Measurement of sap flow in plant stems

Transpiration rates for whole plants, individual branches ortillers can be determined by techniques which measure the rateat which sap ascends stems. All of these methods use heat asa tracer for sap movement, but they are fundamentally differentin their operating principles. Two methods commonly employed,the stem heat balance and trunk sector heat balance methods,use the heat balance principle; the stem is heated electricallyand the heat balance is solved for the amount of heat takenup by the moving sap stream, which is then used to calculatethe mass flow of sap in the stem. In the heat-pulse method,rather than using continuous heating, short pulses of heat areapplied and the mass flow of sap is determined from the velocityof the heat pulses moving along the stem. In addition, ratesof sap flow can be determined empirically, using the thermaldissipation technique, from the temperature of sapwood neara continuously-powered heater implanted in the stem. Users mustunderstand the theory underlying each of these methods, so thatthey can select the method most appropriate to their applicationand take precautions against potential sources of error. Whenattempting to estimate transpiration by stands of vegetationfrom measurements of sap flow in individual plants, users mustalso select an appropriate sampling strategy and scaling method. Key words: Sap flow, transpiration, stem heat balance, heat pulse velocity, review     

Components of ecosystem evaporation in a temperate coniferous rainforest, with canopy transpiration scaled using sapwood density

Here we develop and test a method to scale sap velocity measurements from individual trees to canopy transpiration (E(c)) in a low-productivity, old-growth rainforest dominated by the conifer Dacrydium cupressinum. Further, E(c) as a component of the ecosystem water balance is quantified in relation to forest floor evaporation rates and measurements of ecosystem evaporation using eddy covariance (E(eco)) in conditions when the canopy was dry and partly wet. Thermal dissipation probes were used to measure sap velocity of individual trees, and scaled to transpiration at the canopy level by dividing trees into classes based on sapwood density and canopy position (sheltered or exposed). When compared with ecosystem eddy covariance measurements, E(c) accounted for 51% of E(eco) on dry days, and 22% of E(eco) on wet days. Low transpiration rates, and significant contributions to E(eco) from wet canopy evaporation and understorey transpiration (35%) and forest floor evaporation (25%), were attributable to the unique characteristics of the forest: in particular, high rainfall, low leaf area index, low stomatal conductance and low productivity associated with severe nutrient limitation.     

Experiments on the water balance of individual attached twigs of Picea abies (L .) Karst. in pure and ozone-enriched air

 Rates of transpiration and xylem sap flow were continuously measured in individual twigs in the upper crown of an 18-year-old spruce. Two gas exchange chambers were run simultaneously under identical conditions. One of two equivalent twigs was exposed to pure air whereas the other received the ozone-enriched air of the site. A third gas exchange chamber in mid-crown ran independently with normal outside air and was used for basic experiments. At certain times needles were sampled for water potential measurements. Chamber humidity was reduced step by step and the transpiration and xylem sap flow rates were permanently compared. It turned out that sap flow keeps up with transpiration without lagging as long as chamber humidity is high and the twigs outside the chamber are not subject to substantial evaporative demand. However, in warm summer weather and with high flow rates sap flow is no longer sufficient. As the balance quotient (uptake/release) of 0.8 was reached the stomata began to close and water balance improved. The flux quotient increased far above 1.0 without water potential of the needles, which had decreased before, increasing significantly. The balance quotient of the twig in ozone-enriched air fell to relatively low values and only increased again correspondingly slowly due to lagging stomatal closure. Despite increased water uptake after the light phase, the ozone-treated twig ran into a water deficit in the daily balance during the course of an uninterrupted drought period. Water deficit increased from day to day and only disappeared with the next rainfall. For providing insight into the ozone effect shown in this study it was thought necessary to discuss comprehensively the controversial views of the stomatal control mechanism. The results show that the stomata react to small scale changes in water status as expected for an effective negative feedback system. The less sensitive reaction of the stomata under the influence of ozone confirms earlier results. Increased severity and duration of the daily drought stress loads were attributed to a lag in adjustment due to poor stomatal control. The demonstrated function mechanism shows that the detrimental effect of ozone is dependent on weather conditions. Received: 12 April 1996 / Accepted: 24 June 1996     

Environmental limits to the distribution of Scaevola plumieri along the South African coast

