Propagule and nutrient limitations on vegetation recovery after high severity wildfire in dry coniferous forests

Severe wildfires kill vegetation and consume surface litter, exposing bare soil and increasing soil erosion hazards, particularly during intense rainfall events. Erosion hazards decline as vegetation recovers and litter covers bare soil, but the recovery rate may be limited by various factors, including propagule availability and soil nutrients. We studied seeding and fertilization effects on live plant and bare soil cover during the first two years following the 2004 Pot Peak wildfire in dry mixed-conifer forests of Washington State. We assessed the effectiveness of four seeding treatments and three fertilization levels in factorial combination at eight sites using a generalized randomized complete block design. Fertilization alone significantly increased live plant cover and reduced bare soil in the first two growing seasons following wildfire. Seeding treatments alone also increased plant cover, but the effects were much smaller. Two native perennial forbs, yarrow (Achillea millifolium) and fireweed (Chamerion angustifolium), produced most of the added plant cover. Combined, seeding and fertilization reduced bare soil by up to 13% in 2005 and 21% in 2006. Our results suggest that soil nutrients and propagule availability both limit plant cover and litter production somewhat following high severity wildfires in dry forests. Seeding treatments can increase plant cover by augmenting natural post-fire colonization, which is particularly important on sites with sparse understory vegetation prior to the fire. Fertilization increases plant cover by stimulating growth of both surviving and colonizing species.