Effects of fire and harvesting on tree regeneration and ground-layer vegetation in forested peatland communities in northern Minnesota

Abstract

Large-scale, catastrophic fires have historically been rare in peatland ecosystems in the Great Lakes region with surface fires, windthrow, and insects being the main disturbance agents. With climate change, these disturbances are predicted to become more common due to increases in the frequency and severity of drought. Boreal peatlands at their southern range margin, such as those in northern Minnesota, are expected to be particularly vulnerable to these impacts. In 2012, a wildfire burned over 10,000 ha of a large forested peatland complex in north-central Minnesota providing a rare opportunity to document the early regeneration response of vascular and nonvascular plant species in black spruce (Picea mariana) and tamarack (Larix laricina) dominated communities. Post-fire vegetation data were also used to compare with data collected from clearcuts, the most commonly used management approach for these systems. Selected clearcut sites were harvested the same year as the fire and within the same community types. Distinct differences in tree regeneration and vegetation composition were observed between burned and harvested sites. Community response to disturbance was also varied, largely reflecting the amount of disturbance to the peat layer. Burned areas had a greater abundance of tree seedlings in black spruce-dominated stands relative to harvests, however, the reverse was true for tamarack-dominated communities due to a complete removal of suitable seed beds and seed sources by the fire. In these tamarack communities, fire disturbance shifted plant community composition towards marsh-like conditions with cattails (Typha spp.) now the dominant species creating challenging conditions for future tree regeneration in these areas. In contrast, fire increased the availability of favorable seedbed conditions in black spruce by reducing the depth of peat and the abundance of Labrador tea (Rhododendron groenlandicum (Oeder) Kron & Judd), an ericaceous shrub known to compete with conifer seedling regeneration and growth. These divergent responses indicate that tamarack-dominated systems may be more vulnerable to increases in the frequency and severity of fire due to the potential for phase shifts towards non-forested conditions. The increased levels of black spruce regeneration following fire relative to harvests suggest that application of prescribed fire should be considered for enhancing regeneration efforts in these important communities.