Abstract. Scaevola plumieri is an important pioneer on many tropical and subtropical sand dunes, forming a large perennial subterranean plant with only the tips of the branches emerging above accreting sand. In South Africa it is the dominant pioneer on sandy beaches along the east coast, less abundant on the south coast and absent from the southwest and west coasts. Transpiration rates (E) of S. plumieri are predictably related to atmospheric vapour pressure deficit under a wide range of conditions and can therefore be predicted from measurement of ambient temperature and relative humidity. Scaling measurements of E at the leaf level to the canopy level has been demonstrated previously. Using a geographic information system, digital maps of regional climatic variables were used to calculate digital maps of potential transpiration from mean monthly temperature and relative humidity values, effectively scaling canopy level transpiration rates to a regional level. Monthly potential transpiration was subtracted from the monthly median rainfall to produce a map of mean monthly water balance. Seasonal growth was correlated with seasonal water balance. Localities along the coast with water deficits in summer corresponded with the recorded absence of S. plumieri, which grows and reproduces most actively in the summer months. This suggests that reduced water availability during the summer growth period limits the distribution of S. plumieri along the southwest coast, where water deficits develop in summer. Temperature is also important in limiting the distribution of S. plumieri on the southwest coast of South Africa through its effects on the growth and phenology of the plant.     

Long-term transpiration in two eucalypt species in a native woodland estimated by the heat-pulse technique

Abstract The heat-pulse method was used to estimate transpiration rates continuously for periods up to 2 years in mature trees of Eucalyptus wandoo and Eucalyptus salmonophloia at two topographic locations in a remnant native woodland in the Western Australian wheatbelt. Annual transpiration per tree ranged from about 11400 to 18000 L per tree. Highest transpiration rates occurred in late spring or early summer, depending on rainfall distribution. The trees were able to rapidly utilize water following heavy rain outside the agricultural growing season. Extrapolating transpiration rates from single trees to an area of woodland showed that annual transpiration at the ridge site was 150 mm and 168 mm at a site alongside a drainage line. Scaling up transpiration from individual trees requires caution and should allow for variability in trees and soils. The role of trees in curtailing salinization is discussed.     

祁连山区青海云杉林蒸腾耗水估算

青海云杉(Picea crassifolia)林在祁连山区水量平衡中起着重要的调节作用,合理并准确地估算其冠层蒸腾量,对于了解该区的水分循环和水分平衡具有重大意义。以位于祁连山中段大野口关滩森林站的青海云杉纯林为研究对象,结合研究区微气象梯度观测塔自动记录的常规气象资料(气温、空气相对湿度、风速等)以及太阳辐射、大气中CO₂ 浓度、土壤热通量等,应用改进的Penman-Monteith修正式对研究区青海云杉林2008年生长季的冠层蒸腾量进行估算,并采用单变量敏感性分析法对模拟结果进行分析。结果表明:青海云杉林2008年生长季(5-9月)的总蒸腾量为148.8 mm,平均日蒸腾量为0.97 mm;冠层蒸腾量主要受太阳辐射的影响,使得日蒸腾量在典型晴天高达2.19 mm,而在阴雨天仅为0.016 mm;气温对冠层蒸腾量的作用也十分重要,随气温的变化,日蒸腾量从5月开始逐渐增大,在7月中上旬达最大值,从7月下旬到生长季结束其值逐渐减小;模型输入参数对模拟蒸腾量的影响强弱顺序为:林冠层截获的净辐射＞叶面积指数＞气温＞风速＞大气相对湿度;当模型输入参数在±10%变动时,模拟结果的变化范围均在±10%以内,说明该模型的模拟结果比较稳定。在积累了大量相关基础数据的条件下,通过该改进的Penman-Monteith修正式估算森林蒸腾耗水量仍是一种获得满意结果的便捷方法。     

