As the original native forest has been dissected by roads and replaced by farms or towns, the amount of forest edge relative to the area of forest interior has greatly increased. Today forest edge communities are widespread in many man-modified landscapes. However, the role the edge community plays in the regional landscape, and the extent of and variation in the edge community are relatively undefined. The purpose of this study is to determine the nature of forest edge communities. The specific objectives of the study are to: (1) quantitatively survey and assess the forest edge community; (2) determine how forest edge vegetation differs from that of the forest interior; (3) determine if the directional aspect affects the edge vegetation and (4) present guidelines and suggestions for the management of forest habitat islands, particularly as they relate to the edge community

Bruner, Marc C

Levenson, James B

English

University of Wisconsin-Milwaukee

University of Wisconsin MilwaukeeUWM Digital CommonsField Station Bulletins UWM Field StationSpring 1977Vegetation forest island edges: A preliminary reportMarc C. BrunerUniversity of Wisconsin-MilwaukeeJames B. LevensonUniversity of Wisconsin-MilwaukeeFollow this and additional works at: https://dc.uwm.edu/fieldstation_bulletinsPart of the Forest Biology Commons, and the Zoology CommonsThis Article is brought to you for free and open access by UWM Digital Commons. It has been accepted for inclusion in Field Station Bulletins by anauthorized administrator of UWM Digital Commons. For more information, please contact open-access@uwm.edu.Recommended CitationBruner, M.C. and J.B. Levenson. 1977. Vegetation forest island edges: A preliminary report. Field Station Bulletin 10(1): 12-19.12VEGETATION FOREST ISLAND EDGES:A PRELIMINARY REPORTINTRODUCTIONAs the original native forest has been dissected by roads and replaced byfarms or towns, the amount of forest edge relative to the area of forest interiorhas greatly increased. Today forest edge communities are widespread in manyman-modified landscapes. However, the role the edge community plays in theregional landscape, and the extent of and variation in the edge community arerelatively undefined.In many regions, forested landscapes have been reduced to woodlots,often widely dispersed, fencerows, and urban forests (Pollard 1973). Oneexample of this declining forest base was described for southcen tral Wisconsinby Curtis (1956). Only 0.8 per cent of the original forest cover of Cadiz Town-ship (Green County) remained in a seminatural state in 1950. Forest remnantsrepresent the seed source on which the native forest depends for continuedexistence and for seeding into abandoned and naturally regenerating sites.The importance of edge to wildlife has long been accepted. Leopold(1933) outlined the concept of "edge effect" as it relates to wildlife, anddeveloped the principles of the "law of interspersion" of habitat types as relatedto wildlife abundance and distribution. Th is concept evolved in to a quan titativemeasure of edge effect using the ratio of edge length to in terior area. This ratiois still widely used, although somewhat modified (Patten 1975).Most phytosociological studies are not designed to examine the edgecommunity. In fact, most explicitly exclude the edge. For example, Borman andBuell (1964) studied an old-growth hemlock-hardwood forest in Vermont, butlocated all sample points at least 20m from the forest boundary. In samplingWisconsin plant communities, Curtis (1959) specifically excluded a boundaryzone 30m in width at the periphery of stands being studied. This effort to avoidthe vegetation at the forest margin implies that the edge community is differentfrom the forest interior.Relatively few studies have dealt with the vegetation of the forest edge.Barick (1945, 1950) examined the vegetation of ten different types of forestedges with particular concern for their comparative habitat values. He interpretedforest edge as the ecotone between two forest types. A different interpretationof edge, a forest-field transition, was used by Gysel (1951) in his study of theencroachment of woodlot borders in to surrounding open fields in sou thernMichigan. Wales (1972) studied the north and south edges of the Hutcheson13Memorial Forest in New Jersey. Statistical analyses led him to conclude that theforest edge was 5-10m wide on the north exposure and la-20m wide on thesouth.Recen t studies have applied the concepts of island biogeograph y to habitatislands in terrestrial ecosystems (Diamond 1975, Forman et aI., 1976). Othershave investigated