Regeneration Dynamics in Mountain Pine Beetle-Disturbed Forests: Lessons From the Current and the 1978-82 Flathead Epidemics

There are two dominant mechanisms for development of a new tree layer and subsequent canopy recruitment after major canopy mortality events. First, regeneration may develop from a pulse of new post-disturbance recruitment. Alternatively, regeneration can be from the existing seedling bank that survived the canopy mortality event. The timing and extent of post-disturbance recruitment from seed and the relative importance of the existing seedling bank is poorly understood in MPB-disturbed forests. The recruitment of post-MPB seedlings is a function of seed-source availability, seedbed substrate, overstory structure, and time since MPB attack. In the northern interior, post-MPB recruitment was sparse in stands impacted by beetle 3-10 years earlier. Subalpine fir comprised the majority of the post-MPB recruitment. It increased with local parent tree basal area and increased strongly with proximity to a major seed source, resulting in a patchy distribution. Lodgepole pine post-MPB recruitment was limited by overstory shading; pine regeneration decreased as the total overstory basal area increased. Spruce regeneration was similarly limited by total overstory basal area. Seedbed substrates were dominated by undisturbed moss layers and changed little (in the 3-10 yr post-MPB attacked stands). There was a weak trend to increased regeneration in older stands (7-10 years). Based on destructive sampling and stand reconstruction, regeneration post-beetle attack in the Flathead was often, but not always, delayed by 5-10 years. In most stands, this was followed by a strong pulse of regeneration for about 10 years. Twenty years after the outbreak there was an abrupt decrease in recruitment. In 2007, densities of post-beetle regeneration in all but 2 of the 22 stands were adequate for stocking. Based on age of understory trees in 2007, there was little advance regeneration in these forests at the time of the beetle attack. Virtually all stands are well-stocked with regeneration today