Predawn Shoot Water Potential Research Articles

Multi-level physiological and morphological adjustment of Haloxylon ammodendron related to groundwater drawdown in a desert ecosystem

Climate change and anthropogenic exploitation will increase groundwater lowering rates and aggravate groundwater shortage, particularly in desert ecosystems that rely heavily on groundwater. We conducted detailed studies about morphological and ecophysiological traits of two Haloxylon ammodendron communities across different groundwater depths in the Gurbantonggut Desert in China during an extreme drought period. When photosynthetic twig temperatures exceeded 45°C, photosynthesis was suppressed and dropped to zero, while stomatal conductance and transpiration rate increased rapidly. This decoupled behavior may cool photosynthetic tissues to prevent permanent damage. The optimum temperature decreased by ∼3°C in H. ammodendron at deep groundwater sites with less soil-water availability, which contributed to improving water use efficiency (WUE) at lower temperatures. H. ammodendron at the deep groundwater site had lower photosynthetic twig size and plant height while higher proportion of twig and branch biomass, which induced a denser canopy structure, and they had greater leaf dry mass content, photosynthetic pigments, and non-structural carbohydrates, and more negative predawn shoot water potential. Overall, our study indicates that H. ammodendron could alleviate adverse effects of reduced soil water availability and maintain photosynthetic capacity through multiple morphological and physiological adjustments. This establishes the basis for predicting responses of desert shrubs to groundwater drawdown.

Drought affects the performance of native oak seedlings more strongly than competition with invasive crested wattle seedlings.

Two of the most important processes threatening vulnerable plant species are competitive displacement by invasive alien species and water stress due to global warming. Quercus lusitanica, an oak shrub species with remarkable conservation interest, could be threatened by the expansion of the invasive alien tree Paraserianthes lophantha. However, it is unclear how competition would interact with predicted reductions in water availability due to global climate change. We set up a full factorial experiment to examine the direct interspecific competition between P. lophantha and Q. lusitanica seedlings under control and water-limited conditions. We measured seed biomass, germination, seedling emergence, leaf relative growth rate, biomass, root/shoot ratio, predawn shoot water potential and mortality to assess the individual and combined effects of water stress and interspecific competition on both species. Our results indicate that, at seedling stage, both species experience competitive effects and responses. However, water stress exhibited a stronger overall effect than competition. Although both species responded strongly to water stress, the invasive P. lophantha exhibited significantly less drought stress than the native Q. lusitanica based on predawn shoot water potential measurements. The findings of this study suggest that the competition with invasive P. lophantha in the short term must not be dismissed, but that the long-term conservation of the native shrub Q. lusitanica could be compromised by increased drought as a result of global change. Our work sheds light on the combined effects of biological invasions and climate change that can negatively affect vulnerable plant species.

Investigating the relationship between various measuring methods for determination of establishment success of urban trees

Establishment is a key concept in urban forestry but it is currently inconsistently defined and measured. Thus, several different methods are being used to determine establishment success but their consequences and applications are rarely discussed. With this paper we would like to stimulate an increased discussion regarding these concepts both in relation to a theoretical definition but also to their practical use. The problem was approached through an experiment using sweet cherry (Prunus avium L.) and northern red oak (Quercus rubra L.) trees and the most common methods used for determination of establishment success. The trees were studied during the first three years after transplant and the association between the different measuring methods was examined. A Principal Component Analysis showed that terminal and lateral shoot length were strongly correlated, and that midday- and pre-dawn shoot water potential, and stomatal conductance were strongly correlated. We developed an index for nightly recovery of water status, which showed that terminal shoot growth was not related to nightly recovery until the third year after transplanting. Our results suggest that successful tree establishment is determined differently depending on which method is used for determination but that the differences might decrease with time. The lack of a firm definition of the term establishment may complicate communication, both within the scientific community and in practice.

Post-fire environments are favourable for plant functioning of seeder and resprouter Mediterranean shrubs, even under drought.

