Effects of xylem sap flow on carbon dioxide efflux from stems of birch (Betula pendula Roth)

The effect of xylem sap flow in stems of mature Betula pendula Roth on radial CO₂ efflux was studied from April to October 2001. Temperature-controlled respiration cuvettes allowed measurements of CO₂ efflux without interference from temperature gradients between stem surface and sapwood. Variations of sap flow in different stem sectors, and in a given sector at different heights were analysed. Daytime reduction of CO₂ efflux caused by sap flow was expressed as the difference between gross and apparent CO₂ release. Gross CO₂ release was calculated from Arrhenius-equations derived from night-time data records of the same day, which were free from interference by sap flow. In mid-July, daytime reductions of CO₂ efflux reached 1.8–3.9 μmol CO₂ m⁻² g⁻¹ xylem sap transpired. Assuming tree-specific maximum transpiration rates of 30 kg H₂O d⁻¹ this is up to 40% of gross CO₂ release. In relation to photosynthetic CO₂ fixation the endogenous supply of dissolved CO₂ to the leaves acccounted for 0.5–3.7%. This study indicates a negative correlation between sap flow velocity and radial CO₂ efflux from B. pendula stems. Periods of unbalanced CO₂ partial pressures between aqueous and gaseous pathways during increase and decrease of sap flow seem to affect gaseous CO₂ release through lenticels. It is concluded that CO₂ efflux rates are not simply equivalent to respiration rates because of the interference of aqueous CO₂ transport by xylem sap flow in the wood-body of trees.     

Gas-exchange and sap flow measurements of Salix viminalis trees in short-rotation forest

Simultaneous field measurements of transpiration and sap flow were performed on short-rotation Salix viminalis trees ranging in diameter from 1.5 to 3.5 cm (2-year-old shoots on 8-year-old stumps). Transpiration was measured using an open-top ventilated chamber enclosing the whole foliage of a tree. Sap flow was measured using a tree-trunk heat balance (THB) technique with a constant temperature difference and variable heat input. Both the instantaneous and daily values of water flux measured by the two absolute techniques agreed well with a difference of up to about 5%. In July, the hourly transpiration reached a maximum of about 0.2 kg m^–2 (leaf area) or 0.45 kg tree^–1, whereas maximum daily integrals reached 4 kg tree^–1. The response of sap flow rate to abrupt flux change when inducing emboli by cutting-off the stem was very rapid: the registered signal dropped by 85% within 10 min for a specimen with a projected leaf area of 2 m². For S. viminalis trees, transpiration was linearly correlated with stem cross-sectional area and with leaf area.     

Maximum sustainable xylem sap tensions in Rhododendron and other species

The acoustic technique was used in conjunction with the pressure chamber to determine the tensions causing cavitation of xylem sap in leaves of five woody angiosperms (Acer pseudoplatanus L., Alnus glutinosa L. Gaertn., Eucalyptus globulus Labill., Fraxinus excelsior L. and Rhododendron ponticum L.) and three species of herbs (Lycopersicum esculentum Mill., Plantago major L. and Ricinus communis L.). The results showed leaves of most species to suffer considerably from cavitation at sap tensions of 1.6-3 MPa. Two of the herbs, Lycopersicum and Ricinus, cavitated extensively at sap tensions below 1 MPa. Additional evidence is presented that clicks, detected by acoustic amplification, are caused by cavitation of sap in the xylem conduits. A rapid method is suggested for the determination of sap tensions in cavitating leaves and which is suitable for surveys of the critical sap tension in a large number of species.     

