DeFriese (1880) noted that topography, especially the "change in height above drainage," was a major factor in determining the distribution and general character of the forests in the Jackson Purchase Region of Kentucky (JPR). He also considered exposed gravel beds, either where loess deposits were thin or where erosion had removed the loess, as influencing vegetation patterns. Over 40 years later, Davis (1923) identified seven geographic subdivisions of the JPR (Figure 1) on the basis of differences in topography, soil, vegetation, and other factors or combinations of these.

      Since that time, follow-up studies to determine whether the early observations of DeFriese (1880) and Davis (1923) were correct have not been undertaken. Analyses of specific ecological communities, e.g., bottomland hardwoods (Bryant 1990), mixed mesophytic forests (Bryant 1993), swamps (Bryant 1997), and flatwoods (Bryant 1999), and numerous floristic surveys, e.g., Bryan (1977), Fuller (1980), Funk and Fuller (1978, 1980), Grubbs and Fuller (1991), Wilson (1976), and Woods (1983) have revealed much about vegetation structure and composition in the JPR. However, these studies were subjective and communities based primarily on leading dominants; we believe that a more objective means is needed when considering vegetation or community patterns for the entire JPR.

      Ordination methods may represent an improvement on vegetation description based on dominants (Dooley and Collins 1984) and perhaps reveal some of the underlying relationships between vegetation and environment. The specific objective of this study was to elucidate vegetational gradients in the JPR and their possible environmental bases by the use of ordination. In particular, we were interested to see whether topographic or soil related gradients occur that might explain vegetation patterns.

     The JPR, 6144 km² in size, is that portion of Kentucky west of the Tennessee River. This area is bounded by three large rivers, the Tennessee on the east, the Ohio on the north, and the Mississippi

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Figure 1. Map of the Jackson Purchase Region of Kentucky with subdivisions after Davis (1923): A. The Big Bottoms; B. The Second Bottoms; C. The Cane Hills; D. The Barrens; E. The Oak and Hickory Uplands; F. The Flatwoods; G. The Breaks of the Tennessee River.

on the west. The state of Tennessee forms the southern boundary at 36E30' north latitude. The JPR was purchased from the Chickasaw Indians in 1818 and was the last area added to the state of Kentucky. This was the only region of Kentucky to be surveyed in sections using General Land Office methods (Bryant and Martin 1988).

     For the most part, the JPR is an undulating plain with elevations usually between 100-125 m. Local relief is not much in excess of 15 m except in the vicinity of major streams (McFarlan 1950). There are three general patterns of relief in the JPR: nearly level to hilly uplands; gently sloping to steep uplands; and nearly level to gently sloping bottomlands (Forsythe 1997).

     The JPR differs from the rest of Kentucky geologically since it is located in the Gulf (or Mississippi) Embayment, a Coastal Plain region. This region is the area of outcrop of the unconsolidated and only semi-consolidated sediments of the Cretaceous and Tertiary (McFarlan 1950). Cherty residuum and limestone of the Mississippian formation are exposed in a narrow strip along the Tennessee River where erosion has removed the Coastal Plain sediments. A thick mantle of loess covers much of the coastal plain sediments, except where removed by erosion. This loess cover thins across the JPR from west to east. Loess deposits may exceed 30 m along the Mississippi River.

     Soil surveys have been produced for each of the eight counties of the JPR: Ballard and McCracken (Humphrey 1976), Calloway (Leighty et al. 1945), Calloway and Marshall (Humphrey, et al. 1973), Carlisle and Hickman (Forsythe 1997), Fulton (Newton and Sims 1987), and Graves (Leighty 1953). Soils in the region differ widely in physical characteristics and primarily have formed from deep or thin loess, coastal plain sediments, river alluvium, alluvium washed from loessal uplands, and cherty limestone. Soils are seldom completely dry and are frozen for only short periods. Nearly continual leaching has moved many of the soluble bases and clay minerals from upper to lower horizons (Forsythe 1997). Fragipan development is typical for many of the soils on level to gently rolling topography. Upland soils are mature and floodplain soils are young.

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     Climate for the JPR is temperate with hot humid summers and occasional cold spells in winter. Rainfall averages 120 cm annually (Newton and Sims 1987) and the mean annual temperature is 15E C (Forsythe 1997).

METHODS

Field Sampling

     Sixty-seven sites were visited and sampled and these data were used in the ordination. A minimum of five plots/stands was sampled, but more than five plots were sampled in most stands. Plots were 0.04 ha in size and spaced at 30 m intervals through the stands. All woody stems >10 cm diameter-breast-height (dbh) were measured in plots. Collected data were analyzed to relative density (RD), relative dominance or basal area (RDo), and importance value (IV=RD+RDo). Density (trees/ha), H' (species diversity) and other variables such as percentage canopy vs. subcanopy also were determined.