Understanding how drought affects seeder and resprouter plants during post-fire regeneration is important for the anticipation of Mediterranean vegetation vulnerability in a context of increasing drought and fire caused by climate change. A Mediterranean shrubland was subjected to various drought treatments (including 45% rainfall reduction, 7months droughtyr-1 ), before and after experimental burning, by means of a rainout-shelter system with an irrigation facility. Predawn shoot water potential (Ψpd ), relative growth rate (RGR), specific leaf area (SLA) and bulk leaf carbon isotopic composition (δ13 C) were monitored in the main woody species during the first 3yr after fire. Cistus ladanifer seedlings showed higher Ψpd , RGR and SLA, and lower δ13 C, than unburned plants during the first two post-fire years. Seedlings under drought maintained relatively high Ψpd , but suffered a decrease in Ψpd and RGR, and an increase in δ13 C, relative to control treatments. Erica arborea, E.scoparia and Phillyrea angustifolia resprouts had higher Ψpd and RGR than unburned plants during the first post-fire year. Resprouters were largely unaffected by drought. Overall, despite marked differences between the two functional groups, post-fire environments were favourable for plant functioning of both seeder and resprouter shrubs, even under the most severe drought conditions implemented.

High-intensity thinning treatments in mature Pinus halepensis plantations experiencing prolonged drought

Pinus halepensis plantations are widespread throughout semiarid–subhumid landscapes of the Mediterranean. Recently, drought-induced decline has often been reported raising concerns with regard to the future of these man-made ecosystems. The study was set out to investigate thinning as a means to alleviate water stress and improve performance of mature P. halepensis plantations experiencing prolonged drought. The study was conducted in a 40-year-old P. halepensis forest in the Jerusalem Mountains of Israel (rainfall: 550 mm year−1). Declining stands (stand basal area increment, BAIstand ≈ −3 % year−1) were treated by thinning: (1) intense thinning—tree density, BAstand, and leaf area index (LAI) reduced by 81, 68, and 59 %, respectively; (2) moderate—56, 48, and 26 %, respectively; (3) control (~560 tree ha−1). Plots of 70 × 70 m were used in four replicates. Individual tree- to stand-level variables were monitored during 4 years through stem and leaf area metrics alongside with predawn shoot water potential (ψ PD) and tree mortality. Thinning ameliorated drought stress, reduced mortality, and improved individual tree growth (ψ PD = −1.7, −1.8, and −2.0 MPa; mortality = 0.2, 2, and 5 % year−1; BAItree = 3.4, 2.0, and 1.4 % year−1 in intense, moderate, and control treatments, respectively). Thinning effects became more pronounced with time. LAI and individual tree leaf area (LAtree) fluctuated with association to annual rainfall. Higher LAtree caused by thinning reflected a “selection effect” while increased leaf area efficiency (BAI per unit LAtree) was attributed to a “release effect” of thinning.

Visual assessments of establishment success in urban Prunus avium (L.) and Quercus rubra (L.) in relation to water status and crown morphological characteristics

Assessing the establishment of newly planted urban trees is important to ensure that establishment management is maintained as long as needed. Such assessment is often based on visual appraisal of the post-transplant vitality two or three years after planting. How visual assessments relate to water status in the trees, or which morphological characteristics that are driving forces in such assessments have received limited attention in past research. In this interdisciplinary study, several leaf characteristics, such as leaf shape, size and colour were determined on 50 trees each of sweet cherry (Prunus avium L.) and red oak (Quercus rubra L.), transplanted to two Swedish sites three seasons before the study. In addition, shoot length was measured, and crown density, crown shape and die-back were estimated. Visual establishment assessments were performed by professional evaluators, who also completed a questionnaire on their background knowledge and priorities during assessments. The study investigated which of these leaf and crown characteristics had the greatest impact on the visual assessments. In addition, it compared the characteristics and evaluator assessments against the tree water status, determined by pre-dawn shoot water potential measurements. Visual assessments proved to be unrelated to shoot water potential for both species but, for red oak the assessments were strongly related to leaf colour and leaf shape. Shoot water potential was related to leaf size but not to leaf colour, indicating that one of the most influential parameters for visual assessments of red oak were not well-suited for determining tree establishment success, if defined by water status. The assessments for sweet cherry were best explained by leaf area. Our findings suggest that leaf size is a good indicator of tree establishment and that leaf shape, the character considered least important by tree evaluators, is influential on tree assessments. These findings are important for the development of more accurate tree expert assessments.

