Impacts of mixed severity wildfire on exotic plants in a Colorado ponderosa pine–Douglas-fir forest

Abstract

The 2002 Hayman Fire burned with mixed severity across 55,800 ha of montane Colorado forest, including pre-existing plots that were originally measured for understory plant composition and cover in 1997. We examined the influence of the Hayman Fire on exotic plants by remeasuring these plots annually from 2003 to 2007. We found that (1) exotic richness and cover generally increased as fire severity and time since fire increased; (2) the exotic species present in a plot before the fire were also largely present in the plot postfire, regardless of fire severity; (3) most of the new postfire species in a plot were present elsewhere in the study area before the fire, although some new species were truly new invaders that were not found in prefire surveys; (4) lightly burned riparian forests were not more susceptible to exotic invasion than surrounding uplands that burned with similar severity; and (5) native and exotic richness and cover were positively correlated or uncorrelated for all fire severities and years. Our findings indicate that exotics were stimulated by the Hayman Fire, especially in severely burned areas. However, exotic richness and cover remain low as of 2007, and correlations between native and exotic richness and cover suggest that exotics have not yet interfered with native understory development. Therefore, we conclude that exotic plants are not a major ecological threat at present, but recommend that monitoring be continued to evaluate if they will pose a threat in future years.

Evolution of growth but not structural or chemical defense in Verbascum thapsus (common mullein) following introduction to North America

“…Mullein flourishes in response to disturbance, and therefore may depress recruitment by co-occurring natives in early-seral communities (Pitcairn 2000). Although this species tends to be fugitive, infestations can persist for many years in the introduced range, especially following fire (Fornwalt et al 2011) or in areas subject to chronic disturbance (e.g., black-tailed prairie dog [Cynomys ludovicianus] colonies; Alba, pers. obs.).…”

Overstory Structure and Surface Cover Dynamics in the Decade Following the Hayman Fire, Colorado

“…Diameter at breast height (DBH), species, and live or dead status were also recorded for each tree. In 2003, one year post-fire, we relocated all pre-fire overstory trees, retagged them as necessary, and noted their status as either live, dead and standing, dead and downed, or dead and missing [32,39,40]. Dead and downed trees were those that were either uprooted or broken below breast height.…”

“…Total cover could therefore exceed 100% due to overlap among these elements. Measurements were repeated in 2003, 2004, 2005, 2006, 2007, and 2012 [32,39,40]. In 2003, we also assessed direct fire effects on organic surface material by noting the degree of scorch and consumption for pre-fire litter, duff, and wood in the subplots.…”

“…We classified each plot as burning with low-, moderate-, or high-severity by utilizing our direct fire effects data [32,39,40]. We categorized plots where <50% of the overstory trees were killed and where tree crown and organic surface material consumption were generally slight as burning with low-severity (i.e., burned by light surface fire; ten plots).…”

“…The Hayman Fire was contained on July 2, after having impacted more than 52,000 ha. In 2003, one year post-fire, we successfully reestablished all original plots [32,39,40]. Plot locations were precisely identified using pre-fire data such as plot coordinates and overstory tree stem maps, as well as from finding the remains of plot corner stakes and tree tags.…”