马占相思夜间树干液流的分配及其对整树蒸腾估算的影响

采用Granier热消散探针测定了马占相思(Acacia mangium)的树干液流,结合Li-6400光合测定系统测定的夜间叶片气孔导度和蒸腾,将夜间液流区分为夜间树干水分补充和叶片气孔蒸腾。叶片的蒸腾作用微弱,因此,夜间液流主要用于补充贮水部位的水分亏缺。马占相思夜间水分补充量年内和年际的变化不明显,树形特征的差异是解释夜间水分补充量变化的重要因子,夜间水分补充量对于整树蒸腾量的贡献因季节和树木径级的不同而有明显变化,但对整树总蒸腾量计算造成的误差可以忽略。     

整树水力导度协同冠层气孔导度调节森林蒸腾

冠层气孔导度决定森林的蒸腾效率,它对驱动水汽移动的水汽应力的响应受树木水力结构的影响,并随水汽压亏缺上升和水力导度下降而降低,维持水势在最低阈值之上,避免出现水力灾变,调控冠层蒸腾。由于叶形和树冠结构的特点,部分脱耦联反映了湿润地区阔叶林冠层与大气的水汽交换特征,单纯以气孔导度的变化难以完整描述水分通量的调节规律,因而,需要考虑冠层气孔导度与水力导度协同控制冠层蒸腾的潜在机理。通过整合叶片气孔气体交换、树干液流、冠层微气象和其他环境因子的野外观测值,估测不同时间尺度的森林冠层气孔导度与大气的脱耦联系数和变异范围,以基于树干液流的冠层蒸腾,结合叶片/土壤水势梯度计算的水力导度,分析水力导度影响冠层气孔导度响应水汽压亏缺的敏感性,可以揭示和阐明水力导度和冠层气孔导度联合调节森林蒸腾的机理,对准确估测全球变化背景下森林对水资源利用的潜在生态效应有明显的理论意义。     

树干液流对环境变化响应研究进展

随着大气中CO2浓度和其它温室气体的上升,预计全球和区域尺度的温度会增加,由于增温导致地球上一些地区降水增加,一些地区可能面临干旱的加剧.要分析气候、环境变化对植被的影响,需要深入了解植被和大气之间能量、水汽和CO2交换,蒸腾是这个交换过程的一个重要组成部分,是水分和能量离开森林生态系统的主要途径.目前,树干液流测定技术已经发展得比较成熟,能比较可靠的估计整树蒸腾,逐步被应用于研究树木水分利用对环境变化的响应.介绍比较成熟的树木(林分)蒸腾估算方法,就树木(林分)水分利用对环境变化响应研究中的几个热点问题进行了总结:(1) 大气中CO2浓度升高对树木水分利用、气孔导度和冠层结构的影响,环境条件决定树木水分利用对CO2的响应幅度.(2) 树木蒸腾对降雨的响应类型,降雨格局改变导致的土壤干旱对林分蒸腾的影响.(3) 树体储存水的生理意义.随着液流技术的发展和推广,其作为一种科学研究的技术与手段将会受到更多学者的重视,也必将推进树木水分利用对环境变化响应的研究.     

广州地区荷木夜间树干液流补水的影响因子及其对蒸腾的贡献

明确树木夜间水分补充现象有助于提高总蒸腾量和冠层气孔导度估算的精确度,进一步认识冠层蒸腾与树干液流之间存在的时滞关系.本研究采用热消散探针法测定了广州地区的荷木树干液流密度,同步监测了主要的环境因子,从不同时间尺度分析了树干夜间液流的水分补充现象.结果表明：与白天相比,荷木夜间液流密度较小,旱季变化幅度比湿季大;夜间水分补充的时间段主要在前半夜(18:00-22:00);年内各季节夜间水分补充量之间没有显著差异,与环境因子之间的偏相关关系不显著,但与胸径、树高、冠幅、树干生物量、冠层生物量的回归曲线拟合很好,表明树形特征和生物量能更好地解释夜间补水的变化;各季节夜间水分补充量对总蒸腾量的贡献有显著差异,旱季明显高于湿季.     

