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Page 95 ï~~2003 THE MICHIGAN BOTANIST 95 TWENTY-FIVE YEARS OF CHANGE IN A DRY-MESIC FOREST OF SOUTHEASTERN WISCONSIN Christopher D. Tyrrell 1208 Wisconsin Street Stevens Point, Wisconsin 54481 715. 342. 8789 Christopher.D.Tyrrell@uwsp.edu ABSTRACT This study assesses successional changes in a southern dry-mesic forest (Curtis 1971) at the Wehr Nature Center in Franklin, Wisconsin, after 25 years of static management. The site was historically maintained by periodic fire. I sampled using techniques that duplicated a 1976 study of the same area. I then calculated importance values as per Curtis (1971) for the 2001 data and compared the results to those from 1976. Among the trees and saplings, I found a decline of importance in oak species, while species of ash and maple increased. I observed a rise in the importance of black cherry within the sapling stratum. Most notably, I found a drastic change in the composition of the shrub layer. Common buckthorn, an invasive exotic, virtually dominates the shrub stratum in 2001 and displaces gray dogwood, which was prevalent in 1976. In the herb layer, I found a rise in importance of jack-in-the pulpit and two of the most common herb species in 1976, enchanter's nightshade and white avens, remained the most prevalent in 2001. Garlic mustard increased in occurrence from 1976 to 2001. I conjecture the majority of the changes are due to the cessation of fire in the community. If left undisturbed, the dominant overstory may well shift from oak to maple, and the understory community will continue to be dominated by invasive exotic species. INTRODUCTION In presettlement times, the northwestern part of Franklin, Milwaukee County, Wisconsin was dominated by oak forest (SEWRPC 1997). According to Abrams (1992), oak forests are midsuccessional communities developed and maintained by periodic fires. These fires open the canopy, allowing for oak recruitment. Studies cited by Abrams (1992) point toward the potential replacement of oaks by shade-tolerant species in the absence of fire. Laatsch and Anderson (2000) found the suppression of fire in unmanaged oak woodlands correlates with an increase in density and cover of invasive exotic species. Invasive exotics often outcompete natives, becoming the prevalent species. This study assesses changes that have occurred after 25 years of static management in a southern dry-mesic forest at the Wehr Nature Center in Franklin, Wisconsin. Data were collected in 2001 following protocols from a 1976 study and the changes are described. The 1976 study is compared to the 2001 results and an explanation of the changes is hypothesized. Predictions made in the 1976 study are evaluated.
Page 96 ï~~96 THE MICHIGAN BOTANIST Vol. 42 DESCRIPTION OF STUDY AREA The study surveyed a woods, hereafter referred to as College Woods, in the northwest part of Wehr Nature Center in Franklin, Milwaukee County, Wisconsin. College Avenue bisects College Woods into north and south portions. These parts are located at SW X SE Y Section 32, T6N, R21E and NW Y NE X Section 5, T5N, R21E respectively. Franklin's average growing season is 175 days. Lake Michigan heavily influences the climate: high temperatures average 28 Â~C in July and -4 Â~C in January, lows average 15.3 Â~C in July and -13.8 Â~C in January. Most of the precipitation falls in the growing season and normal annual precipitation is 78.44 cm and average snowfall is 103.1 cm (NOAA 2001). Ownership of College Woods has undergone a number of changes over the past 155 years. According to property maps, College Woods was under private ownership until 1929 (Sheperd & Weiler 1988). In 1929 and 1930, Milwaukee County Park Commission acquired the land from August Powers and Hugo Koch who owned the north and south portions respectively (Park Land Acquisition Book, Milwaukee County Park Commission, undated). The Wehr Nature Center was established as a self-managed park unit in 1975. College Woods has remained under static management since that time. The large, spreading lower branches of the oaks in College Woods suggests that these trees were open grown, a condition probably maintained by fire (Abrams 1992). The overstory is characterized by a high density of these large oaks. The construction of College Avenue in the 1920s caused a change in the hydrology of College Woods. A large area is seasonally inundated with ponded water, due to poor drainage. METHODS The survey used 20 nested plots to sample shrubs