Understory responses to fire and artificial seeding in an eastern Cascades Abies grandis forest, U.S.A.

Abstract

To mitigate erosion after fire, land managers often seed non-native grasses onto burned slopes. To assess how post-fire seeding affects plant recovery, we compared areas that were either unseeded or artificially seeded after high-intensity fire in a dry Abies grandis (Dougl.) Lindl. forest in the northeastern Cascades. Seeding with a mix of non-native grasses and a legume significantly reduced the cover of native plants and shifted patterns of relative abundance after 2 years. Although seeding did not significantly affect total cover or native species richness, it reduced overall native plant cover by 47%. Species that recolonize via wind-dispersed seeds (e.g., Epilobium angustifolium L., Lactuca serriola L., and Arenaria macrophylla Hook.), species with long-lived seeds that germinate after fire (e.g., Ceanothus velutinus Dougl.), and species with wide successional amplitudes that resprout after fire (e.g., Apocynum androsaemifolium L. and Salix scouleriana Barratt) all declined steeply in cover on seeded plots. In addition, conifer seedlings were only half as abundant on seeded plots. As seeding after fire does not boost total plant cover and limits conifer tree establishment on the study area, it appears to do little to reduce the risk of soil erosion. It also appears to inhibit native shrub and herb re-establishment. These substantial effects on native species appear to alter plant communities well beyond the life of the seeded species.