the vegetation of forest islands in agricultural landscapes(Levenson and Matthiae 1975, Elfstrom 1976, Suhrweir and Tramer 1976). Eachstudy examined the effects of island size on species richness using mathematicalmodels and assumptions. These studies have stimulated reevaluation of the im-portance and role of forest edge communities.As forest islands get smaller, younger or more disturbed, functionally theentire island becomes edge (Schmid 1975, Levenson 1976). At some point, theforest island becomes su fficien t1y small that the edges from all sides merge at theisland center and the forest edge and forest interior communities are the same(Levenson 1976).Edge communities are not restricted to the forest border. Gaps within theforest function in much the same manner as edge. A high frequency of gaps canmake even a large island no more than a highly convoluted edge. In the metro-politan Milwaukee region most upland forest islands are of relatively small size.When a large tree dies or is windthrown, the island's structure and compositionmay change rapidly. This condition can result from selective cutting or naturalevents. When holes are created in the canopy, the mesophytic environment with-in the stand is degraded.If the concept of edge is expanded to include the canopy, isolated forestislands are completely surrounded by a protective edge. When examining theinteractions between the isolated forest islands, the environment and theregional ecosystem, an analogy with living cell seems appropriate. The edgefunctions much like a semipermeable membrane (Levenson 1976). The edge andcanopy moderate the extremes of temperature, humidity, and air circulationwhile filtering ligh t levels in much the same manner as a cell membrane metersincoming compounds. Geiger (1966) reported daily maximum temperatures'justabove the ground surface were much higher along the forest edge than in theforest interior. Conversely, by reducing outgoing radiation at night, the forestedge and canopy form an effective screen against extreme minimum tempera-tures (Ch risty 1952).The drying effect of wind influences forest edges more than interiors(Curtis 1959, Geiger 1966, Lowry 1967). The extent of the drying is dependenton the edge. Small or heavily disturbed forest stands may be su fficien t1ypermeable to wind so that xeric conditions extend throughout the stand. Theseconditions may be evidenced by changes in species composition (Auclair andCottam, 1971).14The amount of light which penetrates to the forest interior helps to deter-mine stand characteristics; i.e., species composition, stem density, etc. Lightpenetration decreases with increasing stem and leaf density and the number ofstructural strata (Geiger ]966, Wales ]967).The purpose of this study is to determine the nature of forest edge com-munities. The specific objectives of the study are to: (1) quantitatively surveyand assess the forest edge community; (2) determine how forest edge vegetationdiffers from that of the forest interior; (3) determine if the directional aspectaffects the edge vegetation and (4) present guidelines and suggestions for themanagement of forest habitat islands, particularly as they relate to the edgecommunity.STUDY AREAMetropolitan Milwaukee, an area of about 525 km 2, includes MilwaukeeCounty, southern Ozaukee County, and the eastern portions of Waukesha andWashington Counties. Located in southeastern Wisconsin (Fig. 1), the metropoli-tan region lies in the Great Lakes section of the Cen tral Lowlands Province(Fenneman ] 938). The entire area is east of the Saint Lawrence-Mississippi sub-continental divide.Ozauk•• County1N•W••hlngton CountyW.uke.ha County•PortW••hlnlton••t Cra ton, ceduLrl•..Fig. 1 The location of the study sitesin the metropolitan Milwaukeeregion of southeastern Wisconsin .15Southern mesic and dry-mesic forest is the dominant native vegetation onthe uplands of metropolitan Milwaukee now and was prior to settlement. Thegeneral characteristics of these Wisconsin forest communities have been describedby Curtis (1959). Whitford and Salamun (1954) and Levenson (1976) have madedetailed analyses of southern forest in the Milwaukee area. The nature of thisplant community, its role in the regional landscape, and the availabili ty of highquality information suggested the type as ideal for an edge study.Primary study sites were limited to those stands previously sampled byLevenson (1976). The criteria used in selecting stands for that study requiredthat the stands be of the southern mesic forest type, isolated from neighboringstands, and original remnan ts of the upland