and herbs, 10 plots north and 10 south of College Avenue. As described by Nowak (1976), the shrub plots are 2.0 by 8.0 meter rectangles. The herb sampling plot was nested inside the northeast corner of the shrub plot and measured 0.4 by 2.5 m. Trees were sampled using point-quarter sampling (Brower & Zar 1984), with 10 points north and 10 south of College Avenue. To prevent overlap, the plots and points were located randomly along 3 striated north-south transects lines that evenly divided College Woods. All plots and points maintained a 12 pace buffer from the road or trails to reduce influence from edge effect. Relative frequency, relative density and relative dominance were calculated for trees, saplings and shrubs, while only relative frequency and relative cover were calculated for herbs. All importance value (I.V.) calculations followed Curtis & McIntosh (1951) except the herbs, which follows Kershaw (1964), summing only relative frequency and cover. RESULTS A comparison of the 2001 to 1976 data shows a loss of oak importance in the past 25 years. This is consistent with Nowak's (1976) prediction that "oaks will decline in the future." In 1976, Quercus rubra (red oak) was the most important species in the tree stratum. In 2001, red oak has declined to a rank of third in im
Page 97 ï~~2003 THE MICHIGAN BOTANIST 97 portance (Table 1). There are few oak saplings and fewer seedlings for replacement (Tables 2 and 4). The mature individuals of red oak are a small number of large diameter trees, expressed in their high relative dominance. As the oaks have declined, Acer saccharum (sugar maple) has risen in importance. A multitude of small-diameter sugar maples generates the high I.V. In 1976 there was a low I.V. for sugar maple saplings (Table 2). A higher I.V. for sugar maple saplings is expected as the current trees have just recently grown to tree diameter. The trees and seedlings of the Fraxinus (ash) spp. in College Woods were difficult to distinguish. Often the twigs were either inaccessible or immature but those found showed possible hybridization of E americana (white ash) and E pennsylvanica (green ash). Even so, ash species have maintained a high importance in all strata since 1976. Ash has been the second most important species in the tree stratum for the past 25 years (Table 1) and most important tree in the herb layer (Table 4). Additionally, white ash has held the highest I.V. for saplings at 99.1 in 1976 and 88.4 in 2001 (Table 2). Among the trees, Prunus serotina (black cherry) has shown no increase in importance since 1976 (Table 1). This is contrary to Nowak's (1976) prediction that "black cherry will probably increase in importance." Black cherry has been second in importance among saplings and currently accounts for more than 25% of the frequency, density, and dominance (Table 2). Four of seven species present in the shrub layer were exotics. Rhamnus cathartica (common buckthorn), the leading shrub exotic, increased in importance from 44.7 in 1976 to 228.3 in 2001 (Table 3). This dramatic increase may be contrasted with Cornus racemosa (gray dogwood), which declined from an I.V. of 103.0 in 1976 to 5.4 presently (Table 3). The remaining exotics include Rhamnus frangula (glossy buckthorn), Viburnum opulus (European cranberrybush) and various escaped honeysuckles: Lonicera maackii, L. xbella, L. morrowi and L. tartarica. Arisaema triphyllum (jack-in-the-pulpit), Circaea lutetiana (enchanter's nightshade) and Geum canadense (white avens) were the three most important herbs in 2001 with values of 17.9, 17.5 and 15.6 respectively (Table 4). The most important, jack-in-the-pulpit, has recently become prominent. Enchanter's nightshade and white avens sequentially held the 2 highest I.V. in 1976 at 20.8 and 12.6 (Table 4). All these herbs reach their highest presence in southern dry-mesic forests (Curtis 1971). The shrubs under 0.5 m tall in the herb layer show a trend similar to that of the shrub stratum, where common buckthorn replaces gray dogwood. Presently, common buckthorn has an I.V. of 30.7, and gray dogwood is sparsely present in the stand, but does not appear in sample plots as an herb (Table 4). In 1976 as shrubs under 0.5 m tall, gray dogwood had the highest I.V. of 24.1, and common buckthorn had an I.V. of 3.7 (Table 4). The most important seedlings were ash spp., sugar maple, and black cherry with I.V. of 27.8, 23.6 and 18.8 respectively (Table 4). In 1976, ash spp. had the greatest I.V. of 16.1 with black cherry second at 8.7.
