Changes in the water relations of balsam fir and white birch saplings after thinning

In the autumn of 1987, young balsam fir (Abies balsamea (L.) Mill.) and white birch (Betula papyrifera Marsh.) trees were thinned and their water relations followed during the next two growing seasons. At the beginning of the first summer following treatment, thinned trees of both species had lower osmotic potentials at full saturation (Psi(pi,sat)) and at turgor loss point (Psi(pi,tlp)) compared with controls. At this time, Psi(pi,sat) was linearly related to the percentage of full sunlight reaching the trees. A higher sugar concentration in leaves was an important component of the lower Psi(pi,sat) of thinned trees. For the other two sampling dates during the first growing season after treatment and all three sampling dates during the second growing season after treatment, little osmotic adjustment of the thinned trees relative to the control tress was observed in either species. The absence of osmotic adjustment during the second growing season following thinning suggests that other mechanisms were responsible for the acclimation of the treated trees to the higher atmospheric evaporative demand. Sapwood permeability (k) of white birch was higher than that of balsam fir, but no differences in k or in sapwood area were found between treated and control trees of either species. Predawn water potentials (Psi(pred)) of treated trees were less negative than those of controls.     

Relationship between photosynthesis and leaf nitrogen concentration in ambient and elevated [CO2] in white birch seedlings

To study the effects of elevated CO2 concentration ([CO2]) on relationships between nitrogen (N) nutrition and foliar gas exchange parameters, white birch (Betula papyrifera Marsh.) seedlings were exposed to one of five N-supply regimes (10, 80, 150, 220, 290 mg N l(-1)) in either ambient [CO2] (360 micromol mol(-1)) or elevated [CO2] (720 micromol mol(-1)) in environment-controlled greenhouses. Foliar gas exchange and chlorophyll fluorescence were measured after 60 and 80 days of treatment. Photosynthesis showed a substantial down-regulation (up to 57%) in response to elevated [CO2] and the magnitude of the down-regulation generally decreased exponentially with increasing leaf N concentration. When measured at the growth [CO2], elevated [CO2] increased the overall rate of photosynthesis (P(n)) and instantaneous water-use efficiency (IWUE) by up to 69 and 236%, respectively, but decreased transpiration (E) and stomatal conductance (g(s)) in all N treatments. However, the degree of stimulation of photosynthesis by elevated [CO2] decreased as photosynthetic down-regulation increased from 60 days to 80 days of treatment. Elevated [CO2] significantly increased total photosynthetic electron transport in all N treatments at 60 days of treatment, but the effect was insignificant after 80 days of treatment. Both P(n) and IWUE generally increased with increasing leaf N concentration except at very high leaf N concentrations, where both P(n) and IWUE declined. The relationships of P(n) and IWUE with leaf N concentration were modeled with both a linear regression and a second-order polynomial function. Elevated [CO2] significantly and substantially increased the slope of the linear regression for IWUE, but had no significant effect on the slope for P(n). The optimal leaf N concentration for P(n) and IWUE derived from the polynomial function did not differ between the CO2 treatments when leaf N was expressed on a leaf area basis. However, the mass-based optimal leaf N concentration for P(n) was much lower in seedlings in elevated [CO2] than in ambient [CO2] (31.88 versus 37.00 mg g(-1)). Elevated [CO2] generally decreased mass-based leaf N concentration but had no significant effect on area-based leaf N concentration; however, maximum N concentration per unit leaf area was greater in elevated [CO2] than in ambient [CO2] (1.913 versus 1.547 g N m(-2)).     

Axial and radial water flow in the trunks of oak trees: a quantitative and qualitative analysis

Axial water flow in the trunks of mature oak trees (Quercus petraea (Matt.) Liebl. and Q. robur L.) was studied by four independent techniques: water absorption from a cut trunk, sap flowmeters, heat pulse velocity (HPV) and thermoimaging. Estimation of the total water flow with sap flowmeters, HPV and water absorption yielded comparable results. We concluded from dye colorations, thermograms and axial profiles of sap flow and heat pulse velocity that, in intact trunks, most of the flow occurred in the current-year ring, where early-wood vessels in the outermost ring were still functional. Nevertheless, there was significant flow in the older rings of the xylem. Total water flow through the trunk was only slightly reduced when air embolisms were artificially induced in early-wood vessels, probably because there was little change in hydraulic conductance in the root-leaf sap pathway. Embolization of the current-year vessels reactivated transport in the older rings.     