Effect of water availability and fertilization on water status, growth, vigour and the resistance of Scots pine to fungal mass inoculation with Ophiostoma ips

The effect of water and nutrient availability on the performance of Scots pine (Pinus sylvestris L.) against Ophiostoma ips (Rumb.), a bark beetle-associated phytopathogenic blue-stain fungus, was investigated. Field-grown trees were subjected for 18 months to water-stress and/or fertilization, and the effects of such treatments on the needle nutrient status, tree vegetative growth and vigour were examined. At the end of the experimental period, the trees were mass-inoculated (800 inocula m−2) with the fungus, and the relationship between resource availability and tree performance against pathogen attack was also tested. Predawn shoot water potential (ΨPD) of irrigated trees was significantly higher than that of water-stressed trees, and fertilized trees had a significantly lower C/N ratio. The ΨPD values and needle nitrogen content suggest that resource-limited trees were under moderate stress. Improved nutrient availability significantly increased tree growth and tree vigour. However, no evidence for an effect of improved nutrient availability on tree fungal resistance was found in our study.

Variations physiologiques dans des populations de thuyas géants (Thuja plicata Donn ex D. Don) en réponse à une sécheresse courte

• Variation in the ability of western redcedar ( Thuja plicata Donn ex D. Don) populations to withstand water stress may exist because this species is found in coastal and interior biogeoclimatic subzones representing the full range of precipitation regimes in British Columbia, Canada.• Seven western redcedar populations from locations in British Columbia, representing a wide range of habitat types, were assessed for their gas exchange and water relations response to controlled drought. • Before drought, population variation occurred in stomatal conductance, net CO 2 assimilation rate and intrinsic water use efficiency and the relative water content at turgor loss point. During drought, populations had different responses of net CO 2 assimilation to decreasing predawn shoot water potential. After drought, populations differed in stomatal conductance and intrinsic water use efficiency, plus osmotic potential at turgor loss point, osmotic potential at saturation and apparent cuticular transpiration. Western redcedar populations from drier-inland habitats had a lower osmotic potential at turgor loss point, lower relative water content at turgor loss point and lower apparent cuticular transpiration in response to drought than populations from coastal origin with temperate maritime habitat.• Reduction of cuticular water loss and adjustments of cellular water relations in response to drought was found to occur among seven western redcedar populations originating along a precipitation gradient while; there were minimal population differences in the gas exchange response to drought.

Water stress responses of two Mediterranean tree species influenced by native soil microorganisms and inoculation with a plant growth promoting rhizobacterium

Soil microorganisms, such as plant growth-promoting rhizobacteria (PGPR), play crucial roles in plant growth, but their influence on plant water relations remains poorly explored. We studied the effects of native soil microorganisms and inoculation with the PGPR strain Aur6 of Pseudomonas fluorescens on water stress responses of seedlings of the drought-avoiding Pinus halepensis Mill. and the drought-tolerant Quercus coccifera L. Plant growth, nutrient concentrations and physiology (maximum photochemical efficiency of photosystem II (PSII; F(v)/F(m)), electron transport rate (ETR), stomatal conductance (g(s)) and predawn shoot water potential (Psi(PD))) were measured in well-watered plants, and in plants under moderate or severe water stress. Inoculation with PGPR and native soil microorganisms improved tree growth, and their interactions had either additive or synergistic effects. Both F(v)/F(m) and ETR were significantly affected by PGPR and native soil microorganisms. Marked differences in g(s) and Psi(PD) were found between species, confirming that they differ in mechanisms of response to water stress. A complex tree species x treatment interactive response to drought was observed. In P. halepensis, F(v)/F(m) and ETR were enhanced by PGPR and native soil microorganisms under well-watered conditions, but the effects of PGPR on Psi(PD) and g(s) were negative during a period of water stress. In Q. coccifera, F(v)/F(m) and ETR were unaffected or even reduced by inoculation under well-watered conditions, whereas Psi(PD) and g(s) were increased by PGPR during a period of water stress. Our results indicate that microbial associates of roots can significantly influence the response of tree seedlings to drought, but the magnitude and sign of this effect seems to depend on the water-use strategy of the species.

Resprout characteristics of creosote bush ( Larrea tridentata) when subjected to repeated vehicle damage

Studies were conducted in the central Mojave Desert to quantify how creosote bushes ( Larrea tridentata) respond to physical damage during large-scale military training exercises. Creosote bush possesses a resilient growth form that recovers from repeated physical damage via resprouts arising from meristems in stem bark below severed or crushed canopy units. At high levels of disturbance by heavy vehicles, nearly all individuals showed measurable breakage, but without additional damage each plant can regain a full canopy within 5 years under arid field conditions. Resprouts exhibited more vigorous growth and doubled the biomass accumulation stimulated by high rainfall of 1998, an El Niño year, vs. a normal year. New shoots of resprouted individuals were markedly different in morphological traits than canopy old growth and had slightly higher predawn shoot water potentials. The natural ability of this evergreen species to recover from cutting and crushing bodes well for re-establishment of creosote bush desert scrub communities following episodes of severe damage by vehicles.

