Table 1. Comparative differences of visual obstruction and height measurements of residual vegetation among stands 
dominated by tall wheatgrass, stands dominated by other wheatgrasses, and all stands, 1977-79. 



Number of Number of 
Number of obstruction - height i. 
Year and category stands measurements Xx measurements xX 
1977 
Tall wheatgrass 16 1,448 2.6 362 8.7 
Other wheatgrasses 67 6,008 2.1 1,408 7.2 
All 83 7,456 2.2 1,770 7.5 
1978 
Tall wheatgrass 26 2,564 1.0 641 6.4 
Other wheatgrasses 112 10,407 0.8 2,620 4.9 
All 138 12,971 0.8 3,261 Bie 
1979 
Tall wheatgrass 32 3,100 775 7.4 
Other wheatgrasses 112 11,140 2,785 5.7 
All 144 14,240 3,560 6.1 

ardized precipitation appeared related and thus was the 
only precipitation variate used in the analysis. 
A repeated measures analysis of covariance was per- 
formed with the computer program BMDP2V (BMDP 
1977) to relate the dependent variable visual obstruction 
with standardized precipitation, soil class, percentage 
grass, and age. Repeated measures analysis was used be- 
cause 58 stands were measured on the same sites in 3 con- 
secutive years; consequently, the measurements were not 
independent from year to year. The visual obstruction 
values were transformed by taking the square root to 
stabilize the variance. 
An analysis of variance program (SAS Institute Inc. 
1979) was used to test the relation of the dependent vari- 
able’s visual obstruction and height with the stand age and 
sample location within each transect and by year. 
Results 
Attributes of Residual Vegetation 
In 1977 mean visual obstruction per stand ranged from 
0.6 to 4.3 dm and averaged 2.2 dm, and mean height per 
stand ranged from 3.2 to 17.0 dm and averaged 7.5 dm 
(Table 1). The 4-year-old stands of SNC in 1977 averaged 
higher visual obstruction than did stands in other age 
classes (Fig. 4A). Except for the 2-year-old stands, plant 
height was also greatest in the 4-year-old age class 
(Fig. 4B). Greater height occurred in the 2-year-old stands 
mainly because sweet clovers, very tall species, were an 
important component of the vegetation in this age class. 
In 1978 mean visual obstruction per stand ranged from 
0.2 to 2.2 dm and averaged 0.8 dm, and mean height per 
stand ranged from 1.9 to 9.9 dm and averaged 5.2 dm 
(Table 1). In 1978, when sweet clovers were predominant, 

the visual obstruction averaged higher in the 3-year-old 
stands (Fig. 4C) and plant height averaged higher in the 
2-year-old stands (Fig. 4D). 
In 1979 mean visual obstruction per stand ranged from 
0.4 to 3.4 dm and averaged 1.2 dm, and mean height per 
stand ranged from 1.9 to 11.7 dm and averaged 6.1 dm 
(Table 1). The 5-year-old stands of SNC averaged higher 
visual obstruction than did stands in other age classes 
(Fig. 4E). Except for the 2- and 3-year-old stands, plant 
height was also greatest in the 5-year-old age class stands 
(Fig. 4F). 
There are four plausible reasons for the dissimilar trends 
of cover height and obstruction between the 1977, 1978, 
and 1979 data sets: (1) an early and heavy snowfall oc- 
curred over much of the region during 7-9 November 1977 
and remained all winter (some areas in Minnesota and 
South Dakota accumulated more than 100 cm); (2) an ice 
storm occurred in central North Dakota during 15-19 De- 
cember 1977; (3) most of the older stands had just endured 
nearly 2 years of extreme drought during 1976 and 1977; 
and (4) the 1979 measurements were somewhat reduced 
because of the effects of a heavy snow cover and the har- 
vesting of vegetation by rodents, mainly meadow voles, 
Microtus spp. (Fig. 5). Storms and drought are believed 
largely responsible for the lower residual cover and the re- 
duced structure of the vegetation in SNC stands through- 
out much of the study region in 1978. 
Factors Affecting Visual Obstruction Measurements 
Tall Wheatgrass Versus Other Wheatgrasses 
Fields with predominantly more tall wheatgrass with- 
stood the effects of winter and climatic elements better 
than fields dominated by other wheatgrasses (Fig. 6). In 
1977-79, stands dominated by tall wheatgrass averaged 
