Short-term understory plant community responses to salvage logging in beetle-affected lodgepole pine forests

Recent bark beetle outbreaks in western North American subalpine forests have prompted managers to salvage log some beetle-affected stands. We examined the short-term (i.e., two to three years post-treatment) consequences of such salvage logging on vascular understory plant (i.e., graminoid, forb, and shrub) communities. At 24 lodgepole pine (Pinus contorta) sites in Colorado, USA, that had been attacked by mountain pine beetles (Dendroctonus ponderosae), we evaluated how logging operations impacted understory plant species richness, cover, and composition by comparing paired unlogged and logged stands. At half of the sites, we also evaluated how logging-related slash management and site preparation activities impacted understory plant cover by comparing experimentally-implemented treatments. Average total species richness increased 18% following logging due to an increase in graminoids and forbs, while average total cover decreased 33% due to a decrease in shrubs. Experimental treatments showed that, within logged stands, average total and shrub cover were greatest where slash was retained on-site and lowest where slash was taken off-site and the soil was scarified. Average exotic species richness more than doubled after logging but values were low even in logged stands, while average exotic cover was unaffected by logging. Taken together, our results suggest that salvage logging following beetle outbreaks altered understory plant communities in the short-term by making them richer, sparser, more graminoid- and forb-dominated, less shrub-dominated, and somewhat more invaded by exotics. Our results also suggest that slash management and site preparation activities can impact the magnitude of some of the understory plant community changes brought about by logging.