广州地区荷木夜间树干液流补水的影响因子及其对蒸腾的贡献

明确树木夜间水分补充现象有助于提高总蒸腾量和冠层气孔导度估算的精确度,进一步认识冠层蒸腾与树干液流之间存在的时滞关系.本研究采用热消散探针法测定了广州地区的荷木树干液流密度,同步监测了主要的环境因子,从不同时间尺度分析了树干夜间液流的水分补充现象.结果表明:与白天相比,荷木夜间液流密度较小,旱季变化幅度比湿季大;夜间水分补充的时间段主要在前半夜(18:00-22:00);年内各季节夜间水分补充量之间没有显著差异,与环境因子之间的偏相关关系不显著,但与胸径、树高、冠幅、树干生物量、冠层生物量的回归曲线拟合很好,表明树形特征和生物量能更好地解释夜间补水的变化;各季节夜间水分补充量对总蒸腾量的贡献有显著差异,旱季明显高于湿季.     

热脉冲技术3种方法组合在测量树干液流中的应用

利用树干液流方法获取树木蒸腾特征对理解树木水分生理、森林生态和森林系统水分交换具有重要意义.利用广泛应用于土壤热参数和土壤蒸发测量的
三针热脉冲探头,基于热比率法(HRM)、最大温度法(T_Max)和单针热脉冲法(SHPP)同时实现了旱柳液流密度的测定,并与热扩散探针(TDP)测量结果进行对比分析.结果表明： 三针热脉冲探头安装约5周后进入稳定测量阶段,3种方法初期测量结果比稳定测量阶段高135%~220%,HRM、T_Max和SHPP法与TDP测量结果具有显著的线性相关性,R2分别为0.93、0.73和0.91,SHPP与HRM法测定结果的R²达到0.94.HRM在低速和逆向液流时测量具有较高的精度;SHPP探头配置简单、测量精度高,但无法甄别液流方向,是测定液流非常有前途的方法;T_Max测量液流误差较大,无法测量<5 cm³·cm^-2·h^-1的液流,不建议单独用于液流测量,但其能够准确测定树干热扩散系数,并可用于其他方法液流计算.建议根据试验目的,选取不同方法或者几种方法组合进行树干液流测量.     

Granier树干液流测定系统在马占相思的水分利用研究中的应用

介绍了Granier热消散探针在树干液流测定中的工作原理,并利用该系统长期监测广东鹤山马占相思林14株样树的液流密度,分析了树木个体内和个体之间液流密度的差异、整树和林段水分利用的量化特征.由于树木边材结构以及周围微环境的差别,树木内和个体间的液流密度差异非常明显,变异系数的平均值分别为15.51%-37.26%、37.46%-50.73%.尽管液流密度的差异较大,但同一株树木不同方位的液流密度之间却呈现明显的线性相关（p＜0.0001）,这是重要的特征值,使得只需测定某一方位的液流密度经尺度外推计算整树和林段蒸腾成为可能.树木液流对环境因子响应的变化规律取决于所参照的时间尺度,日变化主要受光辐射、水汽压差等气候因子的控制,而土壤水份对液流的季节变化影响较大.形态特征明显影响树木的液流,高大树木由于边材较厚、树干粗壮和冠幅较宽而承载较多的辐射能量,因而水分蒸腾较高.对树木液流密度在径向和方位上进行适当的整合,可较准确地计算整树和林段蒸腾.由液流估测的马占相思整树和林段蒸腾的结果显示,该群落的水分利用在时间和空间上均有明显的分化.     

