(Curtis 1956, Burgess and Sharpe 1981, Zipperer et al. 1990). An understanding of how different deforestation patterns influence habitat quality of forest patches ...
Landscape Ecology vol. 8 no. 3 pp 177-184 (1993) SPB Academic Publishing bv, The Hague
Deforestation patterns and their effects on forest patches Wayne C. Zipperer USDA Forest Service NEFES, c/o SUNY-CESF, Syracuse, N Y 13210, USA Keywords: deforestation, fragmentation, forest edge, forest interior
Abstract Five identifiable patterns of deforestation are recognized - internal, indentation, cropping, fragmentation, and removal - and each has a distinct effect on habitat quality of forest patches in the eastern United States. By overlaying land use maps from 1973 and 1981 for three counties in the State of Maryland (Prince Georges, Anne Arundel, and Wicomico), changes in the interior core area and edge length of individual patches were measured. Forest interior declined by 23.8 km2 in Anne Arundel, 16.3 km2 in Prince Georges, and 8.4 km2 in Wicomico. Within Anne Arundel and Prince Georges Counties, deforestation increased edge length by 52.1 km and 31.2 km, respectively, whereas, within Wicomico, it decreased edge length by 8.7 km. Differences among counties resulted from current land use patterns, percentage of forest cover, and the dominant deforestation pattern.
Introduction Habitat fragmentation is considered by many to be the most serious threat to biological diversity, and is the primary cause of the present extinction crisis (Wilcox and Murphy 1985, Lovejoy et al. 1986, Wilcove et al. 1986). Yet, when considering the entire dimension of deforestation, fragmentation is only one pattern of the process. Deforestation, the process of clearing forest cover so that the land can be used for a different purpose (SAF 1983), is an ongoing disturbance within human-dominated landscapes. The extent of human activities on the landscape determines the type, intensity, and frequency of deforestation (Curtis 1956, Burgess and Sharpe 1981, Zipperer et al. 1990). An understanding of how different deforestation patterns influence habitat quality of forest patches is essential to managing this resource
within a region or a landscape. In this paper, I identify five deforestation patterns, describe their effects on the forest interior core and edge length, and measure their cumulative effects within three counties in the State of Maryland, U.S.A.
Forest fragments The term ‘forest fragment’ refers to an isolated patch of forest cover created through patchy or disconnected deforestation of a contiguous forest. As the external matrix is physiognomically (and ecologically) different from the forest patch, an induced edge is formed (Yahner 1988). This edge, however, denotes more than just the transition between the external matrix and the forest; it represents an outer band that is environmentally and
180 Table 1. Net changes in forest cover between 1973 and 1981 for Anne Arundel, Prince Georges, and Wicomico Counties, Maryland, U.S.A.
Forest cover changes between 1973 and 1981
County Anne Arundel Prince Georges Wicomico
Forest cover in 1973 (km2)
Area deforested (km2)
Area reforested (km2)
583.1 679.0 479.8
19.9 19.5 9.9
0.6 4.7 0.3
U.S.A. - Anne Arundel, Prince Georges, and Wicomico (Fig. 2). In 1973, the counties had nearly identical percentages of forest cover. Anne Arundel County, 1091.9 km2 in total area, was 53.4% forested; Prince Georges County, the largest of the three (1269.1 km2), was 53.5% forested; and Wicomico County, 906.9 km2 in total area, was 52.9% forested. These counties were chosen because of contrasting changes in land use trends. Anne Arundel and Prince Georges Counties are located between Baltimore, Maryland, and Washington, Distinct of Columbia, a corridor that is rapidly becoming urbanized. In contrast, Wicomico County does not neighbor a major metropolitan area and is being developed more slowly. County census data provided the rates of human population change. Between 1960 and 1970, the approximate period preceding the land use change analysis, Anne Arundel’s human population grew 44.0% (from 207,000 to 298,000), Prince Georges’s population grew 85.4% (from 357,000 to 622,000), and Wicomico’s population increased only 10.2% (from 49,000 to 54,000) (U.S. Department of Commerce 1982). Between 1970 and 1980, the approximate period during the land use change analysis, Anne Arundel’s population grew 24.5% (from 298,000 to 371,OOO), Prince Georges’s population increased 0.5% (from 662,000 to 665,000), whereas Wicomico’s population grew 20.4% (from 54,000 to 65,000) (US. Department of Commerce 1982). The land use change analysis was conducted by overlaying land use maps for 1981 onto those from 1973. These maps, scaled to 1:63360, were created by the Office of State Planning, Maryland, and in-
Net change (VO)
-3.31 -2.18 -2.00
cluded five categories of land use: urban, agriculture, forest, wetlands and bare ground. Resolution limit was 2.5 hectares (ha). The analysis identified forest patches at both time periods and revealed deforested and reforested areas. By comparing forest patches from each study period, I identified the type of deforestation pattern and measured the resulting changes in the forest edge length and interior core area. To account for deforestation, I included the addition of forest cover into the total values for forest cover, forest edge length, and interior core. For this paper, deforestation effect refers to the loss of forest land, and reforestation effect refers to the addition of forest cover. An analysis of variance (ANOVA) and Tukey’s standardized range test (Sokal and Rohlf 1969) were used to reveal differences among counties for changes in forest edge length and interior core area.
