Xylem Pressure Potential Research Articles

Water sourcing by trees in a mesic savanna: Responses to severing deep and shallow roots

The co-dominance of trees and grasses is a much-debated topic in savanna ecology. One of the proposed mechanisms facilitating the co-dominance of these two different life-forms is root niche partitioning. Specifically, the rooting zone is hypothesized to have two layers: grasses are thought to be the superior competitors for water in the upper soil horizons, whereas trees avoid competitive exclusion by sourcing water from soil layers beyond the vertical reach of grasses. We examined this two-layer hypothesis through a field experiment in which we severed deep and shallow roots of Terminalia sericea saplings, 6 weeks before the onset of the new growing season, using a novel technique. During the course of the growing season, physiological responses to the root severing were measured. Severing lateral (shallow) roots resulted in rapid abscission of the old leaves, smaller leaves after the onset of the following growing season and lower xylem pressure potentials – indicating higher water stress – compared to the other treatment groups. Effects of severing deep-growing taproots were small, demonstrating the importance of lateral roots for maintaining the trees’ water balance, notably in drier times. We have shown that saplings of the common and locally dominant savanna tree T. sericea manage to coexist with grasses without avoiding competition through spatial root separation. The results of our study therefore do not support the two-layer hypothesis for explaining tree–grass coexistence in this mesic savanna.

Photosynthetic and growth responses of Camelina sativa (L.) Crantz to varying nitrogen and soil water status

Water and nitrogen (N) deficiency are two major constraints limiting the yield and quality of many oilseed crops worldwide. This study was designed to assess the response of Camelina sativa (L.) Crantz to the availability of N and water resources on photosynthesis and yield parameters. All the measured variables, which included plant height, root and shoot dry matter, root:shoot ratio, xylem pressure potential (XPP), yield components, photosynthetic parameters, and instantaneous water-use efficiency (WUE) were remarkably influenced by water and nitrogen supply. Net photosynthetic rate (P N) and yield components were significantly decreased more by water deficit than by N deficiency. XPP, stomatal conductance (g s), and intercellular CO2 concentration (C i) decreased substantially as the water deficit increased irrespective of the level of N application. WUE at the high N supply [100 and 150 kg(N) ha-1] dropped in a large degree as the increased water deficit due to a larger decrease in P N than transpiration rate (E). The results of this study suggest that the regulative capacity of N supply on photosynthetic and plant growth response is significantly affected by soil water status and C. sativa is more sensitive to water deficit than N supply.

Endogenous and exogenous ethylene induces needle abscission and cellulase activity in post-harvest balsam fir (Abies balsamea L.)

Post-harvest needle loss is a major problem for balsam fir and other Christmas tree species. Recent evi- dence has implicated ethylene as a signal responsible for post-harvest needle abscission, but enzymological changes remain unknown. The objective of this study was to identify and quantify cellulase activity associated with endogenous and exogenous ethylene-induced abscission. An experiment was designed with three treatments (control, endogenous ethylene, or exogenous ethylene) with five replicates. Key response variables include needle retention duration, xylem pressure potential, ethylene evolution rate, and cellulase activity. Two complimentary methods were used to assess cellulase activity: a cellulose plate digestion and zymog- raphy. The results confirm ethylene as a signal for post-harvest abscission and identify ethylene-induced cel- lulase. Ethylene evolution was typically between 15 and 16 l Lg -1 h -1 , but there was no difference among the three treatments. However, exogenous ethylene significantly decreased needle retention by 60% and resulted in a sixfold decrease in xylem pressure potential. In addition, cellulase activity increased by 8- and 12-fold in endogenous and exogenous ethylene-induced abscission, respectively, compared to the control. Identification of ethylene-induced cellulase activity has increased our understanding of the post-harvest needle abscission process and confirms ethyl- ene's role as a signal molecule.

