How trees thrive in a dry climate: diurnal and seasonal hydrology and water relations in a riparian cottonwood grove

Abstract

Abstract In semi-arid ecoregions, trees are restricted to river valley floodplains where river water supplements the limited precipitation. To characterize the associated diurnal and seasonal dynamics in hydrology and water relations, we studied narrowleaf cottonwoods (Populus angustifolia) along a prairie river in Canada. From June through August, the shallow soil moisture was depleted but moisture remained higher above the alluvial groundwater table, which dropped to 2.3 m along with river recession. Throughout the summer, with the daily rise in temperature and insolation, foliar stomatal conductance (gs) and transpiration (E) increased to midday and then fell, thus maintaining the midday leaf water potential (Ψmd) above ~−1.7 MPa. This Ψmd approximated the water potential associated with 12% loss of xylem conductivity due to cavitation for branches (P12); the Ψmd and P12 varied independently across eight trees, providing differences in relative hydraulic risk. Sap flux density (Fd) was measured with thermal dissipation probes, and revealed diurnal patterns similar to foliar E. In contrast to our expectation, the daily Fd maxima were consistent through the summer despite the seasonal recession in water supply. Canopy conductances (GS), derived from Fd, sapwood area and canopy area, declined with vapor pressure deficit (D) and fell slightly in late summer along with stomatal sensitivity, which reflects the magnitude of decrease in GS with increasing D. For spatial up-scaling, satellite-derived near-infrared reflectance of vegetation revealed the woodland phenology, with leaf expansion from May through June and gradual decline in photosynthetic productivity through the summer. Thus, the phreatophytic cottonwoods: (i) sustained substantial water use and productivity through the warm and dry summer, by (ii) accessing shallow soil moisture and then deeper alluvial groundwater, and (iii) providing diurnal stomatal regulation, to (iv) avoid xylem cavitation and (v) maintain fairly constant hydraulic conductance.