树干液流径向分布格局研究进展

树干液流径向分布的不均匀性是引起热技术估算单株乃至林分蒸腾误差的主要来源。因此, 了解树干液流径向分布格局并将其定量化, 成为利用热扩散和热脉冲技术准确估算森林蒸腾的必要条件。该文详细介绍了树干液流径向分布格局的研究方法, 总结了目前4种常见的树干液流的径向分布格局, 分析了影响树干液流径向分布格局的内部结构因素和外部环境因素, 阐明了树干液流径向分布格局的时间动态特征及其影响因素, 并提出目前研究中存在的问题和可能的解决方法。     

马占相思夜间树干液流特征和水分补充现象的分析

研究马占相思树干的夜间水分补充现象有助于提高总蒸腾量、冠层气孔导度估算的准确度,同时可以深化对冠层蒸腾与树干液流所代表的日蒸腾之间时滞现象的认识。在广东鹤山马占相思林,采用热扩散探针法测定液流密度,同步测定主要的环境因子,从不同的时间尺度分析其树干夜间水分补充现象。结果表明:与白天相比较,夜间马占相思液流密度较小;夜间液流密度的变化幅度旱季比雨季大,树干夜间水分补充的主要时间段是前半夜;年内各个月份夜间水分补充量之间没有显著差异,它与环境因子之间相关关系不显著,但与胸径、树高、冠幅的回归曲线拟合较好;干季夜间补充的水分对总蒸腾量的贡献与白天蒸腾量、总的蒸腾量、水蒸气压亏缺(VPD)、光合有效辐射(PAR)以及大气温度(T)显著相关;湿季夜间补充的水分对总蒸腾量的贡献与白天蒸腾量、总蒸腾量显著相关。     

马占相思林冠层气孔导度对环境驱动因子的响应

利用Granier热消散探针在2003年10月测定了广东鹤山丘陵地马占相思林14株样树的树干液流,同时监测林冠上方的光合有效辐射、空气湿度和气温,结合树木的形态和林分的结构特征,计算马占相思的整树蒸腾(E)、林分总蒸腾(E_t)以及冠层平均气孔导度(g_c),分析树形特征与整树水分利用的关系、冠层气孔导度对光合有效辐射(PAR)和空气水汽压亏缺(D)的响应.结果表明,整树蒸腾与胸径(P＜0.0001)、边材面积(P＜0.0001)和冠幅(P=0.0007)以自然对数的形式、与树高(P=0.014)以幂函数的形式呈现显著正相关.冠层气孔导度最大值(g_cmax)随D的上升呈对数函数下降(P＜0.0001),对光合有效辐射的响应则呈双曲线函数增加(P＜0.0001).液流测定系统能提供连续和准确的整树和林分蒸腾速率值,经严格数学推导公式计算,最终可求出冠层气孔导度,是研究森林水分利用与环境因子相互关系的有效方法.     

应用Granier热消散探针（TDP）法测定树木的木质部液流：理论与实践

Granier热消散探针法是目前研究树木生理生态和森林水文最常用的测定整树水分利用的方法之一。然而,已有的相关综述文献中,还没有专门介绍利用Granier热消散探针研究木质部液流的综述文章。本文重点介绍了Granier热消散探针测定系统的理论基础,对有关该探针的校准和改进的最新研究进展进行了综述,并还深入探讨了零液流信号值的确定、自然热梯度、损伤效应、液流的逆格型和将木质部液流密度外推至整树蒸腾耗水量等重要的实际问题。     

Nighttime sap flow of Acacia mangium and its implications for nighttime transpiration and stem water storage

