Environmental control of whole-plant transpiration, canopy conductance andestimates of the decoupling coefficient for large red maple trees

There is a strong need to extend whole-tree measurements of sap flow into b road-leaved forests where characteristics of stand structure, surface rough ness, leaf dimension, and aerodynamic and canopy conductance may interact t o partially decouple the canopy from the atmosphere. The implications parti al decoupling to understanding the environmental control of canopy transpir ation 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 Row (Q) for 12 red maple (Acer rubrum L.) trees growing in an upland oak f orest of eastern Tennessee. Whole-tree Q was calculated as the product of m easured sap velocity, sapwood area and the fraction of sapwood functional i n water transport. Daily canopy transpiration (E-c) was calculated from who le-tree Q and projected crown area, whereas average daily conductance (g(c) ) was derived by inverting the Penman-Monteith equation. Maximum Q averaged 73 kg per tree per day and varied between 45 and 160 kg per day for trees that ranged in stem diameter (DBH) from 17 to 35 cm, and from 19 to 26 m in height. Canopy transpiration peaked at 3.0 mm per day in early July and av eraged 1.5 mm per day over the 3-month measurement period. Tree-to-toe vari ability for E-c was high. Maximum rates of E-c varied from 1.9 mm per day f or the tree with the smallest projected crown area to 5.7 mm per day for on e of the largest trees. Day-to-day variation in E-c was a function of daily differences in net radiation (R-n) and atmospheric humidity deficit (delta (e)). Increases in daily R-n and delta(e) led to linear increases in canopy transpiration and there was no indication that a plateau-style relationshi p existed between E-c and average daily delta(e). Mean daily g(c) ranged fr om 1.4 to 6.7 mm s(-1), and averaged 3.4 mm s(-1) across the 12 study trees . Some of the tree-to-tree variation observed for E-c and g(c) was related to the fact that not all trees occupied the same vertical position within t he stand. Variation in estimates of the daily decoupling coefficient (0 les s than or equal to Omega less than or equal to 1) was also considerable and for individual trees the seasonally-averaged Omega varied from 0.12 to 0.3 7, and averaged 0.23 for the 12 study trees. An Omega of this magnitude ind icates that red maple canopies are partially decoupled from the atmosphere and suggests that significant vertical gradients of air temperature and del ta(e) from the canopy surface to the bulk air several meters above the cano py are possible. Model analysis of hourly data indicated that simulated sur face temperatures in mid-July were 3.6-5.8 degrees C higher than above-cano py reference temperatures, and delta(e) at the canopy surface was 0.3 kPa h igher than that of the bulk atmosphere. These calculations were partially s upported by leaf-level measurements taken on one of the trees from a 20-m c anopy-access tower. The implications of this partial decoupling to understa nding and modeling the environmental control of canopy transpiration are di scussed. (C) 2000 Elsevier Science B.V. All rights reserved.