辽西农林复合系统中杨树冠层导度特征

利用Granier热扩散式探针法对辽西杨树-玉米复合系统的杨树树干液流进行连续测定,并对环境因子(空气温度、空气湿度、净辐射、风速、土壤温度和土壤湿度)进行同步观测,结合Penman-Monteith方程计算冠层导度值.结果表明:研究区杨树冠层导度日变化呈“单峰型”曲线,季节变化表现为波动式下降趋势;冠层导度随着饱和水汽压差增加呈负对数下降,5-9月,冠层导度对水汽压差变化的敏感性逐渐下降;冠层导度与太阳辐射呈正相关关系;太阳辐射越大,冠层导度曲线下降幅度越大.不同月份,相同环境因子与冠层导度的相关程度不同.从整个生长季来看,与冠层导度相关性最显著的环境因子是饱和水汽压差.     

北京山区侧柏林冠层-大气蒸腾导度模拟及环境因子响应

蒸腾导度模型是衡量冠层-大气界面水汽输出的重要阻力模型,研究其特征及对环境因子的响应,为揭示森林冠层-大气界面水汽输出阻力机制提供理论依据。以首都圈森林生态系统定位观测研究站侧柏林为研究对象,采用TDP热探针法测定侧柏林树干液流密度,同步监测光合有效辐射、饱和水汽压差、气温、风速等主要环境因子,分析冠层导度和空气动力学导度的动态变化,构建冠层-大气蒸腾导度模型并模拟,明确冠层-大气蒸腾导度对各环境因子的响应关系。结果表明:蒸腾导度季节变化表现为非生长季与冠层导度趋势一致,生长季与空气动力学导度趋势一致,全年均为单峰趋势。冬季蒸腾导度与冠层导度保持较稳定差值(45 mol m^(-2 )s^-1左右),其他季节蒸腾导度与冠层导度、空气动力学导度的最大差值,均在各季节冠层导度、空气动力学导度的峰值水平。全年日均蒸腾导度冬季最大(86.92 mol m^(-2 )s^-1),其他季节较小且稳定(40—50 mol m^(-2 )s^-1之间)。在非生长季各环境因子对蒸腾导度的影响与对冠层导度的影响基本一致,温度为主要影响因子(r=-0.198),其他环境因子影响较小(r<0.1);在生长季中风速为主要影响因子(r=0.488),光合有效辐射(r=0.228)和饱和水汽压差(r=-0.299)的影响明显升高,温度的影响降低(r=0.114)。蒸腾导度模型较好的模拟了冠层-大气界面侧柏蒸腾不同季节的变化规律,阐明了各环境因子和冠层导度、空气动力学导度对蒸腾导度的影响机制,证实在生长季应重视空气动力学导度对蒸腾的影响。     

城市绿地冠层导度季节变化及其环境调控

冠层导度(canopy conductance,gc)是生态系统对环境响应的敏感性指标,探讨冠层导度对环境因素的响应模式对了解生态系统生产力的变化模式至关重要。城市绿地作为人为设计的复合生态系统,其冠层导度变化规律及其对环境因子的响应亟待明确。基于涡度相关方法,本研究调查了北京奥林匹克森林公园2012-2016年连续5年的冠层导度的季节动态变化及其对空气温度(Ta)、光合有效辐射(PAR)、饱和水汽压差(VPD)以及土壤含水量(VWC)等环境因子的响应,利用统计回归方法分析了环境因子对冠层导度的影响,最后分析了冠层导度对总生态系统生产力(GEP)的影响。结果表明:2012-2016年的冠层导度依次为3.97、3.28、2.13、3.95和5.07 mm·s^-1,5年平均值为3.69±1.99 mm·s^-1;季节变化表现为从4月开始逐渐升高,在7、8月达到最大值后逐渐降低;在季节尺度上,VWC和Ta是影响冠层导度的主要环境因子,冠层导度随它们的增加而增大;而PAR和VPD对冠层导度的影响存在年际间的差异;5年间,GEP随着冠层导度的升高显著增大;城市绿地中,季节尺度上土壤水分的增加和气温的升高显著增加冠层导度,从而促进GEP。     