     At each sampling site, soils and topography were recorded. Sites were selected by the senior author and by field personnel of the Natural Resource Conservation Service to reflect habitats and topographic diversity for each county. Only stands with no obvious major disturbance and that were large enough to allow a minimum of five sampling plots were considered. The location of sampling sites is shown in Figure 2. A total of 5871 trees was measured.

     Stand relationships were summarized as scores on the first and second axes of the Principal Components Analysis (Kovach 1998) based on square root transformed importance value data, which included all species in each stand. Scores on the second axis did not show "arching" (Pielou,

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1984) and thus the ordination was considered a satisfactory description of the relationship between the stands sampled and environmental variables.

     Axis-1 of the ordination was interpreted to be a moisture gradient and Axis-2 a topographic gradient. Nine broad vegetation types segregated out on the two gradients: swamps, bottomland hardwoods, two types of flatwoods, two types of oak-hickory, mixed mesophytic forest, transitional (successional-disturbed), and former barrens (Figure 3).

     Swamps and bottomland hardwood forests clustered together at the wet end of the moisture gradient. However, microtopographical differences on the floodplain were not of sufficient magnitude to further separate the various community types (Figure 4). Swamps were nearly monospecific stands dominated by Taxodium distichum and/or Nyssa sylvatica. These stands were generally located on first bottoms and were covered by water throughout much of the year. Swamps were of low tree species diversity, but had high basal area values and high density (Bryant 1997). Bottomland hardwood communities, which occupied the alluvial plains of the major rivers and, in the interior of the JPR, were present along streams that have nearly reached grade and flowed sluggishly in wide, flat floodplains. These are on first and second bottoms. The floodplains are microtopographically complex and here species sort out individualistically (Robertson et al. 1978)

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indicating that flooding frequency and duration of flooding are important factors. Celtis-Fraxinus-Liquidambar-Quercus-Acer-Carya are the important genera that sort out on the alluvial floodplains. Sandbar communities that form along the Mississippi River were not sampled.

     The two types of flatwoods that Bryant (1999) suggested for the JPR segregated out at the flat end of the topographic gradient. The two types appeared to be a wet phase and a dry phase. Both flatwoods types were found on level to slightly concave surfaces that appeared to be old lake beds (Leighty et al. 1945). The wet phase was characterized by Q. pagodaefolia and Q. stellata and the dry phase by Q. falcata and Q. stellata. Both types were on Henry silt loam; however, there was one stand on Routon silt loam on the wet phase. Because of alternating wet and dry periods due to the underlying fragipan on Henry and Routon soils, the flatwoods communities tend to be clustered around the mesic portion of the moisture gradient.

     Two types of oak-hickory forest sorted out toward the dry end of the moisture gradient, but one was on flat to rolling topography and the other on more rolling to rough, sloping topography. Quercus stellata and Q. falcata were indicator species, but not community dominants on the flat to rolling lands; Q. alba and/or Q. velutina and Carya ovata and C. glabra were characteristic of the rougher lands. Loess is generally thin or absent where Q. alba and Q. velutina are most prominent.

     Mixed mesophytic forests were nearly confined to the highly dissected loess bluffs although one mixed mesophytic stand in the Sand Hill section of Graves County clustered with those of the bluffs. Braun (1950) and Delcourt and Delcourt (1975) considered these highly diverse communities to be refugial in nature. Their dominants include Acer saccharum, Fagus grandifolia, Liquidambar styraciflua, Q. alba, and Liriodendron tulipifera (Bryant 1993). As expected, the mixed mesophytic communities sorted out near the middle of the moisture gradient.

     Although efforts were made not to sample recently disturbed stands, communities that showed evidence of past disturbances clustered near the intersection of the moisture and topographic gradients. Species in disturbed stands varied; however, the shade intolerant Liquidambar styraciflua (Burns and Honkala 1990) was generally abundant. Their location on the gradients point to their

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transitional character.

     The last grouping that we recognized was not nearly as distinct as some of the others. We recognize it as perhaps the former barrens of the JPR. Quercus stellata, Q. falcata, Sassafras albidum, and Diospyros virginiana were generally present in the stands sampled, which were located in those portions of Ballard, Graves and Calloway counties mapped as barrens by Davis (1923). Andropogon gerardi, Schizachyrium scoparium, and Sorgastrum nutans, characteristic prairie grasses, and several scrub oaks were located together in Graves County near the stands sampled. The former barrens were on flat to gently rolling lands just to the dry side of the moisture gradient.

