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