Results
Between 1973 and 1981, each county lost forest cover to urbanization and agriculture and gained forest cover from reforestation of idle agricultural lands (Table 1). Forest losses in Anne Arundel (1994.1 ha) and Prince Georges (1945.6 ha) were primarily to urbanization, whereas in Wicomico County, forest loss (995.1 ha) was mainly to agriculture. Forest gains from idle agriculture lands were 63.0 ha for Anne Arundel, 467.4 ha for Prince Georges, and only 34.2 ha for Wicomico. Although each county lost forest cover, they differed with respect to net changes in forest edge
181 Table 2 . Mean size of deforested and reforested areas between 1973 and 1981. Reported are mean size, standard error, and number of forest patches measured (n)
County
Deforested area (ha)
Reforested area (ha)
Anne Arundel Prince Georges Wicomico
21.6 f 2.9 (90) 21.9 f 1.9 (89) 15.3* f 1.8 (65)
12.6* f 4.8 (5) 24.6 f 3.3 (19) 34.2 (1)
* Significantly different from the other counties at P
5
0.05.
length and interior core area (Table 2). Overall, both Anne Arundel and Prince Georges Counties showed losses in interior core but gains in edge length. In contrast, Wicomico County showed losses in both the forest edge length and interior core area. The differences between Anne Arundel and Prince Georges in loss of forest cover, interior core, and edge length were related to the number of reforested sites and the deforestation patterns (Table 3 and Fig. 3). In Anne Arundel County, only five agricultural sites reverted to forest cover. The reforestation effect added 63 ha of forest cover, 82 ha of interior core, and 1.5 km of edge length. In contrast, 19 agricultural sites reverted to forest cover in Prince Georges County. This reforestation effect added 467.4 ha of forest cover, and 482.6 ha of interior core, and reduced forest edge length by 15.0 km. Because there were fewer reforested sites, deforestation had a greater influence on forest cover in Anne Arundel County than in Prince Georges County. Anne Arundel and Prince Georges Counties differed with respect to deforestation patterns
(D
-
50 40
AnneArundel
0
c
.-E
-8 a
Prince,Georges
N
20
5
10
P
-
Interior
Indent
Crop
Fragment
Removal
-
Deforestation patterns
Fig. 3. Percentage of forest patches with observed deforestation patterns within Anne Arundel, Prince Georges, and Wicomico Counties, Maryland.