Hydraulic responses of whole vines and individual roots of kiwifruit (Actinidia chinensis) following root severance

Whole vine (K(plant)) and individual root (K(root)) hydraulic conductances were measured in kiwifruit (Actinidia chinensis Planch. var. chinensis 'Hort16A') vines to observe hydraulic responses following partial root system excision. Heat dissipation and compensation heat pulse techniques were used to measure sap flow in trunks and individual roots, respectively. Sap flux and measurements of xylem pressure potential (Ψ) were used to calculate K(plant) and K(root) in vines with zero and ∼80% of roots severed. Whole vine transpiration (E), Ψ and K(plant) were significantly reduced within 24 h of root pruning, and did not recover within 6 weeks. Sap flux in intact roots increased within 24 h of root pruning, driven by an increase in the pressure gradient between the soil and canopy and without any change in root hydraulic conductance. Photosynthesis (A) and stomatal conductance (g(s)) were reduced, without significant effects on leaf internal CO(2) concentration (c(i)). Shoot growth rates were maintained; fruit growth and dry matter content were increased following pruning. The woody roots of kiwifruit did not demonstrate a rapid dynamic response to root system damage as has been observed previously in monocot seedlings. Increased sap flux in intact roots with no change in K(root) and only a moderate decline in shoot A suggests that under normal growing conditions root hydraulic conductance greatly exceeds requirements for adequate shoot hydration.

Ethylene Exposure Duration Affects Postharvest Needle Abscission in Balsam Fir (Abies balsamea L.)

Needle loss after harvest is a major problem for Atlantic Canada's Christmas tree and greenery industry. Ethylene is a signal for abscission in balsam fir, but preliminary studies have suggested that the role of ethylene may be influenced by length of exposure. Short-term and long-term ethylene exposure experiments were conducted. Branches were exposed to ethylene for 24 h (short-term) or continuously (long-term) at concentrations of 0 to 1000 ppm. The response variables measured were needle retention duration (NRD), average water use (AWU), and xylem pressure potential (XPP). Short-term exposure to any concentration of ethylene delayed needle abscission by 30 to 40 days. In contrast, long-term exposure to all concentrations of ethylene accelerated abscission, most evident by a 21-day decrease in NRD at 1000 ppm ethylene. There was a 60% decrease in NRD, 160% decrease (more negative) in XPP, and 80% increase in AWU as a result of long-term exposure to ethylene. Overall, our results demonstrate an opposite effect of short-term and long-term ethylene exposure, which suggests that short-term exposure to ethylene might help to precondition balsam fir and delay needle abscission during postharvest handling.

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.

Sulphate as a xylem-borne chemical signal precedes the expression of ABA biosynthetic genes in maize roots

Recent reports suggest that early sensing of soil water stress by plant roots and the concomitant reduction in stomatal conductance may not be mediated by root-sourced abscisic acid (ABA), but that other xylem-borne chemicals may be the primary stress signal(s). To gain more insight into the role of root-sourced ABA, the timing and location of the expression of genes for key enzymes involved in ABA biosynthesis in Zea mays roots was measured and a comprehensive analysis of root xylem sap constituents from the early to the later stages of water stress was conducted. Xylem sap and roots were sampled from plants at an early stage of water stress when only a reduction in leaf conductance was measured, as well as at later stages when leaf xylem pressure potential decreased. It was found that the majority of ABA biosynthetic genes examined were only significantly expressed in the elongation region of roots at a later stage of water stress. Apart from ABA, sulphate was the only xylem-borne chemical that consistently showed significantly higher concentrations from the early to the later stages of stress. Moreover, there was an interactive effect of ABA and sulphate in decreasing maize transpiration rate and Vicia faba stomatal aperture, as compared to ABA alone. The expression of a sulphate transporter gene was also analysed and it was found that it had increased in the elongation region of roots from the early to the later stages of water stress. Our results support the suggestion that in the early stage of water stress, increased levels of ABA in xylem sap may not be due to root biosynthesis, ABA glucose ester catabolism or pH-mediated redistribution, but may be due to shoot biosynthesis and translocation to the roots. The analysis of xylem sap mineral content and bioassays indicate that the anti-transpirant effect of the ABA reaching the stomata at the early stages of water stress may be enhanced by the increased concentrations of sulphate in the xylem which is also transported from the roots to the leaves.

An Analysis to the Driving Forces for Water and Salt Absorption in Roots of Maize Seedlings Under Salt Stress