Aims Nighttime sap flow of trees may indicate transpiration and/or recharge of stem water storage at night. This paper deals with the water use of Acacia mangium at night in the hilly lands of subtropical South China. Our primary goal was to reveal and understand the nature of nighttime sap flow and its functional significance.Methods Granier's thermal dissipation method was used to determine the nighttime sap flux of A. mangium. Gas exchange system was used to estimate nighttime leaf transpiration and stomatal conductance of studied trees.Important findings Nighttime sap flow was substantial and showed seasonal variation similar to the patterns of daytime sap flow in A. mangium. Mean nighttime sap flow was higher in the less precipitation year of 2004 (1122.4 mm) than in the more precipitation year of 2005 (1342.5 mm) since more daytime transpiration and low soil water availability in the relatively dry 2004 can be the cause of more nighttime sap flow. Although vapor pressure deficit and air temperature were significantly correlated with nighttime sap flow, they could only explain a small fraction of the variance in nighttime sap flow. The total accumulated water loss (E L) by transpiration of canopy leaves was only ~2.6–8.5% of the total nighttime sap flow (E t) during the nights of July 17–18 and 18–19, 2006. Therefore, it is likely that the nighttime sap flow was mainly used for refilling water in the trunk. The stem diameter at breast height, basal area and sapwood area explained much more variance of nighttime water recharge than environmental factors and other tree form features, such as tree height, stem length below the branch, and canopy size. The contribution of nighttime water recharge to the total transpiration ranged from 14.7 to 30.3% depending on different DBH class and was considerably higher in the dry season compared to the wet season.     

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     

Interactive effects of ozone and climate on tree growth and water use in a southern Appalachian forest in the USA

* A lack of data on responses of mature tree growth and water use to ambient ozone (O(3)) concentrations has been a major limitation in efforts to understand and model responses of forests to current and future changes in climate. * Here, hourly to seasonal patterns of stem growth and sap flow velocity were examined in mature trees from a mixed deciduous forest in eastern Tennessee (USA) to evaluate the effects of variations in ambient O(3) exposure and climate on patterns of stem growth and water use. * Ambient O(3) caused a periodic slowdown in seasonal growth patterns that was attributable in part to amplification of diurnal patterns of water loss in tree stems. This response was mediated by statistically significant increases in O(3)-induced daily sap flow and led to seasonal losses in stem growth of 30-50% for most species in a high-O(3) year. * Decreased growth and increased water use of mature forest trees under episodically high ambient O(3) concentrations suggest that O(3) will amplify the adverse effects of increasing temperatures on forest growth and forest hydrology.     

Laurel forests in Tenerife, Canary Islands

The efficiency of the conductive system in about 40-year-old Laurus azorica trees growing in a laurel forest was evaluated by comparing main stems and leaves (petioles) on the basis of theoretical sap flow values (1) calculated from vessel anatomy (taking vessels as ideal capillaries), (2) derived from measured dye velocity and (3) data taken from direct sap flow measurements. It was found that actual sap flow rate per wood area increases in stems from the pith towards the cambium. The outermost part of the stem is the most important part of the tree for conducting water. Maximum actually measured transpiration (sap flow rate) for the stand was practically identical to the theoretical rate calculated based on petiole anatomy, but it was about 45 times lower than that calculated based on stem anatomy. This illustrates the safety features of stem wood, which due to its high vessel density, is capable of transporting all the water required even when only a small area of its vessels is working. In the petioles, xylem is more efficiently used, but almost all vessels must work in order to supply water to leaves and any disturbance may cause leaf loss.     

荒漠防护林典型树种液流特征及其对环境因子的响应

利用基于热补偿理论的SF300分体液流仪对干旱荒漠区人工防护林典型树种(俄罗斯杨、胡杨、榆树、沙枣)树干液流全天候监测,自动气象站同步记录相关环境因子变化。研究表明:①4种防护林树种茎干液流日变化除沙枣树外均存在明显昼夜节律,液流速度在同属种间差异较小,在不同属种间差异显著,俄罗斯杨的日平均液流速度可以达到沙枣的13.8倍,耗水量排序为俄罗斯杨胡杨榆树沙枣树。②水分充足条件下,增加实验地灌溉量使4种树木蒸腾受到抑制,液流流速降低,水分利用效率降低。③液流流速因所处树干径向位点不同而存在差异,俄罗斯杨、榆树、沙枣液流速度表现出由形成层到髓心的递减趋势,胡杨树干径向位点液流没有表现一定规律。④树干液流流速与环境因子进行相关分析,通过逐步回归分析建立了4个典型树种茎干液流速度与环境因子关系估算模型,分析认为4种树木的环境敏感性排序为俄罗斯杨榆树胡杨沙枣。     

