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Instream wood loads and channel complexity in headwater southern Rocky Mountain streams under alternative states




Gonzalez, Bridget Livers, author
Wohl, Ellen, advisor
Covino, Tim, committee member
Gooseff, Michael, committee member
Rathburn, Sara, committee member

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Small, forested streams are symbiotic with riparian processes, and thus land use changes to their riparian forests can have lasting effects on stream channel form and function. The first part of this dissertation evaluates the legacy effects of land use on stream channels of forested, subalpine streams of the Southern Rockies, with particular interest in the correlations between stream geomorphic complexity and characteristics of the adjacent riparian forest, valley geometry, and land use history. The study uses field data from the Southern Rocky Mountains of Colorado and Wyoming in streams flowing through old-growth forests (OU), younger-growth, naturally disturbed forests (YU), and forests that have undergone past land use changes (YM, management) such as logging. Field sites also have varied valley geometry (lateral confinement). Field data are used to evaluate measures of geomorphic complexity based on cross-sectional, planform, and instream wood piece and logjam variables. Significant differences in geomorphic stream complexity between OU, YU, and YM result primarily from differences in wood characteristics, which correlate strongly with pool volume and organic matter storage. Unconfined OU streams have the largest wood loads and the greatest complexity in form and function, whereas legacy effects of logging, tie-drives, and channel simplification create lowest complexity in YM streams. The second part of this dissertation proposes that the geomorphic concepts of thresholds, river metamorphosis, and complex response are the geomorphic analog to alternative states in ecology, which recognize that biotic community structure and function can exist in multiple states under the same environmental conditions. This concept is used in conjunction with field data from relatively laterally unconfined valley bottoms in the first part of this dissertation, in addition to wood data from the montane zone, to demonstrate how land use can drive streams across a threshold to induce an alternative state of significantly reduced complexity of stream form, function, and carbon storage in large wood and instream particulate organic matter. This is illustrated by threshold differences between unmanaged and managed stream segments, regardless of current forest stand age, implying that the legacy effects of past land use on riparian forest characteristics result in an alternative state of reduced stream complexity and retention. High complexity can maintain aquatic-riparian ecosystem functions through positive feedbacks, and the reduced state of managed watersheds implies an alternative ecologic state with reduced carbon storage, ecosystem productivity, and biotic diversity. The cumulative effects of reduced carbon storage in mountainous environments experiencing analogous human alteration may have large implications for global carbon budgets. Alternative states driven by land use changes likely apply to watersheds in other forested, mountain environments. Maintenance of riparian forest buffers around streams in laterally unconfined valley segments is a recommended first-order restoration technique for physical and ecological recovery.


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