Plants in the Eloise Butler Wildflower Garden
Will Their Dominance Last in Minnesota?
by Gary Bebeau
Oaks, concentrated in a single genus, Quercus, make up more forest tree biomass than any other woody plant genus in North America and Mexico. There are over 400 known species worldwide today.(1) When did they become so dominant and what will happen to them as the climate warms?
It is uncertain if the oaks arose first in Europe or in North America. There are fossil pollen grains found in Austria dating to 53 million years ago. There is a fossil acorn from Oregon dating to 48 million years ago.(1) What is known is that as the tropical seas and forests of the Eocene epoch (c56 to 33.9 million years ago) moved southward in North America the cooling climate allowed oaks to spread southward, eventually stopping only when the tropics south of Panama were reached, diverging into 90 species 6 of which are native to Minnesota along with a few rare hybrids.(2)(3) Ice ages have come and gone and the oaks have always re-forested the territory. Today in Minnesota there are 1.5 million acres that are predominately oak out of 15.7 million acres of timberland.(4) This is split between the 6 species so its best here to list the Minnesota species and how oaks are grouped.
Oaks are divided into 8 lineages of which 3 exist in North America: the reds, the whites and the southern live oaks of the Southeast. The red group has leaves with awns on the lobes (bristles) and matures acorns in two years. The white group has leaves without awns and matures acorns in one year. In Minnesota we have the Red Oak, Q. rubra, Black Oak, Q. velutina, and the Northern Pin Oak, Q. ellipsoidalis, in the red group; the White Oak, Q. alba, Swamp White Oak, Q. bicolor, and the Bur Oak, Q. macrocarpa, in the white group. The Wildflower Garden hosts five, excepting the Black which is historical.
Oak acorns are important for wildlife, but carbon storage is important for all life and of all the forest trees oak, along with maple, hickory and beech sequester more pounds of carbon than any other tree species. A typical oak of 20 inches diameter will store over 2,000 pounds and tuck in an additional 20 to 25 pounds per year.(5)
Continued below the graphs.
Below: 1st graph - carbon storage levels of various Minnesota tree species groups. Chart source -see note 5. 2nd graph - carbon storage of a single oak tree depending on age. Chart G D Bebeau from source 5.
A standard hypothesis is that warming climate will expand the current northern limit of growth of a species and retard it at its southern limit of growth. Thus, the coniferous boreal forests of northern Minnesota will be pushed northward and the hardwoods will move north. Minnesota’s climate has moved north 70 miles in the last 50 years and is estimated to move another 125-250 miles in the next 50 years.(6) Current experiments indicate the firs , tamaracks and spruces will have great difficulty but the temperate oaks and maples may not, which means that oaks will become even more dominant in areas like Minnesota although the southern growth limit may move north. The chart(7) shown here gives the range and tree concentration in Minnesota of Bur Oak. That of Red Oak is quite similar whereas White Oak and Northern Pin cover much less territory. Estimates by the Minnesota Department of Natural Resources indicate that the White Oak may be able to gain territory in NE Minnesota where it is rare today, the Bur Oak will maintain its territory but the Red Oak will move out of SE Minnesota where it has areas of dominance today.
Several different scenarios on the severity of climate change have been modeled. Between the low and high models the oaks in Minnesota fall into two groups when we look at changes in suitable habitat. Those that should see large increases in suitable habitat are the Black Oak, Northern Pin Oak, White Oak and Swamp White Oak. Little change is expected for the Bur Oak and also for the Northern Red Oak unless the high-end of change is reached, then, there should be a decrease in suitable habitat.
The Wildflower Garden is situated in an area that should remain habitable to all the Oaks still present. These is a third group of oaks - those not currently growing in the wild in Minnesota but are expected under either the low or high scenarios to gain new habitat in Minnesota. Therein rises the possibility of introducing three species that are historical to the Wildflower Garden. The Chinkapin Oak, Q. muehlenbergii, which is considered historical to southern Minnesota may be able to move northward from Iowa. The Pin Oak, which is in Wisconsin and Iowa and the Scarlet Oak which is in southern Wisconsin could also acquire habitat.(8)
The Minnesota DNR has published a chart showing which native trees are suitable for yard planting as the climate changes. Access the chart here. (PDF)
Red Oak, Q. rubra,Indigenous
Black Oak, Q. velutina, Eloise Butler planted some acorns on October 17, 1913 that she had obtained from the Arnold Arboretum. In 1917 she planted 13 acorns collected in Providence Rhode Island. Gardener Cary George replanted the species in 1994.
Northern Pin Oak, Q. ellipsoidalis, Indigenous.
White Oak, Q. alba, Indigenous.
Swamp White Oak, Q. bicolor, Indigenous and planted in 2017.
Bur Oak, Q. macrocarpa, Indigenous.
Chinkapin Oak, Q. muehlenbergii, introduced by Eloise Butler May 3, 1922 with three trees from Andrew’s Nursery, in Boulder Co. Not extant.
Scarlet Oak, Q. coccinea, introduced by Eloise Butler October 6, 1928 from Kelsey’s Nursery in Salem MA. Planted again 1994. Not extant.
Pin Oak, Q. palustris , planted at the memorial ceremony for Eloise Butler, May 5, 1933. Not extant.
Three Oak species that may find new habitat in Minnesota in the next 70 years.
This article is an expanded version of that published in The Fringed Gentian™, Vol. 69 No. 3, Fall 2021.
(1) A. L. Hipp, P. S. Monor & J. C. Bares, "Ascent of the Oaks," Scientific American, Aug. 2020
(2) Flora of North America, Volume 3.
(3) Anita Cholewa, Flora of Minnesota, 2018.
(4) S. Hillard, K. Bergstrand, S. Burns, D Deckard, "Minnesota Forest Resources," 2018, Minnesota Dept. of Natural Resources
(5) "Carbon in Minnesota Trees and Woodlands", University of Minnesota Extension 2020.
(6) "Boreal Forest Warming," Department of Forest Resources, University of Minnesota, St. Paul
(7) See note 4.
(8) Climate Change Field Guide for Northern Minnesota Forests, USDA Northern Forests Climate Hub Technical Report #2.