三温模型—基于表面温度测算蒸散和评价环境质量的方法IV. 植被蒸腾扩散系数

 作为蒸散量的测算和环境评价的一种方法,通过近年来对三温模型的研究,该文详细探讨了植被蒸腾扩散 系数（hat）,并通过实验验证了它在不同环境条件下的特性和应用前景。在该模型中,hat的表示式为 hat=（Tc-Ta）/( Tp-Ta),式中Tc、Tp和Ta分别为冠层温度、没有蒸腾（蒸腾量为零）的冠层温度和气温 。理论上,植被蒸腾扩散系数的取值范围为hat≤1,hat的取值范围可以决定植被蒸腾量的大小,该系数 越小, 蒸腾量越大。为了证明hat的这些特性,在1994～1999年的5年间,用3种作物（高粱(Sorghum bicolor),番茄( Lycopersicon esculentum)和甜瓜(Cucumis melo)）进行了5次试验。实验结果表明: hat值与感热通量比率（H/Hp）的值近似相等,二者之间回归线的斜率接近为1,截距接近0,回归系数为 r2=0.70。此外,hat值不仅能较好地反映植物根系区的土壤水分状况、也能较好地反应天气状况。在缺水 条件下,hat主要受根部区域的水分状况影响。 所以,hat可作为作物水分亏缺的指标。当植被受到其它 环境胁迫（污染、高温等）时,hat可作为评价环境质量的指标。植被蒸腾扩散系数的主要优点不仅是能 很好地反映蒸腾过程和确定蒸腾量,而且容易测得,便于遥感应用。     

小叶锦鸡儿灌丛群落蒸腾耗水量估算方法

为了确定沙地小叶锦鸡儿 (Caragana microphylla) 人工林的蒸腾速率, 于2006年6月运用热平衡茎流测量技术, 对科尔沁沙地一处15a生人工小叶锦鸡儿群落的分枝液流动态进行了监测。根据生物统计结果选取被测标准枝, 标准枝基径在0.4~1cm范围内。同时, 分别用基径总断面积推算法和叶片密度推算法对灌丛叶面积进行了估算。以叶面积为扩展纯量, 利用标准枝液流对灌丛群落耗水量进行尺度转换, 在转换过程中, 假设叶面积与蒸腾耗水量之间具有很强的相关性。该尺度转换方法经与大型称重式Lysimeter测值对比验证, 误差小于14.3%, 可望准确估算小叶锦鸡儿灌丛群落的蒸腾耗水量。     

Stem flow and porometer measurements of transpiration from honey mesquite (Prosopis glandulosa)

The objective of this study was to compare stem flow and porometermethods of measuring transpiration of honey mesquite (Prosopisglandulosa) trees on a semiarid site. Stem flow was measuredusing heat balance stem flow gauges. Porometer measurementsof leaf stomatal conductance (g_s) were made within foliage layersof each stem and scaled to transpiration values for the entirestem (E_stem) using stem leaf area. Simultaneous measurementsusing both methods were made diurnally and under artificiallyimposed stem shading or defoliation in June and October 1990.Stem flow and E_stem had similar diumal patterns except on 2d in June when E_stem increased during the afternoon while stemflow declined relative to midday values. During October, E_stemwas greater than stem flow throughout the day. This was attributedto sampling error in which only undamaged leaves were used forporometer measurements yet, by this time in the growing season,many leaves on each stem were damaged from insects or wind andlikely had lower transpiration rates. A regression coefficientbetween E_stem and stem flow of 0.79 in June and 0.91 in Octobersuggested the two methods were comparable, but there was considerablevariation between methods during peak transpiration rates. Bothtechniques detected that artificial shading or defoliation causedsimilar relative declines in transpiration. Results imply thatestimates of stem transpiration can be obtained by scaling porometermeasurements of leaves but accuracy declines at higher transpirationrates. Key words: Sap flow, evapotranspiration, stomatal conductance, scaling, water relations     

依据野外实测的蒸腾速率对几种沙地灌木水分平衡的初步研究（英文）

 本文旨在将毛乌素沙地植被建设的水分平衡与半固定沙丘持续发展原则应用于治沙造林的实践中。毛乌素沙地是一个灌木“王国”,然而沙地灌丛植被的发育常常受到水分亏缺的严重制约。为此,根据水分平衡的原则与方法确立适宜的植物种植密度,对沙地植被的经营管理具有重要的指导意义。在水分平衡研究中,蒸散是最难估计的一项。本文提供了一种根据叶面积指数的季节变化与蒸腾速率的观测资料计算蒸腾耗水量的方法,并根据沙地水分平衡的要求估算丁几种优势灌木的适宜种植密度。结果表明,毛乌素沙地灌丛生态系统的蒸发散主要来自植物蒸腾作用;在所研究的植物当中,除沙地柏(Sabina vulgaris)可以形成很大的密度外(因其强的蒸腾控制能力),其它灌木的适宜种植密度应控制在使沙丘处于半固定状态的水平上。     

A model for nutrient and water flow and their uptake by plants grown in a soilless culture