Effects of a nursery CO2 enriched atmosphere on the germination and seedling morphology of two Mediterranean oaks with contrasting leaf habit

The use of an enriched CO₂ atmosphere in tree nurseries has been envisaged as a promising technique to increase productivity and to obtain seedlings with a higher root/shoot ratio, an essential trait to respond to water stress in Mediterranean-type ecosystems. In that framework, we have analyzed the effects of three levels of atmospheric CO₂ concentration (350, 500 and 700ppm) on the germination rate, growth and morphology of seedlings of two Mediterranean oaks used in reforestation programs: the evergreen Quercus ilex L. and the deciduous Quercus cerrioides Wilk. et Costa. CO₂ enrichment increased the germination rate of Q. cerrioides (from 70±7 to 81±3%) while it decreased that of Q. ilex (from 71±10 to 41±12%). Seedlings of both species increased approximately 60% their total biomass in response to CO₂ enrichment but at two different CO₂ concentrations: 500ppm for Q. cerrioides and 700ppm for Q. ilex. This increase in seedlings biomass was entirely due to an augmentation of root biomass. Considering germination and biomass partitioning, an enriched CO₂ atmosphere might not be appropriate for growing Mediterranean evergreen oaks, such as Q. ilex, since it reduces acorn germination and the only gains in root biomass occur at a high concentration (700ppm). On the other hand, a moderate CO₂ enrichment (500ppm) appears as a promising nursery technique to stimulate the germination, growth and root/shoot ratio of deciduous oaks, such as Q. cerrioides.     

Effects of elevated concentrations of atmospheric CO2 and tropospheric O3 on leaf litter production and chemistry in trembling aspen and paper birch communities

Human activities are increasing the concentrations of atmospheric carbon dioxide ([CO2]) and tropospheric ozone ([O3]), potentially leading to changes in the quantity and chemical quality of leaf litter inputs to forest soils. Because the quality and quantity of labile and recalcitrant carbon (C) compounds influence forest productivity through changes in soil organic matter content, characterizing changes in leaf litter in response to environmental change is critical to understanding the effects of global change on forests. We assessed the independent and combined effects of elevated [CO2] and elevated [O3] on foliar litter production and chemistry in aspen (Populus tremuloides Michx.) and birch-(Betula papyrifera Marsh.) aspen communities at the Aspen free-air CO2 enrichment (FACE) experiment in Rhinelander, WI. Litter was analyzed for concentrations of C, nitrogen (N), soluble sugars, lipids, lignin, cellulose, hemicellulose and C-based defensive compounds (soluble phenolics and condensed tannins). Concentrations of these chemical compounds in naturally senesced litter were similar in aspen and birch-aspen communities among treatments, except for N, the C:N ratio and lipids. Elevated [CO2] significantly increased C:N (+8.7%), lowered mean litter N concentration (-10.7%) but had no effect on the concentrations of soluble sugars, soluble phenolics and condensed tannins. Elevated [CO2] significantly increased litter biomass production (+33.3%), resulting in significant increases in fluxes of N, soluble sugars, soluble phenolics and condensed tannins to the soil. Elevated [O3] significantly increased litter concentrations of soluble sugars (+78.1%), soluble phenolics (+53.1%) and condensed tannins (+77.2%). There were no significant effects of elevated [CO2] or elevated [O3] on the concentrations of individual C structural carbohydrates (cellulose, hemicellulose and lignin). Elevated [CO2] significantly increased cellulose (+37.4%) input to soil, whereas elevated [O3] significantly reduced hemicellulose and lignin inputs to soil (-22.3 and -31.5%, respectively). The small changes in litter chemistry in response to elevated [CO2] and tropospheric [O3] that we observed, combined with changes in litter biomass production, could significantly alter the inputs of N, soluble sugars, condensed tannins, soluble phenolics, cellulose and lignin to forest soils in the future.     