When maize seedlings were subjected to salt stress, a decline in root xylem pressure was observed within seconds, followed by a gradual increase in Na + deposition in the seedlings. The magnitude of xylem pressure response was positively correlated with, but not proportional to the intensity of the stress. A continuous recording of the xylem pressure profile showed that self-regulation of the xylem pressure existed before and after the imposition of salt stress when the environmental conditions were relatively stable. The salt induced increase in xylem tension dominated the total water potential of the plant when the salt stress was mild, but the osmotic potential became more prominent when the NaCl concentration in the root bathing solution was raised to over 100 mol m −3. The average transpiration rate of the seedlings dropped by 40% when the NaCl concentration in the root ambient was increased to 150 mol m −3. Although salt stress resulted in the decline of both the xylem pressure potential and the osmotic potential in the root xylem, the changes in the total water potential of the root xylem solution were always smaller than the changes in the water (osmotic) potentials of the solution bathing the root. An analysis to the water relations of maize seedlings showed that not only the water potential components, but the radial reflection coefficient of the roots was also dependent on the level of salinity. When the NaCl level in the root bathing solution was raised from 25 to 150 mol m −3, the radial reflection coefficient of the root declined from 0.43 to 0.31. This small change resulted in a remarkable increase in the normalised relative NaCl absorption by 2.4 times, indicating that the radial reflection coefficient of root played a very important role in regulating the absorption of NaCl in maize seedlings under salt stress.

Effects of Moisture and Mycorrhiza on Stomatal Conductance and Xylem Pressure Potential of Faidherbia albida (Del.) A. Chev

The effects of moisture and mycorrhizal inoculation on stomatal conductance and xylem pressure potential of Faidherbia albida (Del.) A. Chev. were investigated in the semi arid environment of Sokoto, Nigeria. A complete randomized block design with five replications and factorical combination of three watering regimes (daily for unstressed plant, twice weekly for partially stressed plants and once a week for moisture stressed plants) and three mycorrhizal treatments namely; endomycorrhizal plants, ectomycorrhizal plants and non mycorrhizal plants were used. Both the ecto and endo mycorrhizae significantly (P

A quantitative comparison of two extremes in chaparral shrub phenology

We quantitatively compared phenology and water relations of a fully deciduous shrub, Styrax officinalis, and an evergreen shrub, Arctostaphylos glauca, in shared microsites in a sandstone outcrop in southern California during a multi-year drought. Pre-dawn xylem pressure potentials, Ψ pd, were similar for the two species during most months of 2 years, but occasional differences and watering experiment results suggest S. officinalis may have phreatophytic roots that tap water in deep rock cavities and joint traces, while A. glauca may have primarily shallow roots. Neither species varied in maximum or minimum Ψ pd between years of very different rainfall totals. Twig elongation and leaf production of S. officinalis began earlier during spring, and its leaves matured more quickly and more synchronously than A. glauca. Leaves lived a mean of 180 days for S. officinalis and 849 days for A. glauca. Leaf life spans varied among years in both species. S. officinalis leaf senescence occurred mostly in August and September and was not discernibly related to Ψ pd or drought avoidance. A. glauca leaf senescence occurred throughout the year, but especially coincided with leaf production. In A. glauca most senescence occurred at the beginning of a leaf cohort's third growing season, but numbers of retained older leaves increased during 4 years of drought. Timing of twig elongation and leaf and flower production appeared to be related to current rainfall, but amount of twig growth and numbers of leaves and flowers produced appeared to be related to rainfall of the previous year or years for S. officinalis and, more complexly, A. glauca. Because of an interrupted pattern of flower production, number of flowers produced by A. glauca may be responsive to rainfall amounts during two environmental periods. Morphological differences, including much higher above ground allocation, many more leaves/twig, lower allocation to stem mass, and longer duration of leaves/year in A. glauca, are probably responsible for A. glauca having >6 times more above ground biomass per plant than S. officinalis. During the multi-year drought S. officinalis changed little, indicating either superb adaptation or growth pattern rigidity, while A. glauca underwent extensive phenological and morphological changes, indicating either stress or adaptive flexibility.

A comparative study of oak (Quercus, Fagaceae) seedling physiology during summer drought in southern California

Natural recruitment of oaks appears to be declining throughout the northern hemisphere. Summer drought poses a potentially important barrier to oak recruitment in southern California. To evaluate this barrier, we grew evergreen Quercus agrifolia and deciduous Q. lobata from seeds near parental trees. We measured water relations, chlorophyll fluorescence, and gas exchange during these seedlings' fourth and fifth summers and compared them to neighboring adults. Most seedlings had substantially lower values for predawn xylem pressure potential (Ψ(pd)), minimum photosystem II (PSII) quantum efficiency (Φ(PSIIMIN)), maximum quantum efficiency for PSII under dark-adapted leaf conditions (Fv/Fm), and maximum photosynthetic assimilation (Amax), and higher values for maximum nonphotochemical quenching (NPQmax) than did conspecific adults. The high, unvarying Ψ(pd) values of the adults suggest they use perennially available groundwater. Quercus lobata seedlings commonly had lower values for Ψ(pd) than did Q. agrifolia, and values for Ψ(pd) and Φ(PSIIMIN) were significantly related to size in Q. lobata but not in Q. agrifolia. These data suggest important interspecific differences in root architecture. Lower values for Φ(PSIIMIN), Fv/Fm, and higher NPQmax in Q. agrifolia indicate that Q. agrifolia seedlings were usually under more stress than Q. lobata, which typically had higher Amax rates than did Q. agrifolia seedlings. Diurnal photosynthesis curves were quite flat for Q. agrifolia, but they peaked in the morning for Q. lobata. Established seedlings appeared to be under more stress than adults, but this stress did not appear severe enough to cause death. Access to perennially available groundwater may be crucial for the seedling to sapling transition.

