Leaf traits and tree rings suggest different water-use and carbon assimilation strategies by two co-occurring Quercus species in a Mediterranean mixed-forest stand in Tuscany, Italy

We compared the water-use characteristics of co-occurring mature Quercus cerris L. and Quercus pubescens Willd. trees growing in resource-limited (mainly water) hilly habitats in Tuscany, Italy. The species differed in their distribution along soil water gradients and in their access to, and use of, water, even though the study year was wetter than average, though with a summer drought. Compared with Q. cerris, Q. pubescens had greater access to soil water (less negative predawn water potentials) and a more conservative water-use strategy based on its relatively low stomatal conductance, high instantaneous water-use efficiency, less negative midday water potential and high soil-to-leaf hydraulic conductance. Quercus cerris had less conservative water-use characteristics than Q. pubescens, exhibiting relatively high stomatal conductance, low instantaneous water-use efficiency, more negative midday water potentials and low soil-to-leaf hydraulic conductance; however, Q. cerris had higher photosynthetic rates than Q. pubescens. Photosynthesis and stomatal conductance were positively correlated in both species. Although a strong correlation between ring widths and precipitation patterns was not found, some dry periods influenced ring-width growth. Quercus pubescens has always grown faster than Q. cerris, probably because of more efficient water use, although stand dynamics (driven by exogenous disturbance factors, including coppicing, browsing and competition) cannot be excluded. Ring-width variability, as well as tree-ring growth in dry years, which should be unaffected by stand dynamics, were higher in Q. pubescens than in Q. cerris. Moreover, Q. pubescens recovered completely after the drought in the seventies, even showing higher tree-ring growth than in the recent past, whereas Q. cerris showed a minor growth decline followed by a recovery to values comparable with those observed before the 1970s drought. Beginning in the early eighties, tree-ring growth decreased in both species, though Q. pubescens showed consistently higher values than Q. cerris. These differences can be explained by differences in water-use efficiency. Despite differences between the species in water use and water status, the results are consistent with the interpretation that both are drought tolerant, but that Q. pubescens is at an advantage on xeric ridges because of its greater ability to access soil water and use it more conservatively compared with Q. cerris.     

Water-use strategies in two co-occurring Mediterranean evergreen oaks: surviving the summer drought

In the Mediterranean evergreen oak woodlands of southern Portugal, the main tree species are Quercus ilex ssp. rotundifolia Lam. (holm oak) and Quercus suber L. (cork oak). We studied a savannah-type woodland where these species coexist, with the aim of better understanding the mechanisms of tree adaptation to seasonal drought. In both species, seasonal variations in transpiration and predawn leaf water potential showed a maximum in spring followed by a decline through the rainless summer and a recovery with autumn rainfall. Although the observed decrease in predawn leaf water potential in summer indicates soil water depletion, trees maintained transpiration rates above 0.7 mm day(-1) during the summer drought. By that time, more than 70% of the transpired water was being taken from groundwater sources. The daily fluctuations in soil water content suggest that some root uptake of groundwater was mediated through the upper soil layers by hydraulic lift. During the dry season, Q. ilex maintained higher predawn leaf water potentials, canopy conductances and transpiration rates than Q. suber. The higher water status of Q. ilex was likely associated with their deeper root systems compared with Q. suber. Whole-tree hydraulic conductance and minimum midday leaf water potential were lower in Q. ilex, indicating that Q. ilex was more tolerant to drought than Q. suber. Overall, Q. ilex seemed to have more effective drought avoidance and drought tolerance mechanisms than Q. suber.     

