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

Abstract

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 <2.5cm in diameter) for all three forest types, as evidenced by previous measurements conducted in our mulched and untreated stands 2–4years post-treatment. The total richness and cover of understory plant species in mulched stands 2–4years post-treatment were either similar to, or greater than, the richness and cover in untreated stands for the three forest types; however, by 6–9years post-treatment, total understory plant richness and cover in mulched stands were always greater. The stimulatory effect of mulching on understory plants was largely driven by the response of graminoids and forbs; mulching had little effect on shrub richness or cover. The increases in total understory plant richness and cover in mulched stands 6–9years post-treatment occurred despite the fact that understory plants tended to be heavily suppressed in localized areas where the forest floor layer was deep, because such areas were rare. Exotic plant richness and cover were commonly higher in mulched than untreated stands in both sampling periods, but nonetheless understory plant communities remained highly native-dominated. Taken as a whole, our findings suggest that mulching treatments promoted denser and more diverse native understory plant communities in these three Colorado coniferous forest types, particularly over the longer-term.