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Alluvial rivers are dynamic elements of the landscape in the Pacific Northwest. They expand, contract, and migrate across the bottom of valleys in response to changing flow and vegetation. Channel dynamics fundamentally structure river and floodplain ecosystems. Human activities that affect channel migration—including river regulation, channel revetments, and land use—have the potential for impacting river ecosystems. Active channel width and lateral movement of the active channel centerline was analyzed in 17 km valley segment of the middle Green River in western Washington from aerial photographs for 26 years of unregulated flows (1936–1961) and 41 years of flood regulation by Howard Hansen Dam. Area-based measures proved more robust for characterizing channel dynamics than cross-sectional measurements, though cross-sectional measurements are useful for resolving local processes. Prior to regulation from 1936 to 1961, the active channel width varied from 82 m to 120 m (median 94 m) and the channel migrated laterally a total of 68 m. After flood regulation from 1961 to 2002, the active channel width was generally smaller, varying from 84 m to 52 m (median 69 m) and the channel migrated 48 m. Streamflow greater than about 250 m3/s are most effective for forcing migration and have been reduced since dam construction. The river has re-occupied areas with increasing frequency since dam construction. High flows are essential to create new channel and floodplain habitats in the middle Green River, but land cover/use and revetments in the river corridor are also influential factors for maintaining channel dynamics.
Scheduled to begin in 2011, the removal of two dams on the Elwha River, Washington, will be one of the largest dam removal and river restoration projects undertaken in the United States. One challenge associated with this project is to understand how exposure and downstream deposition of sediments presently detained behind the dams will influence patterns of revegetation and invasive species colonization following dam removal. We conducted two greenhouse experiments assessing the potential effects of reservoir sediments on germination success and growth of (1) propagules dispersed naturally via seed rain, and (2) seeds of selected native and invasive species. Observed summer seed rain density was relatively low (<125 seeds m-2). This suggests slow recolonization in the initial years following dam removal, although these results may have been influenced by sampling methods and timing. In the selected species experiment, four out of five tested species exhibited reduced germination and growth on fine reservoir sediments, while the invasive forb Cirsium arvense was unaffected. Though reduced compared to more typical alluvial sediments, germination and growth of the natives Artemisia suksdorfii and Rubus parviflorus were comparable to that of C. arvense on reservoir sediments. These native species may be useful for revegetating exposed sediments. However, depending on such factors as source population sizes, seed production and dispersal rates, growth rates, water availability and competition during establishment, control of C. arvense and other exotics in the years following dam removal may be necessary to prevent a relative increase in invasive species populations on the new post-dam substrates.
Fire exclusion for more than a century in Olympic National Park changed old-growth Douglas-fir forest stand structure and species composition from an open structure of fire-adapted species to a more crowded, complex structure formed by fire-avoiding species. A previous study identified an historical mean fire return interval of 21 yr for these forests in the eastern Olympics prior to fire exclusion, with frequent, small surface fires maintaining an open forest. We tested whether Douglas-fir/salal forests would support low intensity prescribed fires, and monitored community responses for 3 yr post-fire in a randomized complete block ANOVA. Compared to pre-fire values, total fuels, 1000-hr fuels, 1-hr fuels, duff depth, total tree density, tree species density, sapling density, understory cover, and understory frequency of five prominent species were significantly lower one month post-fire. Differences in tree basal area, Douglas-fir sapling density, and western redcedar tree density were not significant after the fire. Lower sapling density was an important result. Salal, a known resprouter, was the only understory species to return to at least 50% of pre-fire cover within 3 yr. This first use of prescribed fire in Olympic National Park demonstrated that Douglas-fir/salal forests would support low intensity surface fire. Although community structure changed significantly immediately after fire, the tree canopy was little affected, and the understory will eventually recover to pre-fire values. The data from this study will contribute to a fire management plan that will incorporate prescribed fire with fire suppression, non-suppressed fires, and other active management to maintain forest health in the eastern Olympic Mountains.
