Tree regeneration and soil responses to management alternatives in beetle-infested lodgepole pine forests

Abstract

Recent mountain pine beetle (Dendroctonus ponderosae; MPB) outbreaks have caused one of the most widespread and dramatic changes in forest condition in North American forests in more than a century and highlighted challenges facing resource managers. To address uncertainty regarding the consequences of post-harvest woody residue management on soil productivity and tree regeneration following MPB outbreaks in lodgepole pine-dominated forests we compared three treatment prescriptions (bole-only harvest, whole-tree harvest, and whole-tree harvest with scarification) and uncut stands. The study was replicated at twelve sites across a range of operational project areas and stand conditions in northern Colorado. Salvage logging generated a new cohort of lodgepole pine at densities far above the threshold considered adequate to develop into well-stocked stands (1700–2300 t ha−1 in logged compared to 537 t ha−1 in uncut areas). Regeneration density was generally highest in whole-tree harvested areas. Growth of planted and naturally regenerating lodgepole pine recruits was best in the bole-only, residue-retention treatment, where soil moisture and inorganic nitrogen supply was also highest. However, we found no indication that whole-tree harvesting lowered soil moisture, soil nitrogen supply or pools relative to uncut stands. The density of trees regenerating beneath uncut stands indicates that post-outbreak forest structure should recover without management in these forests. The cohort of trees that regenerated following MPB-related overstory mortality, but prior to harvesting, comprise the fastest-growing component of the growing stock and 30% of its density. The broader watershed-scale outcomes of these treatments and their implications for wildfire behavior and other effects remain uncertain. However, the soil and tree patterns we report during the initial post-treatment period inform on-going decisions regarding harvest and residue retention and create a platform to guide future forest management research.