Recent large-scale, severe wildfires in the western United States have prompted extensive fuel treatment programs to reduce potential wildfire size and severity. Often, unmerchantable material is mechanically masticated because removing the material is cost-prohibitive.  Mastication treatments involve shredding, chopping, or chipping small trees and/or shrubs into small chunks and leaving the material on site. However, the effects of leaving this material on forest ecosystems are poorly understood. We examined how mastication treatments: 1) alter the distribution of woody material; and 2) impact plant functional group cover 2-4 years post-treatment by comparing data collected within paired masticated and untreated sites in lodgepole (Pinus contorta) and ponderosa (Pinus ponderosa) pine forests. 

Results/Conclusions

The reduction in overstory resulted in greater coverage of fine (<7.62 cm diameter) woody fuels in masticated areas of both forest types.  Fine woody fuel coverage increased in masticated areas to 52% in lodgepole pine and 36% in ponderosa pine compared to 6% in untreated areas of both forest types.  Coarse woody (>7.62 cm diameter) fuels also increased in masticated areas in ponderosa pine forests, but did not increase in lodgepole pine treatments.  Surface needle litter coverage decreased from 87% in the control areas to 42% in masticated areas of lodgepole pine forests and from 78% to 42% in ponderosa pine forests.  Forest floor depth increased after mastication by 1 cm for both forest types. However, needle litter mass was similar among treatments, indicating that needles are still a component of the forest floor complex, but mixed in or buried. The increased surface woody fuel component in treated areas resulted in a shift from a needle fuel bed in control areas to a compact woody/needle fuel bed in the treated areas.  This change in the fuel bed composition and orientation will likely influence fire behavior and effects. Although overstory density decreased, we did not detect an effect of mastication treatment on the cover of understory functional groups (graminoid, forb, and shrub).  The lack of vegetative response to overstory removal might represent an offsetting effect of the woody materials acting as a physical barrier to plant sprouting or establishment moderated by variability in mulch distribution, depth, and time since treatment.