Environmental control of whole-plant transpiration, canopy conductance and estimates of the decoupling coefficient for large red maple trees
Environmental control of whole-plant transpiration, canopy conductance and estimates of the decoupling coefficient for large red maple trees
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DOI：
10.1016/s0168-1923(00)00152-0
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发表时间：
2000-08
影响因子：
6.2
通讯作者：
S. Wullschleger;K. Wilson;P. Hanson
中科院分区：
文献类型：
--
作者：
S. Wullschleger;K. Wilson;P. Hanson
有一个强烈的需要，以扩大到阔叶林的林分结构，表面粗糙度，叶片尺寸，空气动力学和冠层传导特性的树干液流的测量可能会相互作用，部分脱离大气的冠层。这种部分解耦的影响，以了解冠层蒸腾的环境控制和森林用水的建模是很多的。因此，热耗散探针被用于超过三个月的时间内（1997年6月至8月），以量化全树液流（Q）的一天到一天和树到树的变化12红枫（Acerrubrum L.）生长在田纳西州东部高地橡树林的树木。全树Q计算为测得的液流速度，边材面积和边材功能的水运输的分数的产品。每日冠层蒸腾量（Ec）计算全树Q和投影冠面积，而平均每日电导率（gc）是通过反转Penman-Monteith方程。最大干重平均为73 kg/株·d，胸径17 ~ 35 cm、树高19 ~ 26 m的林木最大干重为45 ~ 160 kg/d。冠层蒸腾量在7月上旬达到最大值3.0mm/d，在3个月的测量期内平均为1.5mm/d。树与树之间的变异系数很高。最大的Ec速率变化从1.9mm每天的树与最小的投影冠面积为5.7mm每天的最大的树之一。Ec的日变化是净辐射（Rn）和大气湿度差（δe）日差的函数。日蒸腾速率δ的增加导致冠层蒸腾速率线性增加，Ec与日平均δe之间不存在平台型关系。平均每日gcranged从1.4到6.7千克-1，平均3.4千克-1在12个研究树木。Ec和gc的一些树与树之间的变化与并非所有树木在林分内占据相同的垂直位置有关。每日去耦系数（0≤Ω≤1）的估计值也有相当大的变化，对于单个树木，季节平均Ω在0.12到0.37之间变化，对于12棵研究树木，平均Ω为0.23。这种量级的Ω表明红枫树冠与大气部分解耦，并表明从树冠表面到树冠上方几米处的大量空气的空气温度和δ e的显著垂直梯度是可能的。小时数据的模型分析表明，7月中旬模拟的地表温度比冠层参考温度高3.6-5.8°C，冠层表面δ比大气大0.3kPa。这些计算得到了从20米高的树冠进入塔上对其中一棵树进行的叶子水平测量的部分支持。这种部分解耦的理解和模拟冠层蒸腾的环境控制的影响进行了讨论。
There is a strong need to extend whole-tree measurements of sap flow into broad-leaved forests where characteristics of stand structure, surface roughness, leaf dimension, and aerodynamic and canopy conductance may interact to partially decouple the canopy from the atmosphere. The implications of this partial decoupling to understanding the environmental control of canopy transpiration and to the modeling of forest water use are many. Therefore, thermal dissipation probes were used over a three-month period (June through August, 1997) to quantify day-to-day and tree-to-tree variation in whole-tree sap flow (Q) for 12 red maple (Acerrubrum L.) trees growing in an upland oak forest of eastern Tennessee. Whole-tree Q was calculated as the product of measured sap velocity, sapwood area and the fraction of sapwood functional in water transport. Daily canopy transpiration (Ec) was calculated from whole-tree Q and projected crown area, whereas average daily conductance (gc) was derived by inverting the Penman–Monteith equation. Maximum Q averaged 73kg per tree per day and varied between 45 and 160kg per day for trees that ranged in stem diameter (DBH) from 17 to 35cm, and from 19 to 26m in height. Canopy transpiration peaked at 3.0mm per day in early July and averaged 1.5mm per day over the 3-month measurement period. Tree-to-tree variability for Ecwas high. Maximum rates of Ecvaried from 1.9mm per day for the tree with the smallest projected crown area to 5.7mm per day for one of the largest trees. Day-to-day variation in Ecwas a function of daily differences in net radiation (Rn) and atmospheric humidity deficit (δe). Increases in daily Rnand δeled to linear increases in canopy transpiration and there was no indication that a plateau-style relationship existed between Ecand average daily δe. Mean daily gcranged from 1.4 to 6.7mms−1, and averaged 3.4mms−1across the 12 study trees. Some of the tree-to-tree variation observed for Ecand gcwas related to the fact that not all trees occupied the same vertical position within the stand. Variation in estimates of the daily decoupling coefficient (0≤Ω≤1) was also considerable and for individual trees the seasonally-averaged Ω varied from 0.12 to 0.37, and averaged 0.23 for the 12 study trees. An Ω of this magnitude indicates that red maple canopies are partially decoupled from the atmosphere and suggests that significant vertical gradients of air temperature and δefrom the canopy surface to the bulk air several meters above the canopy are possible. Model analysis of hourly data indicated that simulated surface temperatures in mid-July were 3.6–5.8°C higher than above-canopy reference temperatures, and δeat the canopy surface was 0.3kPa higher than that of the bulk atmosphere. These calculations were partially supported by leaf-level measurements taken on one of the trees from a 20-m canopy-access tower. The implications of this partial decoupling to understanding and modeling the environmental control of canopy transpiration are discussed.