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Compound hot–dry events cause more severe impacts on terrestrial ecosystems than dry events, while the differences in recovery time (ΔRT) between hot–dry and dry events and their contributing factors remain unclear. Both remote sensing observations and eddy covariance measurements reveal that hot–dry events prolong the recovery time compared with dry events, with greater prolongation of recovery time in drylands than in humid regions. Random forest regression modeling demonstrates that the difference in vapor pressure deficit between hot–dry and dry events, with an importance score of 35%, is the major factor contributing to ΔRT. The severity of stomatal restriction exceeds that of non-stomatal limitation, which restricts the vegetation productivity that is necessary for the recovery process. These results emphasize the negative effect of vapor pressure deficit on vegetation recovery during hot–dry events and project an extension of drought recovery time considering elevated vapor pressure deficit in a warming world. [ABSTRACT FROM AUTHOR]

Water-use strategies play a crucial role in the adaptive capabilities of mangroves to the saline intertidal conditions, yet the intricacies of daily water-use patterns in mangrove species, which are pivotal for maintaining water balance, remain poorly understood. In this comprehensive study, we aimed to clarify the water use strategies of three co-occurring mangrove species, Avicennia marina , Aegiceras corniculatum and Kandelia obovata , through stem sap flow monitoring, leaf gas exchange and stem diameter change measurements. Our findings revealed that the daily sap flow density of Avicennia and Aegiceras reached the peak about 1 h earlier than that of Kandelia. When transpiration was strong, Kandelia and Aegiceras used stem storage to meet water demand, while Avicennia synchronized stem water storage. These three mangrove species adopted cross-peak water used and unique stem water storage to regulate their water balance. In Kandelia , the daily sap flow in per sapwood area was significantly lower, while water-use efficiency was significantly higher than those of Avicennia and Aegiceras , indicating that Kandelia adopted a more conservative and efficient water-use strategy. Sap flow in Avicennia was the most sensitive to environmental changes, while Kandelia limited water dissipation by tightly controlling stomata. Meteorological factors (photosynthetically active radiation, vapor pressure deficit and air temperature) were the main driving factors of sap flow. The increase of soil temperature can promote the water use of mangrove species, while the increase of salinity resulted in more conservative water use. Our results highlight the diversity of daily water-use strategies among the three co-occurring mangrove species, pinpointing Kandelia as the most adaptive at navigating the changing conditions of intertidal habitats in the future climate. In conclusion, our findings provide a mesoscale perspective on water-use characteristics of mangroves and also provides theoretical basis for mangroves afforestation and ecological restoration. [ABSTRACT FROM AUTHOR]

Increases in hydrological extremes, including drought, are expected for Amazon forests. A fundamental challenge for predicting forest responses lies in identifying ecological strategies which underlie such responses. Characterization of species-specific hydraulic strategies for regulating water-use, thought to be arrayed along an 'isohydric–anisohydric' spectrum, is a widely used approach. However, recent studies have questioned the usefulness of this classification scheme, because its metrics are strongly influenced by environments, and hence can lead to divergent classifications even within the same species. Here, we propose an alternative approach positing that individual hydraulic regulation strategies emerge from the interaction of environments with traits. Specifically, we hypothesize that the vertical forest profile represents a key gradient in drought-related environments (atmospheric vapor pressure deficit, soil water availability) that drives divergent tree water-use strategies for coordinated regulation of stomatal conductance (gs) and leaf water potentials (ΨL) with tree rooting depth, a proxy for water availability. Testing this hypothesis in a seasonal eastern Amazon forest in Brazil, we found that hydraulic strategies indeed depend on height-associated environments. Upper canopy trees, experiencing high vapor pressure deficit (VPD), but stable soil water access through deep rooting, exhibited isohydric strategies, defined by little seasonal change in the diurnal pattern of gs and steady seasonal minimum ΨL. In contrast, understory trees, exposed to less variable VPD but highly variable soil water availability, exhibited anisohydric strategies, with fluctuations in diurnal gs that increased in the dry season along with increasing variation in ΨL. Our finding that canopy height structures the coordination between drought-related environmental stressors and hydraulic traits provides a basis for preserving the applicability of the isohydric-to-anisohydric spectrum, which we show here may consistently emerge from environmental context. Our work highlights the importance of understanding how environmental heterogeneity structures forest responses to climate change, providing a mechanistic basis for improving models of tropical ecosystems. [ABSTRACT FROM AUTHOR]

The water deficit experienced by crops is a function of atmospheric water demand (vapor pressure deficit) and soil water supply over the whole crop cycle. We summarize typical transpiration response patterns to soil and atmospheric drying and the sensitivity to plant hydraulic traits. We explain the transpiration response patterns using a soil–plant hydraulic framework. In both cases of drying, stomatal closure is triggered by limitations in soil–plant hydraulic conductance. However, traits impacting the transpiration response differ between the two drying processes and act at different time scales. A low plant hydraulic conductance triggers an earlier restriction in transpiration during increasing vapor pressure deficit. During soil drying, the impact of the plant hydraulic conductance is less obvious. It is rather a decrease in the belowground hydraulic conductance (related to soil hydraulic properties and root length density) that is involved in transpiration down-regulation. The transpiration response to increasing vapor pressure deficit has a daily time scale. In the case of soil drying, it acts on a seasonal scale. Varieties that are conservative in water use on a daily scale may not be conservative over longer time scales (e.g. during soil drying). This potential independence of strategies needs to be considered in environment-specific breeding for yield-based drought tolerance. [ABSTRACT FROM AUTHOR]

