Predawn Xylem Water Potential Research Articles

From comfort zone to mortality: Sequence of physiological stress thresholds in Robinia pseudoacacia seedlings during progressive drought.

Parameterizing the process of trees from the comfort zone to mortality during progressive drought is important for, but is not well represented in, vegetation models, given the lack of appropriate indices to gauge the response of trees to droughts. The objective of this study was to determine reliable and readily available tree drought stressindices and the thresholds at which droughts activate important physiological responses. We analyzed the changes in the transpiration (T), stomatal conductance, xylem conductance, and leaf health status due to a decrease in soil water availability (SWA), predawn xylem water potential (ψpd), and midday xylem water potential (ψmd) in Robinia pseudoacacia seedlings during progressive drought. The results showed that ψmd was a better indicator of drought stress than SWA and ψpd, because ψmd was more closely related to the physiological response (defoliation and xylem embolization) during severe drought and could be measured more conveniently. We derived the following five stress levels from the observed responses to decreasing ψmd: comfort zone (ψmd > -0.9 MPa), wherein transpiration and stomatal conductance are not limited by SWA; moderate drought stress (-0.9 to -1.75 MPa), wherein transpiration and stomatal conductance are limited by drought; high drought stress (-1.75 to -2.59 MPa), wherein transpiration decreases significantly (T< 10%) and stomata closes completely; severe drought stress (-2.59 to -4.02 MPa), wherein transpiration ceases (T< 0.1%) and leaf shedding orwilting is > 50%; and extreme drought stress (< -4.02 MPa), leading to tree mortality due to xylem hydraulic failure. To our knowledge, our scheme is the first to outline the quantitative thresholds for the downregulation of physiological processes in R. pseudoacacia during drought, therefore, can be used to synthesize valuable information for process-based vegetation models.

Non-structural carbohydrate dynamics associated with antecedent stem water potential and air temperature in a dominant desert shrub.

Non-structural carbohydrates (NSCs) are necessary for plant growth and affected by plant water status, but the temporal dynamics of water stress impacts on NSC are not well understood. We evaluated how seasonal NSC concentrations varied with plant water status (predawn xylem water potential, Ψ) and air temperature (T) in the evergreen desert shrub Larrea tridentata. Aboveground sugar and starch concentrations were measured weekly or monthly for ~1.5 years on 6-12 shrubs simultaneously instrumented with automated stem psychrometers; leaf photosynthesis (Anet ) was measured monthly for 1 year. Leaf sugar increased during the dry, premonsoon period, associated with lower Ψ (greater water stress) and high T. Leaf sugar accumulation coincided with declines in leaf starch and stem sugar, suggesting the prioritization of leaf sugar during low photosynthetic uptake. Leaf starch was strongly correlated with Anet and peaked during the spring and monsoon seasons, while stem starch remained relatively constant except for depletion during the monsoon. Recent photosynthate appeared sufficient to support spring growth, while monsoon growth required the remobilization of stem starch reserves. The coordinated responses of different NSC fractions to water status, photosynthesis, and growth demands suggest that NSCs serve multiple functions under extreme environmental conditions, including severe drought.

Drought stress responses of seedlings of two oak species (Quercus cerris and Quercus robur)

Effects of drought stress on growth, predawn xylem water potential (ψ pd ), osmotic solutes (soluble sugar and proline), and stomatal conductance were assessed in two oak ( Quercus cerris L. and Quercus robur L.) seedlings. Seedlings of both species were subjected to three drought treatments with the following irrigation intervals: well-watered (control: irrigation every 2-3 days), moderate drought stress (irrigation every 15 days), and severe drought stress (irrigation every 30 days). Drought-stressed seedlings of Q. cerris and Q. robur had more negative predawn xylem water potential than their well-watered seedlings. In Q. cerris , root collar diameter and root dry weights were negatively influenced by drought, while height, shoot dry weight, and root:shoot ratios were unaffected. Height and shoot dry weight of drought-stressed Q. robur were decreased, while root:shoot ratio increased. Drought caused significant decreases in stomatal conductance of both species. Q. cerris seedlings sustained higher stomatal conductance compared with Q. robur . Proline and soluble sugar increased in response to drought stress. Q. robur had a higher proline accumulation than Q. cerris . These findings suggested that the Q. cerris and Q. robur seedlings showed a drought stress adaptive mechanism either by restricting their growth or increasing root:shoot ratio or by decreasing water loss (reduced stomatal conductance) and accumulating of osmotic solutes.