樱桃冠层导度特征及模拟

为了揭示樱桃冠层蒸腾、冠层导度对环境因子的响应规律,评价Jarvis模型在樱桃冠层尺度上应用的适用性,利用Granier热消散式探针连续监测了北京四季青果林所试验地3年生盆栽樱桃(Prunus avium L.)4-8月份蒸腾动态变化,同步监测了气象与土壤水分数据。以实测液流为基础,利用Penman-Monteith方程反推方法获取了长期连续冠层导度,在分析樱桃冠层蒸腾、冠层导度的动态变化规律的基础上,采用十字交叉法对多元回归模型与Jarvis模型进行参数率与误差分析,结果显示盆栽樱桃冠层蒸腾规律性强、时滞效应小,不同辐射条件下,冠层导度随水汽压亏缺增加呈负指数函数下降趋势,采用水汽压亏缺、光合有效辐射、气温的不同组合方式构建了多元回归和Jarvis冠层模型,模拟结果显示Jarvis模型精度高于多元回归模型,环境因子对模型精度的影响程度依次为:水汽压亏缺光合有效辐射气温,考虑了水汽压亏缺和太阳辐射的Jarvis模型精度最高,最低相对误差仅为12.12%,均方根误差为0.271。     

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。     

科尔沁草甸生态系统水分利用效率及影响因素

生态系统水分利用效率(WUE)是衡量碳水循环耦合程度的重要指标。利用科尔沁温带草甸草地碳水通量观测数据,对该生态系统总初级生产力水分利用效率(WUEGPP)的日季变化规律及对环境和生理因子的响应进行分析。结果表明:(1)WUEGPP日变化呈下降-稳定-上升的变化趋势,最大值出现在日出后1—2 h,阴天条件下WUEGPP高于晴天,生长中期WUEGPP高于生长初期和末期;(2)总初级生产力、总蒸散和WUEGPP季节变化均呈夏季高、春秋低的形式,生长季平均值分别为0.57 mg m-2s-1、0.08 g m-2s-1和5.97 mg/g,最大值分别为1.49 mg m-2s-1、0.16 g m-2s1和13.62 mg/g;(3)总初级生产力与饱和差、气温和叶面积指数均呈二次曲线关系,与冠层导度呈对数曲线关系;总蒸散与气温呈二次曲线关系,与饱和差、叶面积指数和冠层导度相关性均不显著;(4)WUEGPP与饱和差、气温和叶面积指数均呈二次曲线关系,与冠层导度呈对数曲线关系,饱和差、冠层导度和叶面积指数分别为2.0 k Pa、0.0015 m/s和4.2是控制WUEGPP增加的阈值;(5)净生态系统生产力水分利用效率(WUENEP)和净初级生产力水分利用效率(WUENPP)季节变化规律与WUEGPP一致,均值分别为3.47和5.47 mg/g。     

苹果树冠层空气温差变化及其与环境因子的关系

于2002—2005年,采用红外测温仪观测得到苹果树主要生长季节冠层温度数据,结合同步观测得到冠层净辐射(Rn)、风速(V)、空气温度(Ta)和湿度(RH)等冠层微气象要素数据及0～80cm土壤含水量(SW),分析苹果树冠层-空气温差(△T)变化规律及其与环境因子的关系.结果表明:苹果树主要生长季节(萌芽期—果实迅速膨大期)晴天△T日变化呈多峰曲线分布,△T最高值都出现在12:00—13:00左右;阴天△T日变化呈多峰曲线分布,但△T绝对值明显低于晴天日.2003年和2004年晴天日14:00△T与Rn、V、RH及SW具有较好的复相关关系:ΔT=7.159-0.002Rn-0.061V-0.7RH-46.0SW(r=-0.825),与Rn、RH、V及SW的偏相关系数分别为0.125、-0.078、-0.036和-0.874,逐步回归方程式为ΔT=5.317-43.1SW,说明土壤水分对△T的影响程度相对最大.经2002年和2005年观测数据验证,△T观测值与计算值吻合关系较好,二者线性相关系数可达0.9083.这说明采用晴天日14:00时刻数据分析△T的影响机制,预测土壤水分含量具有很好的可行性.     