Stomatal limitation to CO2 assimilation and down-regulation of photosynthesis in Quercus ilex resprouts in response to slowly imposed drought

Holm oak (Quercus ilex L.) is native to hot, dry Mediterranean forests where limited water availability often reduces photosynthesis in many species, and forest fires are frequent. Holm oaks resprout after a disturbance, with improved photosynthetic activity and water relations compared with unburned plants. To better understand the role of water availability in this improvement, watering was withheld from container-grown plants, either intact (controls) or resprouts after excision of the shoot, to gradually obtain a wide range of soil water availabilities. At high water availability, gas exchange rates did not differ between controls and resprouts. At moderate soil dryness, net photosynthesis of control plants decreased as a result of increased stomatal limitation, whereas gas exchange rates of resprouts, which had higher midday and predawn leaf water potentials, were unchanged. Under severe drought, resprouts showed a less marked decline in gas exchange than controls and maintained photosystem II integrity, as indicated by chlorophyll fluorescence measurements. Photosynthesis was down-regulated in both plant types in response to reduced CO2 availability caused by high stomatal limitation. Lower non-stomatal limitations in resprouts than in control plants, as evidenced by higher carboxylation velocity and the capacity for ribulose-1,5-bisphosphate regeneration, conferred greater drought resistance under external constraints similar to summer conditions at midday.     

Regulation of transpirational water loss in Quercus suber trees in a Mediterranean-type ecosystem

Sap flux density in branches, leaf transpiration, stomatal conductance and leaf water potentials were measured in 16-year-old Quercus suber L. trees growing in a plantation in southern Portugal to understand how evergreen Mediterranean trees regulate water loss during summer drought. Leaf specific hydraulic conductance and leaf gas exchange were monitored during the progressive summer drought to establish how changes along the hydraulic pathway influence shoot responses. As soil water became limiting, leaf water potential, stomatal conductance and leaf transpiration declined significantly. Predawn leaf water potential reflected soil water potential measured at 1-m depth in the rhizospheres of most trees. The lowest predawn leaf water potential recorded during this period was -1.8 MPa. Mean maximum stomatal conductance declined from 300 to 50 mmol m(-2) s(-1), reducing transpiration from 6 to 2 mmol m(-2) s(-1). Changes in leaf gas exchange were attributed to reduced soil water availability, increased resistances along the hydraulic pathway and, hence, reduced leaf water supply. There was a strong coupling between changes in soil water content and stomatal conductance as well as between stomatal conductance and leaf specific hydraulic conductance. Despite significant seasonal differences among trees in predawn leaf water potential, stomatal conductance, leaf transpiration and leaf specific hydraulic conductance, there were no differences in midday leaf water potentials. The strong regulation of changes in leaf water potential in Q. suber both diurnally and seasonally is achieved through stomatal closure, which is sensitive to changes in both liquid and vapor phase conductance. This sensitivity allows for optimization of carbon and water resource use without compromising the root-shoot hydraulic link.     

Stomatal responses to drought at a Mediterranean site: a comparative study of co-occurring woody species differing in leaf longevity

We studied stomatal responses to decreasing predawn water potential (Psipd) and increasing leaf-to-air water vapor pressure difference (VPD) of co-occurring woody Mediterranean species with contrasting leaf habits and growth form. The species included two evergreen oaks (Quercus ilex subsp. ballota (Desf.) Samp. and Q. suber L.), two deciduous oaks (Q. faginea Lam. and Q. pyrenaica Willd.) and two deciduous shrubs (Pyrus bourgaeana Decne. and Crataegus monogyna Jacq.). Our main objective was to determine if stomatal sensitivity is related to differences in leaf life span and leaf habit. The deciduous shrubs had the least conservative water-use characteristics, with relatively high stomatal conductance and low stomatal sensitivity to soil and atmospheric drought. As a result, Psipd decreased greatly in both species during the growing season, resulting in early leaf abscission in the summer. The deciduous oaks showed intermediate water-use characteristics, having maximum stomatal conductances and CO2 assimilation rates similar to or even higher than those of the deciduous shrubs. However, they had greater stomatal sensitivity to soil drying and showed less negative Psipd values than the deciduous shrubs. The evergreen oaks, and especially the species with the greatest leaf longevity, Q. ilex, exhibited the most conservative water-use behavior, having lower maximum stomatal conductances and greater sensitivity to VPD than the deciduous species. As a result, Psipd decreased less during the growing season in the evergreens than in the deciduous species, which may contribute to greater leaf longevity by avoiding irreversible damage during the summer drought. However, the combination of low maximum CO2 assimilation rates and high stomatal sensitivity to drought must have a negative impact on the final carbon budget of leaves with a long life span.     