Water relations link carbon and oxygen isotope discrimination to phloem sap sugar concentration in Eucalyptus globulus.

A strong correlation was previously observed between carbon isotope discrimination (Delta(13)C) of phloem sap sugars and phloem sap sugar concentration in the phloem-bleeding tree Eucalyptus globulus Labill. (J. Pate, E. Shedley, D. Arthur, M. Adams [1998] Oecologia 117: 312-322). We hypothesized that correspondence between these two parameters results from covarying responses to plant water potential. We expected Delta(13)C to decrease with decreasing plant water potential and phloem sap sugar concentration to increase, thereby maintaining turgor within sieve tubes. The hypothesis was tested with analyses of E. globulus trees growing on opposite ends of a rainfall gradient in southwestern Australia. The Delta(13)C of phloem sap sugars was closely related to phloem sap sugar concentration (r = -0.90, P < 0.0001, n = 40). As predicted, daytime shoot water potential was positively related to Delta(13)C (r = 0.70, P < 0.0001, n = 40) and negatively related to phloem sap sugar concentration (r = -0.86, P < 0.0001, n = 40). Additional measurements showed a strong correspondence between predawn shoot water potential and phloem sap sugar concentration measured at midday (r = -0.87, P < 0.0001, n = 30). The Delta(13)C of phloem sap sugars collected from the stem agreed well with that predicted from instantaneous measurements of the ratio of intercellular to ambient carbon dioxide concentrations on subtending donor leaves. In accordance, instantaneous ratio of intercellular to ambient carbon dioxide concentrations correlated negatively with phloem sap sugar concentration (r = -0.91, P < 0.0001, n = 27). Oxygen isotope enrichment (Delta(18)O) in phloem sap sugars also varied with phloem sap sugar concentration (r = 0.91, P < 0.0001, n = 39), consistent with predictions from a theoretical model of Delta(18)O. We conclude that drought induces correlated variation in the concentration of phloem sap sugars and their isotopic composition in E. globulus.

Comparative field water relations of three Mediterranean shrub species co-occurring at a natural CO(2) vent.

Annual variations in the water relations and stomatal response of Erica arborea, Myrtus communis and Juniperus communis occurring at a natural CO(2) vent were analysed under Mediterranean field conditions. A distinct gradient of CO(2)concentration ([CO(2)]) exists between two sites near a natural CO(2)-emitting vent, with higher [CO(2)] (700 micromol mol(-1)) in the proximity of the CO(2) spring. Plants at the CO(2) spring site have been growing for generations at elevated [CO(2)]. At both sites, maximum leaf conductance was related to predawn shoot water potential. The effects of water deficits during the summer drought were severe. Leaf conductance and water potential recovered after major rainfalls in September to predrought values. Strong relationships between leaf conductance, predawn water potential, and leaf-specific hydraulic resistance are consistent with the role of stomata in regulating plant water status. Considerable between-species variation in sensitivity of water potentials and stomatal characters to elevated [CO(2)] were observed. Common to all the shrubs were a reduction in leaf conductance and an increase in water potentials in response to elevated [CO(2)]. Elevated [CO(2)] decreased the sensitivity of leaf conductance to vapour pressure deficit. Morphological characters (including stomatal density and degree of sclerophylly) showed site-dependent variations, but degree and sign of such changes varied with the species and/or the season. Measurements of discrimination against (13)C provided evidence for long-term decreases of water use efficiency in CO(2) spring plants. Analysis of C isotope composition suggested that a downward adjustment of photosynthetic capacity may have occurred under elevated [CO(2)]. Elevated [CO(2)] effects on water relations and leaf morphology persisted in the long term, but the three shrubs growing in the same environment showed species-specific responses.

Physiological responses of black spruce layers and planted seedlings to nutrient addition.