Morphological, physiological and biochemical responses to drought stress of Stone pine (Pinus pinea L.) seedlings

In this study, the effect of irrigation intervals (drought stress) on growth, predawn xylem water potential (Ψ w), the osmotic potential at full turgor (Ψπ 100), the osmotic potential at the turgor loss point (Ψπ TLP), osmotic adjustment and osmotic solutes (soluble sugars and proline) of Pinus pinea L. seedlings were examined. An experiment was carried out under greenhouse conditions using four watering treatments (control, 7-, 14- and 21-day irrigation intervals) in the first growth season; from mid-July to early November. Results showed that irrigation interval had significant effect on growth characteristics, Ψ w, water relation parameters, and osmotic solutes. The increasing irrigation interval significantly decreased the seedling height, root collar diameter, root, stem and needle dry weight, number of lateral branches, root percentage, root:shoot ratio and diameter:height ratio. Ψ w and total soluble sugars decreased while proline content increased with the increase of drought stress. The Ψπ 100 and Ψπ TLP significantly decreased in drought-stressed seedlings compared to control (no stress) seedlings. The results suggest that the impact of drought stress increased with the increase of irrigation interval. Therefore, in the drought-stressed P. pinea seedlings were indicated osmotic adjustment by increasing the proline content and decreasing Ψπ 100 and Ψπ TLP during drought stress. Growth decreased under drought stress conditions in P. pinea seedlings.

Physiological and Proteomic Analyses of Drought Stress Response in Holm Oak Provenances

Responses to drought stress by water withholding have been studied in 1 year old Holm oak (Quercus ilex subsp. ballota [Desf.] Samp.) seedlings from seven provenances from Andalusia (southern Spain). Several physiological parameters, including predawn xylem water potentials and relative water content in soil, roots, and leaves as well as maximum quantum efficiency and yield of PSII were evaluated for 28 days in both irrigated and nonirrigated seedlings. The leaf proteome map of the two provenances that show the extreme responses (Seville, GSE, is the most susceptible, while Almerı́a, SSA, is the least susceptible) was obtained. Statistically significant variable spots among provenances and treatments were subjected to MALDI-TOF/TOF-MS/MS analysis for protein identification. In response to drought stress, ~12.4% of the reproducible spots varied significantly depending on the treatment and the population. These variable proteins were mainly chloroplastic and belonged to the metabolism and defense/stress functional categories. The 2-DE protein profile of nonirrigated seedlings was similar in both provenances. Physiological and proteomics data were generally in good agreement. The general trend was a decrease in protein abundance upon water withholding in both provenances, mainly in those involved in ATP synthesis and photosynthesis. This decrease, moreover, was most marked in the most susceptible population compared with the less susceptible one.

Water relations of selected wallum species in dry sclerophyll woodland on the lower north coast of New South Wales, Australia