Adaptations to soil drying in woody seedlings of African locust bean, (Parkia biglobosa (Jacq.) Benth.)

Stomatal conductance, transpiration and xylem pressure potential of African locust bean (Parkia biglobosa (Jacq.) Benth.) seedlings subjected from the sixth week after emergence to four weeks of continuous soil drought did not differ from those of well-watered, control plants until two-thirds of the available soil water had been used. In both well-watered and drought-treated plants, stomatal conductance was highest early in the day when vapor pressure deficits were low, but decreased sharply by midday when evaporative demand reached its highest value. There was no increase in stomatal conductance later in the day as vapor pressure deficit declined. The relationship between transpiration rate and xylem pressure potential showed non-linearity and hysteresis in both control and drought-treated plants, which seems to indicate that the plants had a substantial capacity to store water. The rate of leaf extension in African locust bean seedlings subjected to six consecutive 2-week cycles of soil drought declined relative to that of well-watered, control plants, whereas relative root extension increased. It appears that African locust bean seedlings minimized the impact of drought by: (1) restricting transpiration to the early part of the day when a high ratio of carbon gain to water loss can be achieved; (2) utilizing internally stored water during periods of rapid transpiration; (3) reducing the rate of leaf expansion and final leaf size in response to soil drought without reducing the rate of root extension, thereby reducing the ratio of transpiring leaf surface area to absorbing root surface area.     

Ontogenetic changes in stomatal and biochemical limitations to photosynthesis of two co-occurring Mediterranean oaks differing in leaf life span

A quantitative analysis was applied to the stomatal and biochemical limitations to light-saturated net photosynthesis under optimal field conditions in mature trees and seedlings of the co-occurring evergreen oak, Quercus ilex L., and the deciduous oak, Q. faginea Lam. Stomatal limitation to photosynthesis, maximal Rubisco activity and electron transport rate were determined from assimilation versus intercellular leaf carbon dioxide concentration response curves of leaves that were subsequently analyzed for nitrogen (N) concentration, mass per unit area, thickness and percent internal air space. In both species, seedlings had a lower leaf mass per unit area, thickness and leaf N concentration than mature trees. The root system of seedlings during their third year after planting was dominated by a taproot. A lower leaf N concentration of seedlings was associated with lower maximal Rubisco activity and electron transport rate and with assimilation rates similar to or lower than those of mature trees, despite the higher stomatal conductances and potential photosynthetic nitrogen-use efficiencies of seedlings. Consequently, stomatal limitation to photosynthesis increased with tree age in both species. In both seedlings and mature trees, a lower assimilation rate in Q. ilex than in Q. faginea was associated with lower stomatal conductance, N allocation to photosynthetic functions, maximal Rubisco activity and electron transport rate, and potential photosynthetic nitrogen-use efficiency but greater leaf thickness and leaf mass per unit area. Tree-age-related changes differed quantitatively between species, and the characteristics of the two species were more similar in seedlings than in mature trees. Despite higher stomatal conductances, seedlings are more N limited than adult trees, which contributes to lower biochemical efficiency.     