We investigated effects of nutrient addition on several physiological characteristics of 60-cm-tall black spruce (Picea mariana Mill. B.S.P.) layers (i.e., rooted branches of overstory trees) and 20-cm-tall planted seedlings on a clear-cut, N-limited boreal site. After two growing seasons, current-year and one-year-old needles of fertilized trees (layers and seedlings combined) had higher net photosynthetic rates (A(n)) and maximum capacity of Rubisco for CO(2) fixation (V(max)) than unfertilized trees. One-year-old needles of fertilized trees had higher stomatal conductance (g(s)), higher water-use efficiency, and lower intercellular to ambient CO(2) ratio than unfertilized trees. Additionally, fertilized trees had higher predawn and midday shoot water potentials than unfertilized trees. Stomatal conductance of 1-year-old needles was 23% higher in seedlings than in layers, but there were no significant differences in g(s) of current-year needles between the regeneration types. For both needle age-classes, A(n) and V(max) of layers were 25 and 40% higher, respectively, than the corresponding values for seedlings. The higher values of A(n), V(max) and foliar N concentration of layers compared with seedlings after two growing seasons may be associated with the larger root systems of the layers compared with the transplanted seedlings.

Phenological and physiological responses of heavily dusted creosote bush (Larrea tridentata) to summer irrigation in the Mojave Desert

In the Mojave Desert, numerous human activities damage soil structure, often resulting in increased wind erosion and deposition of dust on plant leaves. Especially susceptible to dust coating are the resinous, evergreen leaves of the dominant creosote bush ( Larrea tridentata ) wherever these shrubs grow along unpaved roads and trails and at the margins of dry playas. A yearlong field study was conducted, begun during summer drought, to investigate effects of summer irrigation on heavily dusted plants of L. tridentata . Plant water relations, quantitative phenology, and gas exchange were monitored for sets of five plants under three conditions: dusted nonirrigated, dusted irrigated, and undusted nonirrigated (control). Following watering, dusted irrigated plants experienced rapid shoot growth and had significantly higher predawn shoot water potentials (Ψ = −2.5 MPa) than dusted nonirrigated plants (−5.5 MPa), and midday values were −3.1 to −4.1 MPa versus −5.5 to −6.4 MPa, respectively. For dusted plants, irrigation resulted in marked increases of assimilation (14.0 versus 1.24μmol m −2 s −1 ), stomatal conductance (0.19 versus 0.03 mol m −2 s −1 ), and linear growth, and accompanying new growth the old, dusted leaves were mostly abscised. All measures of water-use efficiency (WUE) were higher in controls than in dusted plants, and during the summer, dust was associated with a 40–90% reduction in shoot growth. Nonirrigated controls showed much greater water stress than dusted irrigated plants, but when irrigation was discontinued, the formerly irrigated plants rapidly converged on the physiological and phenological characteristics of not watered control plants. Dust deposition may reduce plant carbon gain by decreasing WUE and impairing tolerance of water stress. This experiment demonstrated that creosote bushes can recover rapidly from acute heavy dust deposition if irrigation, simulating heavy summer rainfall, is provided, and these results may be useful in measures to minimize the ecological impact of human activities in desert ecosystems.

Diurnal variations of photosynthesis and dew absorption by leaves in two evergreen shrubs growing in Mediterranean field conditions

The effects of summer drought, dew deposition on leaves and autumn rainfall on plant water relations and diurnal variations of photosynthesis were measured in two evergreen shrubs, rosemary (Rosmarinus officinalis) and lavender (Lavandula stoechas), grown in Mediterranean field conditions. Withholding water for 40 d caused a similar decrease in predawn shoot water potential (ψpd) from c.−0.4 to c.−1.3 MPa in both species, but a 50% decrease in the relative leaf water content in L. stoechas compared with 22% in R. officinalis. A similar decrease in CO2 assimilation rates by c. 75% was observed in water‐stressed plants of both species, although L. stoechas showed smaller photosynthesis: stomatal conductance ratio than R. officinalis (35 vs 45 μmol CO2:mol H2O). The relative quantum efficiency of photosystem II photochemistry also decreased by c. 45% at midday in water‐ stressed plants of both species. Nevertheless, neither L. stoechas nor R. officinalis suffered drought‐induced damage to photosystem II, as indicated by the maintenance of the ratio Fv:Fm throughout the experiment, associated with an increase in the carotenoid content per unit of chlorophyll by c. 62% and c. 30%, respectively, in water‐stressed plants. Only L. stoechas absorbed dew by leaves. In this species the occurrence of 6 d of dew over a 15‐d period improved relative leaf water content by c. 72% and shoot water potential by c. 0.5 MPa throughout the day in water‐stressed plants, although the photosynthetic capacity was not recovered until the occurrence of autumn rainfall. The ability of leaves to absorb dew allowed L. stoechas to restore plant water status, which is especially relevant in plants exposed to prolonged drought.