(Fig. 3). The counties lost approximately the same amount of forest land (Table 4); however, they differed in the amount of interior core lost and forest edge gained. On average, cleared areas within Anne Arundel County lost 3.6 ha of additional interior core and gained 43.8 m of edge length per clearing than did Prince Georges. Although these differences are slight, the cumulative effects were 324 ha of lost forest interior and 3.9 km of new edge. Besides having fewer reforested areas, forest interior loss in Anne Arundel also resulted from higher frequencies of internal clearing, fragmentation, and removal than those observed in Prince Georges County (Fig. 3). Wicomico County was unique in that it had a similar percentage of total forest cover in 1973 as the other counties, but lost edge over the eight year period. A comparison with the other counties showed that the mean cleared area (15.3 ha) and lost interior (13.3 ha) were smaller in Wicomico County (Table 3). These values were attributed to
Table 3. Net changes in forest interior core area caused by deforestation and reforestation between 1973 and 1981 in Anne Arundel, Prince Georges, and Wicomico Counties, Maryland, U.S.A. Reported are mean areas, standard error, and number of measured forest patches (n). Changes in forest interior core area between 1973 and 1981
County
Deforestation effect (ha)
Reforestation effect (ha)
Net change (ha)
Anne Arundel Prince Georges Wicomico
-27.3 f 3.0 (90) -23.7 f 2.1 (89) -13.3* k 2.3 (65)
16.4* f 4.0 (5) 25.4 f 3.6 (19) 20.4 (1)
- 2375 .O
* Significantly different from the other counties at P
5 0.05.
- 1626.7 -
844.1
182 Table 4. Net changes in forest edge length caused by deforestation and reforestation between 1973 and 1981 in Anne Arundel, Prince Georges, and Wicomico Counties, Maryland, U S A . Reported are mean length, standard error, and number of measured forest patches (n).
Changes in forest edge length between 1973 and 1981
County
Deforestation effect (m)
Reforestation effect (m)
Anne Arundel Prince Georges Wicomico
562.8 & 131.9 (90) 519.0 f 129.1 (89) --117.8' f 148.4 (65)
298.8* -789.8 1,063.6
* Significantly different from the other counties at P
5
Net change (m)
* 357.1 (5) * 286.8 (19) (1)
52,101.I 31,184.8 - 8,720.6
0.01.
higher frequencies of cropping forest extensions and removal of patches (Fig. 3). Collectively, these patterns removed more edge than was created with the other patterns. In contrast, Anne Arundel and Prince Georges Counties had higher frequencies of fragmentation and interior clearing than Wicomico County, thereby increasing forest edge length. Discussion The landscape patterns described for Anne Arundel, Prince Georges, and Wicomico Counties, Maryland, are similar to those predicted by Godron and Forman (1983) and observed in Georgia (Turner and Ruscher 1988) and New York (Zipperer et al. 1990). In Wicomico County, forest conversion occurred primarily in intensively managed agricultural landscapes resulting in the further reduction of existing forest patches as evident by the lost of forest edge. Biotic effects from further deforestation and interior losses may include higher rates of nest predation and brood parasitism, a more xeric microclimate, and a higher density of shadeintolerant species. In Anne Arundel and Prince Georges Counties deforestation occurred across a continuum of landscapes ranging from intensively managed urban and agricultural lands to forest lands with the majority of the deforestation occurring within the forest landscapes. Consequently, both counties experienced high losses of interior habitat and increases in forest edge. Like Wicomico County, similar biotic effects are expected for the intensively
managed urban and agricultural landscapes, but in the forest landscapes, these effects may be different. Deforestation in a forest matrix does not create small, isolated forest patch but rather openings within a contiguous forest cover, a spatial pattern that is uniquely different from those observed in urban and agricultural landscapes. Likewise, these opening are different from those created by harvesting practices and gap phase dynamics in that openings created by development are more permanent. Hence, across a temporal scale, spatial patterns associated with development in a forest landscape also differ from those observed within natural or managed forest landscapes (Godron and Forman 1983, Franklin and Forman 1987). Fragmentation studies have focused principally on the biotic effects of small, isolated forest patches within intensively managed landscapes (Robbins 1979, Whitcomb et al. 1981, Forman et al. 1976, Wilcove et al. 1986). Because spatial patterns created by development within a forest landscape are different from those in agricultural landscapes, the biotic effects associated with intensively managed agricultural landscapes may be different. For example, nest parasitism by the brown-headed cowbird may be less severe in developed forest landscapes. These birds are associated with agricultural landscapes and may find the spatial matrix created by development in forests unsuitable habitat. Likewise, with an increase in edge caused by internal clearings and the addition of domestic pets from the developments, predation rate may actually be higher in developed forest landscapes than those
183 observed for agricultural landscapes (Calhoon and Haspel 1989). The effects of deforestation patterns within forest landscapes may be mitigated through proper landscape planning. Using existing species, community, and landscape inventorying techniques for evaluating and ranking the biological and landscape importance of forest patches (Noss 1987a, Duever and Noss 1990), landscape architects and planners can identify less critical forested sites and recommend placement of development within these areas. In doing so, the exposure of remaining interior habitat to humans and their activities, to nonnative species, and to the harassment of wildlife species by domestic pets is reduced (Janzen 1983, Noss 1987b).