Page 98 ï~~TABLE 1. Sampling data for trees in College Woods, Wehr Nature Center, with comparison between 2001 and 1976. 2001 1976 Relative Relative Relative Importance Relative Relative Relative Importance Species Frequency Density Dominance Value Frequency Density Dominance Value Acer rubrum 8.0 5.1 6.9 20.0 3.0 2.5 2.7 8.2 Acer saccharum 16.0 23.1 17.7 56.8 7.0 5.0 9.9 21.9 Fraxinus spp. 2.50 35.9 32.7 93.6 7.7 5.1 3.1 15.9 Juglans nigra 10.0 6.4 9.5 25.9 9.0 6.3 5.9 21.2 Prunus serotina 8.0 5.1 4.0 17.1 10.0 10.0 2.2 22.2 Quercus rubra 12.0 11.5 23.7 47.2 17.0 17.5 45.4 79.9 Tilia americana 14.0 15.4 11.5 40.9 7.0 7.5 1.7 16.2 Ulmus americana 1.20 10.3 7.8 30.1 Ulmus rubra 4.0 2.6 1.3 7.9 9.0 7.5 1.2 17.7 TABLE 2. Sampling data for saplings in College Woods, Wehr Nature Center, with comparison between 2001 and 1976. 2001 1976 Relative Relative Relative Importance Relative Relative Relative Importance Species Frequency Density Dominance Value Frequency Density Dominance Value Acer saccharum 5.7 1.9 4.7 12.3 1.3 0.3 0.1 1.7 Crataegus spp 2.9 1.0 6.0 9.9 9.2 7.0 5.9 22.1 Fraxinus americana 22.9 32.7 32.9 88.4 23.7 43.1 32.3 99.1 Fraxinus pennsylvanica 8.6 8.7 2.0 19.2 5.3 2.3 1.4 9.0 Ostrya virginiana 2.9 1.0 2.7 6.5 7.9 5.7 15.7 29.3 Populus tremuloides 2.9 1.9 2.0 6.8 1.3 0.7 0.6 3.9 Prunus nigra 20.0 23.1 16.8 59.9 Prunus serotina 28.6 27.9 24.2 80.6 15.8 22.4 11.6 49.8 Quercus rubra 2.9 1.0 4.7 8.5 6.6 2.3 6.2 15.1 Tilia americana 2.9 1.0 4.0 7.8 7.9 6.0 8.4 22.3 00 n z 0 z H 0
Page 99 ï~~TABLE 3. Sampling data for shrubs in College Woods, Wehr Nature Center, with comparison between 2001 and 1976. 2001 1976 Relative Relative Relative Importance Relative Relative Relative Importance Species Frequency Density Dominance Value Frequency Density Dominance Value Cornus racemosa 3.3 1.1 1.0 5.4 14.8 56.0 32.2 103.0 Lonicera spp. 6.7 1.1 1.0 8.7 1.7 0.6 0.4 2.7 Rhamnus cathartica 53.4 86.9 88.0 228.3 9.6 3.5 31.6 44.7 Rhamnus frangula 6.7 1.1 1.0 8.7 0.9 0.2 0.3 1.4 Ribes missouriense 23.3 7.7 6.7 37.7 10.4 16.0 6.1 32.5 Viburnum opulus 3.3 1.6 1.4 6.4 Zanthoxylum americanum 3.3 0.5 1.0 4.8 6.1 0.7 1.3 8.1 n z 0 z -H
Page 100 ï~~100 THE MICHIGAN BOTANIST Vol. 42 TABLE 4. Sampling data for herb layer (i.e., all plants < 0.5 m tall) in College Woods, Wehr Nature Center, with comparison between 2001 and 1976. 2001 1976 Relative Relative Importance Relative Relative Importance Herb Species Frequency Cover Value Frequency Cover Value Agrimonia gryposepala Alliaria petiolata Arisaema dracontium Arisaema triphyllum Carex blanda Carex hirtifolia Circaea lutetiana Fragaria virginiana Geranium maculatum Geum canadense Glyceria sp. Oxalis stricta Sanicula gregaria Smilacina racemosa Smilax herbacea Solidago flexicaulis Polygonum virginianum Shrub species Cornus racemosa Lonicera spp. Parthenocissus sp. Rhamnus cathartica Rhus toxicodendron Ribes missouriense Rubus occidentalis Viburnum opulus Viburnum rafinesquianum Tree Species Acer saccharum Fraxinus spp. Ostrya virginiana Prunus nigra Prunus serotina Quercus rubra Ulmus americana Ulmus rubra 1.6 10.9 4.7 15.6 1.6 1.6 15.6 1.6 3.1 14.1 1.6 3.1 4.7 10.9 4.7 1.6 1.6 3.4 3.4 24.1 27.6 3.4 10.3 3.4 17.2 6.9 22.7 27.3 4.5 13.6 18.2 4.5 4.5 4.5 0.1 0.5 0.2 2.3 0.1 2.0 1.9 0.1 0.2 1.5 0.1 0.2 0.5 1.1 0.2 0.1 0.2 0.1 0.3 2.2 3.1 0.2 0.2 0.1 0.4 0.1 0.8 0.5 0.1 0.9 0.7 0.1 0.1 0.1 1.7 11.4 4.9 17.9 1.7 3.6 17.5 1.6 3.3 15.6 1.6 3.3 5.1 12.1 4.9 1.6 1.7 3.5 3.7 26.3 30.7 3.7 10.5 3.5 17.7 7.0 23.6 27.8 4.6 14.5 18.8 4.7 4.6 4.6 1.4 1.4 7.5 4.1 6.8 0.7 0.7 1.4 8.2 0.7 8.2 2.7 1.4 0.7 2.1 6.3 7.7 6.3 7.7 13.3 20.8 3.7 7.8 5.8 0.7 0.2 2.3 15.9 0.7 9.6 1.0 2.3 0.2 2.0 12.6 1.4 0.9 3.7 24.1 1.4 17.8 3.7 3.7 0.9 4.1 0.7 0.2 0.9 9.6 6.5 16.1 4.1 0.7 2.1 4.6 0.2 2.2 8.7 0.9 4.3 DISCUSSION The transition of prominent overstory trees in College Woods from oak-dominated to maple-dominated is similar to the changes documented in numerous other studies (Peet & Loucks 1977; Abrams 1992; Curtis & McIntosh 1951). Given the similarity of College Woods to stands in these studies, replacement of oaks by maples is typical. Abrams (1992) attributes the decline of oak in these