     The JPR has had a long history of agriculture. Even though forests have been fragmented, swamps have been drained, woodlands have been grazed, and fires suppressed, most of the plant assemblages recognized in the 1820 GLO Survey of the JPR (Bryant and Martin 1988) were still recognizable, although perhaps modified. These communities generally segregated out in response to moisture and topographic gradients.

     According to Braun (1950), the vegetation of the Mississippi Embayment section of her Western Mesophytic Forest Region in which the JPR lies is a mosaic of unlike types including prairies, oak hickory forest, swamp forest, and mixed mesophytic communities. In general, these were present in our study. Braun (1950) further noted that oak-hickory communities occupied a considerable part of the rolling uplands and that their composition varied in relation to topography and soils. The importance of oaks and hickories as a group in the JPR was shown in the ordination. Flatwoods were dominated by oaks and hickories, as were former barrens. The two upland oak-hickory forest types also reflect the importance and broad areal extent of oak-hickory in the JPR. Braun (1950) wrote that if it were not for the mixed mesophytic communities of the loess bluffs, she would have included the JPR in her Oak-Hickory Forest Region rather than the Western Mesophytic. The fact that the various community types including those dominated by oaks and hickories sorted out along moisture and topographic gradients is of significance since these environmental factors can now be used to determine if other factors might also underlie vegetation patterns in the JPR. Heineke (1987) considered flatwoods to have been largely eliminated from the JPR; however, not only were they present, they were of two types that segregated in response to topography, soil moisture, and especially soil type.

     Other gradients, i.e. an anaerobic gradient (Wharton et al. 1982) may be operational in sorting out species and communities on the alluvial plain. There the interaction of flooding depth and duration with microtopographic differences on the alluvial plain exercise influence on species patterns. Former barrens were now more similar to oak-hickory than to original prairie. Remnant populations of prairie grasses that formerly characterized the barrens at the time of settlement added evidence to our interpretation. However, historic accounts written in the 1880s (e.g. DeFriese 1880, Loughridge 1888) mentioned that 30, 40, or even 60 years earlier, prairies were abundant in portions of the JPR and that these prairies were fire-maintained. Leighty (1945) wrote, "Beginning about 1863, after the removal of the Indians and the increase in white settlers, fires decreased, the forests extended in area, and the prairies became more restricted." This is similar to Baskin and Baskin's (1978) conclusion that the Big Barrens of Kentucky were not part of the Prairie Peninsula, but were fire maintained. In Williamson County, Illinois, Anderson and Anderson (1975) found that oak-hickory was the dominant vegetation type during presettlement times. There prairies or barrens occurred on level tracts of land where fire beat back forests. Rugged landscapes do not carry fires as well as level or rolling ones (Anderson and Anderson 1975) and these maintain oak-hickory. In a similar way, the more rolling and rough lands supported oak-hickory in the JPR. Fire also may have had an influence on the oak-hickory of the level to gently rolling topography. These lands may have formerly been more open and savanna-like, but with fire cessation returned to oak-hickory. The fire tolerant Q. stellata and Q. falcata were considered to be indicator species of this oak-hickory type. Anderson and Anderson (1975) found much of the rolling land in Williamson County, Illinois,

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to support savanna-like vegetation during presettlement times. The lower importance of Q. marilandica today as compared to 1820 (Bryant and Martin 1988) also may be a result of fire suppression.

Factors other than fire, soils, moisture and topography may also influence vegetation patterns. DeFriese (1880) did not feel that geology had as much influence on plant distributions in the JPR as in other parts of Kentucky. However, Acer saccharum appears to be somewhat influenced by geology in its distribution in the JPR. It is found along the Tennessee River where Mississippian limestone is exposed and on the deep loess, but is rare in the interior of the JPR where loess is thin and Coastal Plain deposits are predominant.

Anderson, R.C. and M.R. Anderson. 1975. The presettlement vegetation of Williamson County, Illinois. Castanea 40:345-363.

Baskin, J.M., and C.C. Baskin. 1978. The Big Barrens of Kentucky not a part of Transeau's Prairie Peninsula. Pp. 43-48. In: Stuckey, R. and K.  Reese (eds.). 1981. Ohio Biol. Surv. Biol. Notes 15.

Braun, E.L. 1950. Deciduous Forests of Eastern North America. The Blakiston Co. Philadelphia, PA.

Bryan, C.A. 1977. A floristic survey of the vascular plants in Loess Bluff Ravines in Carlisle, Hickman and Fulton Counties in Kentucky. M.S. thesis. Murray State Univ. Murray, KY.