Perturbed Lignification Impacts Tree Growth in Hybrid Poplar—A Function of Sink Strength, Vascular Integrity, and Photosynthetic Assimilation

The effects of reductions in cell wall lignin content, manifested by RNA interference suppression of coumaroyl 3'-hydroxylase, on plant growth, water transport, gas exchange, and photosynthesis were evaluated in hybrid poplar trees (Populus alba x grandidentata). The growth characteristics of the reduced lignin trees were significantly impaired, resulting in smaller stems and reduced root biomass when compared to wild-type trees, as well as altered leaf morphology and architecture. The severe inhibition of cell wall lignification produced trees with a collapsed xylem phenotype, resulting in compromised vascular integrity, and displayed reduced hydraulic conductivity and a greater susceptibility to wall failure and cavitation. In the reduced lignin trees, photosynthetic carbon assimilation and stomatal conductance were also greatly reduced, however, shoot xylem pressure potential and carbon isotope discrimination were higher and water-use efficiency was lower, inconsistent with water stress. Reductions in assimilation rate could not be ascribed to increased stomatal limitation. Starch and soluble sugars analysis of leaves revealed that photosynthate was accumulating to high levels, suggesting that the trees with substantially reduced cell wall lignin were not carbon limited and that reductions in sink strength were, instead, limiting photosynthesis.

A case-study of water transport in co-occurring ring- versus diffuse-porous trees: contrasts in water-status, conducting capacity, cavitation and vessel refilling

Recent work has suggested that the large earlywood vessels of ring-porous trees can be extraordinarily vulnerable to cavitation making it necessary that these trees maintain a consistent and favorable water status. We compared cavitation resistance, vessel refilling, transport capacity and water status in a study of ring-porous Quercus gambelii Nutt. (oak) and diffuse-porous Acer grandidentatum Nutt. (maple). These species co-dominate summer-dry foothills in the western Rocky Mountains of the USA. Native embolism measurements, dye perfusions and balance pressure exudation patterns indicated that the large earlywood vessels of 2-3-year-old oak stems cavitated extensively on a daily basis as predicted from laboratory vulnerability curves, resulting in a more than 80% reduction in hydraulic conductivity. Maple branches showed virtually no cavitation. Oak vessels refilled on a daily basis, despite negative xylem pressure in the transpiration stream, indicating active pressurization of embolized vessels. Conductivity and whole-tree water use in oak were between about one-half and two-thirds that in maple on a stem-area basis; but were similar or greater on a leaf-area basis. Oak maintained steady and modest negative xylem pressure potentials during the growing season despite little rainfall, indicating isohydric water status and reliance on deep soil water. Maple was markedly anisohydric and developed more negative pressure potentials during drought, suggesting use of shallower soil water. Although ring porosity may have evolved as a mechanism for coping with winter freezing, this study suggests that it also has major consequences for xylem function during the growing season.

Stem girdling manipulates leaf sugar concentrations and anthocyanin expression in sugar maple trees during autumn

To better understand the effects of sugar accumulation on red color development of foliage during autumn, we compared carbohydrate concentration, anthocyanin expression and xylem pressure potential of foliage on girdled versus non-girled (control) branches of 12 mature, open-grown sugar maple (Acer saccharum Marsh.) trees. Half of the study trees were known to exhibit mostly yellow foliar coloration and half historically displayed red coloration. Leaves from both girdled and control branches were harvested at peak color expression (i.e., little or no chlorophyll present). Disruption of phloem export by girdling increased foliar sucrose, glucose and fructose concentrations regardless of historical tree color patterns. Branch girdling also increased foliar anthocyanin expression from 50.4 to 66.7% in historically red trees and from 11.7 to 54.2% in historically yellow trees, the latter representing about a fivefold increase compared with control branches. Correlation analyses indicated a strong and consistent relationship between foliar red coloration and sugar concentrations, particularly glucose and fructose, in both girdled and control branches. Measures of xylem pressure potentials confirmed that girdling was a phloem-specific treatment and had no effect on water transport to distal leaves. Results indicate that stem girdling increased foliar sugar concentrations and enhanced anthocyanin expression during autumn in sugar maple foliage. Native environmental stresses (e.g., low autumn temperatures) that reduce phloem transport may promote similar physiological outcomes.