Despite the mounting attention being paid to vegetation growth and their driving forces for water-limited ecosystems, the relative contributions of atmospheric and soil moisture dryness stress on vegetation growth are an ongoing debate. Here we comprehensively compare the impacts of high vapor pressure deficit (VPD) and low soil water content (SWC) on vegetation growth in Eurasian drylands during 1982–2014. The analysis indicates a gradual decoupling between atmospheric dryness and soil dryness over this period, as the former has expanded faster than the latter. Moreover, the VPD–SWC relation and VPD–greenness relation are both non-linear, while the SWC–greenness relation is near-linear. The loosened coupling between VPD and SWC, the non-linear correlations among VPD–SWC-greenness and the expanded area extent in which SWC acts as the dominant stress factor all provide compelling evidence that SWC is a more influential stressor than VPD on vegetation growth in Eurasian drylands. In addition, a set of 11 Earth system models projected a continuously growing constraint of SWC stress on vegetation growth towards 2100. Our results are vital to dryland ecosystems management and drought mitigation in Eurasia. [ABSTRACT FROM AUTHOR]

Interannual variability of the terrestrial ecosystem carbon sink is substantially regulated by various environmental variables and highly dominates the interannual variation of atmospheric carbon dioxide (CO2) concentrations. Thus, it is necessary to determine dominating factors affecting the interannual variability of the carbon sink to improve our capability of predicting future terrestrial carbon sinks. Using global datasets derived from machine-learning methods and process-based ecosystem models, this study reveals that the interannual variability of the atmospheric vapor pressure deficit (VPD) was significantly negatively correlated with net ecosystem production (NEP) and substantially impacted the interannual variability of the atmospheric CO2 growth rate (CGR). Further analyses found widespread constraints of VPD interannual variability on terrestrial gross primary production (GPP), causing VPD to impact NEP and CGR. Partial correlation analysis confirms the persistent and widespread impacts of VPD on terrestrial carbon sinks compared to other environmental variables. Current Earth system models underestimate the interannual variability in VPD and its impacts on GPP and NEP. Our results highlight the importance of VPD for terrestrial carbon sinks in assessing ecosystems' responses to future climate conditions. [ABSTRACT FROM AUTHOR]

Direct selection for yield under drought has resulted in the release of a number of drought-tolerant rice varieties across Asia. In this study, we characterized the physiological traits that have been affected by this strategy in breeding trials across sites in Bangladesh, India, and Nepal. Drought- breeding lines and drought-tolerant varieties showed consistently longer flag leaves and lower stomatal density than our drought-susceptible check variety, IR64. The influence of environmental parameters other than drought treatments on leaf traits was evidenced by close grouping of treatments within a site. Flag-leaf length and width appeared to be regulated by different environmental parameters. In separate trials in the Philippines, the same breeding lines studied in South Asia showed that canopy temperature under drought and harvest index across treatments were most correlated with grain yield. Both atmospheric and soil stress strengthened the relationships between leaf traits and yield. The stable expression of leaf traits among genotypes and the identification of the environmental conditions in which they contribute to yield, as well as the observation that some breeding lines showed longer time to flowering and higher canopy temperature than IR64, suggest that selection for additional physiological traits may result in further improvements of this breeding pool. [ABSTRACT FROM AUTHOR]

Photosynthetic carbon uptake by tropical forests is of critical importance in regulating the earth's climate, but rising temperatures threaten this stabilizing influence of tropical forests. Most research on how temperature affects photosynthesis focuses on fully sun-exposed leaves, and little is known about shade leaves, even though shade leaves greatly outnumber sun leaves in lowland tropical forests. We measured temperature responses of light-saturated photosynthesis, stomatal conductance, and the biochemical parameters V CMax (maximum rate of RuBP carboxylation) and J Max (maximum rate of RuBP regeneration, or electron transport) on sun and shade leaves of mature tropical trees of three species in Panama. As expected, biochemical capacities and stomatal conductance were much lower in shade than in sun leaves, leading to lower net photosynthesis rates. However, the key temperature response traits of these parameters—the optimum temperature (T Opt) and the activation energy—did not differ systematically between sun and shade leaves. Consistency in the J Max to V CMax ratio further suggested that shade leaves are not acclimated to lower temperatures. For both sun and shade leaves, stomatal conductance had the lowest temperature optimum (~25  ° C), followed by net photosynthesis (~30 °C), J Max (~34  ° C) and V CMax (~38  ° C). Stomatal conductance of sun leaves decreased more strongly with increasing vapor pressure deficit than that of shade leaves. Consistent with this, modeled stomatal limitation of photosynthesis increased with increasing temperature in sun but not shade leaves. Collectively, these results suggest that modeling photosynthetic carbon uptake in multi-layered canopies does not require independent parameterization of the temperature responses of the biochemical controls over photosynthesis of sun and shade leaves. Nonetheless, to improve the representation of the shade fraction of carbon uptake dynamics in tropical forests, better understanding of stomatal sensitivity of shade leaves to temperature and vapor pressure deficit will be required. [ABSTRACT FROM AUTHOR]

