DETERMINING WATER-USE BY TREES AND FORESTS FROM ISOTOPIC, ENERGY-BALANCE AND TRANSPIRATION ANALYSES - THE ROLES OF TREE SIZE AND HYDRAULIC LIFT

Use of soil water and groundwater by open-grown Acer saccharum Marsh. (sugar maple) tree canopies and forests was estimated by measuring tra nspiration (E) rates using porometry, sap flow methods, and the Bowen ratio method. The Bowen ratio and sap flow methods showed the best agr eement; porometer measurements scaled to whole canopies always underes timated E by 15-50%. Trees of different sizes showed very different ra tes of E. I hypothesized that these differences were due to the differ ential 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 s table isotopic composition (delta D). Soil water delta D varied betwee n -41 and -16 parts per thousand seasonally (May to September), wherea s groundwater delta D was -79 +/- 5 parts per thousand during the enti re growing season. Daily transpiration rates of large (9-14 m tall) tr ees 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)). Sm all trees also showed greater variation in E during the growing season than large trees. In addition, compared to the large trees, small tre es demonstrated greater sensitivity to environmental factors that infl uence E, such as soil water deficits and increased evaporative demand. Over the entire growing season, large trees and forest stands compose d 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 hyd raulically lifted water reservoir enabled large trees to use more pote ntial transpirational water during daylight hours than small trees, le ading to a greater total water flux. In contrast, small trees and fore st stands composed of younger trees almost exclusively used soil water , except during two dry periods when their transpirational water was c omposed of between 7 and 17% groundwater. Thus groundwater discharge f rom sugar maple trees and forest stands of different sizes (ages) diff ers significantly, and large trees and older forest stands have a grea ter impact on the hydrologic balance of groundwater than small trees a nd 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 wate r and groundwater reservoirs and thus can substantially increase total water discharge on scales from tens to hundreds of hectares.