Page 101 ï~~2003 THE MICHIGAN BOTANIST 101 communities to disease and cessation of fire, which is what appears to have happened in College Woods as well. The low I.V. of sugar maple saplings in 1976 compared to the high IV of present day sugar maple trees is possibly a result of favorable growing conditions. Under such conditions, sugar maple could germinate, grow, and reach the sizes observed in College Woods within 25 years. However, no data were collected between 1976 and the present to support this. Since no permanent plots exist in College Woods it is also possible that the boundaries of my 2001 study could have extended into an adjacent area containing a higher density of sugar maple which was not part of the 1976 study area. Nowak (1976) cites two studies that demonstrated white ash being more important in urban settings with higher human use than sugar maple (Forman & Elfstrom 1975; Levenson & Matthiae 1976, cited in Nowak 1976). Nowak suggests the abundance of ash spp. in College Woods is the result of the stand's proximity to the Village of Hales Corners and surrounding suburban area. She correctly predicted an increase in ash spp. I propose the increase in ash is a function of both the proximity to an urban area and the seasonal ponding that inundates College Woods. The slow growth of black cherry, resulting in a small size class in College Woods, is a characteristic of southern Wisconsin black cherry populations, as they tend to remain suppressed for 40-60 years (Auclair & Cottam 1971) until canopy gap opportunities allow for accelerated growth. The rise of jack-in-the-pulpit is, as in the case of the maples, presumably due to favorable growing conditions. However, the abundance could be a result of deer browsing on the surrounding species. Enchanter's nightshade and white avens have remained important for 25 years and are prevalent species in southern dry-mesic forest communities (Curtis 1971). They are likely to remain important unless out-competed by invasive exotic species. As seen in the data, exotic invasive species are abundant in College Woods. Deer avoid browsing common buckthorn; and, with the exclusion of fire (Laastch & Anderson 2000), common buckthorn out-competes the native shrub species. A visual inspection of College Woods shows the stand dominated by invasive species. Areas where Alliaria petiolata (garlic mustard) exists have become virtual monocultures of this species. I predict that patches of garlic mustard will follow the same trend as common buckthorn and continue to expand throughout the entire herb stratum, eliminating native species. McCune and Cottam (1985) credit successional outcome to stochastic events. They concluded that no model could predict the changes that occurred in their study area. This holds true in College Woods; however, in a general sense, College Woods follows a rather predictable pattern of overstory replacement. The decline of red oak created canopy gaps permitting opportunist species like black cherry to establish and quickly reach a sapling stage. The cessation of fire has allowed fire-intolerant, shade-tolerant species like sugar maple and ash to encroach and shade out the less shade-tolerant species. The suppression of the shade-intolerant species reinforces the shade-tolerant species' ability to outcompete the intolerant species in a continuing cycle. Within the mix, invasive exotic species are out-competing native species and dominating the shrub and
