Date of Award

Spring 5-2024

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Ecology and Environmental Sciences

Advisor

Shawn Fraver

Second Committee Member

Anthony W. D'Amato

Third Committee Member

Jay W. Wason III

Abstract

Picea rubens is a cold-adapted, sub-boreal species whose existence along Maine’s coast is enabled by the moderating effect of the Gulf of Maine. Picea rubens, and the larger spruce-fir forest type, are highly vulnerable to climate change with studies predicting near complete loss of P. rubens suitable habitat in the USA as early as 2060. However, despite these dire future predictions, P. rubens populations have recently seen positive trends across a range of metrics. In addition, coastal Maine may have served as a refugium for P. rubens during the mid-Holocene warming, remaining cooler and wetter than inland regions. Therefore, significant uncertainty remains regarding P. rubens’ future. A detailed understanding of local P. rubens forest dynamics and a clear picture of historic change is needed to provide context for land managers planning for a changing climate. We resampled seven historic inventory sites established by Dr. Ron Davis in mature coastal P. rubens forests in 1959 to (1) assess the current structure and composition of these sites; (2) quantify changes in forest structure and composition of mature coastal spruce forests over the last sixty years; and (3) reconstruct the disturbance history of each site using dendrochronological methods. Importantly, our results revealed that P. rubens maintained or increased dominance in the overstory at all sites, despite presumed past harvesting (six sites), near stand-replacing blowdown (two sites), and partial blowdown (one site). P. rubens also dominated the abundant advance regeneration layer, suggesting future persistence of this species. Two sites on exposed headlands with large P. glauca components in their 1959 overstories transitioned to P. rubens dominated overstories following windstorms that occurred during the 60-year period. Large site-to-site variability confounded generalizable trends regarding changes in stand structure; this variability can be attributed to periodic wind disturbance to the canopy. However, the structural metrics at four sites now resemble those of Western Mountain, the only old-growth (never harvested) site remaining on Mount Desert Island. Reconstruction of past disturbance and cohort structure revealed that five of these seven sites exhibited clearly defined cohort age structures, with recruitment peaks in the mid-1800s, likely following harvests, which were widespread at that time. Stand structure has since been shaped by low- to moderate-severity wind disturbance, including at the Western Mountain old-growth site whose age structure shows continuous recruitment in nearly every decade since 1730. Taken together, these findings suggest compositional resilience to disturbance, and they are encouraging for the persistence of coastal spruce, at least under current climatic conditions.

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