Sap flow of three co-occurring Mediterranean woody species under varying atmospheric and soil water conditions

We studied the seasonal patterns of water use in three woody species co-occurring in a holm oak forest in northeastern Spain. The three species studied, Quercus ilex L., Phillyrea latifolia L. and Arbutus unedo L., constitute more than 99% of the total basal area of the forest. The study period included the dry seasons of 1999 and 2000. Water use was estimated with Granier-type sap flux sensors. Standard meteorological variables, soil water content and leaf water potentials were also monitored. All monitored individuals reduced leaf-related sap flow (Q(l)) during the summer, concurrent with an increase in soil moisture deficit (SMD). Despite similar maximum Q(l) between species, the decline in Q(l) with increasing SMD was species-dependent. The average reduction in Q(l) between early summer and the peak of the drought was 74% for A. unedo (n = 3), 58% for P. latifolia (n = 3) and 87% for Q. ilex (n = 1). The relationship between canopy stomatal conductance (G(s)) and vapor pressure deficit (D) changed during the course of the drought, with progressively lower G(s) for any given D. Summertime reductions of Q(l) and G(s) were associated with between-species differences in vulnerability to xylem embolism, and with the corresponding degree of native embolism (lowest in P. latifolia and highest in Q. ilex). Our results, combined with previous studies in the same area, outlined differences among the species studied in manner of responding to water shortage, with P. latifolia able to maintain water transport at much lower water potentials than the other two species. In an accompanying experiment, A. unedo responded to an experimental reduction in water availability by reducing Q(l) during the summer. This species also modified its water use between years according to the different seasonal patterns of precipitation. These results are discussed in relation to the possible impacts that climate change will have on Q. ilex-dominated forests.     

Gas exchange, leaf structure and nitrogen in contrasting successional tree species growing in open and understory sites during a drought

Seasonal ecophysiology, leaf structure and nitrogen were measured in saplings of early (Populus grandidentata Michx. and Prunus serotina J.F. Ehrh.), middle (Fraxinus americana L. and Carya tomentosa Nutt.) and late (Acer rubrum L. and Cornus florida L.) successional tree species during severe drought on adjacent open and understory sites in central Pennsylvania, USA. Area-based net photosynthesis (A) and leaf conductance to water vapor diffusion (g(wv)) varied by site and species and were highest in open growing plants and early successional species at both the open and understory sites. In response to the period of maximum drought, both sunfleck and sun leaves of the early successional species exhibited smaller decreases in A than leaves of the other species. Shaded understory leaves of all species were more susceptible to drought than sun leaves and had negative midday A values during the middle and later growing season. Shaded understory leaves also displayed a reduced photosynthetic light response during the peak drought period. Sun leaves were thicker and had a greater mass per area (LMA) and nitrogen (N) content than shaded leaves, and early and middle successional species had higher N contents and concentrations than late successional species. In both sunfleck and sun leaves, seasonal A was positively related to predawn leaf Psi, g(wv), LMA and N, and was negatively related to vapor pressure deficit, midday leaf Psi and internal CO(2). Although a significant amount of plasticity occurred in all species for most gas exchange and leaf structural parameters, middle successional species exhibited the largest degree of phenotypic plasticity between open and understory plants.     

Responses to water stress in two Eucalyptus globulus clones differing in drought tolerance

We evaluated drought resistance mechanisms in a drought-tolerant clone (CN5) and a drought-sensitive clone (ST51) of Eucalyptus globulus Labill. based on the responses to drought of some physiological, biophysical and morphological characteristics of container-grown plants, with particular emphasis on root growth and hydraulic properties. Water loss in excess of that supplied to the containers led to a general decrease in growth and significant reductions in leaf area ratio, specific leaf area and leaf-to-root area ratio. Root hydraulic conductance and leaf-specific hydraulic conductance decreased as water stress became more severe. During the experiment, the drought-resistant CN5 clone maintained higher leaf water status (higher predawn and midday leaf water potentials), sustained a higher growth rate (new leaf area expansion and root growth) and displayed greater carbon allocation to the root system and lower leaf-to-root area ratio than the drought-sensitive ST51 clone. Clone CN5 possessed higher stomatal conductances at moderate stress as well as higher hydraulic conductances than Clone ST51. Differences in the response to drought in root biomass, coupled with changes in hydraulic properties, accounted for the clonal differences in drought tolerance, allowing Clone CN5 to balance transpiration and water absorption during drought treatment and thereby prolong the period of active carbon assimilation.     

