The Vegetation of Wisconsin: An Ordination of Plant Communities

6. Southern forests - mesic

CHAPTER 6 Southern forests-mesic GENERAL DISTRIBUTION OF MESIC FORESTS The mesic forests of the prairie-forest floristic province are not present in a single contiguous area but rather are scattered quite widely in separate islands (Figure VI-2). The largest areas in the province are in the eastern counties near Lake Michigan (Milwaukee, Ozaukee, \,yashington, Sheboygan, and Calumet), in an area centering on Richland County and in the St. Croix, Pierce, Dunn county region. Smaller concentrations are to be seen in Grant, Green, and Sauk counties. In Dane County, only a few very small, isolated areas are known, with four of these in small pockets along the northeast shores of the four Madison lakes (Ellarson, 1949). The total amount of the mesic forest type in presettlement times was about 3,432,000 acres, or 9.80 per cent of the total land area of the state. Copyrighted Material 104 so UTHERN FORES T S COMPOSITION AND STRUCTURE The average importance values for all trees found in the 47 P.E.L. stands (Figure VI-I and the map at the opening of this chapter) of southern mesic forests are shown in Table VI-I, together with their constancies as determined by the number of stands in which they appeared in the sample of measured trees. Sugar maple (Acer saccharum) is in a class by itself. The next most important trees are basswood (Tilia americana), beech (Fagus grandifolia), slippery elm (Ulmus rubra), red oak (Q. borealis), and ironwood (Ostrya virginiana), while all 19 other species are of relatively minor importance. Notice that beech is in third place with respect to average importance value, although it ranks tenth in constancy. This is a result of its very limited geographical range within the province. Within that range, however, it may achieve high levels of importance, equaling or exceeding sugar maple in this respect. Of the 26 trees found in this forest, eight species reach their optimum development in Wisconsin in the type, including sugar maple, basswood, ironwood, yellowbud hickory (Carya cordiformis), and Kentucky coffeetree (Gymnocladus dioica). The numerical structure of a typical mesic forest is shown in Table VI-2. Again sugar maple stands out by itself, with a terrific preponderance over all other species in the three smallest size classes. Slippery elm, ironwood, yellowbud hickory, and basswood are the only other species which are clearly reproducing themselves; butternut (Juglans cinerea), black walnut (J. nigra), white oak (Q. alba), and American elm (Ulmus americana) are totally lacking in the seedling classes. The value of 15 small seedlings per acre for red oak was based upon the presence of germinated acorns. The seedlings depend solely upon the stored food within the acorn, and die when all of it is used. Nevertheless, their ability to germinate and persist for even a few years gives them an advantage if an opening appears in the canopy. Life-history of sugar maple (Acer saccharum) The great importance of sugar maple in the southern mesic forests justifies an examination of its life-history characteristics in some detail, since the unique features of this forest are dependent almost entirely on the behavior of this species. Potentialities for achieving high levels of dominance are based upon several qualities, which include high reproduction rates, great tolerance of shade, and long life. The reproduction rate is evident from the figures in Table VI-2, which show nearly 20,000 seedlings less than 12 inches in height per acre. This measure Copyrighted Material MESIC 105 is regularly at a high level in all the mesic stands, but is subject to quite wide fluctuations from year to year in anyone stand; these fluctuations depend upon the size of the seed crops of previous years as well as on the annual vagaries of the weather. In early spring the flowers of sugar maple are produced in umbels from primordia formed during the previous growing season. Most of the flowers are male and some trees produce only male flowers. Pol· lination is brought about largely through the aid of bees and other Table 5 Annual variation in seed yield of sugar maple in a mesic forest in Green County No. oj seeds % oj seeds No. oj seeds % oj seed" Year fier acre* with embryos Year per acre* with embryos 1947 4 ,300 ,000 ? 1953 5,270,760 50.0 1948 914,000 15 .5 1954 248,290 10.5 1949 423,600 12.2...