Ecotonal Dynamics of the Invasion of
Woody Plants into the Tallgrass Prairie
Thomas A. Eddy
Abstract—Invasion of woody plants into eastern Kansas has resulted in the loss of thousands of acres of productive tallgrass
prairie. Historical and ecological events related to the decline of the
prairie in a four-county area were assessed. Woody plants invaded
all unburned sites on the study areas, while only 5% of the burned
area was infested. Five woody species were primary invaders.
Frequency of occurrence of trees and shrubs on burned sites was
one-third that of unburned sites, and percent foliar cover on burned
sites was one-fifth that of unburned sites. Economic losses to the
ranching industry in the area was estimated at $317,000 annually.
Good range management practices and selected control strategies
can reduce woody plant populations.
This study traced land use, vegetation, and related ecological changes that have caused the widening of the ecotone
between the tallgrass prairie and the adjacent deciduous
forest in eastern Kansas. The study examined: (1) the
historic and cultural events associated with the conversion
of grassland to shrubland/woodland; (2) the composition and
foliar cover of the woody plants in burned and unburned
prairie; (3) the influence of slope on the occurrence of shrub
and trees in burned and unburned prairie; (4) reduction of
livestock forage by invading woody plants and the resulting
economic losses to the ranching industry; and (5) management techniques useful in reducing the invasion of woody
plants into the area grasslands.
Study Areas ____________________
The four-county study area is located in east central
Kansas, with Morris and Chase counties in the interior
upland of the Flint Hills and Lyon and Greenwood counties
on the eastern edge of the region (fig. 1). The two latter
counties are within the extreme eastern border of the continental forest-tallgrass prairie transition.
Methods _______________________
This study was accomplished in four parts. The preliminary phase examined early settlement newspapers to trace
changes in agricultural and cultural events in the rural
communities of the study area. Journals of military expeditions and diaries by early residents gave insights into the
vegetation of the mid to late 1900’s. Interviews with a
Figure 1—Location of Flint Hills Region Four County Study Area.
In: McArthur, E. Durant; Ostler, W. Kent; Wambolt, Carl L., comps. 1999.
Proceedings: shrubland ecotones; 1998 August 12–14; Ephraim, UT. Proc.
RMRS-P-11. Ogden, UT: U.S. Department of Agriculture, Forest Service,
Rocky Mountain Research Station.
Thomas A. Eddy is Professor, Division of Biological Sciences, Emporia
State University, Emporia, KS 66801.
56
USDA Forest Service Proceedings RMRS-P-11. 1999
ranching family whose great grandfather had homesteaded
in the area was valuable in understanding early land use
philosophy and practice.
Secondly, the history of the occurrence of woody vegetation in the area was assembled by examining section line
surveys by the Federal Land Office in 1856 and 1857, aerial
photographs taken in 1937-1939 and later in 1950, 1956,
1969, and current aerial photographs and soil maps. Fifty
sites (200 m sq) were selected in each of the four counties as
sites to compare the vegetation changes over the past 15
decades. Woody vegetation composition and foliar cover
(Daubenmire method) was determined for each 200 m sq
site. The burning history of 157 sites was collected from
ecological records and interviews of land owners and managers. A comparison was made of the woody vegetation on
slopes (hill top to bottom slope) on burned and unburned
sites on the study area in Chase county.
The third aspect of the study evaluated the economic
impact of the loss of the forage to invading woody plants and
the subsequent loss of income to the ranching industry.
Analysis of vegetation in the transects of the study sites and
1⁄2 cm overlay grids on early and recent aerial photographs
were used to calculate the extent of the woody invasion into
the four-county study area and to estimate the percent
closure of the canopy of woody plants over the grass and
forbs. Each acre lost to grazing was assigned the current
rental value of $18.00/acre/year.
The fourth objective was to develop management strategies to reduce losses in livestock forage from weedy plant
invasions and maintain the productive herbaceous species.
This was accomplished by examination of the scientific
literature, interviews with range management specialists,
and observations based on my field experiences in the study
areas.
Results ________________________
Woody plants have invaded all unburned prairie sites in
the study area. Burned pasture sites (where burning has
occurred in at least once every 4 years) showed a slow
increase in woody plants from 0 to 5% through the period
(150 years) (table 1). Distribution of woody plants on slopes
that burned regularly was linear from hill top (1%) to bottom
slope (20%) (table 2). On unburned areas woody plant infestations occurred on 3% of the hill top sites to 100% of the
slower slope sites.
Table 1—Tree and shrub invasion of burned and
unburned prairie sites in the central Flint
Hills study areas in Kansas, 1860-1998.
Year
1860
1900
1940
1960
1980
1998
Average percent sites/woody plants
Burned
Unburned
0
0
1
2
3
5
1
2
24
40
64
100
USDA Forest Service Proceedings RMRS-P-11. 1999
Table 2—Effects of slope on occurrence of woody
plants on burned and unburned sites in
the central Flint Hills study areas in
Kansas, 1998.
