Native Species Replace Introduced Grass Cultivars Seeded following Wildfire

Abstract

Seeding of non-native species following wildfires to stabilize soils and prevent erosion has become a controversial practice because it risks inhibiting tree regeneration, introducing noxious weeds, and permanently replacing native species. This paper reports the fate of non-native seeded species during 31 years of postfire forest succession in northcentral Washington state, USA. In that region, catastrophic wildfires in grand fir (Abies grandis)/pinegrass (Calamagrostis rubescens) associations characteristically result in a flush of lodgepole pine (Pinus contorta) regeneration that creates a dense tree monoculture. In the study, seven grass cultivars were broadcast seeded to prevent erosion, limit tree regeneration, and increase forage production for wildlife and livestock. Tree regeneration, native and non-native species cover, and aboveground biomass were evaluated in 1971, 1975, 1980, 1989, and 2001. The seeded species quickly established dominant cover with levels of biomass production two to three times the level of native species. Density of tree regeneration was inversely correlated with perennial grass cover during the first 10 years. Between 1980 and 1989, the seeded grasses disappeared, long before tree canopy closure. Within 15 to 20 years native species had regained dominance, and after 30 years the last remnants of the non-native cultivars were replaced in the seeded areas by a diverse mixture of native graminoids, forbs, shrubs, and trees. In contrast, a monoculture of lodgepole pine dominated the unseeded areas. This study showed that non-native grasses seeded after wildfires do not always persist and can serve as a transition to restoring a more diverse seral community.