Combining sap flow and trunk diameter measurements to assess water needs in mature olive orchards

Sap flux (Q) and trunk diameter variation (TDV) are among the most useful plant-based measurements to detect water stress and to evaluate plant water consumption. The usefulness of both methods decreases, however, when applied to species that, like olive, have an outstanding tolerance to drought and a remarkable capacity to take up water from drying soils. Evidence shows that this problem is greater in old, big trees with heavy fruit load. Our hypothesis is that the analysis of simultaneous measurements of Q and TDV made in the same trees is more useful for assessing irrigation needs in old olive orchards than the use of any of these two methods alone. To test our hypothesis, we analysed relations between Q, TDV, midday stem water potential (Ψ_stem), relative extractable water and atmospheric demand in an olive orchard of 38-year-old ‘Manzanilla’ trees with heavy fruit load. Measurements were made during one irrigation season (May-October), in fully irrigated trees (FI, 107% of the crop evapotranspiration, ET_c, supplied by irrigation), and in trees under two levels of deficit irrigation (DI60, 61% ET_c; DI30, 29% ET_c). Time courses of Q and TDV measured on days of contrasting weather and soil water conditions were analysed to evaluate the usefulness of both methods to assess the crop water status. We calculated the daily tree water consumption (E_p) from Q measurements. For both DI treatments we calculated a signal intensity by dividing daily E_p values of each DI tree by those of the FI tree (SI−Ep). We did the same with the maximum daily shrinkage (MDS) values (S_I−MDS). Neither SI−Ep nor S_I−MDS rendered useful information for assessing the crop water needs. On the contrary, the daily difference for maximum trunk diameter (MXTD) between each of the DI trees and the FI tree (D_MXTD) clearly indicated the onset and severity of water stress. A similar analysis with the E_p values, from which DEp values were derived, showed the effect of water stress on the water consumption of the trees. We concluded that the simultaneous use of D_MXTD and DEp values provides more detailed information to assess water needs in mature olive orchards than the use of Q or TDV records alone.     

Changes with seasonal flooding in sap flow of the tropical flood-tolerant tree species, <Emphasis Type="Italic">Campsiandra laurifolia</Emphasis>

In order to determine how flooding affects sap flow and hydraulic conductivity of the tolerant species, Campsiandra laurifolia, trees growing in a tropical seasonally flooded forest in Venezuela were studied. We hypothesized that trees respond to rising-waters with a decrease in root-water absorption, caused by hypoxia, and stomatal conductance, and that this is reverted later on through a process of acclimation that involves improvement in water absorption. We followed the seasonal changes, of trees with the whole or part of the canopy exposed to air, in sap flow density, leaf stomatal conductance, leaf transpiration rate and xylem water potential. The highest daytime sap flow density occurred at noon and its proportion relative to the yearly maximum (drainage at falling-waters) was 41 (dry season), 15 (flooding by rising-waters for 2 weeks), 54 (2 months of flooding) and 41% (6 months of flooding). Since at rising-waters dawn xylem water potential remained high, it became apparent that the initial stages of flooding imposed a restriction to sap flow unrelated to water deficit. The decrease at rising-waters in highest daytime sap flow density was due to reduced leaf-specific hydraulic conductivity, whereas the recovery observed 1.5 months later was correlated to an increase in hydraulic conductivity, and attributed to acclimation. Sap flow density was highly and positively correlated with radiation at all seasons but rising-waters; also, the relationship with air water vapor saturation deficit was high and significant on dates other than at rising-waters. Results suggest that early flooding inhibited water absorption by roots and that this inhibition was overcome later on at a higher water column through an acclimation process involving the improvement of internal aeration by adventitious roots.     

Effects of the hydraulic coupling between xylem and phloem on diurnal phloem diameter variation

Measurements of diurnal diameter variations of the xylem and phloem are a promising tool for studying plant hydraulics and xylem-phloem interactions in field conditions. However, both the theoretical framework and the experimental verification needed to interpret phloem diameter data are incomplete. In this study, we analytically evaluate the effects of changing the radial conductance between the xylem and the phloem on phloem diameter variations and test the theory using simple manipulation experiments. Our results show that phloem diameter variations are mainly caused by changes in the radial flow rate of water between the xylem and the phloem. Reducing the hydraulic conductance between these tissues decreases the amplitude of phloem diameter variation and increases the time lag between xylem and phloem diameter variation in a predictable manner. Variation in the amplitude and timing of diameter variations that cannot be explained by changes in the hydraulic conductance, could be related to changes in the osmotic concentration in the phloem.     