华北低丘山地人工林蒸散的控制因子

人工林蒸散的影响机制研究对指导我国林业生态工程建设有重要意义。基于涡度相关技术,对华北低丘山区30年生栓皮栎-刺槐-侧柏人工混交林进行了连续2a的观测,以探讨蒸散的控制因子。结果表明:退耦系数(Ω)与冠层导度的季节动态有很好的一致性。2007、2008年的快速生长季Ω变化范围分别为0.12—0.62、0.08—0.54,平均值分别为0.37、0.23,快速生长季蒸散主要受气孔控制。2008年比2007年偏旱,该年快速生长季的退耦系数、冠层导度和蒸散低于2007年,发生干旱时蒸散主要受气孔限制。气孔对蒸散的限制作用在大气湿度较低时较高,并且这种限制作用在发生干旱时会进一步加剧。快速生长季内辐射是影响蒸散的主要环境因子,辐射资源丰富的年份气孔对蒸散的控制程度更高。     

华北平原冬小麦冠层导度的环境响应及模拟

通过引入叶面积指数，将叶片水平的气孔导度组合模型扩展到冠层水平，建立了冠层导度环境响应组合模型，组合模型所需参数较少，且均可在冠层水平直接测量，便于应用；模型由潜在气孔导度（PSC）和相对气孔开度（RDO）组成，二者分别由环境变量的日际（inter-day）和日间（intra-day）的值决定。分析表明，冠层导度在日际尺度和日间尺度上对环境变量具有多尺度响应特性，在日际尺度上，温度是影响冠层导度的主要因子，在日间尺度，光是影响气孔开闭的主要因素。利用以温度和光合有效辐射为输入变量构建的组合模型，模拟了华北平原冬小麦农田生态系统的冠层导度，并用Penman-Monteith方程估算的表面导度进行验证。结果显示，在不同天气情况下，二者的日变化均具有较好的一致性；将组合模型与电学类比模型结合，进一步估算了拔节．灌浆期的冠层潜热通量，利用涡度相关系统观测的潜热通量数据进行验证，结果表明对冬小麦冠层潜热通量模拟精度较高，直线回归斜率为0．7054，R^2=0．7894。     

大豆和玉米冠层光合有效辐射各分量日变化

通过实测大豆和玉米冠层光合有效辐射各分量并计算其反射率、透射率,分析了各分量日变化规律及其影响因素。结果表明:光合有效辐射分量(FPAR)在一天中均接近于常数,特别是在8:00—11:00和14:00—16:00相对稳定;晴天大豆冠层入射光合有效辐射变化曲线较阴天平滑,反射率和透射率曲线没有阴天平滑;由于云层的吸收和散射作用,阴天中光合有效辐射(PAR)最大值的出现时间比晴天晚1h左右;植被冠层空间异质性对光合有效辐射各分量影响较大,不同作物类型的各分量之间有较大差异;大豆冠层空间异质性较玉米小,其光合有效辐射各个分量曲线较平滑;线性光量子传感器与太阳入射方向垂直投影线成30°时,冠层入射光合有效辐射平均偏离度值最小,为0.657%。     

Spatiotemporal variations of T/ET (the ratio of transpiration to evapotranspiration) in three forests of Eastern China

Evapotranspiration (ET), which is comprised by evaporation from soil surface (E), transpiration (T) and evaporation from the intercepted water by canopy (EI), plays an important role in maintaining global energy balance and regulating climate. Quantifying the spatiotemporal variations of T/ET (the ratio of T to ET) can improve our understandings on the role of vegetation ecophysiological processes in climate regulation. Using eddy covariance measurements at three forest ecosystems (Changbaishan temperate broad-leaved Korean pine mixed forest (CBS), Qianyanzhou subtropical coniferous plantation (QYZ) and Dinghushan subtropical evergreen mixed forest (DHS)) in north–south transect of Eastern China (NSTEC), we run the revised Shuttleworth–Wallace model (S–W model), validated its performance with the water vapor fluxes measured at two layers, and quantified the spatiotemporal variations of T/ET. The S–W model performed well in simulating ET and T/ET. The mean value of annual T/ET at three forests during the observation period all exceeded 0.6. The diurnal variation of canopy stomal conductance (G_c) dominated that of T/ET. The seasonal dynamics of T/ET was mainly shaped by that of leaf area index (LAI), vapor pressure deficit (VPD) and air temperature (T_a) through altering G_c and the portion that the energy absorbed by canopy (P_EC) at temperate forest (CBS), while the seasonal dynamics of T/ET at subtropical forests (QYZ and DHS) were mainly affected by T_a, net radiation, VPD, and soil water content through altering G_c and soil surface conductance (G_s). The variation of mean annual G_c governed the interannual varaition and spatial variation of T/ET. Therefore, forests in Eastern China played an important role in regulating climate through T and G_c primarily affected the spatial and temproal variations of the role of forest T in regulating climate.     

