Steve Wondzell is a research ecologist with the US Forest Service’s Pacific Northwest Research Station. Much of his research focuses on stream-groundwater interactions, how they are controlled by hydrology and channel morphology and the ways that these interactions influence stream ecosystem processes, especially stream thermal regimes and the cycling of carbon and nitrogen. His work also extends to examining riparian management issues more broadly, including studies of the effects of forest harvest and riparian buffer designs on stream thermal regimes, influence of future climate change and channel restoration on stream ecosystem processes.
Abstract
Riparian restoration strategies to mitigate future changes in climate – or – How long does it take to grow a tree?
The upper Middle Fork John Day River (MFJD) has been the focus of extensive restoration and monitoring. Over that past 150 years the channel was straightened, valley floors drained, and native riparian vegetation converted into expansive dry meadows. Today, the MFJD is poorly shaded. The river supports spring Chinook Salmon, but in some summers, maximum stream temperatures exceed lethal thresholds, killing as much as 60% of returning adult salmon before they can spawn. Future increases in air temperature will further threaten these cold-water fishes. Potential riparian restoration strategies were examined using the mechanistic stream temperature model, Heat Source. Current conditions with effective shade at 19% were compared to future conditions in which air temperatures were 4° C hotter than today but with shade ranging from 7% (post-wildfire) to 79% (mature riparian forest). Shade from riparian vegetation dramatically influenced future maximum stream temperatures. In fact, with. 79% effective shade, daily maximum temperatures were predicted to decreased by 7° C despite, the much warmer air temperatures.
Thousands of seedlings of native woody riparian species were planted in 2006 in several meadow-dominated reaches of the MFJD to restore riparian vegetation and shade the stream. These planting had limited success, even in areas fenced to exclude cattle. By 2009 most cottonwood had died; those species that survived had not grown substantially. Consequently, fenced exclosures were established to measure the effects of elk and deer browsing. The results showed that browsing was preventing the growth of most hardwoods. Only ponderosa pine and thinleaf alder showed consistent growth without protection from browsing.
These two studies pose a critical question: “Can shade be grown fast enough to mitigate effects of climate change, given the rate at which such changes are projected to occur?” The answer is a qualified maybe. Ponderosa pine grows slowly in northeastern Oregon; trees growing in riparian zones require, on average, 120 years to reach 30-m height. Cottonwood may grow much faster, but the browse study showed high mortality and growth was severely limited by browsing. Clearly, substantial hurdles remain when attempting to mitigate impacts of future climate changes by growing trees to provide shade.