Drought frequency is increasing in many parts of the world and may enhance tree decline and mortality. The underlying physiological mechanisms are poorly understood, however, particularly regarding chronic effects of long-term drought and the response to increasing temperature and vapor pressure deficit (VPD). We combined analyses of radial growth and stable carbon isotope ratios (δ13C) in tree rings in a mature Scots pine (Pinus sylvestris L.) forest over the 20th century to elucidate causes of tree mortality in one of the driest parts of the European Alps (Pfynwald, Switzerland). We further compared trees that have recently died with living trees in a 10-year irrigation experiment, where annual precipitation was doubled. We found a sustained growth increase and immediate depletion of δ13C values for irrigated trees, indicating higher stomatal conductance and thus indeed demonstrating that water is a key limiting factor for growth. Growth of the now-dead trees started declining in the mid-1980s, when both mean temperature and VPD increased strongly. But growth of these trees was reduced to some extent already several decades earlier, while intrinsic water-use efficiency derived from δ13C values was higher. This indicates a more conservative water-use strategy compared with surviving trees, possibly at the cost of low carbon uptake and long-term reduction of the needle mass. We observed reduced climatic sensitivity of raw tree-ring δ13C for the now-dead in contrast to surviving trees, indicating impaired stomatal regulation, although this difference between the tree groups was smaller after detrending the data. Higher autocorrelation and a lower inter-annual δ13C variability of the now-dead trees further indicates a strong dependence on (low) carbon reserves. We conclude that the recent increase in atmospheric moisture demand in combination with insufficient soil water supply was the main trigger for mortality of those trees that were weakened by long-term reduced carbon uptake. [ABSTRACT FROM AUTHOR]

Atmospheric vapor pressure deficit (VPD) is a key component of drought and has a strong influence on yields. Whole-plant transpiration rate (TR) response to increasing VPD has been linked to drought tolerance in wheat, but because of its challenging phenotyping, its genetic basis remains unexplored. Further, the genetic control of other key traits linked to daytime TR such as leaf area, stomata densities and -- more recently -- nocturnal transpiration remains unknown. Considering the presence of wheat phenology genes that can interfere with drought tolerance, the aim of this investigation was to identify at an enhanced resolution the genetic basis of the above traits while investigating the effects of phenology genes Ppd-D1 and Ppd-B1. Virtually all traits were highly heritable (heritabilities from 0.61 to 0.91) and a total of mostly trait-specific 68 QTL were detected. Six QTL were identified for TR response to VPD, with one QTL (QSLP.ucl-5A) individually explaining 25.4% of the genetic variance. This QTL harbored several genes previously reported to be involved in ABA signaling, interaction with DREB2A and root hydraulics. Surprisingly, nocturnal TR and stomata densities on both leaf sides were characterized by highly specific and robust QTL. In addition, negative correlations were found between TR and leaf area suggesting trade-offs between these traits. Further, Ppd-D1 had strong but opposite effects on these traits, suggesting an involvement in this trade-off. Overall, these findings revealed novel genetic resources while suggesting a more direct role of phenology genes in enhancing wheat drought tolerance. [ABSTRACT FROM AUTHOR]

'White oaks'—one of the main groups of the genus Quercus L.—are represented in western Eurasia by the 'roburoid oaks', a deciduous and closely related genetic group that should have an Arcto-Tertiary origin under temperate-nemoral climates. Nowadays, roburoid oak species such as Quercus robur L. are still present in these temperate climates in Europe, but others are also present in southern Europe under Mediterranean-type climates, such as Quercus faginea Lam. We hypothesize the existence of a coordinated functional response at the whole-shoot scale in Q. faginea under Mediterranean conditions to adapt to more xeric habitats. The results reveal a clear morphological and physiological segregation between Q. robur and Q. faginea, which constitute two very contrasting functional types in response to climate dryness. The most outstanding divergence between the two species is the reduction in transpiring area in Q. faginea, which is the main trait imposed by the water deficit in Mediterranean- type climates. The reduction in leaf area ratio in Q. faginea should have a negative effect on carbon gain that is partially counteracted by a higher inherent photosynthetic ability of Q. faginea when compared with Q. robur, as a consequence of higher mesophyll conductance, higher maximum velocity of carboxylation and much higher stomatal conductance (gs). The extremely high gs of Q. faginea counteracts the expected reduction in gs imposed by the stomatal sensitivity to vapor pressure deficit, allowing this species to diminish water losses maintaining high net CO2 assimilation values along the vegetative period under nonlimiting soil water potential values. In conclusion, the present study demonstrates that Q. faginea can be regarded as an example of adaptation of a deciduous oak to Mediterranean-type climates. [ABSTRACT FROM AUTHOR]