Photosynthetic responses to water deficit in six Mediterranean sclerophyll species: possible factors explaining the declining distribution of Rhamnus ludovici-salvatoris,an endemic Balearic species

We sought to explain the declining distribution in the Balearic Islands of the endemic shrub Rhamnus ludovici-salvatoris R. Chodat, by comparing its photosynthetic response to drought with that of several widely distributed, competing Mediterranean species (R. alaternus L., Quercus ilex L., Pistacia lentiscus L., Q. humilis Mill. and P. terebinthus L.). All of the study species, except for the two Rhamnus species, avoided desiccation by rapidly adjusting their stomatal conductance at the onset of drought, and maintaining constant leaf relative water content. The two Rhamnus species showed desiccation-tolerant behavior; i.e., as drought progressed, their predawn leaf relative water content decreased simultaneously with stomatal closure. All four desiccation-avoiding species showed a significant positive correlation between leaf thermal dissipation (estimated by the fluorescence parameter NPQ (non-photochemical quenching)) and the de-epoxidation state of the xanthophyll cycle (DPS). The two Rhamnus species exhibited maximum DPS regardless of treatment, but only R. alaternus increased NPQ in response to drought. Rhamnus ludovici-salvatoris had a high ratio of photorespiration to photosynthesis and a low intrinsic water-use efficiency; traits that are likely to be unfavorable for plant productivity under arid conditions. It also had the lowest DPS and thermal dissipation among the six species. We conclude that the photosynthetic traits of R. ludovici-salvatoris account for its limited ability to compete with other species in the Mediterranean region.     

Localized ozone fumigation system for studying ozone effects on photosynthesis, respiration, electron transport rate and isoprene emission in field-grown Mediterranean oak species

We used a localized ozone (O3) fumigation (LOF) system to study acute and short-term O(3) effects on physiological leaf traits. The LOF system enabled investigation of primary and secondary metabolic responses of similarly and differently aged leaves on the same plant to three different O3 concentrations ([O3]), unconfounded by other influences on O3 sensitivity, such as genetic, meteorological and soil factors. To simulate the diurnal cycle of O3 formation, current-year and 1-year-old Quercus ilex (L.) and Quercus pubescens (L.) leaves were fumigated with O3 at different positions (and hence, different leaf ages) on the same branch over three consecutive days. The LOF system supplied a high [O3] (300+/-50 ppb) on leaves appressed to the vents, and an intermediate, super-ambient [O3] (varying between 120 and 280 ppb) on leaves less than 30 cm from the vent. Leaves more than 60 cm from the O3 vent were exposed to an [O3] comparable with the ambient concentration, with a 100 ppb peak during the hottest hours of the day. Only leaves exposed to the high [O3] were affected by the 3-day treatment, confirming that Mediterranean oak are tolerant to ambient and super-ambient [O3], but may be damaged by acute exposure to high [O3]. Stomatal and mesophyll conductance and photosynthesis were all reduced immediately after fumigation with high [O3], but recovered to control values within 72 h. Both the intercellular and chloroplast CO2 concentrations ([CO2]) remained constant throughout the experiment. Thus, although treatment with a high [O3] may have induced stomatal closure and consequent down-regulation of photosynthesis, we found no evidence that photosynthesis was limited by low [CO2] at the site of fixation. One-year-old leaves of Q. ilex were much less sensitive to O3 than current-year leaves, suggesting that the low stomatal conductance observed in aging leaves limited O3 uptake. No similar effect of leaf age was found in Q. pubescens. Dark respiration decreased during the treatment period, but a similar decrease was observed in leaves exposed to low [O3], and therefore may not be an effect of O3 treatment. Light respiration, on the other hand, was mostly constant in ozone-treated leaves and increased only in leaves in which photosynthesis was temporarily inhibited by high [O3], preventing them from acting as strong sinks that recycle respiratory CO2 in the leaves. There was no evidence of photochemical damage in Q. ilex leaves, whereas Q. pubescens leaves exposed to a high [O3] showed limited photochemical damage, but recovered rapidly. Biochemical markers were affected by the high [O3], indicating accumulation of reactive oxygen species (ROS) and increased denaturation of lipid membranes, followed by activation of isoprene biosynthesis in Q. pubescens leaves. We speculate that the high isoprene emissions helped quench ROS and normalize membrane stability in leaves recovering from O3 stress.     