The response of sap flow to pulses of rain in a temperate Australian woodland

In water-limited systems, pulses of rainfall can trigger a cascade of plant physiological responses. However, the timing and size of the physiological response can vary depending on plant and environmental characteristics, such as rooting depth, plant size, rainfall amount, or antecedent soil moisture. We investigated the influence of pulses of rainfall on the response of sap flow of two dominant evergreen tree species, Eucalyptus crebra (a broadleaf) and Callitris glaucophylla (a needle leaved tree), in a remnant open woodland in eastern Australia. Sap flow data were collected using heat-pulse sensors installed in six trees of each species over a 2 year period which encompassed the tail-end of a widespread drought. Our objectives were to estimate the magnitude that a rainfall pulse had to exceed to increase tree water use (i.e., define the threshold response), and to determine how tree and environmental factors influenced the increase in tree water use following a rainfall pulse. We used data filtering techniques to isolate rainfall pulses, and analysed the resulting data with multivariate statistical analysis. We found that rainfall pulses less than 20 mm did not significantly increase tree water use (P > 0.05). Using partial regression analysis to hold all other variables constant, we determined that the size of the rain event (P < 0.05, R ² = 0.59), antecedent soil moisture (P < 0.05, R ² = 0.29), and tree size (DBH, cm, P < 0.05, R ² = 0.15), all significantly affected the response to rainfall. Our results suggest that the conceptual Threshold-Delay model describing physiological responses to rainfall pulses could be modified to include these factors. We further conclude that modelling of stand water use over an annual cycle could be improved by incorporating the T-D behaviour of tree transpiration. Responsible Editor: Stephen S.O. Burgess     

Stem and leaf water potentials, gas exchange, sap flow, and trunk diameter fluctuations for detecting water stress in lemon trees

The sensitivity of continuous (on a whole-day basis) and discretely (at midday) measured indicators of the plant water status in adult lemon trees in response to a cycle of water deprivation and recovery, and the feasibility of obtaining baselines for tree water status indicators was investigated in 30-year-old Fino lemon trees (Citrus limon (L.) Burm. fil.) grafted on sour orange (C. aurantium L.) rootstocks. Control plants (T0) were irrigated daily above their crop water requirements in order to obtain non-limiting soil water conditions, while T1 plants were subjected to water stress by withholding irrigation for 50 days, after which time irrigation was restored and plant recovery was studied for 16 days. In T0 plants the water relations and the plant symptoms confirmed that they had not suffered waterlogging. In contrast, T1 plants showed a substantial degree of water stress, which developed very slowly. Maximum daily trunk shrinkage (MDS) increased in response to water stress during the first 15 days of the experiment, but when the stem water potential (Ψ_stem) fell below −1.8 MPa, the MDS signal intensity decreased. However, Ψ_stem and sap flow (SF) signal intensities progressively increased during the water stress period. The results showed that MDS is a very suitable plant-based indicator for precise irrigation scheduling in adult lemon trees. Reference or baseline relationships for MDS, Ψ_stem, and SF measurements as a function of several parameters related to the evaporative demand of the atmosphere were obtained. This fact open up the possibility of considering a plant-based indicator measurement at a given time relative to the expected value under non-limiting water conditions, which can be calculated from the reference relationships.     

The effect of short-term flooding on the sap flow, gas exchange and hydraulic conductivity of young apricot trees