Conclusion Previous studies of forest cover and edge length focused on the entire forest cover (see Turner and Ruscher 1988) and neglected the effects of deforestation on individual forest patches and on interior habitat. In this study, I identified different deforestation patterns and their effects on forest edge length and interior. The analysis showed that forest edge and interior dynamics were related not only to the amount of forest cover within a region (see Franklin and Forman 1987), but also to the type of deforestation patterns. These different deforestation patterns must be considered when modeling landscape dynamics to understand fully deforestation effects on landscape attributes and biodiversity. Likewise, the spatial patterns created by development within these forest landscapes may provide research opportunities in identifying forest clearing thresholds as they relate to nest parasitism, nest predation, introduction of exotics, and the development of xeric communities.
Acknowledgments I thank Robert L. Burgess and the anonymous reviewers for their valuable comments. The research was funded by the U.S. Forest ServiceNortheastern Forest Experimental Station.
References Ambuel, B. and Temple, S.A. 1983. Area-dependent changes in the bird communities and vegetation of southern Wisconsin forests. Ecology 64: 1057-1068. Brittingham, M.C. and Temple, S.A. 1983. Have cowbirds caused forest songbirds to decline. BioScience 33: 31-35. Burgess, R.L. and Sharpe, D.M. (eds.) 1981. Forest Island Dynamics in Man-Dominated Landscapes. Springer-Verlag, New York. 310 pp. Calhoon, R.E. and Haspel, C. 1989. Urban cat populAons compared by season, subhabitat, and supplemental feeding. Journal of Animal Ecology 58: 321-328. Curtis, J.T. 1956. The modification of mid-latitude grasslands and forests by man. In: Man’s Role in Changing the Face of the Earth. pp. 721-736. Edited by W.L. Thomas. University of Chicago Press, Chicago. Duever, L.C. and Noss, R.F. 1990. A computerized method of priority ranking for natural areas. In: Ecosystem Management: Rare Species and Significant Habitats. pp. 22-33. Edited by R.S. Mitchell, C.J. Sheviak, and D.L. Leopold. New York State Museum Bulletin 471, Albany. Franklin, J.F. and Forman, R.T.T. 1987. Creating landscape patterns by forest cutting: Ecological consequences and principles. Landscape Ecology 1: 5-18. Forman, R.T.T., Galli, A.E. and Leck, C.F. 1976. Forest size and avian diversity in New Jersey woodlots with some land use implications. Oecologia 26: 1-8. Forman, R.T.T. and Godron, M. 1981. Patches and structural components for a landscape ecology. BioScience 31: 733-740. Forman, R.T.T. and Godron, M. 1986. Landscape Ecology. John Wiley and Sons, New York. 619 pp. Gates, J.E. and Gysel, L.W. 1978. Avian nest dispersion and fledging success in field-forest ecotones. Ecology 59: 871-883. Godron, M. and Forman, R.T.T. 1983. Landscape modification and changing ecological characteristics. In: Disturbance and Ecosystems. pp. 12-28. Edited by H.A. Mooney and M. Godron. Springer-Verlag, New York. Janzen, D.H. 1983. No park is an island: Increase in interference from outside as park size increases. Oikos 41: 402-410. Leopold, A. 1933. Game Management. Scribners, New York. 481 pp. Levenson, J.B. 1981. Woodlots as biogeographic islands in southeastern Wisconsin. In: Forest Island Dynamics in ManDominated Landscapes. pp. 14-39. Edited by R.L. Burgess and D.M. Sharpe. Springer-Verlag, New York. Lovejoy, T.E., Bierregaard, R.O., Rylands, A.B., Malcolm, J.R., Quintela, C.E., Harper, L.H., Brown, K.S., Powell, A.H., Powell, G.V.H., Schubart, H.O.R. and Hays, M.B. 1986. Edge and other effects of isolation on Amazon forest fragments. In: Conservation Biology: The Science of Scarcity and Diversity. pp. 257-285. Edited by M.E. SoulC. Sinauer Associated, Inc., Sunderland. Noss, R.F. 1987a. From plant communities to landscapes in
