The conversion of natural grasslands of the Northern 
Great Plains into man-induced and -managed plant com- 
munities began in the 1800's and still continues. If these 
man-induced and -managed plant communities are grown 
on essentially the same physical sites as earlier endemic 
plant communities, they are known as “replacement plant 
communities” (Mueller-Dombois and Ellenberg 1974). As 
early as 1747, red clover (Trifolium pratense) was sown as 
a replacement community on previously tilled soil in 
North America and by the late 1700’s there was consider- 
able interest in forage grasses and legumes from England 
(Heath 1973). The use of forage grasses and legumes in 
North American agriculture continued gradually until the 
late 1920’s and then expanded rapidly in the 1930's and 
early 1940’s during the nationwide soil conservation and 
protection movement in response to drought. 
After World War II small grains gained prominence in 
American agriculture and by the end of the decade, pro- 
duction began to exceed demand. However, starting in the 
mid-1950’s the U.S. Department of Agriculture (USDA) 
initiated several cropland retirement programs whereby 
tame grasses and legumes were sown in efforts to help 
ameliorate a surplus of grain. Two of these programs were 
the Soil Bank Program (Peterson 1956) of the late 1950's 
and early 1960’s and the Cropland Adjustment Program 
(Jaenke 1966) of the late 1960's and early 1970's. Some 
beneficial values of these programs to wildlife in the 
prairie pothole region were reported by Schrader (1960), 
Benson (1964), Jessen et al. (1964), Moyle (1964), Dahl- 
gren (1967), Drewien and Springer (1969), Duebbert 
(1969), Harmon and Nelson (1973), Kirsch et al. (1973), 
Duebbert and Kantrud (1974), Kirsch (1974), and Dueb- 
bert and Lokemoen (1976, 1977, 1980). Stewart (1975) 
reported 37 species of birds that used tame grass—legume 
plant communities to satisfy some or most of their habitat 
requirements; 29 of these plus 5 others are known to use 
sown stands for nesting (Table A-1). 
In 1978, 120,125 ha (Table A-2) of seeded nesting cover 
(SNC) were provided for wildlife on privately owned land 
in the glaciated prairie pothole region through the USDA’s 
Water Bank Program (Phillips 1975; Womach 1977). I es- 
timate that about 40,500 ha are provided for wildlife on 
other State, Provincial, and Federal lands in this region. 
The principal replacement community (SNC) presently 
being established on the Water Bank Program and refuge 
lands is a mixture of introduced cool-season grasses and 
legumes. This mixture generally includes intermediate 
wheatgrass (see Table A-3 for scientific names of plants), 
tall wheatgrass, alfalfa, and sweet clovers. Occasionally, a 
few managers included a supplemental amount of western 
wheatgrass, slender wheatgrass, crested wheatgrass, green 
needlegrass, or switchgrass in the seeding mixture. Recent 
studies of relations of wildlife to combinations of this par- 
ticular mixture have been reported by Duebbert (1969), 
Duebbert and Kantrud (1974), Nelson and Duebbert 
(1974), Cowardin and Johnson (1979), Kaiser et al. (1979), 
and Duebbert and Lokemoen (1976, 1977, 1980). They 
found that (1) the mixture is economical to establish, (2) it 
can be sown or broadcasted on previously tilled land with 
the commonest of commercial farm equipment, (3) it gen- 
erally supports greater nest densities and higher hatch 
rates for many upland nesting bird species than adjacent 
habitat types, (4) it is capable of producing wildlife during 
the first several years of secondary plant succession, and 
(5) the wildlife values seem to have some relation to the 
growth form and composition of the vegetation. 
A recent concern of land managers is whether this type 
of seeded nesting cover will degenerate in composition, 
cover, height, and denseness, and if so, will degeneration 
occur in a consistent pattern or in a time-predictable man- 
ner, The present study was initiated in 1976; its objectives 
were (1) to determine whether any consistent patterns of 
change occur in species composition, canopy cover, plant 
height, and visual obstruction within vegetation cover 
during secondary succession of the seeded mixture (SNC) 
in known age stands and (2) to determine the influence of 
species other than the seeded mixture on the stability of 
SNC stands during early succession. 
Materials and Methods 
Study Areas 
All study areas were either on National Wildlife Refuge 
or Waterfowl Production Area land and had known-aged 
stands of SNC. Their distribution ranged from the south- 
ern edge of eastern South Dakota to the northern bound- 
ary of North Dakota and from west central Minnesota to 
the extreme northeast corner of Montana (Fig. 1). A mois- 
ture gradient represented by a gradual increase in annual 
precipitation from about 30 cm in the Northwest to about 
64 cm in the East and Southeast extends across the region. 
All stands were sown on uplands suitable for cultiva- 
tion, based on Soil Conservation Service guidelines. Study 
area stands were found on a large variety of upland soils 
and all were within the following capability classification 
groups: 
Class II,—Soils with some limitations that reduce 
choice of crops or require moderate conservation treat- 
ments. 
Class III. —Soils with severe limitations for cropland 
use that reduce choice of crops and require special conser- 
vation treatments, particularly for erosion control. 
Class IV. —Soils with very severe limitations for crop- 
land use that limit choice of crops and require very careful 
management, particularly in control of erosion. 
Stand Selection Criteria 
Selection of stands was based on two criteria: (1) each 
stand or field had to consist of a unit of land with a recent 
history of tillage and a sown mixture of grasses and leg- 
umes, and (2) stands could not have been altered by till- 