The effect of short-term flooding was examined in 2-year-old apricot trees (Prunus armeniaca cv. Búlida). Six apricot trees of similar appearance were submitted to two treatments: three were irrigated daily, while the others were flooded for a period of 50 h by submerging the pots in plastic water tanks. The trees were removed from the water, drained and then placed in the same conditions as the control plants. A decrease in transpiration in the flooded trees with respect to the control plants was evident. The daily pattern of soil O₂ concentration and plant hydraulic resistance followed a similar trend during the flooding. However, this relationship was not maintained throughout the experiment, since the O₂ values increased rapidly when the waterlogging ceased, while plant hydraulic resistance only recovered at the end of the experiment when the original root system, damaged by flooded conditions, was replaced with new roots. In flooded trees, the midday leaf water potential decreased progressively from the beginning of flooding, but gradually recovered when the waterlogging ceased. Leaf conductance values of treated plants were slow to recover, reaching values of the control plants 8 days after the leaf water potential had recovered. The close relationship observed during most of the experiment between the leaf water parameters, leaf conductance and plant hydraulic conductance indicate that hydraulic messages are likely to play a dominant role in co-ordinating the observed responses of the shoot.     

The effects of water and multi-nutrient stress on xylem sap chemistry,photosynthesis and transpiration of seedlings of two Eucalypts

Summary Ten seedlings each of Eucalyptus kitsoniana Maiden and Eucalyptus globulus Labill. were subjected to two levels of water stress and two levels of nutrient stress (macro and micro-nutrients) in a greenhouse for 3 weeks. The objectives were to determine the degree to which seedlings show differences in sap chemistry, photosynthesis and transpiration that relate to the environments in which these two species live. Whole plants were then extracted for xylem sap using a pressure chamber and the sap was analyzed for 14 elements using an inductively coupled plasma spectrometer and a nitrometer. For E. kitsoniana water and nutrient stress, applied separately or in combination, significantly reduced leaf conductance, transpiration, photosynthesis and midday water potential. Nutrient stress alone had less effect than water stress on most functions measured. Water stress alone reduced the root/shoot ratio; the combination of water and nutrient stress increased the root/shoot ratio, primarily because of reduced shoot weight. In E. kitsoniana, water stress alone or in combination with nutrient stress increased the xylem sap concentrations of B and Si. Multi-nutrient stress alone, or in combination with water stress, significantly decreased sap Zn and K. For this species, sap N was decreased by nutrient stress, but increased by water stress. E. globulus had significantly lower transpiration rates and less root mass than E. kitsoniana. Slightly lower leaf conductance and photosynthesis were not significant in E. globulus compared to E. kitsoniana. Water and nutrient stress reduced conductance, transpiration (except for nutrient stress) and photosynthesis, and the effects of water stress on E. globulus were greater than the effects of nutrient stress. Midday water potential was reduced by water stress. Water or nutrient stress alone did not alter seedling root/shoot ratio, but the combination of water and nutrient stress significantly increased the root/shoot ratio for both species. For E. globulus, sap concentrations of Mn, Na, Si and K were increased by water stress (alone or in combination with nutrient stress). Sap N increased with water stress or combined stresses, but decreased under nutrient stress alone. When the two species were compared, E. globulus generally had lower or similar nutrient concentrations in the sap, with Ca, Mg, Mn and P significantly lower than in E. kitsoniana. Seedlings of these two species show strong site adaptations to water and nutrient availability.     

A plumber's-eye view of xylem water transport in woody plants

We present a practical for university-level students aimed at measuring and comparing xylem hydraulic properties of co-existing plant species. After sampling branches of several woody species in the field, their main hydraulic properties were measured using a simple set-up. Hydraulic conductivity (K_h ) was calculated as the ratio between water flow through a plant segment and the pressure gradient driving the flow. The percent reduction in conductivity due to xylem embolism (i.e. air-filled conduits) was estimated by comparing K_h before and after flushing the measure segments to remove all native embolism. Raw hydraulic conductivity was standardised by cross-sectional wood area or supported leaf area to obtain more meaningful measures of conducting capacity. The results showed differences among study species, particularly between conifers and angiosperms. These differences are briefly discussed in terms of wood anatomy and the general biology of the species. Overall the practical provides a good opportunity for students to appreciate the main aspects of xylem water transport and the constraints it imposes on plant water relations.     