Midday depression of photosynthesis in <Emphasis Type="Italic">Enkleia malaccensis</Emphasis>, a woody climber in a tropical rainforest

We measured the diurnal changes in net photosynthetic rate (P _N) and stomatal conductance (g _s) of the leaves of a liana, Enkleia malaccensis Griff. (Thymelaeaceae), at the canopy level in the lowland tropical rainforest at Pasoh, Peninsular Malaysia. The measurements were made from a canopy walkway system, 30 m from the ground for 3 d in March 2003. P _N increased with increasing photosynthetically active radiation (PAR) before noon, though P _N was not enhanced by the strong radiation hit in the afternoon. Plotting g _s at saturating PAR (>0.5 mmol m⁻² s⁻¹) against the vapour pressure deficit (VPD) failed to reveal a significant correlation between VPD and g _s, and g _s became very low at VPD >2.5 kPa. The relationship between P _N and g _s was fitted on the same regression line irrespective of measuring day, indicating that this relationship was not influenced by either VPD or leaf temperature (T _L). Therefore, in the liana E. malaccensis, an increase in VPD leads to partial stomatal closure and, subsequently, reductions in P _N and the midday depression of P _N of this plant.     

BP人工神经网络模拟杨树林冠蒸腾

利用2008和2010年的气温、饱和差、总辐射和叶面积指数作为模型输入,液流法观测的蒸腾速率作为模型输出,建立了用于杨树林冠蒸腾模拟的BP人工神经网络模型,利用2009年的观测数据对模型的模拟能力进行了检验,并应用连接权值计算得到的输入变量对输出变量的相对贡献进行了敏感性分析。结果表明:建立的BP人工神经网络蒸腾模型可以很好的模拟林冠蒸腾大小和季节变化,模拟的绝对误差和绝对相对误差的平均值分别为0.11 mm/d和9.5%,纳什效率系数为0.83;输入变量对蒸腾的相对贡献以及蒸腾与输入变量之间的相关性大小顺序相同,均为总辐射叶面积指数饱和差气温。     

西北干旱荒漠绿洲区葡萄园水热通量特征及其主要影响因素

为提高干旱绿洲区农田地表过程的科学认识及改善农业水管理，该研究基于涡度相关技术，运用葡萄园生长季能量平衡观测资料，分析了不同时间尺度的水热通量变化特征，以及干旱平流和冠层导度(G_c)在不同生长阶段上对水热通量的影响，并应用通径分析研究了环境因子对潜热通量(LE)的影响路径和程度。结果表明：(1)日尺度上，LE呈现出不同程度多峰状，其余水热通量总体上呈单峰状。整体上，各生长阶段日间净辐射(R_n)>LE>感热通量(H)>土壤热通量(G)。G相较于R_n呈现出明显的滞后现象。(2)整个生长季上，LE和H分别占白天可利用能量(R_n-G)的86%和14%,表明LE始终是白天葡萄园可利用能量的主要消耗项。干旱平流对白天LE的贡献范围为5%～59%,全生长季平均贡献为28%;G_c对LE的影响在生长季上呈动态变化，且在新梢生长期和落叶期对LE的影响强于中间生长阶段。(3)LE主要受R_n的影响，饱和水汽压差(VPD)和气温(T_a)对LE...     

Seasonal variability in the effect of elevated CO2 on ecosystem leaf area index in a scrub-oak ecosystem

We report effects of elevated atmospheric CO₂ concentration (C_a) on leaf area index (LAI) of a Florida scrub‐oak ecosystem, which had regenerated after fire for between three and five years in open‐top chambers (OTCs) and was yet to reach canopy closure. LAI was measured using four nondestructive methods, calibrated and tested in experiments performed in calibration plots near the OTCs. The four methods were: PAR transmission through the canopy, normalized difference vegetation index (NDVI), hemispherical photography, and allometric relationships between plant stem diameter and plant leaf area. Calibration experiments showed: (1) Leaf area index could be accurately determined from either PAR transmission through the canopy or hemispherical photography. For LAI determined from PAR transmission through the canopy, ecosystem light extinction coefficient (k) varied with season and was best described as a function of PAR transmission through the canopy. (2) A negative exponential function described the relationship between NDVI and LAI; (3) Allometric relationships overestimated LAI. Throughout the two years of this study, LAI was always higher in elevated C_a, rising from, 20% during winter, to 55% during summer. This seasonality was driven by a more rapid development of leaf area during the spring and a relatively greater loss of leaf area during the winter, in elevated C_a. For this scrub‐oak ecosystem prior to canopy closure, increased leaf area was an indirect mechanism by which ecosystem C uptake and canopy N content were increased in elevated C_a. In addition, increased LAI decreased potential reductions in canopy transpiration from decreases in stomatal conductance in elevated C_a. These findings have important implications for biogeochemical cycles of C, N and H₂O in woody ecosystems regenerating from disturbance in elevated C_a.     