vegetation. Only islands that wererelatively undisturbed, containing all structural strata of the southern mesicforest, and (of particular significance for this study) that had a "mature forestedge" at their periphery (Levenson and Matthiae 1975) were included.METHODSFrom within the group of forest islands that met the above criteria, standsdominated by sugar maple or beech were selected. Selection was based on theresults of the cluster analysis and ordination performed by Levenson (1976).Potential sites were surveyed on foot to determine suitability.Specific forest edges were selected using the following six criteria: 1) theedge should face in a cardinal direction; 2) the vegetation bordering the edgeshould be dominated by herbaceous or agriculatural plants; 3) transition fromthe edge to the border should be abrupt; 4) the edge should be well establishedand free from severe human or natural disturbance; 5) the edge should be linearfor a minimum of 100m; and 6) topography should be flat to gently rolling.In each edge selected, the woody vegetation was sampled using a modifica-tion of the line-strip method developed by Lindsey (1955). Canopy, sapling orunderstory, shrub, and groundlayer strata were sampled with a series of nestedplots. Samples were centered on a 30m transect from the extreme edge to theinterior. For trees and saplings, the presence or absence of "edge-growth-form"was noted. The definition of edge tree used follows that of other authors(Wales 1972, Levenson 1976) wh ich sta tes th at edge trees ex hibit an asy mmetri-cal growth form, with many lateral branches on the exterior (open grown) side ofthe bole (Figure 2).RESULTS AND DISCUSSIONObservation suggests that the way in which the edge is managed or main-tained is a critical factor in determining the, nature and extent of the edge com-munity. Wales (1972) defined a "stabilized edge" as one which is well established.and maintained in a permanent position by continued usage of adjacent land.16Fig. 2 An edge tree with the characteristic asymetric growth form./7Very different is the "unstabilized edge" which advances into the surroundinglandscape as a result of abandonment or discontinued maintenance of theadjacent land (Wales 1972). We have observed another form where a new edgehas recently been created. In this case, the edge extends into or encroaches uponthe forest interior. The unstabilized encroaching edge is the type that can mostsignificantly affect the species composition and microclimate of the forestinterior. It appears that the way the edge is maintained is one of the most sig-nificant factors affecting the biota of the entire forest island.Time is another critical factor affecting the composition and structure offorest edges. Settlement of metropolitan Milwau kee began abou t 150 years ago.This places an upper limit on the age of the edges we sampled. Loucks (1970)has suggested that successional patterns may take from 100 to 300 years tofuIly develop. Edges used in this study are still in the process of developing. Inaddition, each structural stratum of the forest responds to disturbance on differ-ent time and space scales. Groundlayer and shrub strata respond to disturbancemuch more quickly than other strata, and are sensitive to weaker or more local-ized types of disturbance, such as windfall openings (Gysel 1951, Bray 1956).In contrast, the tree stratum responds to disturbances of a much different timescale, in periods approaching the length of the life cycle of the arboreal species.The effects of non-catastrophic disturbance on the tree layer may take severaldecades or longer to be manifested while we are trapped at a single point insuccessional time. The observations and conclusions we make abou t the edgesystem and the response of forest communities to isolation and size reductionmust take into account this time factor.Keeping in mind the constraints imposed by the factors of both edgemaintenance and time, our preliminary results and observations follow: (1)species richness in all strata decreases from the edge to the interior; (2) stemdensities of all strata decrease from edge to the interior; (3) species richness anddensity of the sapling layer (2.5-10 cm dbh) are the most efficient indicators ofedge conditions; (4) no precise boundary can be drawn to separate edge frominterior. However, a discontinuity occurs at approximately 10-15 meters fromthe physical edge that appears to separate edge from interior; (5) time and edgemaintenance appear to have more effect on the general structure of the edgethan directional aspect; (6) Basswood (Tilia