Page 102 ï~~102 THE MICHIGAN BOTANIST Vol. 42 herb layers. I predict, based on the trends of the past 25 years, that College Woods (if maintained under static management) will change from an oak-dominated overstory community to a shade-tolerant maple-dominated overstory. The understory, with its current infestation of invasive species, will likely continue to be saturated with exotic shrubs and herbs, perhaps leading to a monoculture condition seen in the localized patches of invasive exotics at College Woods. ACKNOWLEDGMENTS The assistance and contributions of the following individuals and organizations is greatly appreciated and gratefully acknowledged: Daniel Spuhler and Mark Verhagen, for their teachings, plant identification skills, humor and tolerance; Kevin Schiebenes, for his attention to detail; Eric Petersen and Christopher Martin, for their assistance in the field; Ellen Thomson for assistance with calculations; Cheryl Vidergar, Lee George and Emmet Judziewicz for proofreading and critique; The Friends of Wehr for the funding of this project; and The Wehr Nature Center staff for enduring my employment another season. LITERATURE CITED Abrams, M. D. 1992. Fire and the development of oak forests. BioScience 42: 346-353. Auclair A. N. & G. Cottam. 1971. Dynamics of black cherry (Prunus serotina) in southern Wisconsin oak forests. Ecological Monographs 41: 153-176. Brower, J. E. & J. H. Zar. 1984. Field and laboratory methods for general ecology: second edition. Dubuque: Wm. C. Brown. 226 pp. Curtis, J. T. 1971. Vegetation of Wisconsin. Madison: University of Wisconsin Press. 657 pp. Curtis, J. T. & R. P. McIntosh. 1951. An upland forest continuum in the prairie-forest border region of Wisconsin. Ecology 32: 476-496 Forman, R. T. T. & B. A. Elfstrom. 1975. Forest structure comparison of Hutcheson Memorial Forest and eight old woods on the New Jersey Piedmont. The William L. Hutcheson Memorial Bulletin 3: 44-51. Kershaw, K. A. 1964. Quantitative and dynamic ecology. New York: American Elsevier. Laatsch, J. R. & R. C. Anderson. 2000. An evaluation of oak woodland management in northeastern Illinois, USA. Natural Areas Journal 20: 211-220. Levenson, J. & P. Matthiae. 1976. Island biogeography in southeastern Wisconsin-a progress report. The University of Wisconsin-Milwaukee Field Stations Bulletin 8: 6-12. McCune, B. & G. Cottam. 1985. The successional status of a southern Wisconsin oak woods. Ecology 66: 1270-1278 [NOAA] National Oceanic and Atmospheric Administration. 2 August 2001. Milwaukee climate page. <http://www.crh.noaa.gov/mkx/climate.htm>. Accessed 9 August 2001. Nowak, M. M. 1976. The effect of human disturbance on vegetation at the Wehr Nature Center in Whitnall Park [thesis]. Milwaukee: University of Wisconsin-Milwaukee. 71 pp. Park Land Acquisition Book. [undated] Milwaukee County Park Commission. Milwaukee, Wisconsin. Peet, R. K. & O. L. Loucks. 1977. A gradient analysis of southern Wisconsin forests. Ecology 58: 485-499. [SEWRPC] Southeastern Wisconsin Regional Planning Commission. 1997. A regional natural areas and critical species habitat protection and management plan for southeastern Wisconsin. Waukesha. Planning Report No. 42. 531 pp. Sheperd, V. & E. Weiler. 1988. Hales Corners Wisconsin: A history in celebration of 150 years. Hales Corners (WI): Hales Corners Historical Society. 210 pp.