To understand water-use strategies of woody bamboo species, sap flux density (Fd) in the culms of a woody bamboo (Bambusa vulgaris Schrader ex Wendland) was monitored using the thermal dissipation method. The daytime and night-time Fd were analyzed in the dry and rainy seasons. Additionally, diurnal changes in root pressure, culm circumference, and stomatal conductance (gs) were investigated to characterize the mechanisms used to maintain diurnal water balance of woody bamboos. Both in the dry and rainy seasons, daytime Fd responded to vapor pressure deficit (VPD) in an exponential fashion, with a fast initial increase in Fd when VPD increased from 0 to 1 kPa. The Fd and gs started to increase very fast as light intensity and VPD increased in the morning, but they decreased sharply once the maximum value was achieved. The Fd response of this woody bamboo to VPD was much faster than that of representative trees and palms growing in the same study site, suggesting its fast sap flow and stomatal responses to changes in ambient environmental factors. The Fd in the lower and higher culm positions started to increase at the same time in the morning, but the Fd in the higher culm position was higher than that of the lower culm in the afternoon. Consistently, distinct decreases in its culm circumference in the afternoon were detected. Therefore, unlike trees, water storage of bamboo culms was not used for its transpiration in the morning but in the afternoon. Nocturnal sap flow of this woody bamboo was also detected and related to root pressure. We conclude that this bamboo has fast sap flow/stomatal responses to irradiance and evaporative demands, and it uses substantial water storage for transpiration in the afternoon, while root pressure appears to be a mechanism resulting in culm water storage recharge during the night. [ABSTRACT FROM AUTHOR]

Plant species living in the understory increase carbon (C) allocation toward leaf production for maximizing light capture at the expense of roots and stems, with negative consequences for the whole-plant hydraulic conductance. Moreover, under some conditions, the high atmospheric evaporative demand occurring in Mediterranean areas may be not well buffered by the canopy, which might be the case for relict conifer Abies pinsapo Boiss. growing in the forest understory. We hypothesized that acclimation to combined understory shade and high atmospheric dryness can be achieved through the adjustment of water losses to cope with the restriction in water transport. The results reveal high structural plasticity in A. pinsapo that allows light harvesting of this species to maximize light capture in the forest understory, and maintain a positive C balance under low light conditions. However, growth in the understory resulted in reduced leaf-specific conductivity, up to approximately four to five times, implying decreased plant capacity to supply water to the leaves. In order to cope with the high atmospheric evaporative demand in the understory, there is an adjustment of the stomatal conductance to the hydraulic conductivity by means of a reduction in the stomatal density in understory individuals, which is due to the almost complete lack of stomata in the adaxial side of the needles. To the extent of our knowledge, such a drastic phenotypic response found in a conifer when growing under shaded conditions had not been previously reported. [ABSTRACT FROM PUBLISHER]

Stomata control tree transpiration by sensing and integrating environmental signals originating in the atmosphere and soil, and co-occurring species may differ in inherent stomatal sensitivity to these above- and belowground signals and in the types of signals to which they respond. Stomatal responsiveness to environmental signals is likely to differ across species having different types of wood (e.g., ring-porous, diffuse-porous and coniferous) because each wood type differs in the structure, size and spatial distribution of its xylem conduits as well as in the scaling of hydraulic properties with stem diameter. The objective of this study was to evaluate the impact of variation in soil water availability and atmospheric evaporative demand on stomatal regulation of transpiration in seven co-occurring temperate deciduous forest species representing three wood types. We measured whole-tree sap flux and soil and atmospheric variables in a mixed deciduous forest in central Pennsylvania over the course of a growing season characterized by severe drought and large fluctuations in atmospheric vapor pressure deficit (D). The relative sensitivity of sap flux to soil drying was ~2.2-2.3 times greater in the diffuse-porous and coniferous species than in the ring-porous species. Stomata of the ring-porous oaks were only about half as responsive to increased D as those of trees of the other two wood types. These differences in responsiveness to changes in the belowand aboveground environment implied that regulation of leaf water potential in the ring-porous oaks was less stringent than that in the diffuse-porous angiosperms or the conifers. The results suggest that increases in the frequency or intensity of summer droughts in the study region could have multiple consequences for forest function, including altered successional time courses or climax species composition and cumulative effects on whole-tree architecture, resulting in a structural and physiological legacy that would restrict the ability of trees to respond rapidly to more favorable growth conditions. [ABSTRACT FROM AUTHOR]

