Mulching fuels treatments have been increasingly implemented by forest managers in the western United States to reduce crown fire hazard. These treatments use heavy machinery to masticate or chip unwanted shrubs and small-diameter trees and broadcast the mulched material on the ground. Because mulching treatments are relatively novel and have no natural analog, their ecological impacts are poorly understood.
We initiated a study in 2007 to examine the effects of mulching on vascular understory plant species richness (the number of plant species in an area) and cover (the percent of an area covered by plant species). We established 15 study areas in Colorado, which were distributed across three broadly-defined coniferous forest types: pinyon pine – juniper; ponderosa pine and ponderosa pine – Douglas-fir; and lodgepole pine and mixed conifer (lodgepole pine, limber pine, and other conifers). Measurements were conducted along 50 meter transects, 2–4 years post-treatment (2007 or 2008), and again 6–9 years post-treatment (2012), in three mulched and three untreated (not mulched) stands per study area.
Total richness and cover of understory plant species in mulched stands 2–4 years post-treatment were either similar to, or greater than, richness and cover in untreated stands for the three forest types; however, by 6–9 years post-treatment, total understory plant richness and cover in mulched stands were always greater.
The stimulatory effect of mulching on understory plants was largely driven by the response of graminoids and forbs; mulching had little effect on shrub richness or cover.
There were increases in total understory plant richness and cover in mulched stands 6–9 years post-treatment. It was rare to find areas where the forest floor (the mulch + leaf litter + duff layer) was deep. In the rare areas where there was a deep forest floor areas, understory plants tended to be heavily suppressed.
Exotic plant richness and cover were commonly higher in mulched than untreated stands in both sampling periods, but nonetheless understory plant communities remained highly native-dominated.