To illustrate my previous point, I've constructed a few maps of average annual air temperatures that combine the PRISM 2010 normals with interpolated anomalies from the 1881-1910 station records. I plotted the mesic zone as ranging from 7 to 14 °C in annual air temperature, assuming that the soil is 1 degree warmer (8-15 °C).
1910 Normals: https://drive.google.com/open?id=0B3NKeT9nHj0RaFh3NVlRakVwWGs
2010 Normals: https://drive.google.com/open?id=0B3NKeT9nHj0RSmVxeVZadVpRcG8
The mesic-frigid line nicely follows Michigan’s vegetation tension zone near Muskegon in 1910, but expands along the coast 200 km northward to Charlevoix 100 years later. A similar thing happens along the southwest coast of Maine. The 20th Century averages show intermediate values, and plotting the last 20 years of the dataset (1995-2014) show even more migration of the zone.
20th Century Averages: https://drive.google.com/open?id=0B3NKeT9nHj0RbUEydGFPcllDZ1k
1995-2014: https://drive.google.com/open?id=0B3NKeT9nHj0Rd0VzQU5ZYkwxajA
I assume that some land use changes have already taken place, with perhaps a diversification of woody crops (grapes and peaches?) in the cherry belt of northwest Lower Michigan. But the existing mature forests were established during this earlier cooler period. While there is a great deal of momentum to vegetation composition, I might expect that future forests in northern Michigan would have less of a fir component, and an increase in oak recruitment. The southeastern US doesn’t exhibit as much variability.
As an aside, I’ve also produced a climate classification which plots summer averages and winter lows, which I expect to sort different elements of vegetation separately, and better than soil temperature. Plants presumably limited by winter cold (black gum and sassafras) have historically made it further north along Lake Michigan in Michigan, but not Wisconsin. Balsam fir on the other hand appears favored by cooler summer temperatures.
1910 temperature regime: https://drive.google.com/open?id=0B3NKeT9nHj0RcHBlQTcxckpXZmM
2010 temperature regime: https://drive.google.com/open?id=0B3NKeT9nHj0RZnVSMHo3ZnRGY0k
Greg J. Schmidt
Ecological Site Inventory Specialist
Natural Resources Conservation Service
3260 Eagle Park Dr. NE, Suite 104
Grand Rapids, MI 49525
cell - (517) 285-7911
e-mail - greg.schmidt@mi.usda.gov
Ecological Site Descriptions - https://esis.sc.egov.usda.gov/