The present study examined the water relations of wallum dry sclerophyll woodland on the lower north coast of New South Wales (NSW). Wallum is the regionally distinct vegetation of Quaternary dunefields and beach ridge plains along the eastern coast of Australia. Wallum sand masses contain large aquifers, and previous studies have suggested that many of the plant species may be groundwater dependent. However, the extent of this dependency is largely unknown, despite an increasing reliance on the aquifers for groundwater extraction. Fifteen species from five growth-form categories and seven plant families were investigated. The pre-dawn and midday xylem water potential (ψx) of all species was monitored over a 20-month period from December 2007 to July 2009. Pressure–volume curve traits were determined for each species in late autumn 2008, including the osmotic potential at full (π100) and zero (π0) turgor, and bulk modulus of elasticity (ε). Carbon isotope ratios (δ13C) were determined in mid-autumn 2008 to measure water use efficiency (WUE). Comparative differences in water relations could be loosely related to growth forms. A tree (Eucalyptus racemosa subsp. racemosa) and most large shrubs had low midday ψx, π100 and π0, and high ε and WUE; whereas the majority of small and medium shrubs had high midday ψx, π100 and π0, and low ε and WUE. However, some species of similar growth form displayed contrasting behaviour in their water relations (e.g. the herbs Caustis recurvata var. recurvata and Hypolaena fastigiata), and such differences require further investigation. The results suggest that E. racemosa subsp. racemosa is likely to be groundwater dependent, and large shrubs such as Banksia aemula may also utilise groundwater. Both species are widespread in wallum, and therefore have the potential to play a key role in monitoring ecosystem health where aquifers are subject to groundwater extraction.

Phenological and water-use patterns underlying maximum growing season length at the highest elevations: implications under climate change

Consequences of climate change on tree phenology are readily observable, but little is known about the variations in phenological sensitivity to drought between populations within a species. In this study, we compare the phenological sensitivity to temperature and water availability in Abies pinsapo Boiss., a drought-sensitive Mediterranean fir, across its altitudinal distribution gradient. Twig growth and needle fall were related to temperature, precipitation and plant water status on a daily scale. Stands located at the top edge of the distributional range showed the most favourable water balance, maximum growth rates and little summer defoliation. Towards higher elevations, the observed delay in budburst date due to lower spring temperatures was overcome by a stronger delay in growth cessation date due to the later onset of strong water-deficit conditions in the summer. This explains an extended growing season and the greatest mean growth at the highest elevation. Conversely, lower predawn xylem water potentials and early partial stomatal closure and growth cessation were found in low-elevation A. pinsapo trees. An earlier and higher summer peak of A. pinsapo litterfall was also observed at these water-limited sites. Our results illustrate the ecophysiological background of the ongoing altitudinal shifts reported for this relict tree species under current climatic conditions.

Diurnal and seasonal changes in the xylem water potentials of Grevillea and Myoporum plants during drought

SummaryDrought is an irregular feature of World climate and has adverse effects on ornamental plants growing in urban environments. Grevillea ‘Moonlight’ and Myoporum elipticum are attractive native Australian species that are commonly used in parks and gardens and are generally regarded as being drought-tolerant. Diurnal and seasonal water relations during a drought cycle of G. ‘Moonlight’ and M. elipticum, growing together in a non-irrigated garden bed, were monitored during a Summer (wetter)-to-Winter (dryer) transition period using a pressure chamber. G. ‘Moonlight’ generally maintained higher xylem water potentials than M. elipticum. Pre-dawn xylem water potentials in drought-affected M. elipticum dropped to around –2.5 MPa, while G. ‘Moonlight’ maintained pre-dawn xylem water potentials of around –1.0 MPa. This difference suggests that, because pre-dawn xylem water potentials reflect soil water potentials, G. ‘Moonlight’ accessed a different or additional source of soil water to M. elipticum. This study demonstrates the utility of the pressure chamber for describing the diurnal and seasonal water relations of ornamentals in a mixed planting in urban horticulture.

Water Uptake Patterns of an Invasive Exotic Plant in Coastal Saline Habitats

Schinus terebinthifolius Raddi (Schinus) is one of the most widely found woody exotic species in South Florida. This exotic is distributed across environments with different hydrologic regimes, from upland pine forests to the edges of sawgrass marshes and into saline mangrove forests. To determine if this invasive exotic had different physiological attributes compared to native species in a coastal habitat, we measured predawn xylem water potentials (Ψ), oxygen stable isotope signatures (δ18O), and sodium (Na ) and potassium (K ) contents of sap water from plants within: (1) a transition zone (between a mangrove forest and upland pineland) and (2) an upland pineland in Southwest Florida. Under dynamic salinity and hydrologic conditions, Ψ of Schinus appeared less subject to fluctuations caused by seasonality when compared with native species. Although stem water δ18O values could not be used to distinguish the depth of Schinus and native species' water uptake in the transition zone, Ψ and sap Na /K patterns showed that Schinus was less of a salt excluder relative to the native upland species during the dry season. This exotic also exhibited Na /K ratios similar to the mangrove species, indicating some salinity tolerance. In the upland pineland, Schinus water uptake patterns were not significantly different from those of native species. Differences between Schinus and native upland species, however, may provide this exotic an advantage over native species within mangrove transition zones.

