Mulching fuels treatments promote understory plant communities in three Colorado, USA, coniferous forest types

•We examined the impacts of mulching fuels treatments on vascular understory plants.•Total understory plant richness and cover increased by 6–9years post-treatment.•Graminoids and forbs in particular tended to increase following mulching.•Exotic plants were often stimulated by mulching, but were not abundant.•Overall, mulching promoted denser and more diverse native understory communities. Mulching fuels treatments have been increasingly implemented by forest managers in the western USA to reduce crown fire hazard. These treatments use heavy machinery to masticate or chip unwanted shrubs and small-diameter trees and broadcast the mulched material on the ground. Because mulching treatments are relatively novel and have no natural analog, their ecological impacts are poorly understood. We initiated a study in 2007 to examine the effects of mulching on vascular understory plant communities and other ecological properties and processes. We established 15 study areas in Colorado, USA, distributed across three broadly-defined coniferous forest types: pinyon pine – juniper (Pinus edulis – Juniperus spp.); ponderosa pine (P. ponderosa) and ponderosa pine – Douglas-fir (Pseudotsuga menziesii); and lodgepole pine (P. contorta) and mixed conifer (lodgepole pine, limber pine (P. flexilis), and other conifers). Measurements were conducted along 50-m transects 2–4years post-treatment (2007 or 2008), and again 6–9years post-treatment (2012), in three mulched and three untreated stands per study area. Mulching dramatically reduced overstory basal area (i.e., basal area of trees >1.4m tall) and increased forest floor biomass (i.e., the biomass of litter, duff, and woody material