Responses of deciduous broadleaf trees to defoliation in a CO2 enriched atmosphere

Relatively little is known about the implications of atmospheric CO2 enrichment for tree responses to biotic disturbances such as folivory. We examined the combined effects of elevated CO2 concentration ([CO2]) and defoliation on growth and physiology of sugar maple (Acer saccharum Marsh.) and trembling aspen (Populus tremuloides Michx.). Seedlings were planted in the ground in eight open-top chambers. Four chambers were ventilated with CO2-enriched air (ambient + 283 micromol mol-1) and four chambers were supplied with ambient air. After 6 weeks of growth, half of the leaf area was removed on a subset of seedlings of each species in each CO2 treatment. We monitored subsequent biomass gain and allocation, along with leaf gas exchange and chemistry. Defoliation did not significantly affect final seedling biomass in either species or CO2 treatment. Growth recovery following defoliation was associated with increased allocation to leaf mass in maple and a slight enhancement of mean photosynthesis in aspen. Elevated [CO2] did not significantly affect aspen growth, and the observed stimulation of maple growth was significant only in mid-season. Correspondingly, simulated responses of whole-tree photosynthesis to elevated [CO2] were constrained by a decrease in photosynthetic capacity in maple, and were partially offset by reductions in specific leaf area and biomass allocation to foliage in aspen. There was a significant interaction between [CO2] and defoliation on only a few of the measured traits. Thus, the data do not support the hypothesis that atmospheric CO2 enrichment will substantially alter tree responses to folivory. However, our findings do provide further indication that regeneration-stage growth rates of certain temperate tree species may respond only moderately to a near doubling of atmospheric [CO2].     

Tree age effect on fine-root and leaf morphology in a silver birch forest chronosequence

The influence of forest ageing on fine-root morphology and relations between fine-root and leaf characteristics is poorly studied. The aim of this study was to analyse age-driven changes in ectomycorrhizal roots (EcM roots) and leaf morphology in a chronosequence of silver birch (Betula pendula Roth.), which would provide a better understanding of adaptation responses and acclimation capacity of tree roots and leaves. The chronosequence included six age classes (3, 6, 14, 32, 45, and 60 years.). All stands had regenerated naturally and grew in a highly productive Oxalis forest site type in Estonia. Most changes in the morphology of EcM roots and leaves of silver birch occur faster at a young age. The functional parameters—mean specific area of EcM root (SRA) and leaf specific area (SLA) as well as leaf N—decreased with age. EcM root SRA and specific root length (SRL) decreased with stand age as a result of increased mean diameter and tissue density. In age classes of 6, 14, and 32 years, the total number of dominating EcM taxa was 34, and the distribution of four different dominating EcM exploration types (contact-, short-, medium-, long-distance) was similar. We conclude that high values of SRA, SLA, and leaf N measured in young silver birch stands indicate high activity of physiological processes necessary for fast-growing young trees. A decrease of SLA and SRA and N in the chronosequence of fertile stands of silver birch is most probably caused by down-regulation of growth, affecting simultaneously leaves and fine roots.     

Impact of targeted sheep grazing on herbage and holm oak saplings in a silvopastoral wildfire prevention system in south-eastern Spain

Several wildfire prevention programs in southern Europe are currently using livestock grazing for the maintenance of fuelbreaks. This silvopastoral management is valued for being sustainable and effective in reducing fuel loads, but few studies have analyzed other impacts linked to fuelbreak grazing. This paper reports on an experiment performed within the wildfire prevention program in Andalusia (southern Spain) with the aim of clarifying and quantifying the effect of fuelbreak grazing on herbage biomass, ground cover, herbage species composition, and growth of holm oak saplings. The study site, located in a semiarid Mediterranean environment, was grazed by a shepherded sheep flock from February to June in three consecutive years at a similar stocking rate. Livestock consumed between 33 and 68 % of herbage production in the different years, and the greatest fuel reduction (remaining dry matter of 200?kg?ha^?1) was registered in Year 2, when rainfall and herbage production was lowest. Ground cover was significantly affected by grazing: on average, the percentage of bare soil increased three-fold, while herbage cover was reduced by a quarter. The botanical composition of herbage varied remarkably between years, but very little between Grazed and Non-Grazed areas within each year. Non-browsed holm oak saplings became progressively larger than browsed ones, differences only reaching clear statistical significance at the end of the three experimental years. At this time, the volume of browsed saplings was 47–56 % smaller than that of non-browsed holm oaks, even though the former had also grown significantly in the course of the experiment.     

