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  • The Stillaguamish Big Trees ProjectWatershed-Scale Riparian Restoration (Washington)
  • Nate Hough-Snee, Rodney Pond, and Jake Jacobson

“A society grows great when old men plant trees whose shade they know they shall never sit in.”

—Greek proverb

Logging and subsequent agricultural and urban development have left many Puget Sound rivers and riparian forests heavily degraded and unable to support Pacific salmon (Oncorhynchus spp.) at their historic levels of abundance. Given that the root cause of Pacific Northwest riparian and in-stream degradation lies in landscape-scale clearcutting of near-stream mature conifer forests, restoration of ecological processes for salmon recovery should address deforestation at the watershed scale. While numerous entities approach riparian restoration planning from a process-driven, watershed scale, project implementation largely remains at the subbasin and reach scale, emphasizing either in-stream habitat structures or streamside revegetation. This short-term, site-specific implementation insufficiently targets landscape-scale habitat degradation such as lethal in-stream temperatures, loss of large woody debris, and sedimentation. In response to this disconnect, Snohomish County Surface Water Management has undertaken a watershed-scale project to restore riparian conifer forests around the Stillaguamish River, remnant habitat for Chinook salmon (O. tshawytscha).

To understand why watershed-level forest restoration is essential to restore salmon runs, one must consider the relationship between trees, rivers, and fish and how logging has altered the current landscape. Prior to European settlement, frequent flooding scoured and deposited material within forested riparian zones, contributing large woody debris (LWD) to active channels and floodplains, providing nurse logs, in-stream habitat, and structural reinforcement to new landforms (Collins et al. 2003). This disturbance resulted in structurally complex and successionally diverse riparian forests. The component old-growth conifers Sitka spruce (Picea sitchensis), Douglas-fir (Pseudotsuga menziesii), western redcedar (Thuja plicata), and western hemlock (Tsuga heterophylla) were located along river terraces, and early seral hardwoods such as big-leaf maple (Acer macrophyllum), black cottonwood (Populus balsamifera), and red alder (Alnus rubra) were near active channels (Collins et al. 2003).


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Figure 1.

An example of the novel hardwood/invasive community type: an early seral red alder/black cottonwood (Alnus rubra/Populus balsamifera) riparian forest allows for the development of a dense Himalayan blackberry/knotweed (Rubus discolor/Polygonum spp.) understory that prevents conifer seedling recruitment. A stalled successional trajectory results that is common throughout deforested Puget Sound riparian zones. Photo by Rodney Pond

While dam-free rivers still experience flooding, landscape fragmentation and an absence of postlogging conifer seed sources have allowed deciduous species to dominate floodplain forests. Additionally, the prevalence of shade-tolerant invasive species has led to a novel ecotype familiar to local restorationists: deciduous, even-aged gallery forests with non-native Himalayan blackberry (Rubus discolor) and knotweed (Polygonum cuspidatum, P. sachalinense, P. × bohemicum) dominated understories. When short-lived (ca. 60–100 years), fast-growing alder and cottonwood succumb to competition, flooding, windthrow, or age, blackberry and knotweed rapidly infill canopy gaps, precluding canopy seedling recruitment, thus effectively halting forest succession. In the absence of catastrophic disturbance, the remnant riparian canopy declines and a stalled successional trajectory retrogresses into a stable invasive species community (Figure 1) (Walker and del Moral 2009). This trajectory—widespread conifer loss leading to deciduous canopy dominance and invasive species persistence—fails to regenerate long-lived canopy trees that shade streams, stabilize banks, and provide keystone pieces of large woody debris to the channel. Reduced ecosystem services and habitat value result, a tale told by current salmon return numbers.

To restore conifer-mediated processes that aid salmon recovery at the watershed scale, Snohomish County began process-driven successional management under the banner of the Stillaguamish Big Trees Project (SBTP) in 2007. The Stillaguamish River drains an 181,300 ha watershed into Puget Sound and supports several salmon runs, including federally endangered Chinook salmon. While the Stillaguamish was once overwhelmingly covered by [End Page 243] mature forest, land cover analyses show that within the lower Stillaguamish—the area most impacted by logging and agriculture—mature conifer forest comprises only 3%–10% (SIRC 2005). Historic declines in water quality and salmon have been linked to deforestation and land use changes in the watershed (Nehlsen et al...

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