Predicting the light attenuation coefficient through Secchi disk depth and beam attenuation coefficient in a large, shallow, freshwater lake

The diffuse attenuation coefficient of photosynthetically active radiation (PAR) (400–700 nm) (K _d(PAR)) is one of the most important optical properties of water. Our purpose was to create K _d(PAR) prediction models from the Secchi disk depth (SDD) and beam attenuation coefficient of particulate and dissolved organic matter (C _t−w(PAR), excluding pure water) in the PAR range. We compare their performance and prediction precision by using the determination coefficient (r ²), relative root mean square error (RRMSE), and mean relative error (MRE). Our dataset comprised 1,067 measurements, including K _d(PAR), SDD, and C _t−w(PAR) taken in shallow, eutrophic, Lake Taihu, China, from 2005 to 2010. The prediction models of K _d(PAR) were based on the linear model with an intercept of zero, using the inverse SDD, and the nonlinear model using SDD. The linear model generated a slope of 1.369, which was not significantly different from 1.7, the index used worldwide, but significantly lower than the value of 2.26. The nonlinear model gave a slightly more reliable prediction of K _d(PAR) with a r ² of 0.804. Compared to the SDD, C _t−w(PAR) was more significantly correlated to K _d(PAR) based on the linear model, with a significantly higher r ² and lower RMSE and RE. Considering the measurement simplicity of C _t−w(PAR) and data acquisition feasibility from high-frequency autonomous buoys and satellites, our results demonstrated that this prediction model reliably estimates K _d(PAR), and could be used to significantly expand optical observations in an environment where the conditions for underwater PAR measurement are limited.     

Optical properties and optical constituents of the Faroe Islands shelf and off-shelf waters,North East Atlantic

The study comprises a data set of CTD, optical properties—K ₀(PAR), c _p, a(PAR), b(PAR)—and optical constituents—Chl a, SPM, CDOM—from 72 shelf and off-shelf stations in the Faroe Islands (62°N, 7°W) North East Atlantic, in early spring 2005. Results showed that shelf waters surrounding the islands were cold and low saline, whereas off-shelf waters were warmer (~1°C) and more saline (~0.05) PSU. A pronounced oceanographic front separated the two waters, and diffuse light attenuation K ₀(PAR), beam attenuation c _p, Chl a, absorption a(PAR), and scattering coefficient b(PAR) were all significantly higher on the shelf. Analyses showed that off-shelf light attenuation K ₀(PAR) was governed by Chl a, shown by a high (r ² = 0.64) Chl a–K ₀(PAR) correlation, whereas light attenuation on the shelf was governed by both Chl a, SPM, and CDOM in combination. A Chl a specific diffuse attenuation coefficient K₀^* ( \textPAR ) K_{0}^{*} \left( {\text{PAR}} \right) of 0.056 (m² mg⁻¹ Chl a) and a Chl a specific beam attenuation ( c_\textp^* c_{\text{p}}^{*} ) of 0.27 (m² mg⁻¹ Chl a) coefficients were derived for the off-shelf. It is pointed out that Chl a is the single variable that changes over time as no rivers with high SPM and CDOM enter the shelf area. Data were obtained in early spring, and Chl a concentrations were low ~0.5 mg Chl a m⁻³. Spring bloom Chl a are about 10 mg Chl a m⁻³ and estimations showed that shelf K ₀(PAR) will increase about 5 times and beam attenuation about 10 times. The Faroe Islands shelf–off-shelf waters is a clear example where physical conditions maintain some clear differences in optical properties and optical constituents. The complete data set is enclosed.     

