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 (R d ). We applied this method along six rivers draining the Canadian Rocky Mountains and observed tenfold difference in R d . The floodplain forests were dominated by cottonwoods and from mountain to prairie zones we observed progressively deeper roots of Populus trichocarpa (black cottonwood, R d * 0.3 m), P. balsamifera (balsam poplar), P. angustifolia (narrowleaf cottonwood), and P. deltoides (prairie cottonwood, R d * 0.9 m), which had R d similar to P. fremontii (Fremont cottonwood) in Nevada, USA. Roots were shallower for co-occurring facultative riparian trees, with R d * 0.1 m for P. tremuloides (trembling aspen) and Picea glauca (white spruce). Across the Canadian sites, R d for cottonwoods were strongly associated with a growth season moisture index (May through September precipitation-potential evapotranspiration; R 2 = 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.

Root Structure and Belowground Biomass of Hybrid Poplar in Forestry and Agroforestry Systems in Mediterranean France

“…Deep seated second tier allowed survival under drought or other adverse condition. Rood et al (2011) also observed that in drier regions the cottonwood becomes phreatophytic and produces deeper root system to access moisture from ground water. ; Steele et al, 1997 andPinno et al, 2010).…”

“…This understanding does not hold well in the present case since more than 50% of root biomass was distributed beyond this depth. However, there are a few more reports of deep seated coarse root distribution in cottonwood (Rood et al, 2011), loblolly pine (Albaugh et al, 2006), and dehesa vegetation (Moreno et al, 2005).…”

Water use in a riparian cottonwood ecosystem: Eddy covariance measurements and scaling along a river corridor

“…However, the cottonwood forest soil also had a relatively large water storage capacity (field capacity − wilting point, 1577 − 250 = 1327 mm) compared to the shallow Lethbridge grassland soil (420 − 160 = 260 mm; Hufkens et al, 2016). In addition, the riparian cottonwood trees have roots that extend deep enough in the soil (approximately 2.5 m) to reach the capillary fringe, even when the water table is near its maximum depth below the floodplain soil surface (Rood et al, 2011(Rood et al, , 2013. The deep-rooted cottonwood trees should be able to access groundwater (which extends almost horizontally from the river water) for much of their transpiration requirements in years with river flow rates like that observed in 2015 (Rood et al, 2013).…”

Root architecture adaptation of Pistacia atlantica subsp. atlantica according to an increasing climatic and edaphic gradient: case of a north–south transect in Algeria

“…The width of each pit varied according to the length of discovered roots within each unilateral profile. This method corresponds to that of Rood et al (2011); it was far less laborious, and benefited from the erosion of rivers banks through seasonal torrents, to study unilateral root profiles thereby naturally excavated. The limit of 0.8 m depth was mostly imposed on us by limestone slab, which is ubiquitous in these arid regions of Algeria and inherited from the Quaternary (Pouget, 1980).…”