Effects of forest stand age on the characteristics of logjams in mountainous forest streams

We measured longitudinal spacing and wood volume of channel-spanning logjams along 30 1-km reaches of forest streams in the Colorado Front Range, USA. Study streams flow through old-growth (> 200 year stand age) or younger subalpine conifer forest. Evaluating correlations between the volume and longitudinal spacing of logjams in relation to channel and forest characteristics, we find that old-growth forest streams have greater in-stream wood loads and more jams per kilometer than streams in younger forest. Old-growth forests have a larger basal area close to the stream and correlate with larger piece diameters of in-stream wood. Jam volume correlates inversely with the downstream spacing for ramp and bridge pieces that can act as key pieces in jams. Most importantly, old-growth streams have shorter downstream spacing for ramp and bridge pieces (< 20 m). Our results suggest that management of in-stream wood and associated stream characteristics can be focused most effectively at the reach scale, with an emphasis on preserving old-growth riparian stands along lower gradient stream reaches or mimicking the effects of old growth by manipulating the spacing of ramp and bridge pieces. Our finding that average downstream spacing between jams declines as wood load increases suggests that the most effective way to create and retain jams is to ensure abundant sources of wood recruitment, with a particular emphasis on larger pieces that are less mobile because they have at least one anchor point outside the active channel. Copyright (C) 2014 John Wiley & Sons, Ltd.