The impact of catastrophic forest blowdown on large wood loads in headwater streams

Abstract Forest blowdown, or the widespread felling and snapping of trees due to high wind speeds, can substantially increase the amount of downed large wood (LW) on the landscape. Despite high recruitment potential, few studies have investigated the influence of blowdown on in‐channel LW volumes and the subsequent capacity for sediment and water storage. In June 2021, a frontal storm caused widespread blowdown across parts of southeastern Australia, creating an opportunity to better understand interactions between landscape morphology, blowdown intensity, large wood recruitment and in‐channel hydrogeomorphic changes. Blowdown area and density (trees per unit area) were mapped remotely across the Wombat State Forest (WSF), Victoria and paired with field‐based measurements of in‐channel LW and associated sediment and water storage in the Lerdederg River and tributaries. Study reaches were characterized by a range of hillslope gradients, aspects, blowdown intensity and channel and floodplain widths. Eleven percent of the treed area of the WSF was blown down by the June 2021 storm, with winds that exceeded 100 km/hr from a non‐typical direction. The blowdown event was the dominant source of in‐channel LW, delivering 88% of the volume. LW volumes were more strongly influenced by valley morphology, particularly valley bottom width, than blowdown characteristics (affected area or downed wood density). Most LW (85% by volume) accumulated in porous jams, and about 33% of LW stored sediment and/or water, with storage more likely behind pieces that touched the channel bed. Substantial amounts of LW still remained on the floodplain or spanned above the channel, suggesting that LW loads attributed to the June 2021 storm could continue to increase as overbank flows and wood decay continue to recruit wood into the active channel. Catastrophic blowdown like the June 2021 storm could exert significant control on the wood regime and morphology of forested, headwater channels, particularly as extreme wind events are expected to increase in magnitude and severity in the future.