Transpiration and canopy conductance in a pristine broad-leaved forest of Nothofagus: an analysis of xylem sap flow and eddy correlation measurements

Summary Tree transpiration was determined by xylem sap flow and eddy correlation measurements in a temperate broad-leaved forest of Nothofagus in New Zealand (tree height: up to 36 m, one-sided leaf area index: 7). Measurements were carried out on a plot which had similar stem circumference and basal area per ground area as the stand. Plot sap flux density agreed with tree canopy transpiration rate determined by the difference between above-canopy eddy correlation and forest floor lysimeter evaporation measurements. Daily sap flux varied by an order of magnitude among trees (2 to 87 kg day^–1 tree^–1). Over 50% of plot sap flux density originated from 3 of 14 trees which emerged 2 to 5 m above the canopy. Maximum tree transpiration rate was significantly correlated with tree height, stem sapwood area, and stem circumference. Use of water stored in the trees was minimal. It is estimated that during growth and crown development, Nothofagus allocates about 0.06 m of circumference of main tree trunk or 0.01 m² of sapwood per kg of water transpired over one hour.Maximum total conductance for water vapour transfer (including canopy and aerodynamic conductance) of emergent trees, calculated from sap flux density and humidity measurements, was 9.5 mm s^–1 that is equivalent to 112 mmol m^–2 s^–1 at the scale of the leaf. Artificially illuminated shoots measured in the stand with gas exchange chambers had maximum stomatal conductances of 280 mmol m^–2 s^–1 at the top and 150 mmol m^–2 s^–1 at the bottom of the canopy. The difference between canopy and leaf-level measurements is discussed with respect to effects of transpiration on humidity within the canopy. Maximum total conductance was significantly correlated with leaf nitrogen content. Mean carbon isotope ratio was –27.76±0.27 (average ±s.e.) indicating a moist environment. The effects of interactions between the canopy and the atmosphere on forest water use dynamics are shown by a fourfold variation in coupling of the tree canopy air saturation deficit to that of the overhead atmosphere on a typical fine day due to changes in stomatal conductance.This paper is dedicated to Prof. Dr. O.L. Lange on the occasion of his 65th birthday     

基于树干液流技术的北京市刺槐冠层吸收臭氧特征研究

全球范围内加速的城市化导致空气质量严重退化。随着北京市建设范围不断扩大和机动汽车数量迅猛增长,空气污染日益严重。浓度不断增加的近地层臭氧作为影响全球气候变化的重要因素和危害人类健康、动植物生长的二次污染物,受到广泛关注。城市树木能够有效地去除大气污染物,进而提高空气质量。目前已有很多研究关于区域尺度上城市树木吸收臭氧,然而,冠层尺度上城市树木吸收臭氧特征少有研究。因此,本文基于树干液流技术,结合天气变化和大气臭氧浓度分析,研究夏秋季节北京市典型绿化树种刺槐（Robinia pseudoacacia）整树冠层吸收臭氧特征及环境影响因素。结果表明,在日尺度上,刺槐吸收臭氧速率变化呈单峰曲线,于下午15:00左右达到峰值;夏季峰值范围较宽,秋季峰值范围较窄;中午前后累积吸收臭氧量增加最明显。在季节尺度上,夏季刺槐吸收臭氧速率高于秋季;夏季累积吸收臭氧量显著增加,秋季略有增加。刺槐吸收臭氧的时间变化规律取决于大气臭氧浓度和冠层对臭氧的导度。臭氧浓度日变化和季节变化明显,导致刺槐吸收臭氧速率时间变化格局与之接近。在一定的臭氧浓度下,刺槐吸收臭氧速率的变化主要由冠层对臭氧的导度调控,进而受水汽压亏缺和总辐射的影响。随着水汽压亏缺降低,刺槐冠层对臭氧的导度明显下降;总辐射大于600 W/m2,冠层对臭氧的导度迅速下降。研究树种刺槐单位冠层投影面积上年吸收臭氧量约为0.16 g/m2,明显低于基于模型得到的结果,表明评估森林受臭氧危害的风险应考虑树种冠层臭氧通量。     

Perfusion of onion root xylem vessels: a method and some evidence of control of the pH of the xylem sap