184 conservation inventories: a look at the Nature Conservancy (USA). Biological Conservation 41: 11-37. Noss, R.F. 1987b. Protecting natural areas in fragmented Landscapes. Natural Areas. Journal 7: 2-13. Noss, R.F. and Harris, L.D. 1986. Nodes, networks, and MUMS: preserving diversity at all scales. Environmental Management 10: 299-309. Ranney, J.W., Bruner, M.C. and Levenson, J.B. 1981. The importance of edge in the structure and dynamics of forest islands. In: Forest Island Dynamics in Man-Dominated Landscapes. pp. 67-95. Edited by R.L. Burgess and D.M. Sharpe. Springer-Verlag, New York. Robbins, C.S. 1979. Effect of forest fragmentation on bird populations. In: Management of north central and northeastern forests for nongame birds. pp. 198-212. USDA Forest Service, North Central Forest Experiment Station, St. Paul, Minnesota. General Technical Report NC-51. SAF. 1983. Terminology of Forest Science Technology, Practice and Products. Society of American Foresters, Bethesda, Maryland. 370 pp. Sokal, R.R. and Rohlf, F.J. 1969. Biometry. W.H. Freeman and Comp., San Francisco, California, 776 pp. Temple, S.A. 1986. Predicting impacts of habitat fragmentation on forest birds: A comparison of two models. In: Wildlife 2000: Modeling Habitat Relationships of Terrestrial Vertebrates. pp. 301-304. Edited by J. Verner, M.L. Morrison, and C.J. Ralph. The University of Wisconsin Press, Madison, Wisconsin. Turner, M.G. and Ruscher, C.L. 1988. Changes in the spatial patterns of land use in Georgia. Landscape Ecology 1: 241-251. U.S. Department of Commerce. 1982. 1980 Census of Popula-
tion: Number of Inhabitants, Part 22, Maryland. Washington, D.C. pp. 22-8. Wales, B.A. 1972. Vegetation analysis of north and south edges in a mature oak-hickory forest. Ecological Monographs 42: 45 1-471. Whitcomb, R.F., Rohbins, C.S., Lynch, J.F., Whitcomb, B.L., Klimkiewicz, M.K. and Bystrak, D. 1986. Effects of forest fragmentation on avifauna of the eastern deciduous forest. In: Forest Island Dynamics in Man-Dominated Landscapes. pp. 125-206. Edited by R.L. Burgess and D.M. Sharpe. Springer-Verlag, New York. Wilcove, D.S., McLellan, C.H. and Dobson, A.P. 1986. Habitat fragmentation in the temperate zone. In: Conservation Biology: The Science of Scarcity and Diversity. pp. 237-256. Edited by M.E. SoulC. Sinauer Associates, Inc., Sunderland. Wilcox, B.A. and Murphy, D.D. 1985. Conservation strategy: the effects of fragmentation on extinction. American Naturalist 125: 879-887. Williamson, M. 1975. The design of nature preserves. Nature 256: 519. Yahner, R.H. 1988. Changes in wildlife communities near edges. Conservation Biology 2: 333-339. Yaro, R.D., Arendt, R.G., Dodson, H.L. and Brabec, E.A. 1989. Dealing with Changes in the Connecticut River Valley: A Design Manual for Conservation and Development. Lincoln Institute of Land Policy, Cambridge, Massachusetts. 182 pp. Zipperer, W .C., Burgess, R.L. and Nyland, R.D. 1990. Patterns of deforestation and reforestation in different landscape types in central New York. Forest Ecology and Management 36: 103-117.
