Exotic plant response to wildfire severity and time since fire in a Colorado ponderosa pine-Douglas-fir forest

Fire is an important disturbance agent in ponderosa pine-dominated forests, and it historically played important roles such as regulating stand densities, decreasing fuel loads, and sustaining understory communities.  However, today’s fires are increasingly playing a new role— they are contributing to the introduction and spread of exotic plants.  We quantified fire’s role in exotic species establishment and growth by remeasuring understory composition and cover in 20 0.1-ha plots that were originally established in 1996, but that burned with a range of severities in the 2002 Hayman Fire.  Postfire measurements were made annually from 2003 to 2006.  Based on our preliminary analyses, we found that:  (1)  In less severely burned areas, exotic richness and cover increased from prefire levels one to three years after the fire, but returned to prefire levels by the fourth year.  In more severely burned areas, an increase in exotic richness has been sustained for all postfire years, while cover only increased in postfire year three.  (2) The composition of the prefire exotic community greatly influenced the composition of the postfire community, with 62% of a plot’s prefire exotics present in the postfire community in any year.  (3)  Several exotic species became considerably more abundant at some time after the fire, including Conyza canadensis, Lactuca serriola, Taraxacum officinale, Tragopogon dubius, and Verbascum thapsus.  In general, these species were most abundant in severely burned areas.  For some of these species, postfire cover has returned to prefire levels, while for others, postfire cover remains elevated over prefire levels.  Our findings suggest that the most effective exotic control measures within the Hayman Fire and other comparable burns will likely be species-specific and should focus on areas that are severely burned or that had high prefire levels of exotics.