ALTERNATIVE STATES OF STREAM FORM, FUNCTION, AND CARBON STORAGE INDUCED BY HISTORIC LAND USE IN FORESTED WATERSHEDS OF THE SOUTHERN ROCKY MOUNTAINS

The concept of alternative states in ecology recognizes that biotic community structure and function can exist in multiple states under the same environmental conditions. Multiple physical states, induced naturally and from human changes to watersheds, have been demonstrated in streams, but are rarely described explicitly as alternative states or supported by field data. We propose that the geomorphic concepts of thresholds, river metamorphosis, and complex response are the geomorphic analog to alternative states in ecology. Using field data from the Southern Rocky Mountains of Colorado and Wyoming in streams flowing through relatively unconfined valleys with old-growth forests, younger-growth, naturally disturbed forests, and forests that have undergone past land use changes (management) such as logging, we demonstrate how land use can drive streams across a threshold to induce an alternative state of significantly reduced complexity of stream and floodplain form and function, as well as reduced carbon storage in large instream and floodplain wood and instream particulate organic matter. Field data show a threshold of differences between unmanaged and managed stream segments, regardless of current forest stand age, supporting our hypothesis that the legacy effects of past land use on riparian forest characteristics leave affected watersheds in an alternative state of stream complexity and carbon storage. Complexity and retention in streams and across floodplains can maintain aquatic-riparian ecosystem functions, suggesting that the reduced state of managed watersheds can be the physical template for 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. Maintenance of riparian forest buffers around streams in laterally unconfined valley segments is a recommended first-order restoration technique.