Wood in Patagonian headwater streams: A window into the colonial and wildfire caused legacy of large wood in temperate stream ecosystems

Abstract

Forested watersheds with limited human intervention are a vulnerable and diminishing feature in modern temperate latitudes landscapes. They are essential in terms of conserving biodiversity, as reference systems for forest management, restoration, stream ecosystem function, and associated ecosystem services. In Patagonia, some of the largest remaining intact watersheds are juxtaposed among areas heavily impacted from human‐caused fires and clearing, a pattern extending across several forested biomes. We measured in‐stream large wood storage and riparian forest structure across paired reference and impacted headwater watersheds (1–3 km ² ) representing deciduous/dry and evergreen/humid biomes to evaluate baseline conditions and stream geomorphic response to wood storage. Deciduous forested watersheds had significantly higher LW volumes at impacted sites, relative to reference sites, together with higher variability across transects. Meanwhile, evergreen forested streams had significantly higher volumes of LW compared to deciduous sites, which were also consistently higher for impacted streams, although the relative divergence of stream LW volume between reference and impacted decreased in these evergreen sites. Compared to other regions, overall high LW volume may be a function of recent inputs, large amounts of legacy LW following an intense history of recent (50–70 years) wildfire, limited LW removal, low decay rates, and larger diameter species. The riparian forest structural variables that best explained the volumes of wood were live tree density and basal area; however, there was high uncertainty of our GLMM model given the limited number of replicates and high variability within streams. There was also a significant relationship between LW density and stream geomorphic features such as frequency of debris jams, step‐pools and bankfull width for reference condition sites, yet this relationship was not evident in the land‐use impacted sites. Despite clear evidence of land‐use impacts across stream pairs, our observations suggest a less intuitive relation between riparian forest structure, LW and stream geomorphology.