Sand and sandbar willow: a feedback loop amplifies environmental sensitivity at the riparian interface

Abstract

Riparian or streamside zones support dynamic ecosystems with three interacting components: flowing water, alluvia (river-transported sediments), and vegetation. River damming influences all three, and subsequent responses can provide insight into underlying processes. We investigated these components along the 315-km Hells Canyon corridor of the Snake River that included reaches upstream, along, and downstream from three large dams and reservoirs, and along the Salmon River, a free-flowing tributary. Sandbar willow was generally the woody plant at the lowest bank position and was abundant along upstream reaches (53, 45, 67% of transects), sparse along reservoirs (11, 12, 0%), and sparse along the Snake River downstream (11%). It was prolific along the undammed Salmon River (83%) and intermediate along the Snake River below the Salmon inflow (27%), indicating partial recovery with the contribution of water and sediments. Along these rivers, it commonly occurred on sandy substrates, especially on shallow-sloped surfaces, and emerged from interstitial sands between cobbles on steeper surfaces. However, along the Snake River below the dams, sandbars have eroded and willows were sparse on remnant, degrading sand surfaces. We conclude that a feedback loop exists between sands and sandbar willow. Sand favors willow colonization and clonal expansion, and reciprocally the extensively branched willows create slack-water zones that protect and trap sands. This feedback may sustain surface sands and sandbar willows along free-flowing river systems and it amplifies their mutual vulnerability to river damming. Following damming, sediment-depleted water is released downstream, eroding surface sands and reducing willow colonization and expansion. With willow decline, sands are further exposed and eroded, compounding these impacts. From this feedback, we predict the coordinated depletion of surface sands and riparian willows along dammed rivers throughout the Northern Hemisphere.

Groundwater-dependent ecosystems: recent insights from satellite and field-based studies

“…However, specific responses depend upon depth-togroundwater and individual differences amongst functional types; for example, riparian cottonwood trees (P. fremontii) responded to rewetting with growth that was larger and faster than the response of co-occurring willow (S. exigua), a smallstature, thicket-forming shrub that is restricted to streamside areas with very shallow groundwater (Scurlock, 1998;Rood et al, 2011). From an understanding of the relationships between tree growth and depth-to-groundwater, historical periods of sensitivity to hydrological drought (i.e.…”

“…can be very sensitive to groundwater decline, resulting in reductions of ET, productivity and canopy conductance as a consequence of increases in vapour pressure deficit that are correlated with depth-to-groundwater (Gazal et al, 2006;Kochendorfer et al, 2011). Branch sacrifice, partial crown dieback and mortality commonly occur in Populus following substantial groundwater drawdown (Mahoney and Rood, 1991;Kranjcec et al, 1998;Scott et al, 1999;Rood et al, 2000Rood et al, , 2003Cooper et al, 2003). However, stomatal closure and crown dieback in Populus can prevent total hydraulic failure, and thereby minimise mortality rates, by maintaining favourable xylem water potentials within the remainder of the crown (Amlin and Rood, 2003).…”

The Relative Importance of Biotic and Abiotic Controls on Young Conifer Growth After Fire in the Klamath-Siskiyou Region

“…"NPMR generally performs well at both spatial scales and that distributions of non-indigenous species are predicted as well as those of native species." Rood et al (2010) http://dx.doi.org/10.1007/s00442-010-1758-2 quantitative (willow cover) Local mean NPMR with Gaussian weights, regressing willow cover against environmental variables. Illustrations include 2D response curves superimposed on scatterplots.…”

Biomass and root stem production of a colony-forming willow (Salix interior) on highly disturbed, low fertility sites

“…The best growth of INT colonies in this study was found on sites with loose, coarse sand and gravel outwash sediments as opposed to the shale rock overburden that dominates this mine site. Others, working with either INT or the closely related S. exigua [38,46], have also found that INT was generally restricted to coarser-textured sandy soils rather than on heavier-textured clay soils, although in arid areas, a component of finertextured materials can improve the moisture-holding capacity of coarse-textured substrates [45]. By the end of the year of establishment, some plants had already reached heights of 1 m on these gravel outwash deposits and had started to produce RS.…”