Dieback in Juniperus communis as a possible indicator of climate change impacts to Lake Superior sandscapes

Johnson, Sarah; Mead, Jordan

Sandscape habitats are relatively rare and are a valuable resource within the Apostle Islands National Lakeshore on the south shore of Lake Superior. These coastal habitats are inhabited by specialized vegetation adapted to stressful and highly disturbed conditions. While these species naturally evolved in a dynamic system, their responses to rapid shifts in climate and other human-induced changes remain a concern. Prompted by observations by park managers, we investigated patterns and possible causes of foliage dieback in Juniperus communis L. var. depressa Pursh, an important dune stabilizing shrub. We conducted extensive surveys of J. communis on Michigan (n = 524), Outer (n = 266), and Stockton Islands (n = 366) at Apostle Islands National Lakeshore. We quantified shrub size, pattern and degree of dieback, sex, relative elevation, distance from Lake Superior, and distance to nearest conspecific. Dieback on shrubs consisted of tip death, branch death, and seemingly natural defoliation at plant centers. Across all islands, dieback was greatest on large, male shrubs, and was the most extensive on south-facing Michigan Island. The most highly stressed shrubs were positioned at relatively high elevations above Lake Superior and at farther distances from the lake. The healthiest shrubs were more likely to occur at relatively low elevations regardless of distance to the lake. Female biased sex ratios occurred on Michigan and Stockton Islands. Patterns of dieback are suggestive of successional processes and likely climate-induced stress. These sandscape habitats have experienced drought conditions due to lake level declines, precipitation declines, and increased evaporative demand associated with rising temperatures. The proliferation of juniper webworm cocoons and fungal spores on dead foliage suggest possible multi-trophic scale interactions with climate change. Dieback in J. communis may serve as an early indicator for community-wide changes in the sandscape habitat. We recommend targeted monitoring of J. communis to determine if dieback among the shrubs continues and at what rate, and to gain further understanding of the possible drivers of change.