The objective of this study was to develop a sensitive means of control to optimize nutrient concentrations in the root zone of a soilless system, considering plant water and nutrient uptake, and solution circulation rates. A model is proposed to simulate ornamental plants growth in a channel with a non-interacting soilless substrate, irrigated by point sources with constant discharge rates, spaced uniformly along the channel. The model accounts for compensation for transpiration water losses and consequent salinity buildup, and its interactions with plant growth and nutrient uptake. The added water may contain given concentrations of nutrients and/or toxic (saline) compounds, which would cause salinity buildup. Uptake of each solute is specific, according to a Michaelis–Menten kinetics mechanism, but passive uptake by the transpiration stream is also accounted for. Plant growth is affected by time/age and ionic balance in the solution. The model was calibrated with lettuce (Lactuca sativa L.) plants grown in volcanic ash. Simulation of potassium concentration change as a result of discharge rate and emitter spacing revealed that the two parameters could compensate one for the other, once a target lower limit is set. Potassium appeared to be most sensitive to sodium accumulation in the growth medium; this accumulation changed ionic concentration balance, which affected pH and bicarbonate concentration. Passive uptake of calcium by the transpiration stream is highly affected by the root fraction involved, but its calculated contribution is below published values is highly affected by the root fraction involved, but its calculated contribution is below published values.     

Pressure probe technique to study transpiration in phycomyces sporangiophores

The growth equation for the rate of water uptake is augmented with a transpiration term. The obtained augmented growth equations are used to develop methodology which employs the pressure probe to measure transpiration rates from single plant cells. Experiments are conducted on the sporangiophores of Phycomyces blakesleeanus to demonstrate this technique.     

Influence of leaf size,orientation, and arrangement on temperature and transpiration in three high-elevation,large-leafed herbs

Summary The temperature and water relations of the largleafed, high-elevation species Frasera speciosa, Balsamorhiza sagittata, and Rumex densiflorus were evaluated in the Medicine Bow Mountains of southeast Wyoming (USA) to determine the influence of leaf size, orientation, and arrangement on transpiration. These species characteristically have low minimum stomatal resistances (<60 s m^-1) and high maximum transpiration rates (>260 mg m^-2s^-1 for F. speciosa). Field measurements of leaf and microclimatic parameters were incorporated into a computer simulation using standard energy balance equations which predicted leaf temperature (T _leaf) and transpiration for various leaf sizes. Whole-plant transpiration during a day was simulated using field measurements for plants with natural leaf sizes and compared to transpiration rates simulated for plants having identical, but hypothetically smaller (0.5 cm) leaves during a clear day and a typically cloudy day. Although clear-day transpiration for F. speciosa plants with natural size leaves was only 2.0% less per unit leaf area than that predicted for plants with much smaller leaves, daily transpiration of B. sagittata and R. densiflorus plants with natural leaf sizes was 16.1% and 21.1% less, respectively. The predicted influence of a larger leaf size on transpiration for the cloudy day was similar to clear-day results except that F. speciosa had much greater decreases in transpiration (12.7%). The different influences of leaf size on transpiration between the three species was primarily due to major differences in leaf absorptance to solar radiation, orientation, and arrangement which caused large differences in T _leaf. Also, simulated increases in leaf size above natural sizes measured in the field resulted in only small additional decreases in predicted transpiration, indicating a leaf size that was nearly optimal for reducing transpiration. These results are discussed in terms of the possible evolution of a larger leaf size in combination with specific leaf absorptances, orientations and arrangements which could act to reduce transpiration for species growing in short-season habitats where the requirement for rapid carbon fixation might necessitate low stomatal resistances.     

The Relationship between Transpiration, Root Water Uptake, and Leaf Water Potential

The effect of changing the transpiration rate on leaf waterpotential and water balance has been examined to show if permeabilityof the plant (predominantly the roots) is constant or varieswith the transpiration rate. Measurements of leaf effectivethickness, water potential, transpiration, and uptake of waterby roots were made on sunflower, barley, and maize plants grownin solution culture and subjected to a range of atmosphericconditions and root treatments: cooling, low osmotic potential,and removal of part of the root system. Leaf water potential changed little under a wide range of atmosphericconditions and rates of water flux in the three species, sothat the root permeability to water increases as the rate oftranspiration, and therefore flow across the root surface, increases.Equality between uptake and loss of water and thereby maintenanceof constant leaf water potential is assisted by stomatal changes,which appear to be in response to conditions at or in the rootrather than a direct response to changes in bulk leaf waterpotential.     

