Gas exchange, water relations and sap-flow were measured in big leaf maple (Acer macrophyllum) and red alder (Alnus rubra) within the same stand. While big leaf maple had higher transpiration rates, during mid day, red alder developed lower water-potential. Sap-flow data, determined with Granier type probes, indicated that big leaf maple was able to supply greater amounts of water to leaves than red alder. Since xylem structure was similar in both species, and red alder had a higher proportion of area of active xylem to leaf area, it is hypothesized that differences in root structure or root/shoot ratios cause differences between maple and alder in the ability to supply leaves with water. Since red alder is a nitrogen fixer, it may have lower root/shoot ratio, limiting ability to supply water under high demand situations.

Key words: ecophysiology, water-balance, gas-exchange, sap-flow