Response of transpiration to rain pulses for two tree species in a semiarid plantation

Responses of transpiration (E _c) to rain pulses are presented for two semiarid tree species in a stand of Pinus tabulaeformis and Robinia pseudoacacia. Our objectives are to investigate (1) the environmental control over the stand transpiration after rainfall by analyzing the effect of vapor pressure deficit (VPD), soil water condition, and rainfall on the post-rainfall E _c development and recovery rate, and (2) the species responses to rain pulses and implications on vegetation coverage under a changing rainfall regime. Results showed that the sensitivity of canopy conductance (G _c) to VPD varied under different incident radiation and soil water conditions, and the two species exhibited the same hydraulic control (?dG _c/dlnVPD to G _cref ratio) over transpiration. Strengthened physiological control and low sapwood area of the stand contributed to low E _c. VPD after rainfall significantly influenced the magnitude and time series of post-rainfall stand E _c. The fluctuation of post-rainfall VPD in comparison with the pre-rainfall influenced the E _c recovery. Further, the stand E _c was significantly related to monthly rainfall, but the recovery was independent of the rainfall event size. E _c enhanced with cumulative soil moisture change (ΔVWC) within each dry–wet cycle, yet still was limited in large rainfall months. The two species had different response patterns of post-rainfall E _c recovery. E _c recovery of P. tabulaeformis was influenced by the pre- and post-rainfall VPD differences and the duration of rainless interval. R. pseudoacacia showed a larger immediate post-rainfall E _c increase than P. tabulaeformis did. We, therefore, concluded that concentrated rainfall events do not trigger significant increase of transpiration unless large events penetrate the deep soil and the species differences of E _c in response to pulses of rain may shape the composition of semiarid woodlands under future rainfall regimes.     

Assessment of annual carbon exchange in a water-stressed Pinus radiata plantation: an analysis based on eddy covariance measurements and an integrated biophysical model

We used a combination of eddy flux, chamber and environmental measurements with an integrated suite of models to analyse the seasonality of net ecosystem carbon uptake (F_CO2) in an 8-year-old, closed canopy Pinus radiata D.Don plantation in New Zealand (42°52′ S, 172°45′ E). The analyses utilized a biochemically based, big-leaf model of tree canopy photosynthesis (A_c), coupled to multiplicative environmental-constraint functions of canopy stomatal conductance (G_c) via environmental measurements, a temperature-dependent model of ecosystem respiration (R_eco), and a soil water balance model. Available root zone water storage capacity at the measurement site is limited to about 50 mm for the very stony soil, and annual precipitation is only 660 mm, distributed evenly throughout the year. Accordingly the site is prone to soil moisture deficit throughout the summer. G _c and A_c obtained maximum rates early in the growing season when plentiful soil water supply was associated with sufficient quantum irradiance (Q_abs), and moderate air saturation deficit (D) and temperature (T). From late spring onwards, soil water deficit and D confined G_c and A_c congruously, which together with the solely temperature dependency of R_eco resulted in the pronounced seasonality in F_CO2. Reflecting a light-limitation of A_c in the closed canopy, modelled annual carbon (C) uptake was most sensitive to changes in Q_abs. However, Q_abs did not vary significantly between years, and changes in annual F_CO2 were mostly due to variability in summer rainfall and D. Annual C-uptake of the forest was 717 g C m^–2 in a near-average rainfall year, exceeding by one third the net uptake in a year with 20% less than average rainfall (515 g C m^–2).     

Stomatal acclimation to vapour pressure deficit doubles transpiration of small tree seedlings with warming

Future climate change is expected to increase temperature (T) and atmospheric vapour pressure deficit (VPD) in many regions, but the effect of persistent warming on plant stomatal behaviour is highly uncertain. We investigated the effect of experimental warming of 1.9–5.1 °C and increased VPD of 0.5–1.3 kPa on transpiration and stomatal conductance (g_s) of tree seedlings in the temperate forest understory (Duke Forest, North Carolina, USA). We observed peaked responses of transpiration to VPD in all seedlings, and the optimum VPD for transpiration (D_opt) shifted proportionally with increasing chamber VPD. Warming increased mean water use of Carya by 140% and Quercus by 150%, but had no significant effect on water use of Acer. Increased water use of ring‐porous species was attributed to (1) higher air T and (2) stomatal acclimation to VPD resulting in higher g_s and more sensitive stomata, and thereby less efficient water use. Stomatal acclimation maintained homeostasis of leaf T and carbon gain despite increased VPD, revealing that short‐term stomatal responses to VPD may not be representative of long‐term exposure. Acclimation responses differ from expectations of decreasing g_s with increasing VPD and may necessitate revision of current models based on this assumption.