The Effect of Age, Pre-conditioning, and Water Stress on the Transpiration Rates of Douglas-Fir (Pseudotsuga menziesii) Seedlings of Several Ecotypes

Seedlings of Douglas-fir from seed of a number of mesic and xeric origins were grown in growth chambers and a nursery to various ages up to 16 weeks. Measurements were made to determine the effect of seedling age, growth chamber and nursery pre-conditioning, and seed source on transpiration rates under closely controlled laboratory conditions. Additional experiments were conducted on seedlings of two contrasting ecotypes to determine the effect of different pre-conditioning combinations of plant and soil water potential on seedling transpiration rates. Results show that well-watered seedlings of two mesic ecotypes show no decline in transpiration rates per unit leaf area up to 16 weeks of age while corresponding seedlings of three exeric ecotypes do decline. The growth chamber pre-conditioning results in lower seedling transpiration rates and more decline in seedling transpiration rates with increasing plant water stress than for nursery pre-conditioning. In a similar way, the xeric ecotype seedlings have more decline in transpiration rates with increasing plant water stress than do the mesic ecotype seedlings. Soil water potential influences transpiration rates through pre-conditioning effects. Seedlings which have experienced prior soil moisture stress decrease transpiration more in response to low plant water potentials than do plants which have experienced no soil moisture stress. These behavioral characteristics illustrate adaptive means by which seedlings conserve water through the interaction of genetic and preconditioning mechanisms.     

应用热平衡法测定玉米/大豆间作群体内作物的蒸腾量

通过田间试验采用基于热平衡法的茎流计测定玉米/大豆条带间作群体内作物的蒸腾规律.结果表明：间作群体内,玉米和大豆植株的茎流速率在晴天呈单峰曲线,在阴天则呈多峰曲线.植株的茎流受多个环境因子的影响,其中太阳辐射是影响植株茎流最主要的气象因子.玉米和大豆的单株日茎流量与多个气象因子间存在较好的相关关系,达到极显著水平.茎流观测期内（2008年6月1-30日）,间作群体内玉米植株的日均蒸腾量（1.44 mm·d^-1）为大豆（0.79 mm·d^-1）的1.8倍,玉米和大豆植株的蒸腾量分别占间作群体总蒸腾量的64%和36%.考虑到作物的茎直径和叶面积的空间变异,安装一定数量的茎流探头对于准确测定植株茎流是十分必要的.     

Cuticular transpiration and epicuticular lipids of primary leaves of barley (Hordeum vulgare)

Twenty cultivars of barley and 15 eceriferum mutants from one of the cultivars have been analysed for cuticular transpiration and epicuticular lipids of their primary leaves. The relative cuticular transpiration rates of the cultivars ranged from 0.61 to 1.98. In spite of this variation in transpiration most of the cultivars had almost the same amount of epicuticular lipids per leaf area, about 16 μg cm⁻². The eceriferum mutants showed a wider range in amount of epicuticular lipids, from 5.0 to 15.5 μg cm⁻². Nevertheless, most of the mutants transpired almost at the same rate. Only a weak correlation was found between cuticular transpiration and total amount of epicuticular lipids. None of the analysed lipid components (alkanes, aldehydes, primary alcohols, esters or fatty acids) was better correlated to the cuticular transpiration than the total amount of lipids. When the cultivars were exposed to a mild water stress their cuticular transpiration rates decreased by about 11%. This reduction was not accompanied by any corresponding increase in total amount of epicuticular lipids. The most pronounced effect of the water stress treatment was a stimulation in the ester formation and a reduced formation of primary alcohols. This shift in lipid composition could not be correlated to the decreased cuticular transpiration rates of the individual cultivars. From this investigation it is concluded that the cuticular transpiration is poorly correlated to the amount or composition of the epicuticular lipids in this barley material. As a consequence it was not possible to use any characteristic of the epicuticular lipids as a selection criterion in breeding for drought resistance.     