Ecophysiological and morphological responses to shade and drought in two contrasting ecotypes of Prunus serotina

Photosynthesis (A), water relations and stomatal reactivity during drought, and leaf morphology were evaluated on 2-year-old, sun- and shade-grown Prunus serotina Ehrh. seedlings of a mesic Pennsylvania seed source and a more xeric Wisconsin source. Wisconsin plants maintained higher A and leaf conductance (g(wv)) than Pennsylvania plants during the entire drought under sun conditions, and during the mid stages of drought under shade conditions. Compared to shade plants, sun plants of both sources exhibited a more rapid decrease in A or % A(max) with decreasing leaf water potential (Psi). Tissue water relations parameters were generally not significantly different between seed sources. However, osmotic potentials were lower in sun than shade plants under well-watered conditions. Following drought, shade plants, but not sun plants, exhibited significant osmotic adjustment. Sun leaves had greater thickness, specific mass, area and stomatal density and lower guard cell length than shade leaves in one or both sources. Wisconsin sun leaves were seemingly more xerophytic with greater thickness, specific mass, and guard cell length than Pennsylvania sun leaves. No source differences in leaf structure were exhibited in shade plants. Stomatal reactivity to sun-shade cycles was similar between ecotypes. However, well-watered and droughted plants differed in stomatal reactivity within and between multiple sun-shade cycles. The observed ecotypic and phenotypic variations in ecophysiology and morphology are consistent with the ability of Prunus serotina to survive in greatly contrasting environments.     

Interspecific variation in leaf water use associated with drought tolerance in four emergent dipterocarp species of a tropical rain forest in Borneo

By use of tree-tower and canopy-crane systems we studied variations in the water use, including transpiration, stomatal conductance, and leaf water potential, of the uppermost sun-exposed canopy leaves of four emergent dipterocarp species in an aseasonal tropical rain forest in Sarawak, Malaysia. Midday depression in stomatal conductance and leaf water potential was observed in all the species studied. Interspecific differences were clearly observed in the maxima of transpiration rates and stomatal conductance and the minima of leaf water potential among the four dipterocarp species. These interspecific variations were closely related to wood density and to factors affecting ecological patterns of distribution. Specifically, Shorea parvifolia and S. smithiana, both of which have a relatively low wood density for Dipterocarpaceae and are found on clay-rich soil, had a high transpiration rate in the daytime but had a large midday depression and a low leaf water potential. In contrast, Dryobalanops aromatica, which has a high wood density and is found in sandy soil areas, consumed less water even during the daytime. Dipterocarpus pachyphyllus, which has a high wood density and is found on clay-rich soil, stood intermediate between Shorea and D. aromatica in leaf water use. The two Shorea species had higher mortality than the others during the severe drought associated with El Ni?o in 1998, so daily pattern of leaf water use in each dipterocarp species might be correlated with its susceptibility to unusual drought events.     

