The Effects of Mulching Treatments on The Forest Herbaceous Layer of Colorado Coniferous Forests

Novel fire mitigation treatments that chip, shred, or masticate excess forest biomass and distribute it across the forest floor are increasingly prescribed to reduce the density of small diameter trees. We investigated the impacts of mechanical thinning and on-site fuel disposal on forest herbaceous layer productivity in four coniferous forest types (lodgepole pine, mixed-conifer, ponderosa pine, and pinyon juniper) across Colorado. Crews sampled the herbaceous layer in treated stands and in adjacent unthinned control stands across 18 study sites. Treated pinyon-juniper and ponderosa pine forests had twice the graminoid cover and forb cover of control stands. Lodgepole pine and mixed-conifer forests had similar covers of graminoids and forbs in treated and control stands. Within treated ponderosa and pinyon-juniper stands, herbaceous cover was negatively correlated with mulch depth, while in lodgepole and mixed-conifer forests, there was no relationship between mulch depth and herbaceous cover. Several factors may explain the differences observed between the ecosystems. Mulch depths tended to be higher in lodgepole (4.2 cm) and mixed-conifer (6.1 cm) treatments relative to ponderosa (3.7 cm) or pinyon-juniper (1.7 cm) treatments. Mulch depths in lodgepole and mixed-conifer may exceed the threshold where understory vegetation can grow, overriding the influence of increased light availability due to overstory thinning. Alternately, the overstory of lodgepole and mixed conifer are naturally dense compared to ponderosa and pinyon-juniper, and may lack sufficient understory flora to respond to canopy reduction. Consideration of the variability in treatment impacts (e.g., amount and distribution of mulch) combined with differences in overstory and understory species structure and composition are critical for design of “Best Management Practices” guidelines for mulching treatments in western US forests