In this study, we employ a network of thermal dissipation probes (TDPs) monitoring sap flux density to estimate leaf-specific transpiration (EL) and stomatal conductance (GS) in Pinus taeda (L.) and Liquidambar styraciflua L. exposed to +200 ppm atmospheric CO2 levels (eCO2) and nitrogen fertilization. Scaling half-hourly measurements from hundreds of sensors over 11 years, we found that P. taeda in eCO2 intermittently (49% of monthly values) decreased stomatal conductance (GS) relative to the control, with a mean reduction of 13% in both total EL and mean daytime GS. This intermittent response was related to changes in a hydraulic allometry index (AH), defined as sapwood area per unit leaf area per unit canopy height, which decreased a mean of 15% with eCO2 over the course of the study, due mostly to a mean 19% increase in leaf area (AL). In contrast, L. styraciflua showed a consistent (76% of monthly values) reduction in GS with eCO2 with a total reduction of 32% EL, 31% GS and 23% AH (due to increased AL per sapwood area). For L. styraciflua, like P. taeda, the relationship between AH and GS at reference conditions suggested a decrease in GS across the range of AH. Our findings suggest an indirect structural effect of eCO2 on GS in P. taeda and a direct leaf level effect in L. styraciflua. In the initial year of fertilization, P. taeda in both CO2 treatments, as well as L. styraciflua in eCO2, exhibited higher GS with NF than expected from shifts in AH, suggesting a transient direct effect on GS. Whether treatment effects on mean leaf-specific GS are direct or indirect, this paper highlights that long-term treatment effects on GS are generally reflected in AH as well. [ABSTRACT FROM AUTHOR]

Vineyards were planted in the arid region of northwest China to meet the local economic strategy while reducing agricultural water use. Sap flow, environmental variables, a plant characteristic (sapwood-to-leaf area ratio, As/Al) and a canopy characteristic (leaf area index, L) were measured in a vineyard in the region during the growing season of 2009, and hourly canopy stomatal conductance (Gsi) was estimated for individual vines to quantify the relationships between Gsi and these variables. After accounting for the effects of vapor pressure deficit (D) and solar radiation (Rs) on Gsi, much of the remaining variation of reference Gsi (GsiR) was driven by that of leaf-specific hydraulic conductivity, which in turn was driven by that of As/Al. After accounting for that effect on GsiR, appreciable temporal variation remained in the decline rate of GsiR with decreasing vineyard-averaged relative extractable soil water (θE). This variation was related to the differential decline ofθE near each monitored vine, decreasing faster between irrigation events near vines where L was greater, thus adding to the spatiotemporal variation of GsiR observed in the vineyard. We also found that the vines showed isohydric-like behavior whenθE was low, but switched to anisohydric-like behavior with increasingθE. ModeledθE and associated Gs of a canopy with even L (1.9 m2 m−2) were greater than that of the same average L but split between the lowest and highest L observed along sections of rows in the vineyard (1.2 and 2.6 m2 m−2) by 6 and 12%, respectively. Our results suggest that managing sectional L near the average, rather than allowing a wide variation, can reduce soil water depletion, maintaining Gs higher, thus potentially enhancing yield. [ABSTRACT FROM PUBLISHER]

Whiteflies cause major agricultural problems in environments ranging from arid to humid climates on a global scale. A study was conducted on the effects of vapor pressure deficit (VPD) on oviposition, hatching, body size, and survival of immature B-biotype sweetpotato whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae). Experiments were conducted at VPD conditions of 0.5, 1.7, and 2.7 kPa to represent low, medium, and high VPD environments, respectively. The study was conducted at 26°C with insects reared on collard, Brassica oleracea ssp. acephala de Condolle. The rate of oviposition was decreased at the highest VPD treatment. However, percentage egg hatch was not significantly affected by the treatments. More time was required for eggs to hatch at a high (2.7 kPa) compared with a low (0.3 kPa) VPD in a separate experiment. Overall survival to the adult stage was decreased at the highest VPD (≈ 50%) compared with the other two VPD environments (≈75%). Sizes of males and females were different, as is well known, but VPD had little significant effect on adult body size (length and width). The size effect was erased when the F2 generations from the different treatments were reared under a common environment. Insect population models that include the most relevant environmental parameters can offer the best estimation of life-history events. These results help elucidate the ecology of B. tabaci and indicate that extremes in ambient moisture can have a impact on populations. [ABSTRACT FROM AUTHOR]

We investigated effects of heterogeneous stomatal behavior on diurnal patterns of leaf gas exchange in 10 tree species. Observations were made in middle and upper canopy layers of potted tropical rainforest trees in a nursery at the Forest Research Institute Malaysia. Measurements were taken from 29 January to 3 February 2010. We measured in situ diurnal changes in net photosynthetic rate and stomatal conductance in three leaves of each species under natural light. In both top-canopy and sub-canopy species, midday depression of net assimilation rate occurred in late morning. Numerical analysis showed that patchy bimodal stomatal behavior occurred only during midday depression, suggesting that the distribution pattern of stomatal apertures (either uniform or non-uniform stomatal behavior) varies flexibly within single days. Direct observation of stomatal aperture using Suzuki's Universal Micro-Printing (SUMP) method demonstrated midday patchy stomatal closure that fits a bimodal pattern in Shorea leprosula Miq., Shorea macrantha Brandis. and Dipterocarpus tempehes V.Sl. Inhibition of net assimilation rate and stomatal conductance appears to be a response to changes in vapor pressure deficit (VPD). Variable stomatal closure with increasing VPD is a mechanism used by a range of species to prevent excess water loss from leaves through evapotranspiration (viz., inhibition of midday leaf gas exchange). Bimodal stomatal closure may occur among adjacent stomata within a single patch, rather than among patches on a single leaf. Our results suggest the occurrence of patches at several scales within single leaves. Further analysis should consider variable spatial scales in heterogeneous stomatal behavior between and within patches and within single leaves. [ABSTRACT FROM AUTHOR]

