Historical land use as a driver of alternative states for stream form and function in forested mountain watersheds of the Southern Rocky Mountains

Abstract

We demonstrate how land use can drive mountain streams in the Southern Rockies across a threshold to induce an alternative state of significantly reduced physical complexity of form and reduced ecological function. We evaluate field data from 28 stream reaches in relatively laterally unconfined valleys and unmanaged forest that is either old‐growth forest or naturally disturbed younger forest, and 19 stream reaches in managed forest with past land use. We evaluate potential differences in stream form, as reflected in channel planform, cross‐sectional geometry, and in‐stream wood loads, and stream function, as reflected in pool volume and storage of organic carbon. Field data indicate a threshold of differences in stream form and function between unmanaged and managed stream reaches, regardless of forest stand age, supporting our hypothesis that the legacy effects of past land use result in an alternative state of streams. Because physical complexity that increases stream retentiveness and habitat can maintain aquatic‐riparian ecosystem functions, the alternative physical state of streams in managed watersheds creates a physical template for an alternative ecological state with reduced pool volume, organic carbon storage, and ecosystem productivity. We recommend maintaining riparian forests that can supply large wood to streams as a stream restoration technique in historically forested stream segments. Copyright © 2017 John Wiley & Sons, Ltd.

Porosity problems: Comparing and reviewing methods for estimating porosity and volume of wood jams in the field

“…Wood in river corridors (channels, floodplains, and the hyporheic zone) can significantly modify and influence geomorphic processes (Montgomery et al, 2003;Wohl, 2017), provide habitat for fish and biota (Jones et al, 2014), and is a relatively large organic carbon stock (Sutfin et al, 2016;Lininger et al, 2017;Livers et al, 2018). Wood concentrated into accumulations (i.e.…”

Drivers of Low Instream Large Wood Retention and Imprints of Wood Mobility in Mountain Nonperennial Streams of a Mediterranean Semiarid Environment

“…Large instream wood (LW; commonly defined as pieces ≥1 m long with ≥0.1 m diameter) plays an irreplaceable role in the ecological and geomorphic function of streams and rivers draining forested basins (Harmon et al, 1986). Sufficiently large wood pieces or wood jams increase flow resistance and act as obstructions in the channels, which in turn influences in-channel sediment storage (Andreoli et al, 2007;Ryan et al, 2014;Wohl & Scott, 2017) and channel morphodynamics (Davidson & Eaton, 2013;Gurnell et al, 2002) and enhances the spatial heterogeneity of aquatic habitats (Livers et al, 2017;Montgomery et al, 1995). In addition, LW represents an important component of terrestrial carbon budgets (Beckman & Wohl, 2014a;Wohl et al, 2012).…”

The Natural Wood Regime in Rivers

“…The presence of these and other feedbacks between mobile and stored wood and river corridor process and form highlight the importance of considering wood in the context of an integrated wood regime that includes diverse aspects of recruitment, transport, and storage. Feedbacks are particularly important in creating nonlinear behavior in river corridors, including alternative states mediated by the presence or absence of wood (Supplemental Text 3) Livers et al, 2018). Multi-year monitoring of storage from ground-based measurements or airborne imagery can show the potential variability in magnitude, duration, and mode of wood storage at one location and can inform transport dynamics by tracking wood that is imported into the storage zone, remains in storage, or is exported downstream (Boivin et al 2015;Figure 4).…”