Linking leaf chlorophyll fluorescence properties to physiological responses for detection of salt and drought stress in coastal plant species

Effects of salinity and drought on physiology and chlorophyll fluorescence were used to evaluate stress in two coastal plants, Myrica cerifera (L.) and Phragmites australis (Cav.) Trin. ex Steud. Drought and salinity stress were induced and measurements of stomatal conductance, photosynthesis, xylem pressure potential (psi) and fluorescence were conducted following treatment. The onset of stress began at 2 g l(-1) for M. cerifera, and 5 g l(-1) for P. australis, as seen by significant decreases in physiological measurements. Despite the physiological effects of salinity, there was no significant difference in dark-adapted fluorescence (F(v)/F(m), where F(m) is the maximal fluorescence in dark-adapted leaves) for either species at any salinity level. Significant decreases in the light-adapted measurement Delta F/F'(m) (F'(m) is maximal fluorescence in light-adapted leaves) occurred at 10 g l(-1) in M. cerifera and P. australis, days before visible stress was evident. The quantum yield of xanthophyll-regulated thermal energy dissipation (Phi(NPQ), where NPQ is non-photochemical quenching of chlorophyll fluorescence) increased with decreasing Delta F/F'(m). Drought studies showed similar results, with significant decreases in physiological measurements occurring by day 2 in M. cerifera and day 4 in P. australis. Differences in Delta F/F'(m) were seen by day 5 for both species, whereas F(v)/F(m) showed no indication of stress, despite apparent visible signs. Xanthophyll-cycle-dependent energy dissipation may be the underlying mechanism in protecting photosystem II from excess energy in salinity- and drought-treated plants.

Determinants of savanna vegetation structure: Insights from Colophospermum mopane

Abstract Savannas are structurally heterogeneous at the local, community‐level scale due to fine‐scale floristic heterogeneity as well as the responses of individual species to underlying environmental variation. The structure of mopane woodland, an arid savanna of southern Africa, is dictated largely by local variation in the relative dominance of tall, single‐stemmed and shorter, multi‐stemmed forms of the dominant tree species, Colophospermum mopane (Kirk ex Benth) Léonhard. Here we evaluate the hypothesis that the existence of these alternative forms of C. mopane, as well as the factors that dictate their distribution at a local scale, are driven by fine‐scale environmental variability in available water. We surveyed trees at four sites in the Kruger National Park of South Africa, in each instance surveying both forms of the species, from both riparian and non‐riparian zones. A survey of genetic variation across our sample (n = 80 individuals), using inter‐simple sequence repeat (ISSR) amplification profiles, indicates that the two forms are not genetically distinct, instead being environmentally determined. While measurements of xylem pressure potentials, determined using a Scholander pressure chamber, show a significant difference between riparian and non‐riparian zones, there is no significant difference between the two growth forms. Although this seems paradoxical in view of the prevalence of tree and shrub form mopane at riparian and non‐riparian sites, respectively, we speculate that such a pattern may emerge through the interaction of moisture stress and top‐down controls, such as those imposed by large mammal browsing and fire.

Effectiveness of I–V watering devices in maintaining postharvest freshness and quality of cut Christmas trees

Experiments were carried out with cut Douglas-fir ( Pseudotsuga menziezii (Mirb.) Franco) and Leyland cypress [x Cupressocyparis leylandii (A.B. Jacks. & Dallim.)] Christmas trees to evaluate postharvest water uptake, moisture status, needle loss, and tree quality when water was supplied to displayed trees either by an I–V device or a traditional method of placing the base in water. In both experiments, trees consumed about 4× as much water with the traditional method. Based on measurements of twig moisture content, xylem pressure potential ( Ψ), needle loss, and total water consumption, tree freshness and quality was adequately maintained by the traditional method, but not the I–V device.