Japanese barberry (Berberis thunbergii de Candolle) is a thorny, perennial, exotic, invasive shrub that is well established throughout much of the eastern United States. It can form dense thickets that limit native herbaceous and woody regeneration, alter soil structure and function, and harbor increased blacklegged tick (Ixodes scapularis Say) populations. This study examined a potential causal mechanism for the link between Japanese barberry and blacklegged ticks to determine if eliminating Japanese barberry could reduce tick abundance and associated prevalence of Borrelia burgdorferi (Johnson, Schmid, Hyde, Steigerwalt, and Brenner). Japanese barberry was controlled at five study areas throughout Connecticut; adult ticks were sampled over three years. Each area had three habitat plots: areas where barberry was controlled, areas where barberry remained intact, and areas where barberry was minimal or absent. Sampled ticks were retained and tested for B. burgdorferi presence. At two study areas, temperature and relative humidity data loggers were deployed in each of the three habitat plots over two growing seasons. Intact barberry stands had 280 ±51 B. burgdoferiinfected adult ticks/ha, which was significantly higher than for controlled (121 ± 17/ha) and no barberry (30 ± 10/ha) areas. Microclimatic conditions where Japanese barberry was controlled were similar to areas without barberry. Japanese barberry infestations are favorable habitat for ticks, as they provide a buffered microclimate that limits desiccation-induced tick mortality. Control of Japanese barberry reduced the number of ticks infected with B. burgdorferi by nearly 60% by reverting microclimatic conditions to those more typical of native northeastern forests. [ABSTRACT FROM AUTHOR]

Sap flux density was measured continuously during the 1999 and 2000 growing seasons by the heat dissipation method in natural Fagus crenata Blume (Japanese beech) forests growing between 550 and 1600 m on the northern slope of the Kagura Peak of the Naeba Mountains, Japan. Sap flux density decreased radially toward the inner xylem and the decrease was best expressed in relation to the number of annual rings from the cambium, or in relation to the relative depth between the cambium and the trunk center, rather than as a function of absolute depth. The relative influences of radiation, vapor pressure deficit and soil water on sap flux density during the growing season were similar for the outer and inner xylem, and at all sites. Measurements of soil water content and water potential at a depth of 0.25 m demonstrated that sap flux density responded similarly and sensitively to water potential changes in this soil layer, despite large differences in rooting depth at different elevations, localizing one important control point in the functioning of this forest ecosystem. Identification of the relative influences of radiation, vapor pressure deficit and drying of the upper soil layer on sap flux density provides a framework for in-depth analysis of the control of transpiration in Japanese beech forests. In addition, the finding that the same general controls are operating on sap flux density despite climate gradients and large differences in overall forest stand structure will enhance understanding of water use by forests along elevation gradients. [ABSTRACT FROM AUTHOR]

Daily variations in net gas exchange, chlorophyll a fluorescence and water relations of mature, sun-acclimated grapefruit (Citrus paradisi Macfady.) and orange (Citrus sinensis L. Osbeck) leaves were determined in tree canopies either shaded with 50% shade screens or left unshaded (sunlit). Mean daily maximum photosynthetic photon flux density (PPFD) under shade varied from 500 to 700 μmol m−2 s−1 and was sufficient to achieve maximum net CO2 assimilation rates (ACO2). Responses of grapefruit and orange leaves to shading were remarkably similar. At midday, on bright clear days, the temperatures of sunlit leaves were 2–6 °C above air temperature and 1–4 °C above the temperatures of shaded leaves. Although midday depressions of stomatal conductance (gs) and ACO2 were observed in both sunlit and shaded leaves, shaded leaves had lower leaf-to-air vapor pressure differences (D) along with higher gs, ACO2 and leaf water-use efficiency than sunlit leaves. Estimated stomatal limitation to ACO2 was generally less than 25% and did not differ between shaded and sunlit leaves. Leaf intercellular CO2 partial pressure was not altered by shade treatment and did not change substantially with increasing D. Radiation and high temperature stress-induced non-stomatal limitation to ACO2 in sunlit leaves was greater than 40%. Reversible photoinhibition of photosystem II efficiency was more pronounced in sunlit than in shaded leaves. Thus, non-stomatal factors play a major role in regulating ACO2 of citrus leaves during radiation and high temperature stress. [ABSTRACT FROM PUBLISHER]