Cottonwood understory zonation and its relation to floodplain stratigraphy

Abstract Species composition in floodplain vegetation is often related to initial floristics, succession, and disturbances such as flooding and herbivory. However, on a braided, cobble-bedded river with contrasting sediment facies, much of the composition of understory vegetation within a cottonwood forest was related to edaphic factors controlled by stream-deposit stratigraphy. Large shifts in understory species composition were more related to soil texture and water availability than deposit age or hydroperiod. Finer-textured and thicker overbank deposits had greater water availability and supported mesic species, while sites with coarse sediments to the surface supported only xeric species. A strong gradient of site moisture corresponded to a gradient in species composition. Soil water potential (Ψs) indicated by pre-dawn xylem water potential (XWPPD) in a shallow-rooted grass reached −4.2 MPa on xeric-species sites in mid-summer; on mesic-species sites, XWPPD was always greater than −1.5 MPa for the s...

Black spruce seedlings in a Kalmia–Vaccinium association: microsite manipulation to explore interactions in the field

We established a field trial on an ericaceous-dominated clearcut in Quebec to determine the effect of Kalmia angustifolia L., Vaccinium angustifolium (Ait.), and V. myrtilloides (Michx.) on the growth and physiology of black spruce (Picea mariana (Mill.) BSP) seedlings and on soil characteristics over the first two growing seasons. Plots undergoing one of three treatments (shrub removal, humus removal, or undisturbed control) were planted with black spruce seedlings that were either unfertilized or spot fertilized at time of planting. In some of the undisturbed control plots, we also used 15NH415NO3 to compare uptake of broadcast N fertilizer by vegetation. The ericaceous shrubs had a significant negative impact on seedling growth. Growth reductions were not related to water stress, soil temperature, or soil moisture. Extractable NH4-N and P concentrations in mineral soil tended to decrease in the presence of ericaceous shrubs, but effects were not significant. Seedling foliar N concentration was also reduced in the presence of ericaceous shrubs. Of the total amount of 15N fertilizer found in vegetation, 64% was immobilized in Vaccinium spp., 31% in Kalmia, and 5% in black spruce, but spruce took up more 15N per unit of root biomass than the ericaceous shrubs. Kalmia had consistently higher predawn xylem water potentials than black spruce.

Patterns of stomatal conductance among blue oak (Quercus douglasii) size classes and populations: implications for seedling establishment.

Quercus douglasii Hook. & Arn. (blue oak) is a deciduous white oak that is currently failing to regenerate throughout much of its range in California, USA. Patterns of water use were observed in adult trees, saplings and seedlings to determine if ontogenetic changes in water use occur, which might be important in the establishment of this long-lived perennial species in a Mediterranean-type system. Seasonal and diurnal stomatal conductance (g(s)), late-season predawn xylem water potentials (Psi(pre)), carbon isotopic ratio (delta(13)C) and soil water status were compared among the three size classes at three sites differing in mean precipitation and soil water characteristics. Comparisons were also made between microsites with and without regeneration (defined by the presence or absence of saplings). Overall patterns of water use were consistent among the three sites, except that, at the site with the highest rainfall, Q. douglasii plants had higher g(s) and more positive Psi(pre) values. Although no differences in water use patterns were found between regeneration and non-regeneration microsites, the observed ontogenetic differences in water use may have important implications for Q. douglasii establishment. Compared with adult trees and saplings, seedlings had higher gas exchange rates during periods of high soil water content (early in the season and in the morning). Seedling g(s) was correlated with percent extractable soil water (ESW) throughout the season; adult tree and sapling g(s) was correlated with ESW between June and September. Despite experiencing greater water stress (indicated by more negative Psi(pre) values) than older trees, seedlings had more negative delta(13)C values, implying lower water-use efficiencies.

