Root architecture of riparian trees: River cut-banks provide natural hydraulic excavation, revealing that cottonwoods are facultative phreatophytes

Abstract

Plant root architecture reveals the sources of water and nutrients but tree root systems are large and difficult to analyze. With riparian (floodplain) trees, river cut-banks provide natural hydraulic excavation of root systems and this presents a unique study opportunity. Subsequently, we developed the 'Cut-bank Root Method', a simple, quantitative approach for analyzing the distribution of coarse roots, based on analyses of photographs of river cut-banks. These reveal the vertical extent of roots and median root depths (Rd). We applied this method along six rivers draining the Canadian Rocky Mountains and observed tenfold difference in Rd. The floodplain forests were dominated by cottonwoods and from mountain to prairie zones we observed progressively deeper roots of Populus trichocarpa (black cottonwood, Rd ~ 0.3 m), P. balsamifera (balsam poplar), P. angustifolia (narrowleaf cottonwood), and P. deltoides (prairie cottonwood, Rd ~ 0.9 m), which had Rd similar to P. fremontii (Fremont cottonwood) in Nevada, USA. Roots were shallower for co-occurring facultative riparian trees, with Rd ~ 0. 1 m for P. tremuloides (trembling aspen) and Picea glauca (white spruce). Across the Canadian sites, Rd for cottonwoods were strongly associated with a growth season moisture index (May through September precipitation-potential evapotranspiration; R2 = 0.97, P<0.001). Thus, in wetter climates, riparian cottonwoods were shallow-rooted and would be more dependent upon rain than stream flow. Conversely, in the drier semi-arid regions the cottonwoods were phreatophytic, with deeper root systems in the capillary fringe above the alluvial ground-water table. These phreatophytic cottonwoods would be highly dependent upon stream flow and vulnerable to declining river flows due to river regulation or climate change. © 2011 Springer-Verlag.