We describe a method for perfusing the xylem in the stele of excised onion roots with solutions of known composition under a pressure gradient. Tracer studies using [¹⁴C] polyethylene glycol 4000 and the fluorescent dye, Tinopal CBSX, indicated that perfusing solutions passed exclusively through the xylem vessels. The conductance of the xylem was small over the apical 100 mm of the root axis but increased markedly between 100 and 200 mm. Unbuffered perfusion solutions supplied in the range pH 3.7–7.8 emerged after passage through the xylem adjusted to pH 5.2–6.0, indicating the presence of mechanisms for absorbing or releasing protons. This adjustment continued over many hours with net proton fluxes apparently determined by the disparity between the pH of the perfusion solution and the usual xylem sap pH of about 5.5. Mild acidification of the xylem sap by buffered perfusion solutions increased the release of ⁸⁶Rb (K⁺) and ³⁵SO₄ ^2- from the stelar tissue into the xylem stream. The ion-transporting properties of onion roots seemed little changed by excision from the bulbs, or by removal of the apical zones of the root axis. The pH of sap produced by root pressure resembles that found in the outflow solutions of perfused root segments.     

Inter-species variation of photosynthetic and xylem hydraulic traits in the deciduous and evergreen Euphorbiaceae tree species from a seasonally tropical forest in south-western China

The objective of the present study was to examine the functional coordination among hydraulic traits, xylem characteristics and gas exchange rates across three deciduous Euphorbiaceae tree species (Hevea brasiliensis, Macaranga denticulata and Bischofia javanica) and three evergreen Euphorbiaceae tree species (Drypetes indica, Aleurites moluccana and Codiaeum variegatum) from a seasonally tropical forest in south-western China. The deciduous tree species were more vulnerable to water stress-induced embolism than the evergreen tree species. However, the deciduous tree species generally had higher maximal rates of sapwood and leaf-specific hydraulic conductivity (K _S and K _L), respectively. Compared with the evergreen tree species, the deciduous tree species, however, possessed a lower density of sapwood and a wider diameter of xylem vessels. Regardless of leaf phenology, the hydraulic vulnerability and conductivity were significantly correlated with sapwood density and mean vessel diameter. Furthermore, the hydraulic vulnerability was positively correlated with water transport efficiency. In addition, the deciduous tree species exhibited higher maximal photosynthetic rates (A _max) and stomatal conductance (g _max), but lower water use efficiency (WUE). Interestingly, the A _max, g _max and WUE were strongly correlated with K _S and K _L across the deciduous and evergreen tree species. These results suggest that xylem structure, rather than leaf phenology, accounts for the difference in hydraulic traits between the deciduous tree species and the evergreen tree species. Meanwhile, our results show that there is a significant trade-off between hydraulic efficiency and safety, and a strong functional correlation between the hydraulic capacity and gas exchange rates across the deciduous and evergreen tree species.     

Regulation of water flux through trunks, branches, and leaves in trees of a lowland tropical forest

We studied regulation of whole-tree water use in individuals of five diverse canopy tree species growing in a Panamanian seasonal forest. A construction crane equipped with a gondola was used to access the upper crowns and points along the branches and trunks of the study trees for making concurrent measurements of sap flow at the whole-tree and branch levels, and vapor phase conductances and water status at the leaf level. These measurements were integrated to assess physiological regulation of water use from the whole-tree to the single-leaf scale. Whole-tree water use ranged from 379 kg day⁻¹ in a 35 m-tall Anacardium excelsum tree to 46 kg day⁻¹ in an 18 m-tall Cecropia longipes tree. The dependence of whole-tree and branch sap velocity and sap flow on sapwood area was essentially identical in the five trees studied. However, large differences in transpiration per unit leaf area (E) among individuals and among branches on the same individual were observed. These differences were substantially reduced when E was normalized by the corresponding branch leaf area:sapwood area ratio (LA/SA). Variation in stomatal conductance (g _s) and crown conductance (g _c), a total vapor phase conductance that includes stomatal and boundary layer components, was closely associated with variation in the leaf area-specific total hydraulic conductance of the soil/leaf pathway (G _t). Vapor phase conductance in all five trees responded similarly to variation in G _t. Large diurnal variations in G _t were associated with diurnal variation in exchange of water between the transpiration stream and internal stem storage compartments. Differences in stomatal regulation of transpiration on a leaf area basis appeared to be governed largely by tree size and hydraulic architectural features rather than physiological differences in the responsiveness of stomata. We suggest that reliance on measurements gathered at a single scale or inadequate range of scale may result in misleading conclusions concerning physiological differences in regulation of transpiration. Received: 1 October 1997 / Accepted: 6 March 1998     