Stable carbon isotope composition (δ; ‰) was measured on cellulose extracted from maritime pine (Pinus pinaster Aït.) tree rings to investigate inter-tree and interannual variability (7 trees, 20 rings per tree, each ring divided into early and late wood). A model of stand primary production coupled to water balance was used to calculate the stand annual water-use efficiency (WUE).Inter-tree variability in discrimination (maximum 2.88‰ in late wood in 1989, 2.69‰ in early wood in 1983) was as large as interannual variation (maximum 2.72‰ in late wood, 2.05‰ in early wood). Tree size did not explain these differences. Relationships were found between annual discrimination and climate variables such as annual rainfall, summer temperature and vapor pressure deficit (VPD). Higher correlations were found with late wood discrimination. Early wood discrimination was shown to be related to previous-year late wood discrimination. Late wood discrimination was also related to soil water availability. Stand annual WUE was only weakly related to tree ring carbon discrimination. [ABSTRACT FROM PUBLISHER]

Drought stress plays an important role in determining both the structure and function of forest ecosystems, because of the close association between the carbon (C) and hydrological cycles. We used a detailed model of the soil–plant–atmosphere continuum to investigate the links between carbon uptake and the hydrological cycle in a mature, open stand of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) at the Metolius river in eastern Oregon over a 2-year period (1996–1997). The model was parameterized from local measurements of vegetation structure, soil properties and meteorology, and tested against independent measurements of ecosystem latent energy (LE) and carbon fluxes and soil water content. Although the 2 years had very different precipitation regimes, annual uptake of C and total transpiration were similar in both years, according to both direct observation and simulations. There were important differences in ratios of evaporation to transpiration, and in the patterns of water abstraction from the soil profile, depending on the frequency of summer storms. Simulations showed that, during periods of maximum water limitation in late summer, plants maintained a remarkably constant evapotranspirative flux because of deep rooting, whereas changes in rates of C accumulation were determined by interactions between atmospheric vapor pressure deficit and stomatal conductance. Sensitivity analyses with the model suggest a highly conservative allocation strategy in the vegetation, focused belowground on accessing a soil volume large enough to buffer summer droughts, and optimized to account for interannual variability in precipitation. The model suggests that increased allocation to leaf area would greatly increase productivity, but with the associated risk of greater soil water depletion and drought stress in some years. By constructing sparse canopies and deep rooting systems, these stands balance reduced productivity in the short term with risk avoidance over the long term. [ABSTRACT FROM PUBLISHER]

We examined the effects of increased transpiration demand on xylem hydraulic conductivity and vulnerability to cavitation of mature ponderosa pine (Pinus ponderosa Laws.) by comparing trees growing in contrasting climates. Previous studies determined that trees growing in warm and dry sites (desert) had half the leaf/sapwood area ratio (AL/AS) and more than twice the transpiration rate of trees growing in cool and moist sites (montane). We predicted that high transpiration rates would be associated with increased specific hydraulic conductivity (KS) and increased resistance to xylem cavitation. Desert trees had 19% higher KS than montane trees, primarily because of larger tracheid lumen diameters. Predawn water potential and water potential differences between the soil and the shoot were similar for desert and montane trees, suggesting that differences in tracheid anatomy, and therefore KS, were caused primarily by temperature and evaporative demand, rather than soil drought. Vulnerability to xylem cavitation did not differ between desert and montane populations. A 50% loss in hydraulic conductivity occurred at water potentials between –2.61 and –2.65 MPa, and vulnerability to xylem cavitation did not vary with stem size. Minimum xylem tensions of desert and montane trees did not drop below –2.05 MPa. Foliage turgor loss point did not differ between climate groups and corresponded to mean minimum xylem tensions in the field. In addition to low AL/AS, high KS in desert trees may provide a way to increase tree hydraulic conductivity in response to high evaporative demand and prevent xylem tensions from reaching values that cause catastrophic cavitation. In ponderosa pine, the flexible responses of AL/AS and KS to climate may preclude the existence of significant intraspecific variation in the vulnerability of xylem to cavitation. [ABSTRACT FROM PUBLISHER]

We compared leaf gas exchange and water potential among the dominant tree species and major size classes of trees in an upland, pine–oak forest in northern Arizona. The study included old-growth Gambel oak (Quercus gambelii Nutt.), and sapling, pole, and old-growth ponderosa pines (Pinus ponderosa var. scopulorum Dougl. ex Laws.). Old-growth oak had higher predawn leaf water potential (Ψleaf) than old-growth pine, indicating greater avoidance of soil water stress by oak. Old-growth oak had higher stomatal conductance (Gw), net photosynthetic rate (Pn), and leaf nitrogen concentration, and lower daytime Ψleaf than old-growth pine. Stomatal closure started at a daytime Ψleaf of about –1.9 MPa for pine, whereas old-growth oak showed no obvious reduction in Gw at Ψleaf values greater than –2.5 MPa. In ponderosa pine, Pn and Gw were highly sensitive to seasonal and diurnal variations in vapor pressure deficit (VPD), with similar sensitivity for sapling, pole, and old-growth trees. In contrast, Pn and Gw were less sensitive to VPD in Gambel oak than in ponderosa pine, suggesting greater tolerance of oak to atmospheric water stress. Compared with sapling pine, old-growth pine had lower morning and afternoon Pn and Gw, predawn Ψleaf, daytime Ψleaf, and soil-to-leaf hydraulic conductance (Kl), and higher foliar nitrogen concentration. Pole pine values were intermediate between sapling and old-growth pine values for morning Gw and daytime Ψleaf, similar to sapling pine for predawn Ψleaf, and similar to old-growth pine for morning and afternoon Pn, afternoon Gw, Kl, and foliar nitrogen concentration. For the pines, low predawn Ψleaf, daytime Ψleaf, and Kl were associated with low Pn and Gw. Our data suggest that hydraulic limitations are important in reducing Pn in old-growth ponderosa pine in northern Arizona, and indicate greater avoidance of soil water stress and greater tolerance of atmospheric water stress by old-growth Gambel oak than by old-growth ponderosa pine. [ABSTRACT FROM PUBLISHER]

