Background
Climate change is affecting global ecosystems in a variety of ways; however, temperature rises cause the ranges of cold-adapted species to contract (Ashcroft, 2010). Despite these changes, ecosystems can be buffered from the effects of climate change in areas called refugia (Morelli et al., 2016). These refugia can take different forms, but they provide potentially long-term shelter for organisms when climate is unfavorable (Ashcroft, 2010; Morelli et al., 2016). At northern latitudes, climate change is occurring particularly rapidly and the need to identify climate change refugia in the boreal region of North America, where ecosystems are concurrently affected by a variety of disturbances, is particularly urgent (Stralberg et al. 2020)
Lakes can greatly impact the microclimate of their shorelines, due to the large thermal inertia of water (the ability to resist thermal changes) and advection of the lake air inland (Hinkel and Nelson, 2011). Large and deep lakes with intense winds demonstrate an ability to move cool air further to the interior of adjacent land areas (Hinkel and Nelson, 2011). The climatic factors of North America's Great Lakes create cooler conditions along the shoreline, compared to areas as far as 5 km inland (Hinkel and Nelson, 2011). Such cooling effects have created refugia for a variety of alpine- and arctic-adapted plants that exist in disjunct populations on the bedrock of the north shore of Lake Superior (Given and Soper, 1981). Given the size of Lake Superior and the intensity of its climate (Hinkel and Nelson, 2011), it is possible that refugia for cold-adapted species extends further into the shoreline forests. While it is likely that the effects of the microclimate at Lake Superior alter forest composition, this is still unknown and has yet to be measured. As such, determining whether there is an effect and how strong it is could have implications for areas beyond Lake Superior and the Great Lakes region.
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Figure 1. An inset vector of Lake Superior displaying the variation in its depth across its area.
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Research Objectives
Owing to the lack of research on forest structure in response to its cooling, we set out to measure if Lake Superior's cooling effect could alter forest structure at the shoreline. Shoreline forests are technically forest edges, therefore our study compares the structure of shoreline edges (10 m) with shoreline interior (100 m) and control forest plots (5 km); these three distances were chosen to highlight the magnitude of possible cooling effects on forest structure. Further, we examined how the exposure characteristics of Lake Superior might influence the extent of the cooling effect, where island masses offshore would create topographic interruptions to the cooling effect at the shoreline. As such, we hypothesize that Lake Superior's cooling effect impacts forest structure at the shoreline and that this effect is more pronounced at exposed (without offshore islands) compared to unexposed sites (with offshore islands). The results from our study will help in directing how conservation is carried out for these forests in the future and may suggest additional climate change refugia elsewhere as well.
Expected Results
Because Lake Superior’s environment creates refugia for arctic plants along the shore (Given and Soper, 1981), its size, depth, and intense climate (Hinkel and Nelson, 2011) will have a similar effect on forest composition and structure as well. Further, because islands would buffer the cooling effect from the lake, exposed sites will demonstrate more pronounced changes in forest structure compared to unexposed ones. However, because of individual species life history traits relating to shade tolerance, growth strategies, and moisture requirements, we may not see as strong of an impact from temperature alone.