© 1996 Heron Publishing—Victoria, Canada
Determining water use by trees and forests from isotopic, energy balance and transpiration analyses: the roles of tree size
and hydraulic lift
Todd E. Dawson
Section of Ecology and Systematics, Corson Hall, Cornell University, Ithaca, NY 14853, USA / Received March 2, 1995
Summary
Use of soil water and groundwater by open-grown Acer saccharum Marsh. (sugar maple) tree canopies and forests was estimated by measuring transpiration (E) rates using porometry, sap flow methods, and the Bowen ratio method. The Bowen ratio and sap flow methods showed the best
agreement; porometer measurements scaled to whole canopies always underestimated E by 15–50%. Trees of different sizes showed very different rates of E. I hypothesized that these differences were due to the differential access of large and small trees to groundwater and soil
water, respectively. Transpirational flux was partitioned between soil water and groundwater by tracing the water sources
based on their hydrogen stable isotopic composition (δD). Soil water δD varied between –41 and –16‰ seasonally (May to September),
whereas groundwater δD was –79 ± 5‰ during the entire growing season. Daily transpiration rates of large (9–14 m tall) trees
were significantly higher than those of small (3–5 m tall) trees (2.46–6.99 ± 1.02–2.50 versus 0.69–1.80 ± 0.39–0.67 mm day–1). Small trees also showed greater variation in E during the growing season than large trees. In addition, compared to the large trees, small trees demonstrated greater sensitivity
to environmental factors that influence E, such as soil water deficits and increased evaporative demand. Over the entire growing season, large trees and forest stands
composed of trees > 10 m tall transpired only groundwater. The high rates of water loss from large trees and older forests
were likely a result of the influence of an enhanced “pool” of transpirational water in the upper soil layers caused by hydraulic
lift (see Dawson 1993b). The hydraulically lifted water reservoir enabled large trees to use more potential transpirational water during daylight
hours than small trees, leading to a greater total water flux. In contrast, small trees and forest stands composed of younger
trees almost exclusively used soil water, except during two dry periods when their transpirational water was composed of between
7 and 17% groundwater. Thus groundwater discharge from sugar maple trees and forest stands of different sizes (ages) differs
significantly, and large trees and older forest stands have a greater impact on the hydrologic balance of groundwater than
small trees and younger forest stands. However, mixed stands (small and large trees) may have a greater overall impact on
the regional hydrologic balance than old stands, because trees in mixed stands draw on both soil water and groundwater reservoirs
and thus can substantially increase total water discharge on scales from tens to hundreds of hectares.
Keywords:
Acer saccharum, Bowen ratio, hydrogen stable isotopes, groundwater, sap flow, size class, soil water.
