Fire effects on first-year scotch broom

Chief of Resource Management Redwood National and State Parks

In the summer of 1997, twenty transects were established to evaluate the burn effects on first-year Scotch broom (Cytisus scoparius), an invasive plant. Treatments included 6 dry-grass transects, 6 brush-pile transects to increase fire intensity, 6 water-soaked transects to simulate 7.5 cm of rain, and 2 foamed transects to protect plants from direct flame. Six hundred individual Scotch broom plants were tagged with nails and numbered brass tags. Results of April 1998 monitoring showed that brush-pile plots produced the greatest Scotch broom burn response (there was no evidence of growth/resprouting in 98% of tagged individuals). The only tagged individuals that survived in brush-plots were where the brush failed to burn. Evidence, so far, suggests that the brushing provided a total kill of existing plants. In dry-grass plots 82% of the tagged individuals exhibited no growth/resprouting. Mortality in dry-grass plots was greatest with backing fires. The soaked plots were saturated 3-5 days prior to burning. Even with the presence of subsurface moisture there was no evidence of growth/resprouting in 87% of tagged individuals. The foam treatment was intended to serve as a no-burn control. Although, survival in foamed plots was higher than in other treatments, some individuals were scorched and apparently killed by radiation or hot gases from surrounding flames (53% no growth/resprout) even though no grass or Scotch broom were consumed. The data suggests that use of prescribed grass fire with low surface moisture, low to moderate humidity, warm air temperature, and long duration (generally associated with a backing fire) is 100 percent lethal to young Scotch broom.