Monday, 17 November 2003
Effects of fire and soil conditions on germination and seedling success of diffuse knapweed (Centaurea diffusa) in northern Arizona forests
We hypothesized that diffuse knapweed (Centaurea diffusa) germination and seedling growth respond positively to fire in ponderosa pine (Pinus ponderosa) forests. We are the first to address this hypothesis in pine forests of the Southwestern US. With an increase in the size and number of high severity wildfires in the Southwest, we believe diffuse knapweed may increase, leading to reduced forage and land values, and major ecosystem changes. We used both controlled ex situ pot experiments and field comparisons. We obtained intact soil cores from two forest conditions, severely burned and unburned, from the Coconino National Forest, near Flagstaff, Arizona. We planted knapweed seeds in pots with severely burned and unburned soil, and allowed native plant competitors to grow in half, and clipped competition in the other half. Preliminary measurements showed larger plants in both severely burned treatments, with the largest plants in the clipped competition treatment. For the field experiment, we planted knapweed seeds in sealed nylon packets in four forest conditions: severely burned, moderately burned, unburned with litter removed, and unburned with litter intact. Mean germination rate was highest (61.3%) under the severely burned condition than the other conditions: 44.4% in the unburned with litter intact, 45.2% in unburned with litter removed, and 49.3% in the moderately burned condition. Overall, these preliminary results suggest that severe fire may promote germination and seedling success of diffuse knapweed in northern Arizona ponderosa pine forests.