Sap flow was measured on five branches of two poplar (Populus trichocarpa Torr. & A. Gray × P. tacamahaca L.) trees from June to September 1994 in the south of England with stem-surface, heat balance gauges, and was scaled up to estimate transpiration from single trees on the basis of leaf area. On six days, stomatal conductance and plant water potential were measured simultaneously with a porometer and pressure chamber, respectively. The effects of solar radiation (S), vapor pressure deficit (D) and stomatal conductance on transpiration were evaluated. Sap flow per unit leaf area (Fa) was closely related to the time course of demand attributable to S and D throughout the season, and only slightly affected by the water content of the top 120 cm of soil. Although Fa increased linearly at low values of D, it showed a plateau with increases in D above 1.2 kPa. The canopy coupling coefficient (1 – Ω) ranged from 0.48 to 0.78 with a mean of 0.65 ± 0.01, indicating that transpiration was controlled more by stomatal conductance than by incident radiation. The seasonal pattern of tree water loss followed potential evaporation with a peak in late June or early July. On bright days, daily transpiration over the projected crown area was 3.6 mm early in the season, 3.8 mm in mid-season, and 2.7 mm late in the season. The water balance of the system indicated that poplar trees took 15–60% of water transpired from groundwater, with the proportion increasing as the soil in the unsaturated zone dried out. Access to the water table resulted in high predawn water potentials throughout the season. Estimated hydraulic resistance to water flow in the soil-tree system was in the range of 1.5 to 1.93 × 106 MPa s m−3. [ABSTRACT FROM AUTHOR]

Australian savannas exhibit marked seasonality in precipitation, with more than 90% of the annual total falling between October and May. The dry season is characterized by declining soil water availability and high vapor pressure deficits (up to 2.5 kPa). We used heat pulse technology to measure whole-tree transpiration rates on a daily and seasonal basis for the two dominant eucalypts at a site near Darwin, Australia. Contrary to expectations, transpiration rates were higher during the dry season than during the wet season, largely because of increased evaporative demand and the exploitation of groundwater reserves by the trees. Transpiration rates exhibited a marked hysteresis in relation to vapor pressure deficit, which was more marked in the dry season than in the wet season. This result may be attributable to low soil hydraulic conductivity, or the use of stored stem water, or both. Tree water use was strongly correlated with leaf area and diameter at breast height and there were no differences in transpiration between the species studied. These results are discussed in relation to scaling tree water use to stand water use. [ABSTRACT FROM AUTHOR]

Midday stomatal closure is mediated by the availability of water in the soil, leaf and atmosphere, but the response to these environmental and internal variables is highly species specific. We tested the hypothesis that species differences in stomatal response to humidity and soil water availability can be explained by two parameters: leaf-specific hydraulic conductance (KL) and a threshold leaf water potential (Ψthreshold). We used a combination of original and published data to estimate characteristic values of KL and Ψthreshold for four common tree species that have distinctly different stomatal behaviors: black cottonwood (Populus trichocarpa Torr. & Gray.), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), red alder (Alnus rubra Bong.) and western hemlock (Tsuga heterophylla (Raf.) Sarg.). We used the values to parameterize a simple, nonelastic model that predicts stomatal conductance by linking hydraulic flux to transpirational flux and maintaining Ψleaf above Ψthreshold. The model successfully predicted fundamental features of stomatal behavior that have been reported in the literature for these species. We conclude that much of the variation among the species in stomatal response to soil and atmospheric water deficits can be explained by KL and Ψthreshold. The relationship between Ψthreshold and xylem vulnerability to cavitation differed among these species. [ABSTRACT FROM PUBLISHER]

We examined the influences of selected environmental variables on stomatal behavior and regulation of transpiration in 26-month-old Acacia koa Gray (koa) stands planted at spacings of 1 × 1 m or 2.5 × 2.5 m and grown without irrigation. Field measurements were made during recovery from an extended 60-day dry period with only 38 mm of precipitation. Biomass and leaf area were also measured at 3-month intervals over the first 24 months after planting and again following completion of the transpiration (T) and stomatal conductance (gs) measurements at about 26 months after planting. Transpiration was measured as sap flow through intact branches by a heat balance method. [ABSTRACT FROM PUBLISHER]