275-3 Decadal changes in large wood volume and mobility in headwater streams following catastrophic forest blowdown

Abstract:

Large-scale forest disturbances, including catastrophic forest blowdown and fire, are mechanisms for infrequent yet mass recruitment of large wood (LW) to headwater channels, potentially elevating LW volumes for decades. These long-term effects on wood storage post-disturbance, and subsequent impacts to local channel morphology and sediment storage, depend on wood mobility and decay. Portions of the Routt National Forest in northern Colorado experienced catastrophic blowdown in October 1997 followed by a wildfire in 2002. Following these events, we hypothesize that (1) LW volumes would have increased following the initial blowdown and again following fire, due to compounding disturbance and additional recruitment, but that (2) LW will have decayed in the past two decades, becoming more mobile and returning wood loads to a pre-disturbance state. To test these hypotheses, we compile data on LW volumes, characteristics, and locations collected in 1998, 2010-2011, and 2025-2026 in two headwater streams in the Routt National Forest: South Fork Lost Dog and Agnes Creeks. Wood volumes substantially increased in both creeks in 1998 following the 1997 blowdown (from 69 m³/ha to 402 m³/ha in Agnes Creek and from 30 m³/ha to 613 m³/ha in South Fork Lost Dog) and have been steadily declining in subsequent surveys, suggesting that compounding disturbances may have a diminishing effect on wood recruitment. Wood volumes recorded in 2025 were still 4x and 11x higher than pre-disturbance volumes for South Fork Lost Dog and Agnes Creek, respectively. The different post-disturbance wood load responses at the two sites may reflect differences in logging history between watersheds. Approximately 500 pieces of LW were tagged or retagged per creek (~1000 total) in 2010, of which 338 (33%) were resurveyed in 2026. Approximately 50% of resurveyed LW had moved >1 m and 10% had moved >30 m, with a maximum surveyed movement distance of 350 m. The true maximum travel distance is likely longer, as tagged logs may have been exported out of the original reach and were not resurveyed. Although resurveyed pieces were shorter than in 2010 and fewer were bridged above the channel, approximately 50% of surveyed LW pieces were incorporated in jams, potentially limiting future mobility. Transport limitations may keep LW volumes elevated in post-disturbance streams for decades to come.

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