![]() There is a weak bimodal pattern of CC gain with elevation between foothills shrublands (1.2 mg CC ha(-1)) and the lower montane, ponderosa pine (1.5 mg CC ha(-1)) and Douglas-fir (1.5 mg CC ha(-1)) forest types prior to a mid-elevation decline in upper montane lodgepole pine forests (1.2 mg CC ha(-1)) before increasing again in the spruce/subalpine fir forests (1.5 mg CC ha(-1)). ![]() We found a significant statistical effect of vegetation type on CC pools along this ecological gradient, but not a linear pattern increasing with elevation gain. We measured CC pools in surface soils (0-10 cm) at mid-slope positions on east facing aspects in five continuous foothills shrubland and conifer forest types. Long-term soil charcoal C (CC) pools result from the combined effects of wildland fires, aboveground biomass characteristics and soil transfer mechanisms. Temperate conifer forests in the Colorado Front Range are fire-adapted ecosystems where wildland fires leave a legacy in the form of char and charcoal. Carbon retention has likely decreased as multithread channels have become less common. Patches of old-growth forest and multithread channels were formerly more widespread. Treefall can thus initiate complex behavior that results in a self-enhancing feedback between wood recruitment and channel geometry. Hydrostatic forces that mobilize jams increase more slowly in low-gradient, unconfined channels, facilitating jam persistence. Valleys must have a lower gradient and less lateral confinement to develop multithread channels, but also larger volumes of riparian and instream wood. Thresholds in wood mobility that reflect valley and channel geometry and wood dimensions determine the distribution of multithread channels and the mechanics underlying these thresholds. Analogous processes in mountain streams can substantially increase the residence time of organic matter in streams otherwise characterized by high transport capacity and limited storage. Instream wood increases complexity, particularly where logjams initiate multithread channels, as documented previously for lowland alluvial channels. The geomorphic complexity of streams influences the residence time of carbon by creating storage sites for organic matter. Given the important ecological and physical effects of floodplain LW, efforts to add LW to river corridors as part of restoration activities, and the need to quantify carbon stocks within river corridors, we urge others to quantify floodplain and instream LW volumes in diverse environments. In our study sites, floodplain LW volumes are lower than those in adjacent channels, but are higher than those in upland (i.e., non-floodplain) forests. ![]() Although sediment is the largest floodplain carbon reservoir, floodplain LW stores substantial amounts of organic carbon and can influence floodplain sediment storage. Other forms of disturbance such as fires, insect infestations, and blowdowns can increase LW volumes in the semiarid boreal and semiarid temperate mountain sites, where rates of wood decay are relatively slow compared to the subtropical lowland sites. Lateral channel migration and flooding influence vegetation communities in the semiarid boreal sites, which in turn influences floodplain LW loads. ![]() Floodplain LW volumes differ among vegetation types within the semiarid boreal and semiarid temperate mountain regions, reflecting differences in species composition. Observed patterns support the hypothesis that the largest downed LW volumes occur in the semiarid temperate mountain sites, which is likely linked to a combination of moderate-to-high net primary productivity, temperature-limited decomposition rates, and resulting slow wood turnover time. Average volumes of downed LW are 42.3 m3ha-1, 50.4 m3ha-1, and 116.3 m3ha-1 in the semiarid boreal, subtropical, and semiarid temperate sites, respectively. We compare LW volumes in relatively unaltered floodplains of semiarid boreal lowland, subtropical lowland, and semiarid temperate mountain rivers in the United States. Very few studies, however, have quantified LW volumes in floodplains that are unaltered by human disturbance. Downed large wood (LW) in floodplains provides habitat and nutrients for diverse organisms, influences hydraulics and sedimentation during overbank flows, and affects channel form and lateral migration.
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