Bryant, W.S. 1990. Sugarberry (Celtis laevigata) and its associates in the bottomland forests of the Jackson Purchase of Kentucky. Pp. 93-100. In: Hamilton, S.W. and M.T. Finley (eds.) Proc. of the Third Annual Symposium on Natural History of Lower Tennessee and Cumberland River Valleys. The Center for Field Biology. Austin Peay State University. Clarksville, TN.

Bryant, W.S. 1993. Vegetation of loess bluff ravines in the Jackson Purchase Region of Kentucky. Pp. 281-288. In: Gillespie, A.R., G.R. Parker, and P.E. Pope (eds.). 9th Central Hardwood Forest Conference. USDA, Forest Serv. General Tech. Report NC-161.

Bryant, W.S. 1997. Structure and composition of the swamp forests on the Mississippi alluvial plain in Kentucky's Jackson Purchase Region. Trans. Kentucky. Acad. Sci. 58:85-91.

Bryant, W.S. 1999. Flatwoods of the Jackson Purchase Region, western Kentucky. Pp. 129-134. In: Hamilton, S.W., D.S. White, E.W. Chester and M.T. Finley (eds.). Proc. of the Eighth Symposium on the Natural History of Lower Tennessee and Cumberland River Valleys. The Center for Field Biology, Austin Peay State University, Clarksville, TN.

Bryant, W.S. and W.H. Martin 1988. Vegetation of the Jackson Purchase of Kentucky based on the 1820 General Land Office Survey. Pp. 264-276. In: Snyder, D.H. (ed.). Proc. of the First Annual Symposium of the Natural History of Lower Tennessee and Cumberland River Valleys. The Center for Field Biology, Austin Peay State University Clarksville, TN.

Burns, R.M. and B.H. Honkala (compilers). 1990. Silvics of North America. Volume 2. Hardwoods, USDA, Forest Serv. Agric. Handbook 654.

Davis, D.H, 1923. The Geography of the Jackson Purchase. Kentucky. Geol. Surv. ,Frankfort, KY.

DeFriese, L.H. 1880. Report on the timbers of the district west of the Tennessee River, commonly called the Purchase District. Geol. Surv. of Kentucky 5:125-158.

Delcourt, H.R. and P.A. Delcourt. 1975. The Blufflands: Pleistocene pathway into the Tunica Hills. Amer. Midl. Nat. 94: 385-400.

Dooley, K.L. and S.L. Collins. 1984. Ordination and classification of western oak forests in Oklahoma. Amer. J. Bot. 71:1221-1227.

Forsythe, R. 1997. Soil Survey of Carlisle and Hickman Counties, Kentucky. USDA, NRCS, Washington, DC.

Fuller, M.J. 1980. A preliminary checklist of flowering plants of Backusburg Hill, Calloway County, Kentucky. Trans.

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Kentucky. Acad. Sci. 41:27-34.

Funk, V.A. and M.J. Fuller. 1978. A floristic survey of the seeps of Calloway County, Kentucky. Castanea 43:162-172.

Funk, V.A. and M.J. Fuller. 1980. Analysis of the distribution of southeastern taxa in seeps of Calloway County, Kentucky. Trans. Kentucky Acad. Sci. 41:138-140.

Grubbs, J.T. and M.J. Fuller. 1991. Vascular flora of Hickman County, Kentucky. Castanea 56:193-214.

Heineke, T.E. 1987. The flora and plant communities of the Middle Mississippi River Valley. Ph.D. Dissertation, Southern Illinois University, Carbondale, IL.

Humphrey, M.E. 1976. Soil Survey of Ballard and McCracken Counties, Kentucky. USDA, Soil Conservation Serv., Washington, D.C.

Humphrey, M.E., F.L. Anderson, R.A. Hayes, and J.D. Sims. 1973.Soil Survey of Calloway and Marshall Counties, Kentucky.USDA, Soil Conservation Serv., Washington, D.C.

Kovach, W.L. 1998. MVSP-A multivariate statistical package for Windows, Version 3.0. Kovach Computing Services, Pentraeth, Wales, United Kingdom.

Leighty, W.J. 1953. Soil Survey: Graves County, Kentucky. USDA, Agri. Research Admin. Washington, D.C.

Leighty, W.J., H.W. Higbee, W.S. Reed, and C.E. Wyatt. 1945. Soil Survey: Calloway County, Kentucky. USDA, Agric. Research Admin. Washington, D.C.

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Robertson, P.A., G.T. Weaver and J.A. Cavanaugh. 1978. Vegetation and tree species patterns near the northern terminus of the southern floodplain forest. Ecol. Monogr. 48:249-267.

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Wilson, L.M. 1976. A distributional survey of the vascular plants on Sandy Branch in the Loess Bluff Area of Carlisle County, Kentucky. M.S. thesis. Murray State Univ. Murray, KY.

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