americana) and ash (Fraxinus spp.)playa more significant role in the edge than in the interior. Beech (Fagusgrandifolia) is less importan t in the edge system than in the typical mesic forestinterior.ACKNOWLEDGEMENTSThe authors express appreciation to Jack W. Ranney for aid in data coIlec-tion and for stimulating discussions.Thanks are also ex tended to Marilyn Schaller for her time and effort in18preparing the photograph. A special thanks to Dr. Forest Stearns for his criticalreview of the manuscript.This research was supported through the Landscape Patterns Project ofthe Environmental Sciences Division of the Oak Ridge National Laboratoryunder Subcontract No. 3659, Supplemental Agreement No.7.LITERATURE CITEDAuclair, A. N. and G. Cottam. 1971. Dynamics of black cherry (Prunus serotinaEhrh.) in southern Wisconsin oak forests. Ecol. Monogr. 41:153-77.Barick, F. B. 1945. Environmental analysis of forest edge in relation to wildlife.Trans. Tenth North Amer. Wildl. Conf1126-36.__ . 1950. The edge effect of the lesser vegetation of certain Adirondackforest types with particular reference to deer and grouse. Roosevelt wildl.Bull. 9(1):1-146.Borman, F. H. and M. F. Buell. 1964. Old-age stand of hemlock-northern forestin central Vermont. Bull. Torrey Botan. Club 91:451-65.Bray, J. R. 1956. Gap-phase replacement in a maple-basswood forest. Ecology37:598-600.Christy, H. R. 1952. Vertical temperature gradients in a beech forest in centralOhio. OhioJ. of Sci. 52:199-209.Curtis, J. T. 1956. The modification of mid-latitude grasslands and forests byman. In W. L. Thomas (ed.) Man's role in changing the face of the earth.University of Chicago Press, Chicago. P721-36.__.1959. The vegetation of Wisconsin - an ordination of plant communities.University of Wisconsin Press, Madison. 657p.Diamond, J. D. 1975. The island dilemma: Lessons of modern biogeographicstudies for the design of natural reserves. BioI. Conserv. 7: 129-46.Elfstrom, B. A. 1976. Tree species diversity and forest island size on thePiedmont of New Jersey. M. S. Thesis, Rutgers University, New Bruns-wick, N.J. 73p.Fenneman, N. M. 1938. Physiography of eastern United States. McGraw-HillBook Co., Book Co., New York. 714p.Forman, R. T. T., A. E. Galli and C. F. Leck. 1976. Forest size and aviandiversity in New Jersey woodlots with some land use implications.Oecologia 26: 1-8.Geiger, R. 1966. The climate· near the ground. Harvard University Press,Cambridge, Mass. 61lp.Gysel, L. W. 1951. Borders and openings of beech-maple woodlands in southernMichigan. J. For. 49: 13-19.19Leopold, A. 1933. Game management. Charles Scribner's Sons, New York. 481p.Levenson, J. B. and P. Matthiae. 1975. Island biogeography in sou theasternWisconsin; A progress report. University of Wisconsin-Milwaukee Fld. Sta.Bull. 8(1):6-12.Levenson,J. B. 1976. Forested woodlots as biogeographic islands in an urban-agricultural matrix. Ph.D. Dissertation, University of Wisconsin-Milwaukee.101p.Lindsey, A. A. 1955. Testing the line-strip method against full tallies in diverseforest types. Ecology 36:485-95.Lowry, W. P. 1967. Weather and life; an introduction to biometeorology.Academic Press, New York. 305p.Patten, D. R. 1975. A diversity index for quantifying habitat "edge". WildlifeSoc. Bull. 3(4): 171-73.Pollard, E. 1973. Hedges, VII; woodland relic hedges in Huntington and Pter-borough. J. Ecol. 61 :343-52.Schmid, J. A. 1975. Urban vegetation: A review and Chicago case study. TheUniversity of Chicago Dept. Geog. Res. Pap. No. 161. Chicago, 266p.Suhrweir, D. E. and E.J. Tramer. 1976. Tree species richness: farm woodlots asbiogeographic islands (in press).Wales, B. A. 1967. Climate, microclimate and vegetation relationships on northand south forest boundaries in New Jersey. William L. Hutcheson Mem.For. Bull. 2(3):1-60.__ .1972. Vegetation analysis of north and south edges in a mature oak-hickory forest. Ecol. Monogr. 42 :451-71.Marc C. Bruner and James B. LevensonBotany DepartmentThe University of Wisconsin-MilwaukeeIf you are not already receiving the Bulletin and wish to receive it,contact: Dr. Millicent Ficken, UWM Field Station, 3095 Blue Goose Road,Saukville, Wisconsin 53080.Field Station PersonnelDirectorResident ManagerField Stations CommitteeDouglas CherkauerMillicent Ficken (Chairman)Paul MatthiaeDavid MillerCarroll NordenMillicent FickenPaul MatthiaePeter SalamunForest StearnsCharles WeisePhilip Whitford

Vegetation forest island edges: A preliminary report

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Vegetation forest island edges: A preliminary report

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