Spatial and Temporal Patterns of Xylem Water Potential in Juniperus ashei Seedlings

Juniperus ashei woodlands are found extensively throughout the Edwards Plateau of central Texas. Prior studies indicate that seed germination and seedling survival in J. ashei was greatest under the canopy whereas growth was greatest at the canopy edge. To investigate the cause of these spatial differences, measurements of percent soil moisture and plant predawn xylem water potential (ψ p ) for J. ashei seedlings were made along transects from under a woodland canopy into an intercanopy patch. A soil moisture gradient was detected, with soil moisture highest under the woodland canopy and decreasing into the intercanopy patch. Soil moisture was lowest during the summer drought along this gradient. Mean soil moisture was highest at 68.4 ′ 7.0% (′SE) under the Juniperus canopy in May 1997 and lowest at 12.5 ′ 1.9% in the intercanopy patch during July 1997. Seedling ψ p under the Juniperus canopy ranged from values of -0.51 ′ 0.02 MPa in May to -6.59 ′ 0.00 MPa in August Plant ψ p at the canopy edge was -0.72 ′ 0.03 MPa in May and -6.50 ′ 0.00 MPa in August. No seedlings were found in the intercanopy patch. Spatial and temporal differences in seedling ψ p between the Juniperus woodland and canopy edge did exist, but differences were small (<0.20 MPa). Temporal decreases in soil moisture were followed by more negative seedling ψ p values. Differences in observed ψ p along this woodland to intercanopy patch gradient probably cause reductions in photosynthesis and growth rates in J. ashei, and are likely due to an interaction of several abiotic factors, including soil temperature, soil depth, soil moisture, surface light levels, and soil organic content.

Soil water content and water relations in planted and naturally regenerated Pinus halepensis Mill. seedlings during the first year in semiarid conditions

The capacity of Aleppo pine (Pinus halpensis Mill.) seedlings to overcome the planting shock in dry conditions was assessed by firstly studying the survival and water status during the first year after planting in relation to soil water content. In spite of receiving only 177 mm rainfall during the year studied, survival of planted Aleppo pine was very high (88.5%). Soil water during summer months (after receiving 67 mm rainfall in winter and spring) was only available at 30–60 cm depth, with tension values of −1.1 and −1.3 MPa in July and August respectively; in these conditions, July predawn xylem water potential measured was −2.5 MPa, and midday potential was −3.6 MPa. According to different authors, these values don't jeopardize the survival of Aleppo pine. In addition, acclimation of outplanted seedlings to environmental conditions was followed by comparing their water relations with those of naturally regenerated seedlings on the site. Predawn and midday xylem water potential showed differences in favour of outplanted seedlings since June, indicating an adjustment to this dry site. Compared to naturally regenerated trees, nursery grown stock of the same age before field planting had much more biomass and higher N and P concentrations and contents; although shoot:root balance and Dickson quality index were not significantly different. Finally, planted seedlings acclimation level during first year was also evaluated by Transplant Stress Index, which value (−0.1278) indicated a slight planting impact.

Storage duration and temperature effect on the functional integrity of container and bare-root Pinus radiata D. Don stock-types

We have investigated the effect of storage conditions (duration: 1, 8 or 15 days; temperature: 4°C or 10°C) and root coverage [bare root (BR) or soil-plugged root (PR)] on the water status, electrolyte leakage and root growth potential (number of new roots and new root length) of radiata pine (Pinus radiata D. Don) seedlings. We have also examined the effects that storage causes on plant quality and initial survival potential and their usefulness for detecting physiological damage to seedlings. We observed a gradual decrease of seedling water status and an increase in electrolyte leakage, both in root and shoot, with duration of storage, the latter indicating damage to membranes. Both storage temperature and duration influenced the ability of radiata pine seedlings to initiate and elongate new roots. The effect was more pronounced in BR than in container seedlings, suggesting that the rooting-plug medium provides protection against desiccation. The close relationship (r=0.923, P<0.05) between root growth potential and predawn xylem water potential before transplanting shows that water potential at the time of transplanting is a reliable predictor of the ability to generate new roots. Finally, we have observed a close relationship between survival and parameters such as electrolyte conductivity, post-storage water potential, new root length and root growth potential.