A unified hypothesis of mechanoperception in plants

The perception of mechanical stimuli in the environment is crucial to the survival of all living organisms. Recent advances have led to the proposal of a plant-specific mechanosensory network within plant cells that is similar to the previously described network in animal systems. This sensory network is the basis for a unifying hypothesis, which may account for the perception of numerous mechanical signals including gravitropic, thigmomorphic, thigmotropic, self-loading, growth strains, turgor pressure, xylem pressure potential, and sound. The current state of our knowledge of a mechanosensory network in plants is reviewed, and two mechanoreceptor models are considered: a plasmodesmata-based cytoskeleton-plasma membrane-cell wall (CPMCW) network vs. stretch-activated ion channels. Post-mechanosensory physiological responses to mechanical stresses are also reviewed, and future research directions in the area of mechanoperception and response are recommended.

Tree distribution on a steep environmental gradient in an arid savanna

AbstractAim The structure of savannas ranges from scrub to woodland over broad geographical gradients. Here we examine the hypothesis that water availability is a major determinant of these structural differences by ascertaining the relationship between water availability and tree growth across a steep moisture gradient.Location The study site is a sub‐tropical savanna, with a mean annual precipitation of 400 mm, located in the Phugwane river basin, Kruger National Park, Limpopo Province, South Africa.Methods We determined plant moisture stress using xylem pressure potentials, stem growth using dendrometer bands, and estimated the water sources available to plants using stable hydrogen and oxygen isotope ratios. The primary objective was to understand tree growth relative to available water along an environmental gradient.Results We found that Philenoptera violacea trees growing close to the stream have lower water stress (least negative xylem pressure potentials) and higher cumulative growth than those growing away from the stream. The stem growth of P. violacea was characterized by steady incremental growth and could not be related to antecedent rainfall. Colophospermum mopane trees experienced higher water stress, yet trees growing adjacent to the stream achieved the highest cumulative growth rates over the study period. The growth of C. mopane could be clearly linked to antecedent rainfall, and most growth was achieved during short growth pulses that followed rainfall events. δ18O values become progressively more enriched in the heavier isotope with distance from the stream, suggesting that access to groundwater decreases with distance from the stream side. The isotopic data suggest that P. violacea has access to groundwater, but that C. mopane does not.Main conclusions Our results show that water stress increases with distance from the stream side as a result of reduced access to groundwater. Trees without access to deep water adopt an opportunistic growth strategy.

Differential transpiration by three boreal tree species in response to increased evaporative demand after variable retention harvesting

We compared the change in microclimate and tree water relations between a boreal mixedwood, variable retention (VR), partially harvested stand and an adjacent, unharvested control stand. A nearly three-fold increase in potential evapotranspiration (ET P) at the crown level in the VR site was approximately proportional to a 2.8-fold increase in wind speed ( u), along with subsidiary increases in net radiation ( Q *) and vapour pressure deficit ( D) after harvesting. Soil volumetric moisture content ( θ v) also increased, while there were negligible changes in air temperature ( T a) and relative humidity (RH) after partial harvesting. Whole-tree sap flow response to cutting was measured in white spruce ( Picea glauca), balsam poplar ( Populus balsamifera) and paper birch ( Betula papyrifera) with thermal-dissipation sap flow sensors. After partial harvesting, transpiration per unit leaf area ( Q l) in all three species began earlier in the morning and extended later in the day in VR trees than control trees. Mean maximum sap flow rates per leaf area ( Q l-max) during mid-day for P. glauca in the VR site were about 2.5 times greater than in the control trees, while B. papyrifera Q l-max was approximately 1.6 times higher in the VR site. For P. balsamifera, however, Q l-max was only marginally greater in the VR site than in the control. This suggests stomatal closure by the P. balsamifera and B. papyrifera residual trees likely occurred to prevent excessive xylem cavitation. Xylem pressure potential ( Ψ x) measurements of twigs also indicated water stress in both P. balsamifera and B. papyrifera in the VR stand, but not for P. glauca. However, similarity of sapwood hydraulic conductivity ( K Ψ ) between the two sites, for all three species, showed that xylem cavitation of the main stems was insignificant following VR harvesting. Decoupling coefficients ( Ω) indicated that all three species were more coupled to the atmosphere in the VR site than in the control. Species differences in susceptibility to atmospheric moisture-stress induced cavitation, combined with differences in stress-coping mechanisms and physiology, appeared to influence the response in transpiration to increased ET P following VR harvesting. Species susceptibility to atmospheric moisture-stress due to increased ET P following partial harvesting can be ranked as: P. balsamifera > B. papyrifera > P. glauca.