Structural characteristics of black spruce (Picea mariana) infested with eastern spruce dwarf mistletoe (Arceuthobium pusillum), Minnesota, USA

Disturbances are important processes within forested ecosystems, influencing stand dynamics, species composition, and ecosystem services, with effects across multiple spatial scales. Interacting effects across scales can influence the suitability and results of management actions, necessitating a thorough understanding of disturbances at multiple scales. Eastern spruce dwarf mistletoe (Arceuthobium pusillum Peck, hereafter ESDM) is a native disturbance agent of lowland black spruce (Picea marina (Mill.) B.S.P.) in Minnesota, USA. ESDM results in mortality at the individual tree scale but is commonly managed through stand-level silvicultural treatments; ESDM can also influence landscape-scale forest composition. Despite its role as a mortality agent of black spruce, little is known about the relationship between structure and infestation at scales finer than the stand. Here, our objective was to quantify the relationship between ESDM infestation and structure at the stand, plot, and neighborhood scales. Twenty-five black spruce stands, 12 infested and 13 uninfested, were sampled using fixed radius plots; variables collected relate to ESDM infestation severity, forest structure, and forest composition. The relationship between forest structure and ESDM infestation presence was analyzed at the stand, plot, individual tree, and neighborhood (a hybrid scale focused on individual trees but including variables measured in the plot in which the tree is located) scales. Infested plots and neighborhoods had significantly lower trees per hectare and higher species richness and Shannon's Diversity than uninfested plots and neighborhoods; these differences were not significant the stand scale. At the stand scale, stand structure and composition variables, including trees per hectare and species diversity metrics, shift with ESDM severity. However, at the tree level, the presence or absence of ESDM is more strongly related to structure than is severity. When incorporating ESDM infestations in management planning and decision making, considering the appropriate scale is important. By documenting the effects of ESDM at multiple scales, we can deepen our understanding of stand dynamics and succession in lowland black spruce and use this information to develop new, ecologically based management approaches in these forests that more closely reflect the local disturbance regimes.