Mediterranean climate effects. I. Conifer water use across a Sierra Nevada ecotone

Xylem water potential of the midelevation conifers Pinus jeffreyi, Pinus lambertiana, Abies concolor, and Calocedrus decurrens, the higher elevation Pinus monticola and Abies magnifica, and co-occurring evergreen angiosperm shrubs, together with soil moisture under these plants, were monitored at three sites on the Kern Plateau in the southernmost Sierra Nevada Range of California. Site locations spanned the ecotone between the mid- and upper montane forests at elevations of 2230-2820 m. Measurements were made through a low-snowfall year and a heavy-snowfall year.In the Mediterranean climate of the Sierra Nevada, the heavy winter snowpack persists into late spring, after precipitation has effectively stopped. We found the subsequent depletion of soil moisture due to plant water uptake to result in predawn xylem water potentials for conifers more negative by 0.6-1.4 MPa than those for shrubs or inferred soil potentials. Shrubs generally depleted soil moisture more rapidly and ultimately extracted a greater fraction of the available soil moisture than did the conifers. This depletion of soil moisture by shrubs, particularly Arctostaphylos patula, may limit conifer growth and regeneration by prematurely terminating growth on the shallow soils studied. The conifers all generally showed similar patterns of soil moisture use, except that A. magnifica extracted moisture more rapidly early in the season.

THE WATER RELATIONS OF MATURE `SUNDROP' APRICOT TREES IN RESPONSE TO DIFFERENT VIGOUR CONTROL TECHNIQUES

In the absence of dwarfing rootstocks for apricot, techniques which reduce vegetative growth are important in the orchard management system. A number of techniques may be employed to moderate vegetative vigour of fruit trees. In the orchard in the humid climate of Palmerston North, New Zealand, five-year-old vigorousSundrop' apricot trees on `Golden Queen' peach seedlings trained on Tatura trellis at 2000 tree ha -1 were used. The treatments were regulated deficit irrigation (RDI), dormant root-pruning, soil applied Paclobutrazol (PBZ) and untreated control. The objectives were to examine the effects of these treatments on the internal water relations of apricot. Also, to determine whether the mechanism of response to root pruning appeared similar to the mechanism of response to water stress. Throughout the season treatment effects on vegetative, reproductive growth and leaf physiology of the trees were monitored. Predawn and midday xylem water potential (Ψ xylem) in root pruned and water stressed trees were lower than in control trees, but Cultar treatments did not significantly affect this. PBZ treated trees had the same water status as controls. Root-pruned trees developed an increased internal water deficit in the leaves and fruits especially at the time of highest water demand during fruit stage III.

Effects of Restoration Thinning on Presettlement Pinus ponderosa in Northern Arizona

AbstractIn the 100 years following the arrival of Euro‐American settlers in northern Arizona, Pinus ponderosa (ponderosa pine) forests changed from open, low‐density stands to closed, high‐density stands. The increase in tree density has been detrimental to the vigor of old‐growth trees that established before settlement (presettlement trees). In this study, we examined whether the vigor of presettlement trees could be improved by restoring the original stand structure by thinning the ponderosa pines that established after settlement (postsettlement trees). The restoration treatment caused the following changes in the presettlement trees and their environment in the first year following thinning: an increase in volumetric soil water content between May and August, an increase in predawn xylem water potential in July and August, a decrease in midday xylem water potential in June and August, an increase in net photosynthetic rate in August, an increase in foliar nitrogen concentration in July and August, and an increase in bud and needle size. The results show that the thinning restoration treatment improved the condition of presettlement ponderosa pines by increasing canopy growth and the uptake of water, nitrogen, and carbon.

