Effects of post-fire conditions on germination and seedling success of diffuse knapweed in northern Arizona

Abstract

Introduction of exotic plant species is confounding treatments designed to reduce unnaturally high tree densities in ponderosa pine ( Pinus ponderosa P. & C. Lawson) forests of the western United States. Also, the recent increase in large, stand-replacing wildfires may promote introduction and spread of species, such as diffuse knapweed ( Centaurea diffusa Lam.), which may lead to reduced forage and land values and major ecosystem changes. We hypothesized that diffuse knapweed germination and seedling growth would respond positively to burned conditions in northern Arizona ponderosa pine forests. Our first field study was based on a May 2002 wildfire near Flagstaff, Arizona (Site I). For the field study at Site I, we buried diffuse knapweed seeds in sealed mesh packets in four forest floor conditions: severely burned, moderately burned, unburned bare soil and unburned. We removed packets monthly and measured percent germination. Germination under severely burned conditions at Site I (overall average 76%) was higher than in unburned conditions (59%) ( P = 0.017). Our second field study was based on a fuel reduction treatment with slash piles burned in February 2003 (Site II). Germination in seed packets was higher ( P = 0.003) in pile burn scars (67%) than in adjacent unburned locations (38%) at Site II. In fall 2002, we planted knapweed seeds in pots containing intact soil cores from Site I from unburned and severely burned conditions. We allowed competition to grow in half the pots, and removed competition in the other half. Diffuse knapweed biomass/pot was greater ( P = 0.035) in soil from the severely burned (2.6 g) compared to the unburned condition (0.6 g) from Site I. Knapweed biomass/pot also appeared greater when grown in soil from pile burn scars (3.3 g) compared with unburned locations (2.5 g) from Site II, although the difference was not significant ( P = 0.317). We conclude that severe wildfire or pile burning can promote germination and seedling growth of diffuse knapweed in northern Arizona pine forests. Moreover, diffuse knapweed may also threaten unburned forests, as average germination was 48% in unburned locations. Failure to consider exotic invasive species in fuel reduction and wildfire rehabilitation may result in trading one undesirable condition for another.