Slope
Average percent sites/woody plants
Burned
Unburned
Hill top
Upper slope
Mid slope
Lower slope
Bottom slope
1
4
7
12
20
0
2
3
82
100
Five primary woody species were identified on the study
areas of both burned and unburned sites. Frequency of
occurrence on burned sites was one-third that of unburned
sites, and percent foliar density of burned sites was one-fifth
that of unburned sites (table 3).
The invasion of woody plants into prairie may be attributed to events in the history of the region since the Homestead Act of 1857 (table 4).
Economic losses to the ranching industry in the fourcounty study area from forage destroyed by invading shrubs
was estimated to be $317,000 annually (table 5).
Control of woody species must be based on careful balance
of range forage with numbers of livestock on the pastures
(carrying capacity), selective use of woody plant herbicides,
burning practices that maximize woody plant damage by
allowing for adequate accumulation of fuel, and mowing of
vulnerable plants.
Discussion _____________________
Penetration of woody plants into the remaining tallgrass
prairie in the study area in east central Kansas has resulted
from reduction of the intensity and frequency of prairie fires.
The role of fire in maintaining the integrity of the tallgrass
ecosystem has been described by plant ecologists (Weaver
and Rowland 1952; Bragg and Hulbert 1976; Steinauer and
Collins 1996). Several aspects of the ecology of fire and its
effect on the grassland flora were addressed by this study.
Trees and shrubs successfully invaded all unburned prairie sites examined in the four-county study area. This
infestation of woody plants has degraded the value of the
prairie for forage for livestock and has altered the composition of the native tallgrass communities. On sites burned
regularly there has been a gradual increase in woody plant
invasion over the past 15 decades. Invasion has occurred in
various locations in the prairie where overgrazing by livestock has reduced the fuel (previous years’ dead plant
remains) to levels inadequate to carry fire hot enough to
destroy woody plants establishing on the site. Eventually
the tree and shrub-infested site will convert to a shaded
woodland environment where further growth of prairie
vegetation is suppressed. Entry and establishment of woody
plants occurred most frequently in lower and bottom slopes
because of the deeper, fertile, moist soils near woodland
borders or brushy ravines where sources of woody species
occur.
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Table 3—Composition and foliar density of woody plants on burned and unburned sites in
the central Flint Hills study areas in Kansas, 1998.
Burned
Unburned
% occurrence % foliar cover % occurrence % foliar cover
Species
Cornus drummondii
Maclura pomifera
Rhus glabra
Symphoricarpos
orbiculatus
Ulmus pumila
Other species
10
4
8
6
2
3
3
5
35
15
13
10
18
24
12
11
2
8
1
1
7
4
10
9
Table 4—Historical influences on increase of woody plants on unburned
prairie sites in the central Flint Hills study areas in Kansas,
1998.
Historical event
Beginning
of period
Homestead Act
Small farms on prairie
WWI
Depression
Large farms
Modern farming
1857
1875
1918
1930
1960
1980
% Sites
with woody plants
1
18
28
55
78
100
Table 5—Estimated annual costs of tree and shrub invaded prairie to
ranching interests in the central Flint Hills study areas,
Kansas. 1998.
County
Grazed acres
(non federal)
Chase
Greenwood
Lyon
Morris
Total
404,900
612,900
262,900
262,500
1,543,200
a
4,700
6,100
3,300
3,400
17,500
Annual cost of
invaded acresa
$85,000
$110,000
$60,000
$62,000
$317,000
$18.00/acre/yr
The history of the decline of acreage and quality of the
tallgrass prairie in the four-county area was attributed to six
periods of human activity during the past 150 years. Each of
these brought unique demands on the prairie landscape. The
Homestead Act (1857) opened the prairie for settlement with
the accompanying need to suppress and break native sod for
crops. Small farms (1860-1900) with associated roads, small
fields, orchards, and home sites brought an end to prairie
burning in many areas. Farm failures during the 1930’s
resulted in abandonment of many prairies, where in the
absence of management, pastures and old crop fields were
invaded by woody species. World War II (1940’s) accelerated
the loss of prairie acres to more intensive agriculture as the
nation’s demand for food increased. Large farms and modern
machines (1960’s to present) caused the remaining loss of
acres suited to cropland agriculture. As small farms were
replaced by large farming operations, small pastures and
isolated old crop fields were left to woody plant invasion.
58
Acres lost
of woody
plant invasion
Economic losses due to reduction in forage caused by
woody plant invasion indicates the effect on the families in
the region who depend on ranching for their income. Many
acres of prairie can be returned to productivity if aggressive
control of woody species is practiced.
References _____________________
Bragg, T. B.; Hulbert, L. C. 1976. Woody plant invasion of unburned
Kansas bluestem prairie. Journal of Range Management. 29:
19-23.
Steinauer, E. M.; Collins, S. L. 1996. Prairie ecology: the tallgrass
prairie. In: Prairie Conservation. Sampson, F. B., Knopf, F. L.,
editors. Island Press. Washington DC.
Weaver, J. E.; Rowland, N. W. 1952. Effects of excess natural mulch
on development, yield, and structure of native grassland. Botanical Gazette. 114: 1-19.
USDA Forest Service Proceedings RMRS-P-11. 1999