Repeated insect outbreaks promote multi-cohort aspen mixedwood forests in northern Minnesota, USA
Characterizing the timing, severity, and agents of historic forest disturbances is critical to developing management and conservation strategies based on natural processes. Typically such information is derived from retrospective studies of remnant old-growth forests; however, this approach has limited application in regions dominated by secondary forests heavily influenced by past land-use. One striking example is the secondary aspen mixedwood forests of northern Minnesota, which have risen in both abundance and aerial extent, the result of post-settlement harvesting and subsequent land-use changes. Given their recent rise in abundance, as well as their dominance by relatively short-lived aspen, they have not been the focus of retrospective studies examining pre-settlement conditions. Using methods of dendrochronology, we reconstructed nearly a 90-year history of canopy disturbances and stand development for nine secondary mesic aspen mixedwood forests of northern Minnesota. Results show all stands initiating after near stand-replacing disturbance, presumably harvests, marked by initial recruitment dominated by Populus tremuloides. Ensuing development included an extended recruitment period by shade-tolerant species such as Abies balsamea, Acer rubrum, and Picea glauca. However, some stands experienced defoliation by forest tent caterpillar (Malacosoma disstria) quite early during the stem exclusion stage (major events occurring as early as 23 years after stand initiation), causing tree mortality and giving rise to a new cohort. Subsequent repeated outbreaks of tent caterpillar and spruce budworm (Choristoneura fumiferana) further opened the canopy, resulting in complex mixed-species, multi-cohort stands. This study is one of the first to link chronic defoliation events with long-term community dynamics for this forest type. Our findings highlight the critical roles these events play in structuring aspen mixedwood forests, particularly within the early stages of stand development, resulting in multi-cohort stands. Moreover, our findings support the growing body of literature suggesting that the range of variability in aspen age structures extends beyond the single-cohort model that has guided forest management in this region. Collectively, these findings have important implications for the design of forest management practices that approximate this range of variability and emulate defoliation disturbances, with early harvest entries aimed at increasing stand structural and compositional complexity.