Changes in root growth and relationships between plant organs and root hydraulic traits in American elm (Ulmus americana L.) and red oak (Quercus rubra L.) seedlings due to elevated CO2 level

The relationships between plant organs and root hydrological traits are not well known and the question arises whether elevated CO2 changes these relationships. This study attempted to answer this question. A pseudo-replicated experiment was conducted with two times 24 American elm (Ulmus americana L.) and 23 and 24 red oak (Quercus rubra L.) seedlings growing in ambient CO2 (around 360 μmol·L–1) and 540 ± 7.95 μmol·L–1 CO₂ in a greenhouse. After 71 days of treatment for American elm and 77 days for red oak, 14 American elm and 12 red oak seedlings from each of the two CO2 levels were randomly selected in order to examine the flow rate of root xylem sap, root hydraulic conductance, total root hydraulic conductivity, fine root and coarse root hydraulic conductivity. All seedlings were harvested to investigate total plant biomass, stem biomass and leaf biomass, leaf area, height, basal diameter, total root biomass, coarse root biomass and fine root biomass. The following conclusions are reached: 1) plant organs respond to the elevated CO2 level earlier than hydraulic traits of roots and may gradually lead to changes in hydraulic traits; 2) plant organs have different relationships with hydraulic traits of roots and elevated CO2 changes these relationships; the changes may be of importance for plants as means to acclimatize to changing environments; 3) biomass of coarse roots increased rather more than that of fine roots; 4) Lorentzian and Caussian models are better in estimating the biomass of seedlings than single-variable models.     

Drought and the diurnal patterns of stem CO2 efflux and xylem CO2 concentration in young oak (Quercus robur)

A young potted oak (Quercus robur L.) tree was subjected to drought by interrupting the water supply for 9 days. The tree was placed in a growth chamber in which daily patterns of temperature and radiation were constant. The effects of drought on the water and carbon status of the stem were examined by measuring stem sap flow rate, stem water potential, stem diameter variations, stem CO(2) efflux rate (F(CO2)) and xylem CO(2) concentration ([CO(2)*]). Before and after the drought treatment, diurnal fluctuations in F(CO2) and [CO(2)*] corresponded well with variations in stem temperature (T(st)). Daytime depressions in F(CO2) did not occur. During the drought treatment, F(CO2) still responded to stepwise changes in temperature, but diurnal fluctuations in F(CO2) were no longer correlated with diurnal fluctuations in T(st). From the moment daily growth rate of the stem became zero, diurnal fluctuations in F(CO2) became closely correlated with diameter variations, exhibiting clear daytime depressions. The depressions in F(CO2) were likely the result of a reduction in metabolic activity caused by the lowered daytime stem water status. Xylem [CO(2)*] showed clear daytime depressions in response to drought. When the tree was re-watered, F(CO2) and [CO(2)*] exhibited sharp increases, coinciding with an increase in stem diameter. After resumption of the water supply, daytime depressions in F(CO2) and [CO(2)*] disappeared and diurnal fluctuations in F(CO2) and [CO(2)*] corresponded again with variations in T(st).     

Genotypic variation in drought response of silver birch (Betula pendula): leaf water status and carbon gain

To assess genotypic variation in drought response of silver birch (Betula pendula Roth), we studied the plasticity of 16 physiological traits in response to a 12-14-week summer drought imposed on four clones in two consecutive years. In a common garden experiment, 1-year-old clonal trees from regions with low (550 mm year(-1)) to high rainfall (1270 mm year(-1)) were grown in 45-l pots, and leaf gas exchange parameters, leaf water potentials, leaf osmotic potentials and leaf carbon isotope signatures were repeatedly measured. There were no clonal differences in leaf water potential, but stomatal conductance (gs), net photosynthesis at ambient carbon dioxide concentration, photosynthetic water-use efficiency, leaf carbon isotope composition (delta13C) and leaf osmotic potentials at saturation (Pi0) and at incipient plasmolysis (Pip) were markedly influenced by genotype, especially gs and osmotic adjustment. Genotypes of low-rainfall origin displayed larger osmotic adjustment than genotypes of high-rainfall origin, although their Pi0 and Pip values were similar or higher with ample water supply. Genotypes of low-rainfall origin had higher gs than genotypes of high-rainfall origin under both ample and limited water supply, indicating a higher water consumption that might increase competitiveness in drought-prone habitats. Although most parameters tested were significantly influenced by genotype and treatment, the genotype x treatment interactions were not significant. The genotypes differed in plasticity of the tested parameters and in their apparent adaptation to drought; however, among genotypes, physiological plasticity and drought adaptation were not related to each other. Reduction of gs was the first and most plastic response to drought in all genotypes, and allowed the maintenance of high predawn leaf water potentials during the drought. None of the clones exhibited non-stomatal limitation of photosynthesis. Leaf gs, photosynthetic capacity, magnitude of osmotic adjustment and delta13C were all markedly lower in 2000 than in 1999, indicating root limitation in the containers in the second year.     