The use of stable isotopes has recently grown in studies of trophic structure and the recruitment and migration patterns of consumers. This type of analysis allows the flow of organic matter and trophic relationships to be outlined within complex systems. Although multiple stable isotope analysis is useful in distinguishing linkages between sources and consumers, its efficacy is contingent on the isotopic similarity within each source and the distinctiveness of producer isotope values. As part of a study investigating juvenile Chinook salmon (Oncorhynchus tshawytscha) food webs in the Columbia River estuary we examined the isotopic (δ13C, δ15N, and δ34S) composition of five prominent primary producers. We sought to examine statistical variability of primary producer isotope values and its effects on differentiation between sources. We found that within-group isotopic variability occurs at different scales, related to the heterogeneous landscape in which producers grow. Aquatic and wetland vascular plants displayed the greatest range in isotopic composition while benthic algae and particulate organic matter were more constrained in their isotope signatures. When examining differences between groups we found that although δ13C was the most variable isotope ratio, it was also the most useful in distinguishing sources, especially benthic producers compared to water column and emergent producers. Signatures of δ15N and δ34S were most useful in differentiating benthic algal and vascular marsh plant groups. Isotopic composition along with chlorophyll and elemental composition was also useful in distinguishing phytoplankton samples from particulate organic matter (POM) samples. The type and extent of isotopic variability revealed by this study will help inform future food web studies using isotopes to characterize trophic linkages in large estuaries such as the Columbia River estuary.
Wildfire and debris flows are important physical and ecological drivers in headwater streams of western North America. Past research has primarily examined short-term effects of these disturbances; less is known about longer-term impacts. We investigated wildfire effects on the invertebrate prey base for drift-feeding rainbow trout (Oncorhynchus mykiss, Walbaum) in Idaho headwater streams a decade after wildfire. Three stream types with different disturbance histories were examined: 1) unburned, 2) burned, and 3) burned followed by debris flows that reset channel morphology and riparian vegetation. The quantity of macroinvertebrate drift (biomass density) was more variable within than among disturbance categories. Average body weight and taxonomic richness of drift were significantly related to water temperature and influenced by disturbance history. During the autumn sampling period, the amount of terrestrial insects in rainbow trout diets varied with disturbance history and the amount of overhead canopy along the stream banks. Results indicate that there are detectable changes to macroinvertebrate drift and trout diet a decade after wildfire, and that these responses are better correlated with specific characteristics of the stream (water temperature, canopy cover) than with broad disturbance classes.
The western bumble bee, Bombus occidentalis, is included on the red list of bees by The Xerces Society. It was once a common bumble bee west of the Cascades but in the late 1990s it experienced a dramatic decline along coastal regions. The cause was speculated to be due to the introduction of pathogens from captive-bred bumble bees used for pollination of greenhouse crops. In extensive surveys conducted in western and southern Oregon, 10 individuals have been recorded since 2000. In this note, we report the collection of 49 individual B. occidentalis over two years in the Zumwalt Prairie Preserve of northeastern Oregon. This finding shows that B. occidentalis persists in northeastern regions of the Pacific Northwest, either because of geographic isolation from or potential resistance to the pathogens that decimated populations in the western part of the region. Further research is needed to determine its occurrence in other regions of its historical range to assess the extent of its decline. In addition, conservation efforts are critical for protection of this species in both agricultural ecosystems and in native habitats.
An article by Lyman (2007) in this journal provided an excellent summary of the Holocene paleozoological record of pronghorn in eastern Washington, but the discussion of hunting of herds and specifically of communal hunting prior to the written record does not capture the complexity of this issue. A bone assemblage numerically dominated by pronghorn does not alone provide adequate evidence for hunting herds of multiple animals. There needs to be a convergence of multiple lines of evidence, including indications of human-caused mortality, a single episode of bone deposition, and a single mortality event.