sibirica)和荆条(Vitex negundo var.heterophylla),并对年表中的气候信号进行了统计分析。结果表明:1)人工物种年表中的气候信号较强,主要表现在5—8月份,与温度呈负相关关系(刺槐:r=-0.427—-0.511,P0.05;柠条:r=-0.227—-0.738,P0.05),与降雨则呈正相关关系,但相关系数未达到显著性水平;自然物种年表中的气候信号较弱,与温度和降雨的相关关系均较低;2)不同于自然物种,人工物种树轮年表还与去年夏季(7—9月份)温度(负相关)和降水(正相关)存在相关关系,表明人工物种树木生长对气候因子存在一定滞后性;3)人工物种树轮年表与PDSI干旱指数在各月份均维持正相关关系,在生长季(刺槐4—9月、柠条4—8月)达到显著水平(刺槐:r=0.481—0.704,P0.05;柠条:r=0.314—0.610,P0.05);而自然物种年表与PDSI干旱指数的相关关系较弱,均未达到相关性水平。从各年表与气候要素(温度、降雨、PDSI)响应强度来看,黄土高原人工物种树木生长受水分胁迫显著,且以刺槐最为明显,其次是柠条;自然物种树木生长则没有明显干旱胁迫的影响,仅山杏生长受一定水分胁迫影响,荆条生长则与各气候要素关系较弱,水分胁迫对其生长的影响已很小。本研究的结果表明黄土高原人工物种生长明显受到水分条件限制,而自然恢复物种生长则受水分条件影响较小,能适应黄土高原干旱半干旱气候条件。     

Responses of two semiarid conifer tree species to reduced precipitation and warming reveal new perspectives for stomatal regulation

Relatively anisohydric species are predicted to be more predisposed to hydraulic failure than relatively isohydric species, as they operate with narrower hydraulic safety margins. We subjected co‐occurring anisohydric Juniperus monosperma and isohydric Pinus edulis trees to warming, reduced precipitation, or both, and measured their gas exchange and hydraulic responses. We found that reductions in stomatal conductance and assimilation by heat and drought were more frequent during relatively moist periods, but these effects were not exacerbated in the combined heat and drought treatment. Counter to expectations, both species exhibited similar g_s temporal dynamics in response to drought. Further, whereas P. edulis exhibited chronic embolism, J. monosperma showed very little embolism due to its conservative stomatal regulation and maintenance of xylem water potential above the embolism entry point. This tight stomatal control and low levels of embolism experienced by juniper refuted the notion that very low water potentials during drought are associated with loose stomatal control and with the hypothesis that anisohydric species are more prone to hydraulic failure than isohydric species. Because direct association of stomatal behaviour with embolism resistance can be misleading, we advocate consideration of stomatal behaviour relative to embolism resistance for classifying species drought response strategies.     

Rooting depth,water relations and non‐structural carbohydrate dynamics in three woody angiosperms differentially affected by an extreme summer drought

In 2012, an extreme summer drought induced species‐specific die‐back in woody species in Northeastern Italy. Quercus pubescens and Ostrya carpinifolia were heavily impacted, while Prunus mahaleb was largely unaffected. By comparing seasonal changes in isotopic composition of xylem sap, rainfall and deep soil samples, we show that P. mahaleb has a deeper root system than the other two species. This morphological trait allowed P mahaleb to maintain higher water potential (Ψ), gas exchange rates and non‐structural carbohydrates content (NSC) throughout the summer, when compared with the other species. More favourable water and carbon states allowed relatively stable maintenance of stem hydraulic conductivity (k) throughout the growing season. In contrast, in Quercus pubescens and Ostrya carpinifolia, decreasing Ψ and NSC were associated with significant hydraulic failure, with spring‐to‐summer k loss averaging 60%. Our data support the hypothesis that drought‐induced tree decline is a complex phenomenon that cannot be modelled on the basis of single predictors of tree status like hydraulic efficiency, vulnerability and carbohydrate content. Our data highlight the role of rooting depth in seasonal progression of water status, gas exchange and NSC, with possible consequences for energy‐demanding mechanisms involved in the maintenance of vascular integrity.     

Robinia pseudoacacia as a surrogate for native tree species for saproxylic beetles inhabiting the riparian mixed forests of northern Italy

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