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Fire severity infuences large wood and stream ecosystem responses in western Oregon watersheds

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Background. Wildfre is a landscape disturbance important for stream ecosystems and the recruitment of large wood (LW; LW describes wood in streams) into streams, with post-fre management also playing a role. We used a stratifed random sample of 4th-order watersheds that represent a range of pre-fre stand age and fre severity from unburned to entirely burned watersheds to 1) determine whether watershed stand age (pre-fre) or fre severity afected riparianoverstory survival, riparian coarse wood (CW; CW describes wood in riparian areas), LW, or in-stream physical, chemical, and biological responses; and 2) identify relationships of LW with riparian vegetation and in-stream physical,chemical, and biological factors. Results. At higher fre severities, LW and CW diameter was smaller, but volume did not change in the frst year post- fre. Larger size of CW in riparian areas versus LW in streams suggests potential future recruitment of larger-diameterwood into streams from riparian zones in severely burned watersheds. Fire severity exerted strong control on stream responses across watersheds, explaining more of the variation than stand age. At higher fre severities, riparian tree mortality, salvage logging, light, and dissolved organic matter (DOM) concentrations were higher, whereas canopy cover, LW diameter, macroinvertebrate diversity, and fsh density were lower. In watersheds with older stand ages, elevation and mean annual precipitation were greater but mean annual temperature, specifc ultra-violet absorption at 254 nm, and phosphorus concentrations were lower. Overstory mortality in burned riparian areas was lower for redalder (12%) than western redcedar (69%). Conclusions. Our results link forested streams, fre, and LW by identifying key relationships that change with fire severity and/or watershed stand age. Severe fres burn more overstory riparian vegetation, leading to increased light,DOM concentrations, and macroinvertebrate densities, along with reduced canopy cover, LW diameter, macroinverte-brate diversity, and fsh densities. We highlight an important function of red alder in riparian zones—as a fre-resistant species, it may help facilitate a more rapid recovery for streams in fre-prone landscapes. Continued comprehensive aquatic and riparian ecosystem monitoring of these watersheds will aid in understanding long-term efects of post- fre management activities (salvage logging) on aquatic ecosystems.

A.A. Coble; B.E. Penaluna; L.J. Six; J. Verschuyl

Coble AA, Penaluna BE, Six LJ, Verschuyl J. Fire severity infuences large wood and stream ecosystem responses in western Oregon watersheds. Fire Ecology. 2023 ;19.

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