Evaluation of various heat-pulse methods for estimation of sap flow in orchard trees: comparison with micrometeorological estimates of evaporation

Summary A microcomputer-controlled heat-pulse system for the measurement of sap velocity in trees is described. Several published methods for determining sap velocity from the temperature rise measured either above or below a heater inserted into the stem were compared and evaluated. All methods could be improved by the use of curve-fitting procedures, with a particularly useful approach involving direct estimation of the parameters of the diffusion equation using the non-linear curve-fitting package maximum likelihood programme. An alternative approach that was based on measurement of the value of the maximum temperature was proposed and tested. This was found to be particularly robust and sensitive to changes in flow rate. Although sap-flow velocity varied markedly with depth in the stem (as shown by the rate of dye movement), the maximum temperature at any given flow rate was found to be relatively insensitive to sensor depth. Estimated sap-flow velocities were compared with evaporation rates estimated either by weighing (for potted trees) or by the Penman-Monteith equation. Several independent methods were used for estimating the values of boundary layer resistance and net radiation that were required for application of the Penman-Monteith equation. There was a close relationship between flow and evaporation with some evidence for hysteresis. Although absolute calibration of the sap-flow estimates is difficult, the methods described are all useful for relative studies and all responded rapidly to altered flow rates caused by changing weather conditions.     

应用热技术研究树干液流进展

综述了热技术方法测定树干液流的基本原理和不同适用范围．通过各种热技术方法可以确定树干水分运输格局及其数量;与树木生理指标和环境因素联合测定,可深入分析整树水分导度、气孔导度、边界层导度、水势及树干储水与树木蒸腾之间的关系,探讨树干液流受外界环境因素影响程度及其响应,揭示树木蒸腾内在的调节机制和外在影响因素;热技术可用于长期连续测定地带性森林主要树种蒸腾耗水特征。为正确评价森林的水文效应提供技术支持．     

Stem hydraulic properties of vines vs. shrubs of western poison oak,Toxicodendron diversilobum

Summary This study investigated the effect of mechanical support on water transport properties and wood anatomy of stems of western poison oak, Toxicodendron diversilobum (T. & G.) Greene. This plant grows as a vine when support is present but as a shrub when support is absent. I compared vines and shrubs growing naturally in the field and those produced from cuttings of 11 source plants in a common garden. Huber value (xylem transverse area/distal leaf area) was lower but specific conductivity (water volume · time^-1 · xylem transverse area^-1 · pressure gradient^-1) was higher in supported than unsupported plants both in the field and the common garden. The opposing effects of Huber value and mon garden. The opposing effects of Huber value and specific conductivity resulted in the same values of leafspecific conductivity (LSC, water volume · time^-1 · distal leaf area^-1 · pressure gradient^-1) for supported and unsupported shoots at a given site. Therefore, for the same rates of evapotranspiration, supported and unsupported shoots will have the same pressure gradients in their stems. Vessel lumen composed a higher proportion of stem cross-section in supported than unsupported plants (due to slightly wider vessels and not to greater vessel density). These results suggest that the narrow stems of supported plants are compensated hydraulically by the production of wider vessels: at a given site, poison oak plants co-ordinate their leaf and xylem development such that their stems achieve the same overall conductive efficiencies (LSCs), regardless of support conditions.