Among-tree variation in leaf traits and herbivore attacks in a deciduous oak,Quercus dentata

Abstract

Variations in defensive and some other leaf traits were studied in a population of an oak species, Quercus dentata Thunberg, in northern Japan, with reference to attacks by ectophagous herbivores and leafminers. The oak population showed substantial individual variations in concentrations of total phenolics and condensed tannins, nitrogen content, trichome density, leaf area and budburst timing. With the exception of leaf mass per area, which showed a positive relation with leaf toughness and a negative relation with water content, no significant relation was observed between the plant traits studied, suggesting an absence of trade-off or linkage between them. The oaks also showed substantial individual variations in leaf area loss by ectophagous herbivores, densities of major leafminers (Phyllonorycter and Stigmella species) and survival of Phyllonorycter sap-feeding larvae. The density of trichomes showed a significant, negative relation with leaf area loss by ectophagous herbivores, but significant, positive relations with densities of some leafminers. The other leaf traits seldom showed significant relations with herbivore densities or survival. In this oak population, these traits may not have enough variations to be reflected in the abundance and performance of herbivores.     

Comparison of the measurements of leaf water potential between a hydraulic press and a pressure chamber in six Sahelian woody species

The aim of this methodological study was to quantify differences between water potential measured with a pressure chamber (PC) and with a hydraulic press (HP) in six north Sahelian dominant species of the woody strata across the range of their local environmental conditions in the Malian Gourma. Mean annual rainfall is 372 mm, falling from June to September, followed by 8–10 months of dry season. The daily course of Leaf Water Potential (LWP) was monitored in 2–6 m tall healthy individuals. Water potential measured with the two instruments were statistically comparable (R2 > 40%) except in A. senegal. However, the HP under-estimated LWP and revealed smaller ranges of water potential than the PC. In the Sahelian shrubs studied here, for the precise measurement of a water potential gradient in the soil–plant-atmosphere continuum and for inter-specific comparisons, the PC is more appropriate than the HP. However, the HP may be useful for intra-species comparison in large sampled fields, since calibrations will be checked across a wider range of dates and a large number of sites.     

Environmental and plant-based controls of water use in a Mediterranean oak stand

In this work, new information is reported on water relations of the Mediterranean oak species Quercus pyrenaica based on environmental and physiological measurements carried out during the growing seasons of 2006 and 2007. The interest in this species has increased due to its use in reforestation programs and its impacts on the water resources due to the extensive spontaneous afforestation after the abandonment of forest (firewood, charcoal, livestock, etc.) and agricultural activities, in general in some areas in the Mediterranean region and in particular, in the studied area (Sistema Central range, Spain). The objectives were to evaluate the long-term water use of this stand and its limit and to analyse the specific traits to cope with summer drought, especially the use of stem water storage and deep soil water reserves. Tree water stress associated with depletion of soil water reserves was not observed since the oak trees appeared to avoid a marked water stress using water reserves from deeper soil layers as summer drought progresses. The contribution of mean daily stem water storage to transpiration was low (4%), although it could be greater under dryer conditions. Only at the end of summer of 2006, the transpiration and canopy conductance were reduced due to soil drought. Despite the absence of marked water stress an upper limit was found in transpiration (slightly higher than 3 mm day⁻¹). The heavy use of soil water resources by this species (75% of available soil water in this study) should be considered when evaluating the impact of spontaneous afforestation and reforestation programs on water resources.     

