WHOLE SHOOT HYDRAULIC RESISTANCE IN QUERCUS SPECIES MEASURED WITH A NEW HIGH-PRESSURE FLOWMETER

Whole shoot resistance to water flow was measured in 4 species of oak, Quercus robur L, Q petraea Matt Liebl, Q pubescens Willd, and Q rubra L. Shoots were 1.1 to 1.5 m long with 16-19 mm basal wood diameter an d were 4-8 yr old. Whole shoot resistances accounted for 20-40% of the total resistance to water flow from soils to leaves based on comparis ons with literature values. Leaf blade resistances accounted for 80-90 % of total shoot resistances measured in this study. Stem resistances to water flow were approximate to twice as large in Q robur than in th e other species which had comparable stem resistances. Differences in shoot resistance between Q robur versus Q petrae are discussed in term s of the differential response of these species to drought in mixed st ands.