Sap velocity as an indicator of diurnal and long-term hydraulic resistance changes in mature ‘Walker’ hybrid poplar treesThis article is one of a selection of papers published in the Special Issue on Poplar Research in Canada.

Abstract

Hydraulic resistance directly limits sap flow in trees so that understanding water-use patterns in poplar trees requires knowledge about diurnal and long-term changes in resistances in the soil–plant–atmosphere continuum. Granier thermal dissipation probes were used for continuous measurements of sap velocity (V) changes over time and whole-tree water-use patterns in the main stems of ‘Walker’ hybrid poplar trees. When atmospheric demand was low, V depended only on solar radiation (Rg) and vapour pressure difference (VPD). However, on warm, sunny days, midday V decreased, despite a constant leaf water potential (Ψl) of –1.5 MPa. Because bulk soil water potential (Ψs) did not change significantly, the constant Ψl implied that the water potential difference (ΔΨ) remained diurnally constant. Although stomatal control would explain changes in V when VPD was low, an increase in hydraulic resistance was required to explain the decrease in V when VPD was high. Xylem cavitation was ruled out as a cause because the apparent increase in resistance reversed in late afternoon, while VPD was still high. Mid-August irrigation immediately increased daily V, which was attributed to a change in ΔΨ because of less negative Ψs. A gradual increase in sap flow over two weeks following irrigation implied a decrease in plant hydraulic resistance, most likely attributable to new root growth. It was concluded that hydraulic resistance changes, both diurnal and in the longer term, occurred primarily at the rhizosphere or root level.