Influence of soil texture on hydraulic properties and water relations of a dominant warm-desert phreatophyte

We investigated hydraulic constraints on water uptake by velvet mesquite (Prosopis velutina Woot.) at a site with sandy-loam soil and at a site with loamy-clay soil in southeastern Arizona, USA. We predicted that trees on sandy-loam soil have less negative xylem and soil water potentials during drought and a lower resistance to xylem cavitation, and reach E(crit) (the maximum steady-state transpiration rate without hydraulic failure) at higher soil water potentials than trees on loamy-clay soil. However, minimum predawn leaf xylem water potentials measured during the height of summer drought were significantly lower at the sandy-loam site (-3.5 +/- 0.1 MPa; all errors are 95% confidence limits) than at the loamy-clay site (-2.9 +/- 0.1 MPa). Minimum midday xylem water potentials also were lower at the sandy-loam site (-4.5 +/- 0.1 MPa) than at the loamy-clay site (-4.0 +/- 0.1 MPa). Despite the differences in leaf water potentials, there were no significant differences in either root or stem xylem embolism, mean cavitation pressure or Psi(95) (xylem water potential causing 95% cavitation) between trees at the two sites. A soil-plant hydraulic model parameterized with the field data predicted that E(crit) approaches zero at a substantially higher bulk soil water potential (Psi(s)) on sandy-loam soil than on loamy-clay soil, because of limiting rhizosphere conductance. The model predicted that transpiration at the sandy-loam site is limited by E(crit) and is tightly coupled to Psi(s) over much of the growing season, suggesting that seasonal transpiration fluxes at the sandy-loam site are strongly linked to intra-annual precipitation pulses. Conversely, the model predicted that trees on loamy-clay soil operate below E(crit) throughout the growing season, suggesting that fluxes on fine-textured soils are closely coupled to inter-annual changes in precipitation. Information on the combined importance of xylem and rhizosphere constraints to leaf water supply across soil texture gradients provides insight into processes controlling plant water balance and larger scale hydrologic processes.     

Functional convergence in hydraulic architecture and water relations of tropical savanna trees: from leaf to whole plant

Functional convergence in hydraulic architecture and water relations, and potential trade-offs in resource allocation were investigated in six dominant neotropical savanna tree species from central Brazil during the peak of the dry season. Common relationships between wood density and several aspects of plant water relations and hydraulic architecture were observed. All species and individuals shared the same negative exponential relationship between sapwood saturated water content and wood density. Wood density was a good predictor of minimum (midday) leaf water potential and total daily transpiration, both of which decreased linearly with increasing wood density for all individuals and species. With respect to hydraulic architecture, specific and leaf-specific hydraulic conductivity decreased and the leaf:sapwood area ratio increased more than 5-fold as wood density increased from 0.37 to 0.71 g cm(-3) for all individuals and species. Wood density was also a good predictor of the temporal dynamics of water flow in stems, with the time of onset of sap flow in the morning and the maximum sap flow tending to occur progressively earlier in the day as wood density increased. Leaf properties associated with wood density included stomatal conductance, specific leaf area, and osmotic potential at the turgor loss point, which decreased linearly with increasing wood density. Wood density increased linearly with decreasing bulk soil water potential experienced by individual plants during the dry season, suggesting that wood density was greatest in individuals with mostly shallow roots, and therefore limited access to more abundant soil water at greater depths. Despite their taxonomic diversity and large intrapopulation differences in architectural traits, the six co-occurring species and their individuals shared similar functional relationships between all pairs of variables studied. Thus, rather than differing intrinsically in physiological responsiveness, the species and the individuals appeared to have distinct operating ranges along common physiological response curves dictated by plant architectural and structural features. The patterns of water uptake and access to soil water during the dry season appeared to be the main determinant of wood density, which constrained evolutionary options related to plant water economy and hydraulic architecture, leading to functional convergence in the neotropical savanna trees studied.     

