Influence of Eastern Dwarf Mistletoe and Fungi on Regeneration Health and Composition in Lowland Black Spruce Post-Harvest

Abstract

Black spruce (Picea mariana [Mill] Britton, Sterns & Poggenb) is a dominant and wide-ranging North American boreal forest species. It reaches its southwestern range limit in Minnesota, USA. Forest managers in Minnesota have become increasingly aware of how eastern dwarf mistletoe (Arceuthobium pusillum [Peck]) (EDM) may be influencing the ability of black spruce to regenerate after harvest naturally. This study aimed to explore regeneration dynamics in black spruce stands, quantify residual tree density and impact from EDM, and quantify EDM infestation in neighboring stands. Ten stands were surveyed over three years in northern Minnesota pre-and post-harvest. Eastern larch comprised almost 70% of the regeneration in EDM infected stands and over 50% of the regeneration in the uninfected stands, suggesting that the light intensive environment is favorable for eastern larch regeneration. A significant difference was observed between the total percentage of black spruce regeneration recorded, with uninfected stands having a higher proportion of black spruce (p = 0.06). The diameter at breast height in inches and the square feet of basal area per acre were greater in stands with EDM infections due to residual trees left in clumps. While clear-cuts are a viable option for mitigating EDM, the disease will continue to occur if the 5-foot cutting rule is not followed. Likewise, neighboring stands can be a source of future EMD infections even if the 5-foot cutting rule is efficiently applied within a stand. Management decisions for EDM should consider infection both within the stand and at the broader landscape level (neighboring stands) when developing management recommendations. Additionally, twelve actively managed black spruce stands in Northern Minnesota, USA, were sampled to understand better the diversity and abundance of fungi present in black spruce bogs affected by eastern spruce dwarf mistletoe. The twelve stands were divided evenly between recently harvested (six stands) and unharvested (six stands); additionally, half of the harvested and unharvested stands were infected with eastern spruce dwarf mistletoe, and half were uninfected. Due to the potential for fungal infections to act as secondary disturbance agents that could expedite black spruce mortality, an emphasis was placed on investigating the presence of fungal pathogens in the Basidiomycota. Forty distinct fungal species were identified across all twelve study stands. Of all the species identified, less than half (43%) were determined to be fungal pathogens. No significant differences in fungal diversity were found between the study treatments. Additional work in black spruce systems should increase understanding of potential above and below ground disturbance agents.