Understory vegetation response to mechanical mastication treatments: Evidence for increasing resilience to wildfire in a young ponderosa pine forest

The use of mechanical mastication to reduce the standing biomass of shrubs and small trees within wildland urban interface areas, fuel breaks, and plantations is becoming more widespread in fire-prone western ecosystems. Treatment of these areas is usually approached from a fuels management perspective, the objective being to reduce the height of midstory fuels. However, the resulting increase in canopy openness may also promote understory plant diversity, if the mulch layer does not prevent native species from re-establishing on the site. To test the potential role of mastication as a restoration tool, the effect of vegetation response to mechanical mastication treatments (mastication only, mastication and incorporation, and mastication and prescribed fire) were compared to untreated areas (control) on the Challenge Experimental Forest in northern California. Mastication treatments significantly reduced shrub heights (P < 0.0001) by as much as four times and significantly increased canopy openness (P < 0.01) by an average of approximately 20%. Percent herbaceous cover was not significantly different in any of the treatments compared with the control (P = 0.20), but native understory species richness was significantly higher (P < 0.01) within masticated plots that had supplemental treatments (i.e. prescribed fire, incorporation). Based on the results of this study, reducing midstory fuel levels with mastication can enhance understory species diversity in areas formerly occupied by dense shrubs and/or small non-commercial trees. This reduction in shrub biomass allows greater penetration of sunlight through the canopy and increases the potential of reintroducing fire that can further promote understory plant diversity.