Disturbance dynamics and carbon storage in southern boreal mesic aspen mixedwood forests of northern Minnesota, USA.

Abstract

One emerging objective related to forest management is developing silvicultural systems that increase the levels of carbon storage so as to mitigate or offset atmospheric concentrations of carbon dioxide. Understanding the ecological factors and conditions that led to the development of forest stands with high levels of carbon storage can allow for the formulation of management prescriptions that emulate the frequency, timing, and severity of disturbances leading to these conditions. The aims of this thesis were to (1) generate an understanding of the factors affecting stand-level structural and compositional development in southern boreal mesic aspen mixedwoods (hereafter referred to as ‘aspen mixedwoods’), and (2) identify relationships between carbon storage, stand characteristics (e.g., composition and structure) and disturbance histories. Dendroecological methods were used to detail the mechanisms by which nine aspen mixedwood stands in northern Minnesota developed in terms of composition and structure over the last nine decades. With that knowledge and detailed plot-level measurements of forest carbon pools, relationships between patterns of carbon storage resulting from various disturbance histories and compositional mixtures were examined. Dendroecological reconstructions demonstrated that the development of mature aspen mixedwoods was strongly influenced by the defoliation of trembling aspen (Populus tremuloides) by forest tent caterpillar (FTC: Malacosoma disstria) and of balsam fir (Abies balsamea) by eastern spruce budworm (SBW: Choristoneura fumiferana), resulting in complex multi-aged forests. Notably, disturbance-induced structural and compositional changes began as early as 30 years after stand initiation. Concerning carbon storage, stands with a high proportion of aspen stocking resulted in greater total ecosystem (TEC) and tree carbon (TREEC) storage with an opposite trend observed with proportion of conifer, particularly balsam fir. However, in light of recent disturbance, stands containing a greater diversity of tree species and a greater proportion of conifer stems had higher rates of tree carbon increment over the last two decades than plots with a greater proportion of aspen. Furthermore, lower levels of TEC in plots that had experienced elevated rates of disturbance over the last three decades were documented. Collectively, these findings highlight the influence of low to moderate severity disturbances on the patterns of carbon storage and compositional and structural complexity within these systems. As such, regional patterns of natural disturbance present a challenge within the context of managing for highly productive mature aspen mixedwoods; however, the restoration of historically important species (i.e., Picea glauca, Pinus strobus and Thuja occidentalis), specifically long-lived species resistant to FTC and more importantly SBW, may offer a means to store large amounts of carbon for longer periods.