Effects of ring-porous and diffuse-porous stem wood anatomy on the hydraulic parameters used in a water flow and storage model

Calibration of a recently developed water flow and storage model based on experimental data for a young diffuse-porous beech tree (Fagus sylvatica L.) and a young ring-porous oak tree (Quercus robur L.) revealed that differences in stem wood anatomy between species strongly affect the calibrated values of the hydraulic model parameters. The hydraulic capacitance (C) of the stem storage tissue was higher in oak than in beech (939.8 versus 212.3 mg MPa(-1)). Model simulation of the elastic modulus (epsilon) revealed that this difference was linked to the higher elasticity of the stem storage tissue of oak compared with beech. Furthermore, the hydraulic resistance (R (x)) of beech was about twice that of oak (0.1829 versus 0.1072 MPa s mg(-1)). To determine the physiological meaning of the R (x) parameter identified by model calibration, we analyzed the stem wood anatomy of the beech and oak trees. Calculation of stem specific hydraulic conductivity (k (s)) of beech and oak with the Hagen-Poiseuille equation confirmed the differences in R (x) predicted by the model. The contributions of different vessel diameter classes to the total hydraulic conductivity of the xylem were calculated. As expected, the few big vessels contributed much more to total conductivity than the many small vessels. Compared with beech, the larger vessels of oak resulted in a higher k (s) (10.66 versus 4.90 kg m(-1) s(-1) MPa(-1)). The calculated ratio of k (s) of oak to beech was 2, confirming the R (x) ratio obtained by model calibration. Thus, validation of the R (x) parameter of the model led to identification of its physiological meaning.     

CO2倍增与干旱胁迫交互作用对幼树蒸腾耗水日变化的影响

采用BP3400精密电子天平和Licor-7000开路式CO2/H2O分析仪,于2007年夏季研究了不同CO2浓度梯度和不同干旱胁迫水平下,4种幼树的实际耗水量及蒸腾耗水速率的日变化规律。结果表明,在各干旱胁迫水平下,CO2浓度倍增有效地降低了幼树的蒸腾耗水量和耗水速率。在干旱胁迫后期,白蜡、丁香、黄杨、侧柏的日平均耗水量分别比对照下降了42%、52%、37%和19%;日平均耗水速率平均下降了50%-65%。经过CO2倍增处理后,4种幼树的叶面积都明显增加,并且连续处理时间越长,效果越明显。     

Isotopic carbon discrimination and leaf nitrogen content of Erica arborea L. along a CO(2) concentration gradient in a CO(2) spring in Italy

We studied a Mediterranean species (Erica arborea L.) growing in a CO(2) spring in Italy that was naturally exposed for generations to a gradient of atmospheric CO(2) concentrations. The CO(2) concentration gradient to which different individual plants were exposed was determined by an indirect method based on radioisotope analysis. The stable carbon isotope ratio of sampled leaves was determined by mass spectrometry, and isotopic discrimination was then calculated. Leaf nitrogen, specific leaf area, total soil nitrogen, soil organic matter content and soil pH were also measured. In one group of plants, grown on a homogeneous soil and exposed to moderate CO(2) enrichment, isotopic discrimination was significantly reduced in response to increasing CO(2) concentrations, whereas the intercellular CO(2) concentration and leaf nitrogen content were almost unaffected. In a second group of plants, grown along a gradient of CO(2) concentration and soil nitrogen content, leaf nitrogen content was reduced when nitrogen availability was limiting. However, when soil nitrogen was available in excess, even very high CO(2) concentrations did not result in increased discrimination or reduced leaf nitrogen content in the long term. The results are discussed with respect to current theories about the long-term CO(2) response of plants based on several years of experimentation with elevated atmospheric CO(2) concentrations under controlled conditions.     

Radial velocity profiles of water flow in trunks of Norway spruce and oak and the response of spruce to severing

Trunk-tissue heat balance, volumetric and staining methods were used to study xylem water flow rates and pathways in mature Norway spruce (Picea abies (L.) Karst.) and pedunculate oak (Quercus robur L.) trees. The radial profile of flow velocity was confirmed to be symmetrical in spruce, i.e., maximum flow velocity was in the center of the conducting xylem and tailed with low amplitude (about 30 cm h(-1)) in the direction of the cambium and heartwood. Variability around the trunk was high. In contrast, in oak, the radial profile of flow velocity was highly asymmetrical, reaching a peak of about 45 m h(-1) in the youngest growth ring and tailing centripetally for about 10 rings, but variability around the trunk was less, under non-limiting soil water conditions, than in spruce. In spruce, the flow rate increased abruptly within seconds when the tree was severed while immersed in water, and then decreased gradually, showing significant root resistance. We conclude that water flow through an absorbing cut surface differs from the flow higher in a tree trunk because of the presence of hydraulic capacitances in the conductive pathways. The staining technique always yielded higher estimates of flow velocity than the non-destructive tree-trunk heat balance method.