Stomatal patchiness in the Mediterranean holm oak (Quercus ilex L.) under water stress in the nursery and in the forest

The evergreen holm oak Quercus ilex L. is the most representative tree in Mediterranean forests. Accurate estimation of the limiting factors of photosynthesis for Q. ilex and the prediction of ecosystem water-use efficiency by mechanistic models can be achieved only by establishing whether this species shows heterogenic stomatal aperture, and, if so, the circumstances in which this occurs. Here, we collected gas-exchange and chlorophyll fluorescence data in Q. ilex leaves from a nursery to measure the effects of stomatal oscillations on PSII quantum yield (Φ(PSII)) under water stress. Stomatal conductance (g(s)) was used as an integrative indicator of the degree of water stress. Images of chlorophyll fluorescence showed heterogeneous Φ(PSII) when g(s) was <50 mmol H(2)O m(-2) s(-1), representative of severe drought and corresponding to a container capacity <45%. Stomatal patchiness was related to a coefficient of variation (CV) of Φ(PSII) values >2.5%. A parallel study in the forest confirmed heterogeneous Φ(PSII) values in leaves in response to declining water availability. Three kinds of Q. ilex individuals were distinguished: those resprouting after a clear-cut (resprouts, R); intact individuals growing in the same clear-cut area as resprouts (controls, C); and intact individuals in a nearby, undisturbed area (forest controls, CF). Patchiness increased in C and CF in response to increasing drought from early May to late July, whereas in R, Φ(PSII) values were maintained as a result of their improved water relations since the pre-existing roots were associated with a smaller aerial biomass. Patchiness was related to a % CV of Φ(PSII) values >4 and associated in the summer with mean g(s) values of 30 mmol H(2)O m(-2) s(-1). Under milder drought in spring, Φ(PSII) patchiness was less strictly related to g(s) variations, pointing to biochemical limitants of photosynthesis. The occurrence of heterogenic photosynthesis caused by patchy stomatal closure in Q. ilex during severe drought should be taken into account in ecosystem modelling in which harsher water stress conditions associated with climate change are predicted.     

Stomatal conductance, growth and root signaling in young oak seedlings subjected to partial soil drying

Leaf conductance, water relations, growth, and abscisic acid (ABA) concentrations in xylem sap, root apices and leaves were assessed in oak seedlings (Quercus robur L.) grown with a root system divided between two compartments and subjected to one of four treatments: (a) well watered, WW; (b) half of root system exposed to soil drying and half kept well watered, WD; (c) whole root system exposed to drought, DD; and (d) half of root system severed, RE. Sharp decreases in plant stomatal conductance, leaf water potential, hydraulic conductance and leaf growth were observed during DD treatment. No significant differences in plant leaf water potential and stomatal conductance were detected between the WW and WD treatments. Nevertheless, the WD treatment resulted in inhibition of leaf expansion and stimulation of root elongation only in the well-watered compartment. Abscisic acid concentrations did not change in leaves, root tips, or xylem sap of WD- compared to WW-treated plants. Increased concentrations of ABA were observed in xylem sap from DD-treated plant roots, but the total flux of ABA to shoots was reduced compared to that in WW-treated plants, because of decreases in transpiration flux. Similar plant responses to the WD and RE treatments indicate that the responses observed in the WD-treated plants were probably not triggered by a positive signal originating from drying roots.     

Quercus species differ in water and nutrient characteristics in a resource-limited fall-line sandhill habitat

We compared co-occurring mature Quercus laevis Walt. (turkey oak), Q. margaretta Ashe (sand post oak) and Q. incana Bartr. (bluejack oak) trees growing in resource-limited sandhill habitats of the southeastern United States for water and nutrient characteristics. The Quercus spp. differed in their distribution along soil water and nutrient gradients, and in their access to and use of water, even though the study year was wetter than average with no mid-season drought. Quercus laevis had the greatest access to soil water (least negative pre-dawn water potential, psi(pd)) and the most conservative water-use strategy based on its relatively low stomatal conductance (g(s)), high instantaneous water-use efficiency (WUE), least negative midday water potential (psy(md)) and high leaf specific hydraulic conductance (K(L)). Quercus margaretta had the least conservative water-use characteristics, exhibiting relatively high g(s), low instantaneous WUE, most negative psi(md), and low K(L). Quercus margaretta also had a low photosynthetic nitrogen-use efficiency (PNUE), but a high leaf phosphorus concentration. Quercus incana had the poorest access to soil water, but intermediate water-use characteristics and leaf nutrient characteristics more similar to those of Q. laevis. There were no species differences for photosynthesis (A), leaf nitrogen on an area basis, or seasonally integrated WUE (delta13C). Both A and g(s) were positively correlated for each species, but A and g(s) were generally not correlated with psi(pd), psi(md) or delta psi(pd-md). Although we found differences in resource use and resource status among these sandhill Quercus spp., the results are consistent with the interpretation that they are generally drought avoiders. Quercus laevis may have an advantage on xeric ridges because of its greater ability to access soil water and use it more conservatively compared with the other Quercus spp.     

