Understory plant response to mastication treatments in forested ecosystems of Colorado

Background/Question/Methods

Forest mastication treatments – the disposal of woody biomass by chipping or shredding the material and broadcasting it on the forest floor – have been increasingly utilized by land managers in the last decade to reduce the risk of catastrophic wildfire.  Because mastication treatments are relatively novel and have no natural analog, their potential ecological impacts are poorly understood.  Therefore, we initiated a study in 2007 to examine the short- and long-term effects of mastication treatments on understory plants, including understory plant richness and cover, understory plant species composition, and non-native plants.  We established 17 sites across Colorado, with seven in lodgepole pine – dominated forests, five in ponderosa pine – dominated forests, and five in pinyon pine – juniper forests.  Understory plant sampling at each site occurred in 2007 or 2008 (two to four years post-treatment), and again in 2012 (six to nine years post-treatment), along six 50-m transects located in both masticated and adjacent untreated stands. 

Results/Conclusions

Preliminary analyses of our two to four year post-mastication data revealed several interesting findings.  First, we found that mastication treatments increased herbaceous plant cover overall, although the magnitude of the increase varied with forest type.  At the same time, fine scale assessments showed a decrease in herbaceous cover as the depth of the masticated material increased.  Furthermore, initial herbaceous species composition differed between masticated and untreated areas.  Lastly, we found that although the cover of the non-native plants was low in both masticated and untreated areas, non-natives were favored by the treatments.  In ponderosa pine and lodgepole pine forest types, we found little evidence of non-native plants in the untreated stands, but between 4 and 11 species in the masticated areas. In the pinyon pine - juniper stands, untreated stands had 6 non-native species, but that number increased to 16 species in the masticated areas.  Analyses of 2012 data will clarify whether these findings hold over the longer-term.  Our short- and long-term findings will allow managers, scientists, and others to improve their ability to anticipate understory plant dynamics following mastication treatments, and to determine if, when, and where additional activities are required to meet desired understory conditions in masticated areas.