Mechanical fuel reduction treatments are being widely used across the Western U.S. to reduce wildfire risk.  Often, these treatments focus on the removal of small-diameter trees to lessen the risk of crown fire.  Since it is not economic to harvest small-diameter material, these fuels are often treated by mechanically shredding the harvested biomass and leaving the mulch on site.  Mulching treatments are a relatively new management practice and are currently being implemented across thousands of acres over a variety of ecosystems and site characteristics.  Since the ecological effects of these treatments are poorly understood, we established a network of independent sites that encompasses a wide range of forest ecosystems and geographical conditions in Colorado.  We examined how mastication treatments impact herbaceous plant cover 2-4 years after treatment by comparing data collected within paired mulched and untreated sites in pinyon pine-juniper (PJ: Pinus edulis-Juniperus spp.), ponderosa pine/Douglas-fir (PIPO: Pinus ponderosa / Pseduotsuga menziesii), mixed conifer (MC), and lodgepole pine (PICO: Pinus contorta) forests.  We used this ‘twin-plot’ approach to assess how response to treatment varies across landscape and what information we gained by this landscape approach compared to studying an individual site.

Results/Conclusions

Average herbaceous cover was low within the untreated and mulched sites for each forest type.   Herbaceous cover ranged from 1.7% to 3.4% in the untreated sites and increased slightly in the mulched areas, ranging from 3.6% to 7.2%.  When treatments were compared within an individual study site, the herbaceous cover response varied. For example, in the PJ forest type, the overall herbaceous cover in the mulched area added on average 2.7% more cover.   However, some PJ sites showed very little (<2%) increased cover while others added up to 7% more cover. This same trend appeared in each forest type; some sites increased while others exhibited no response. Our twin-plot approach to assess the herbaceous response to mulching treatments demonstrates some drawbacks of measuring a single stand to extrapolate an ecological response to a landscape scale. Instead, by using the twin-plot approach, we have identified specific stands within a population that responded differently from the average and we can now generate some hypotheses that may explain these differences.  For example, the differences in vegetative response within/among sites 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, time since treatment, and site productivity.