COS 17-1 - Riparian vegetation dynamics after mixed severity fire in the western Cascades, Oregon

Background/Question/Methods

In late summer of 2020, widespread forest fires in the Oregon Cascades burned at different severities, across multiple landownerships, and across different forest types including riparian areas. While vegetation response after fire has been well studied, the response within riparian areas and specifically that of understory riparian vegetation to fire and post-fire management is not well understood. In the absence of an intact overstory, riparian understory vegetation may serve a critical function in the post-fire recovery of the aquatic ecosystem by providing shade. Sites were selected across strata representing sub-basin burn severity and extent, stand age, and landownership. We examined site and vegetation characteristics following mixed severity fire within riparian management zones adjacent to small perennial streams at 30 study sites. We recorded canopy cover, light (as PAR), and stream temperature at each stream reach. In the riparian area adjacent to each stream, we collected data on overstory (species and diameter of live and standing dead snag), shrub (species and cover), and forb (species and cover) vegetation layers.

Results/Conclusions

Overstory canopy cover, shrub layer cover and species richness, and forb layer cover decreased with increasing burn severity, but forb species richness was highest after moderate severity burn. In the overstory, there was a higher proportion of mortality in coniferous species (e.g., Tsuga heterophylla and Thuja plicata) compared to Acer circinatum and Alnus rubra, the most common deciduous species. Overstory mortality across this burn severity gradient was an important predictor of in-stream canopy cover, light, and stream temperature. Across all sites, we encountered 196 species in the forb layer, with each site having 34-74 species. Common species varied with burn severity: in plots with high burn severity, Chamerion angustifolium and Senecio sylvaticus were common; in plots with no evidence of burn, the most common species included Oxalis spp. and Polystichum munitum. Understory (shrub and forb) growth one year after burn provided little shade to streams in the absence of the overstory, but this will likely change with longer-term study. Our results illustrate the complexity of vegetation response after mixed severity fire and its influence of stream temperature and shading, especially within forested buffers which are an ecologically critical area of importance.