Scots pine root distribution derived from radial sap flow patterns in stems of large leaning trees

This study characterizes whole tree root system distribution in a non-destructive way based on its functional parameters, particularly the sap flow patterns in stems. This approach particularly considers sap flow variation across stems, both radial and circumferential patterns of flow that are usually used for a better integration of sap flow density at the whole tree level. We focused at: (1) Showing examples of sap flow variation across stems at a defined situation (high midday values at the period of non-limiting water supply; (2) Analyzing radial flow patterns in terms of root distribution; (3) Validating these results at the stand level (mean data of series of individual trees) using results of classical biometric methods used at the same site; and (4) Applying the results for evaluation of root distribution around leaning trees. Sap flow rate was measured by the heat deformation method on a set of 14 trees at an experimental pine forest stand in Brasschaat (Belgium) during the growing season of 2000. Sap flow variation across stems was measured at a total of 700 points. Amounts of water supplied by superficial (horizontally oriented) and sinker (vertically oriented) roots were estimated from sap flow patterns. The vertical distribution of absorbing roots as derived from the analysis of sap flow patterns in stem sapwood was very similar to the distribution determined by the classical biometric analysis of fine roots. Trees leaning to the East had stem radii at the stump level and crown radii enhanced in the leaning direction. Sinker roots showed higher absorption activities in the leaning direction, but superficial roots were more absorbing in the opposite direction. The application of the above-described method allows for a better evaluation of the whole-tree behavior and facilitates the evaluation of tree and stand properties in traditional forest stands, which are not equipped for detailed scientific research. This may also facilitate practical applications in landscape-level studies.     

北京山区元宝枫夜间液流活动特征及影响因素

树木夜间会维持部分气孔开放,从而能够在一定环境驱动因子的情况下进行夜间蒸腾。夜间液流作为储存水的重要来源,能够补充植物白天的水分亏缺,使其恢复水分储备,对植物生长发育有重要意义。采用TDP热探针法测定了位于八达岭林场的元宝枫树干液流密度,同步监测了主要环境因子,以深入揭示树木夜间蒸腾耗水规律和植被应对环境胁迫的调控机制,为山区植被建设、森林健康经营和挑选节水树种提供理论依据。结果表明:以0:00为界区分前半夜和后半夜,元宝枫夜间液流速率前半夜较后半夜活跃,且前半夜夜间累积液流量占夜间累积液流量的53.85%—64.10%,而后半夜夜间累积液流量占夜间累积液流量的35.9%—46.15%。5月的夜间累积液流量最大,平均夜间液流通量为5月6月8月9月7月。存在水分胁迫的条件下降雨之后夜间液流会增大,而当土壤水分条件较好,土壤水分不再是夜间液流的限制因子时,夜间液流通量并不高。不同树木形态的夜间液流通量有显著差异,在一定范围内,胸径树高冠幅越大的样木,夜间液流通量越大。用于夜间蒸腾的夜间液流通量与饱和水汽压差、温度、空气相对湿度、风速相关,其中夜间蒸腾存在于前半夜,表现为前半夜夜间液流通量与环境因子的相关性相较后半夜相关性较为显著,后半夜则以补水为主,补水量取决于土壤含水量和日蒸腾强度。存在干旱胁迫的条件下,夜间液流既用于夜间蒸腾,又有一部分用来补水;而土壤水分条件好时夜间液流则主要用于补水,此时夜间树干液流与环境因子相关性不高。元宝枫夜间液流通量的日蒸腾贡献率5、6月份大于7、8月份,即干季比湿季贡献率更高。夜间液流通量的日蒸腾贡献率与白天总蒸腾量相关性较高,并与累积太阳辐射成负相关。