Water relations, stomatal response and transpiration of Quercus pubescens trees during summer in a Mediterranean carbon dioxide spring.

Variations in water relations and stomatal response of Quercus pubescens Willd. were analyzed under Mediterranean field conditions during two consecutive summers (1993 and 1994) at two locations characterized by different atmospheric CO(2) concentrations because of the presence at one of them of a CO(2) spring. Trees at the CO(2) spring site have been growing for generations in elevated atmospheric CO(2) concentrations. The heat-pulse velocity technique was used to estimate water use of trees during a 5-month period from June to November 1994. At the end of the sap flow measurements, the trees were harvested and foliage and sapwood area measured. At both sites, maximum leaf conductance was related to predawn shoot water potential. Effects of summer drought on plant water relations, including whole-plant transpiration, were severe, but leaf conductance and water potential recovered to predrought values after major rainfall in September. Leaf conductance, predawn water potential, and sometimes sap flow, decreased in parallel with increases in hydraulic resistance, reaching a minimum in midsummer. Hydraulic resistance was higher in trees at the control site than in trees at the CO(2) spring site. The effect of elevated CO(2) concentration on leaf conductance was less at high leaf-to-air water vapor pressure difference than at low leaf-to-air water vapor pressure difference. Mean and diurnal sap fluxes were consistently higher in trees at the control site than in trees at the CO(2) spring site. During the summer period, plant water use varied between the two sites. Trees at the control site had consistently higher sap flow at corresponding values of sapwood cross-sectional area than trees at the CO(2) spring site. Because trees at the CO(2) spring site supported a smaller foliage area for a corresponding sapwood cross-sectional area than trees at the control site, the overall mean sap flux/foliage area ratio did not differ between sites. The results are discussed in terms of effects of elevated CO(2) concentration on plant water use at the organ and whole-tree scale.

Comparison of growth and physiology of layers and naturally established seedlings of black spruce in a boreal cutover in Quebec

Growth and physiology of layers versus naturally established seedlings of boreal black spruce (Picea mariana (Mill.) BSP) were compared 15 years after a cutover in Quebec. During the first 8 years, height growth of seedlings was greater than that of layers, averaging 10.4 and 7.0 cm/year, respectively. For the last 5 years, annual height growth of layers and seedlings did not differ (25 cm/year; p &gt; 0.05). Over the entire 15-year period, total height growth of seedlings (251 cm) was greater than that of layers (220 cm), although total height did not differ (p &gt; 0.05) over the last 6 years. During the 15th growing season, there were no differences (p &gt; 0.05) for predawn shoot water potential, stomatal conductance, net photosynthesis, intercellular to ambient CO2 ratio, water use efficiency, and hydraulic conductance between layers and seedlings. For diurnal shoot water potential, seedlings showed slightly less stress than layers on two of the four sampling dates. Thus, in the first few years following the cutover, the slower growth observed for layers indicated that they had a longer acclimation period following the cutover. Afterwards, similar height growth, total height, and physiological characteristics of the two regeneration types indicated that layers can perform as well as naturally established seedlings.

Transpiration and stomatal behaviour of Quercus ilex plants during the summer in a Mediterranean carbon dioxide spring

Variations in the water relations and stomatal response of Quercus ilex were analysed under field conditions by comparing trees at two locations in a Mediterranean environment during two consecutive summers (1993 and 1994). We used the heat‐pulse velocity technique to estimate transpirational water use of trees during a 5 month period from June to November 1994. At the end of sap flow measurements, the trees were harvested, and the foliage and sapwood area measured. A distinct environmental gradient exists between the two sites with higher atmospheric CO2 concentrations in the proximity of a natural CO2 spring. Trees at the spring site have been growing for generations in elevated atmospheric CO2 concentrations. At both sites, maximum leaf conductance was related to predawn shoot water potential. The effects of water deficits on water relations and whole‐plant transpiration during the summer drought were severe. Leaf conductance and water potential recovered after major rainfall in September to predrought values. Sap flow, leaf conductance and predawn water potential decreased in parallel with increases in hydraulic resistance, reaching a minimum in mid‐summer. These relationships are in agreement with the hypothesis of the stomatal control of transpiration to prevent desiccation damage but also to avoid ‘runaway embolism’. Trees at the CO2 spring underwent less reduction in hydraulic resistance for a given value of predawn water potential. The decrease in leaf conductance caused by elevated CO2 was limited and tended to be less at high than at low atmospheric vapour pressure deficit. Mean (and diurnal) sap flux were consistently higher in the control site trees than in the CO2 spring trees. The degree of reduction in water use between the two sites varied among the summer periods. The control site trees had consistently higher sap flow at corresponding values of either sapwood cross‐sectional area or foliage area. Larger trees displayed smaller differences than smaller trees, between the control and the CO2 spring trees. A strong association between foliage area and sapwood cross‐sectional area was found in both the control and the CO2 spring trees, the latter supporting a smaller foliage area at the corresponding sapwood stem cross‐sectional area. The specific leaf area (SLA) of the foliage was not influenced by site. The results are discussed in terms of the effects of elevated CO2 on plant water use at the organ and whole‐tree scale.