Seasonal patterns of leaf gas exchange and water relations in dry rain forest trees of contrasting leaf phenology

Diurnal and seasonal patterns of leaf gas exchange and water relations were examined in tree species of contrasting leaf phenology growing in a seasonally dry tropical rain forest in north-eastern Australia. Two drought-deciduous species, Brachychiton australis (Schott and Endl.) A. Terracc. and Cochlospermum gillivraei Benth., and two evergreen species, Alphitonia excelsa (Fenzal) Benth. and Austromyrtus bidwillii (Benth.) Burret. were studied. The deciduous species had higher specific leaf areas and maximum photosynthetic rates per leaf dry mass in the wet season than the evergreens. During the transition from wet season to dry season, total canopy area was reduced by 70-90% in the deciduous species and stomatal conductance (g(s)) and assimilation rate (A) were markedly lower in the remaining leaves. Deciduous species maintained daytime leaf water potentials (Psi(L)) at close to or above wet season values by a combination of stomatal regulation and reduction in leaf area. Thus, the timing of leaf drop in deciduous species was not associated with large negative values of daytime Psi(L) (greater than -1.6 MPa) or predawn Psi(L) (greater than -1.0 MPa). The deciduous species appeared sensitive to small perturbations in soil and leaf water status that signalled the onset of drought. The evergreen species were less sensitive to the onset of drought and g(s) values were not significantly lower during the transitional period. In the dry season, the evergreen species maintained their canopies despite increasing water-stress; however, unlike Eucalyptus species from northern Australian savannas, A and g(s) were significantly lower than wet season values.     

Comparison of drought resistance among Prunus species from divergent habitats

Root and shoot characteristics related to drought resistance were compared among cultivated peach (Prunus persica (L.) Batsch.), P. andersonii (Nevada Desert almond), P. besseyi (western sand cherry), P. maritima (beach plum), P. subcordata (Sierra or Pacific plum), and P. tomentosa (Nanking cherry). In all species, shoot characteristics were more closely associated with drought adaptation than root characteristics. The most xeric species, P. andersonii, had the lowest specific leaf area, smallest leaves, highest stomatal conductance (before stress), highest rate of carbon assimilation (A), high root length/leaf area and root weight/leaf area ratios, and the highest leaf nitrogen content on an area basis. Root hydraulic conductivity was similar for all species, indicating a lack of importance of this parameter for drought resistance. During a 5-7 day drought, water use efficiency (WUE) increased as shoot water potentials (Psi) declined to -3.0 to -4.0 MPa for the xeric P. andersonii and P. subcordata, whereas after an initial increase, WUE decreased with declining Psi in the -1.5 to -3.0 MPa range for the more mesic P. maritima, P. persica and P. tomentosa as a result of non-stomatal limitations to A. Carbon assimilation rate decreased linearly with Psi during drought in all species, but the Psi at which A reached zero was not associated with drought adaptation. We conclude that the variation in leaf characteristics among Prunus species could be exploited to improve the drought resistance of commercial cultivars.     

Patchy stomatal behavior during midday depression of leaf CO₂ exchange in tropical trees

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.