Lake effect on summer cooling of shorelines and adjacent inland forests on Lake Superior’s north shore

Climate refugia, or areas that are projected to remain relatively stable
under climate change, can serve as remnant habitat or steeping stones for
species dispersal. The largest freshwater lake in the world by surface
area, Lake Superior, serves as a model system for understanding
cooling-mediated local refugia, as its cool summer water temperatures and
wave action have maintained shoreline habitats suitable for disjunct
opulations for arctic-alpine plants since deglaciation. It is known to
affect local climates by providing a summer cooling effect near its
shorelines, however, the inland gradient of change and spatial patterns of
cooling has not been quantified. Here, we describe the extent of the
inland cooling gradient, the degree of temperature buffering, and patterns
of cooling for Lake Superior's north shore over a 3-year period.
Specifically, we established 7 transects along Lake Superior's north
shore, with temperature data loggers placed at 10 m, 100 m, 1 km, 10 km,
and 100 km inland. We analyzed temperature data by year, month, summer
maximum, and growing degree days (GDD0) for each site. Summertime cooling
at shore sites (10m) was ~5 °C cooler than inland control sites (100 km),
while sites 10 km from the shore were still 1.6 °C cooler than control
sites. Magnitude of cooling varied geographically, with sites further west
and southeast showing little to no cooling effect. Site on the exposed
north and northeast shore where disjunct arctic-alpine plants are most
common showed the highest degree of temperature buffering. We also found
that shoreline sites on the north and northeast shore had shorter growing
seasons compared to inland control sites. Finally, the number of days per
year warmer than 16 °C, a maximum temperature threshold associated with
arctic-alpine plant occurrences, averaged 65 days per year at 10 m sites
and 82 days per year at control sites. The north shore of Lake Superior
serves as a transition zone between temperate deciduous forests and boreal
forests and is predicted to transition to deciduous forests under climate
change. An understanding of the extent of lake-mediated cooling on
adjacent forests can better inform risk to arctic-alpine disjunct species,
inland forests, and overall vegetation transition models on Lake
Superior's north shore.