Effects of cone-induction treatments on black spruce (Picea mariana) current-year needle development and gas exchange properties

Both drought and root pruning (RP) increased the number of cones induced when black spruce (Picea mariana (Mill.) B.S.P.) grafts were injected with gibberellins A(4/7) (GA), but their effects on predawn shoot water potential and current-year needle development differed. Drought decreased predawn shoot water potential (Psi(pd)), but only during the period when irrigation was withheld, and it had no effect on the growth or gas exchange properties of current-year needles. Conversely, root pruning had little effect on Psi(pd), but it resulted in trees with smaller current-year needles that had lower nitrogen and chlorophyll concentrations and reduced rates of gas exchange up to the later stages of shoot elongation compared with needles of control trees. These findings are discussed in relation to potential effects on the development of induced cones in the following growth cycle.

Growth and water relations of Eucalyptus marginata (jarrah) stands in response to thinning and fertilization.

We studied the effects of five thinning treatments (T1 = 5.5, T2 = 11, T3 = 16.5, T4 = 22.5 and T5 = 28.5 m(2) ha(-1) basal area under bark) x two fertilizer treatments (F0 = unfertilized and F1 = fertilized with 400 kg ha(-1) N plus 229 kg ha(-1) P) on growth and water relations of pole-sized Eucalyptus marginata J. Donn ex Sm. trees growing in southwestern Australia. Thinning reduced leaf area index (LAI) from 2.1 in the T4 and T5 treatments to 0.8 in the T1F0 treatment. Fertilizer had no effect on LAI in the T2, T4 or T5 treatments, but increased LAI by 45 and 20% in the T1 and T3 treatments, respectively. Thinning plus fertilizing increased diameter growth most in the fastest growing trees, from 0.4 cm year(-1) for trees in the T5F0 and T5F1 treatments to 0.7 and 1.2 cm year(-1) for trees in the T1F0 and T1F1 treatments, respectively. In both fertilizer treatments, stand basal area and volume growth increased with increasing stand density up to 15 m(2) ha(-1), and thereafter declined with increasing stand density, such that the growth rate of trees in the T5 treatment was only half of that at a stand density of 15 m(2) ha(-1). In response to fertilizer, growth rates of the slowest and fastest-growing trees increased from 0.35 and 3.5 m(2) ha(-1) year(-1) (F0) to 0.56 and 5.4 m(3) ha(-1) year(-1) (F1), respectively. Stand growth efficiency (growth per unit LAI) increased in response to thinning, and fertilizer increased stand growth efficiency at all stand densities. Throughout the dry season, T5 trees had lower predawn shoot water potentials (Psi(pd)) (minimum of -1.5 MPa) than T1 or T2 trees (minimum of -0.7 MPa). Fertilizer decreased Psi(pd) in T5 trees (by -0.9 and -1.5 MPa, respectively, in F0 and F1), but not in T1 or T2 trees. Stand growth rate was closely related to cumulative midday water stress (CMWS) over the dry season, and volume growth rate declined sharply from 6 m(3) ha(-1) year(-1) at a CMWS of 130 MPa days, to zero at a CMWS of 220 MPa days. Application of fertilizer to thinned stands increased LAI, stand growth efficiency and stand growth. In unthinned stands, fertilizer increased stand growth efficiency and stand growth; however, it also increased tree water stress, which limited the fertilizer-induced increases in LAI and growth. We attribute the increase in tree and stand growth in response to application of fertilizer to increased photosynthetic rates, increased allocation to stem wood, and in thinned stands also to higher LAIs.