The Vegetation of Wisconsin: An Ordination of Plant Communities

7. Southern forests - xeric

CHAPTER 7 Southern forests-xeric GENERAL DISTRIBUTION The xeric forests of southern vVisconsin are here considered to include all of the closed canopy forests on the uplands of the area southwest of the tension zone which are included within the range of 300 to 2300 on the compositional index described in Chapter 5. For purposes of certain discussions, this group will be divided into two segments, the dry forests from 300 to 1300 and the dry-mesic forests from 1300 to 2300, but this division is purely arbitrary. The xeric forests as a whole are predominately oak forests. They are located on well-drained sites on either sandy and porous flat lands, on south and west slopes of hills, or on thin soils on hilltops and ridges. As pointed out by Chamberlin (1877), they frequently adjoin savannas on the dry side and mesic forests on the more protected side, with gradual transitional stages on boundaries of both. Copyrighted Material XERIC 133 The total acreage of oak forest at the time of settlement was 1,386,700 acres or 3.96 per cent of the Janel area of the state (Figure VII-2). This was only abollt 20 per cent of the acreage in oak savanna. No large continuous tracts of closed oak forest, comparable to the massive blocks of mesic forest, were present. The greatest concentration of the type was in the west central counties, toward the northern boundary of the Driftless Area, especially in Monroe, Dunn, and Buffalo counties. Only a single small island was present in Dane County (Ellarson, 1949) and some other southern counties v\'ere completely devoid of stands of this type. COMPOSITION AND STRUCTURE In the 127 xeric stands in the P.E.L. study, which are shown in the figure reproduced at the opening of this chapter, 29 species of trees were found. These are shown in Table VII-I , which gives average importance values and per cent constancies for each of the two segments of the compositional gradient. There are seven species of oak on the list, including the three most important trees. Hickories, which are prominent components of the so-called oak-hickory forest in the southern states, are of minor significance in Wisconsin. From the standpoint of both importance value and constancy, white (Quercus alba), red (Q. borealis), and black oak (Q. velutina), are far in the lead; no other species are even close. Just as sugar maple (Acer saccharum) controlled the mesic forests through its overwhelming dominance, so the oaks control the xeric forests. The other members of the forest are adjusted to the environment as modified and influenced by the oaks. The dry segment is characterized by the prominence of black oak, black cherry (Prunlls seYOtina), bur oak (Q. macyocarpa), Hill's oak (Q. ellipsoidalis ), chinquapin oak (Q. muhlcnbergii Engelm.), trembling aspen (Populus tremuloides), and boxelder (Acer negundo); the dry-mesic segment has red oak, large-toothed aspen (Populus grandidentata), and red maple (Acer rubrum), as characteristic members, plus a number of invaders from the mesic forest, including significant quantities of basswood (Tilia americana), sugar maple, slippery elm (Ulmus rubra), white ash (Fraxinlls americana), and ironwood (Ostrya virginiana). White oak, shagbark hickory (Carya ovata), black walnut (Juglans nigra), and green ash (Fraxinus pennsylvanica var. subintegerrima) are represented about equally well in both segments of the xeric forest. The structure of a typical initial oak forest is well shown in Copyrighted Material 134 SOUTHERN FORESTS Table VIl-2, which is taken from Cottam's detailed study (1949) of a forest near Verona in Dane County (Plate 11). The compositional index of this stand is 1006 so it is near the top of the dry segment. The stand possessed a total density of 123 trees per acre with a dominance of 105 square feet per acre. The canopy coverage was 83 per cent and this allowed sufficient light to reach the forest floor so that reproduction of the dominant trees, white and black oak, could occur. Notice, however, that although the bur oaks are among the largest trees in the forest, they are not reproducing and that black oak has far less reproductive material than white oak. This is in line with the relative shade tolerances of these three species. Red oak, the most tolerant of the major oaks, is not represented in this sample. Table VII-2 also shows the structure of a typical intermediate, or dry-mesic...