Interannual Invariability of Forest Evapotranspiration and Its Consequence to Water Flow Downstream

Although drought in temperate deciduous forests decreases transpiration rates of many species, stand-level transpiration and total evapotranspiration is often reported to exhibit only minor interannual variability with precipitation. This apparent contradiction was investigated using four years of transpiration estimates from sap flux, interception–evaporation estimates from precipitation and throughfall gauges, modeled soil evaporation and drainage estimates, and eddy covariance data in a mature oak-hickory forest in North Carolina, USA. The study period included one severe drought year and one year of well above-average precipitation. Normalized for atmospheric conditions, transpiration rates of some species were lower in drought than in wet periods whereas others did not respond to drought. However, atmospheric conditions during drought periods are unlike conditions during typical growing season periods. The rainy days that are required to maintain drought-free periods are characterized by low atmospheric vapor pressure deficit, leading to very low transpiration. In contrast, days with low air vapor pressure deficit were practically absent during drought and moderate levels of transpiration were maintained throughout despite the drying soil. Thus, integrated over the growing season, canopy transpiration was not reduced by drought. In addition, high vapor pressure deficit during drought periods sustained appreciable soil evaporation rates. As a result, despite the large interannual variation in precipitation (ranging from 934 to 1346 mm), annual evapotranspiration varied little (610–668 mm), increasing only slightly with precipitation, due to increased canopy rainfall interception. Because forest evapotranspiration shows only modest changes with annual precipitation, lower precipitation translates to decreased replenishment of groundwater and outflow, and thus the supply of water to downstream ecosystems and water bodies.     

A coupled model of stomatal conductance, photosynthesis and transpiration

A model that couples stomatal conductance, photosynthesis, leaf energy balance and transport of water through the soil–plant–atmosphere continuum is presented. Stomatal conductance in the model depends on light, temperature and intercellular CO₂ concentration via photosynthesis and on leaf water potential, which in turn is a function of soil water potential, the rate of water flow through the soil and plant, and on xylem hydraulic resistance. Water transport from soil to roots is simulated through solution of Richards’ equation. The model captures the observed hysteresis in diurnal variations in stomatal conductance, assimilation rate and transpiration for plant canopies. Hysteresis arises because atmospheric demand for water from the leaves typically peaks in mid‐afternoon and because of uneven distribution of soil matric potentials with distance from the roots. Potentials at the root surfaces are lower than in the bulk soil, and once soil water supply starts to limit transpiration, root potentials are substantially less negative in the morning than in the afternoon. This leads to higher stomatal conductances, CO₂ assimilation and transpiration in the morning compared to later in the day. Stomatal conductance is sensitive to soil and plant hydraulic properties and to root length density only after approximately 10 d of soil drying, when supply of water by the soil to the roots becomes limiting. High atmospheric demand causes transpiration rates, LE, to decline at a slightly higher soil water content, θ_s, than at low atmospheric demand, but all curves of LE versus θ_s fall on the same line when soil water supply limits transpiration. Stomatal conductance cannot be modelled in isolation, but must be fully coupled with models of photosynthesis/respiration and the transport of water from soil, through roots, stems and leaves to the atmosphere.     

Gas exchange in a 20-year-old stand of Scots pine

The rates of net photosynthesis and transpiration of one-year-old shoots were measured in situ in five different positions within the crown of a young Scots pine ( Pinus sylvestris L.). Measurements were carried out on south- and north-facing shoots on the third and sixth whorls, respectively, and on an east-facing shoot on the ninth whorl. In another investigation the rates of gas exchange of one-year-old shoots on the third whorl of eight different trees were studied. The measurements were made during June and July, 1977, under non-limiting conditions of soil water. The daily rates of net photosynthesis in whorls three and six followed the light conditions closely, with higher rates for the south side of each whorl and higher for whorl three than six. On whorl nine the shoot had a higher light compensation point and a low rate of photosynthesis at light saturation compared to the other shoot positions. The quantum yield for the shoot on the lowest whorl, as estimated from the linear part of the light response curve, was 50% lower than for shoots on whorl three and six.
The variation in transpiration rates was pronounced within the crown as an effect of differences in the absolute value and diurnal course of stomatal conductance. The variation in net photosynthesis was small between different trees while the variation in transpiration was much higher. Thus the variation in water use efficiency was great. It is concluded that it is possible to extrapolate measurements of net photosynthesis from individual trees up to a stand level without introducing large errors in the estimate. More caution must be paid before extrapolating tree transpiration up to stand transpiration. However, before an extrapolation of gas exchange is made from tree to stand level the variation in net photosynthesis and transpiration rate within the crown must be known.