485 
es 
nt 
tion 
of the population, are actively establishing burrow systems. The consequent 
earth-mounding activity for the year is at maximum. Mounds are concentric 
piles of soil on the ground surface that have been pushed to the surface during 
burrow-building activities (Fig. 3). Earth plugs are circular holes approximately 
15 cm in diameter filled with fresh soil. The relationship established by 
Reid et al. (1966), was: 
/ Y X = .6582 /"x log (X + 1) 
where: / Y v = population estimate per unit area 
X = number of signs per unit area 
Northern pocket gopher earth-mounding sign was first observed in 
large-scale (1:2400) aerial photographs during investigations on use of this 
kind of remote sensing for vegetation analysis (Driscoll, 1971). The investigation 
reported here was designed to (1) determine the relationship between ground 
survey and photo interpretations of northern pocket gopher earth mounds, and (2) 
establish the relationship between the population estimate technique developed 
by Reid et al. (1966) and one developed by photo interpretation. The work was 
done on the Black Meas Experimental Forest and Range approximately 45 kilometers 
west of Gunnison, Colorado, U.S.A., at an elevation of approximately 3000 meters. 
The specific study locations were within grassland parks surrounded by quaking 
aspen (Populus tremuloides ) and Engelmann spruce ( Picea engelmannii) forests. 
METHODS 
Ground Data 
At each of three locations, two paired 0.4 ha plots were selected 
within areas of apparent high gopher populations. Each location also represented 
a slightly different vegetation system. At one location, the vegetation was 
primarily a grass-forb mixture, predominantly Thurber fescue ( Festuca thurberi ), 
Idaho fescue (Festuca idahoensis) , hairy golden aster ( Chrysopsis villosa ) and 
aspen daisy (Erigeron macranthus ). The second location included these species 
but much more hairy golden aster, a low, almost prostrate forb, and some Parry 
rabbitbrush ( Chrysothamnus parryi ). The third location contained mostly Parry 
rabbitbrush with a mixture of the previously mentioned and other herbaceous 
species. All plots were grazed by domestic cattle. Each location was selected 
to be approximately 0.5 kilometers away from the forest and on relatively 
level terrain to afford safe airplane navigation and photo scale control. 
The plot pairs at each location were aligned in the direction of 
photo mission flight lines. Within each 0.4 ha plot, twenty 0.004 ha circular 
subplots were selected by restricted randomization such that five occurred in 
each quarter of the large plot. This sampling design was based on that developed 
by Reid et al. (1966) for ground survey technique. 
One plot of each pair at each location was designated as "erased" 
and the other as "natural". On the "erased" plots, in accordance with the 
recommendations of Reid et al. (1966) in which all gopher sign on designated