20 
Conclusions 
Structural changes in species composition, canopy 
cover, plant height, and visual obstruction of cover are 
measurable in stands of SNC of different ages. Evidently 
these changes are not statistically different relative to age 
when stands from over a broad region are pooled, but they 
show general trends of improvement, year of maximum 
growth, and general trends of degeneration. Thus, the 
variability in these changes suggests that any practical 
management decisions, relative to stand age, must be 
made on an individual field basis rather than on all stands 
as a whole. 
The influences of climate and environment on vegeta- 
tional changes and trends are dominant factors. The influ- 
ence of precipitation received in the last year before meas- 
urements and of the deterioration caused by excessive 
snowpack, ice pack, or harvesting by rodents surpasses the 
influences of stand age, production capabilities of soils, and 
species composition on the height and visual obstruction of 
SNC in spring. Under a more moderate and consistent cli- 
mate, detection of age-related differences in residual vege- 
tation structure in spring may be possible, but the erratic 
climate of the glaciated prairie pothole region will negate 
this possibility in many years. 
Species composition, number of species, and canopy 
cover are more consistent from year to year within stands 
than they are among stands, but there is also a high degree 
of annual variation in these same structural characteristics 
within stands. Furthermore, obvious distinctive patterns 
of secondary succession are basically lacking during the 
first nine growing seasons in seeded nesting cover. 
The relations among stand age, vegetation structure, 
climate, and secondary succession are very complex. These 
relations were not fully identified during this study; how- 
ever, their relative importance to measurements of visual 
obstruction and plant height and, indirectly, of potential 
wildlife production, are readily understood when con- 
siderations are made for variable climatic and site condi- 
tions. 
The SNC, as described in this paper, is a manageable 
cover crop that can be used throughout the glaciated 
prairie pothole region of the north central United States 
and throughout much of the Aspen Parkland Biome of Al- 
berta, Saskatchewan, and Manitoba, Canada. It is neither 
the last in a series of possible replacement plant communi- 
ties nor the ultimate cover for the diversity of wildlife 
species that inhabit the Northern Plains; nevertheless, it is 
a crop with a proven ability to produce a stand of man- 
made cover with a structural form that is both attractive 
and secure for many species of birds and mammals. 
Acknowledgments 
Full support for the field research, data collection, and 
data analysis were provided by the U.S. Fish and Wildlife 
Service, Northern Prairie Wildlife Research Center, under 
the direction and encouragement of W. R. Goforth and 
R. C. Stendell. D. Carney, R. F. Johnson, E. Larsen, 
J. M. Callow, R. O. Woodward, and J. Sease assisted in 
field research and data collection. D. A. Davenport and 
A. M. Frank were responsible for computer programs and 
D. H. Johnson offered statistical advice. H. L. Clark con- 
structed the field equipment. An extended thanks goes to 
all the other Fish and Wildlife Service personnel who gave 
help, suggestions, criticisms, and encouragement during 
the extent of the project. A special thanks goes to L. M. 
Cowardin for editorial suggestions and to E. K. Bartels for 
obtaining and checking many references. 
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