Field estimates of root and xylem resistances in Pinus contorta using root excision

A root excision technique was used to estimate the proportion of total resistance to water flux residing in the soil, the root, and the xylem of lodgepole pine (Pinus contorta Dougl ex. Loud.) trees in the field. Root excision at mid-day always resulted in rapid recovery of leaf water potential when water was supplied to the cut stem, suggesting a high soil-root resistance. Transpiration was unaffected if leaf water potential before cutting was not limiting leaf conductance. By mid-June water uptake by the excised stem always exceeded calculated crown transpiration indicating recharge of internal sapwood storage. Predawn leaf water potential before root excision was highly correlated with total soil-plant resistance (r² = 0·89) and calculated root water uptake (r² = 0·92).