Initial size and competing vegetation effects on water stress and growth of Picea mariana (Mill.) BSP seedlings planted in three different environments

Three experimental sites in Québec were planted with four different sizes of containerized black spruce seedlings ( Picea mariana (Mill.) BSP). We examined the water stress experienced by each stock size of black spruce seedlings in relation to different competing vegetation covers and also the effect of the water stress on spruce growth during the first three growing seasons. The sites consisted of one abandoned agricultural field and two forest locations. Containers of sizes 45–110, 45–340, 15–700, and 12–1000 were employed to produce the four different sizes of spruce seedlings. At each site, the experimental protocol used a split-plot in a randomized complete block design, in which the presence of a competing vegetation cover (weedy and bare plots) was assigned to the whole plot, while a specific seedling size was assigned to each subplot. At each experimental site both the predawn xylem water potential Ψ xp and the midday value Ψ xm were measured three times during each of the first three growing seasons. Data were analysed as a completely randomized split-split-plot design, where selection of seedlings in time was considered as the whole plot. The competing vegetation tended to protect the spruce seedlings from excessive water loss, without depressing the soil-water potential (SWP) to the point of reducing the moisture available to the seedlings. Both Ψ xp and Ψ xm were found to decrease significantly with increasing initial seedling size. The increased water stress experienced by the large stock of spruce seedlings had an effect on the absolute growth rate (AGR) in height on only one experimental site. The AGR was impaired by the presence of a competing vegetation cover, but more severely for the smaller stock-size than the larger. The short-term effect of a competition should be based on radial growth; height growth and mortality are not early indicators of such effect. These results emphasize the need to produce a large stock of spruce seedlings with well-developed root systems and root growth capacity, even though only moderate water stress was observed during the first three years of plantation growth.

Elevated atmospheric CO2 and soil N fertility effects on growth, mycorrhizal colonization, and xylem water potential of juvenile ponderosa pine in a field soil

Interactive effects of atmospheric CO2 enrichment and soil N fertility on above- and below-ground development and water relations of juvenile ponderosa pine (Pinus ponderosa Dougl. ex Laws.) were examined. Open-top field chambers permitted creation of atmospheres with 700 µL L-1, 525 µL L-1, or ambient CO2 concentrations. Seedlings were reared from seed in field soil with a total N concentration of approximately 900 µg g-1 or in soil amended with sufficient (NH4)2SO4 to increase total N by 100 µg g-1 or 200 µg g-1. The 525 µL L-1 CO2 treatment within the intermediate N treatment was excluded from the study. Following each of three consecutive growing seasons, whole seedlings of each combination of CO2 and N treatment were harvested to permit assessment of shoot and root growth and ectomycorrhizal colonization. In the second and third growing seasons, drought cycles were imposed by withholding irrigation during which predawn and midday xylem water potential and soil water potential were measured. The first harvest revealed that shoot weight and coarse and fine root weights were increased by growth in elevated CO2. Shoot and root volume and weights were increased by CO2 enrichment at the second harvest, but growth stimulation by the 525 µL L-1 CO2 concentration exceeded that in 700 µL L-1 CO2 during the first two growing seasons. At the third harvest, above- and below-ground growth increases were largely confined to the 700 µL L-1 CO2 treatment, an effect accentuated by high soil N but evident in all N treatments. Ectomycorrhizal formation was reduced by elevated CO2 after one growing season, but thereafter was not significantly affected by CO2 and was unaffected by soil N throughout the study. Results of the xylem water potential measurements were variable, as water potentials in seedlings grown in elevated CO2 were intermittently higher on some measurement days but lower on others than that of seedlings grown in the ambient atmosphere. These results suggest that elevated CO2 exerts stimulatory effects on shoot and root growth of juvenile ponderosa pine under field conditions which are somewhat dependent on N availability, but that temporal variation may periodically result in a greater response to a moderate rise in atmospheric CO2 than to a doubling of the current ambient concentration.