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METHODS
Study Site
The study took place in an active outbreak of the pandora moth in the Deschutes National Forest about 20 km south
of Bend, Oregon. The stand is a ponderosa pine/bitterbrush-manzanitJfescue plant community (Volland 1976). Lodgepole
pine is the only other tree species present and is scarce. The original ponderosa pine was logged in the 1930's and the second
growth stand thinned in the 1970's. The average age of the trees at the time of our study was about 55 years and spacing
averaged 3.6 by 3.6 m, but the latter differed across the area (the study area averaged 165 trees per acre).
Elevation of the study area is 1,250 in, slope varies from 0 to 5 percent, and aspect is slightly south. The soil mantle
is 0.9-m-deep pumice, which resulted from the Mount Mazama volcanic eruption about 6,700 years ago. Bare pumice is
common and there is only a thin organic layer under the trees. Soils are particularly deficient in nitrogen, phosphorus, and
sulfur (Cochran 1978). Annual precipitation is approximately 51 cm and occurs mostly as snowfall from October through
March. Precipitation at Bend was 23 cm in the 1988-1989 season, and first snowfall occurred within a week after we applied
fertilizer.
Experimental Design and Treatments
In fall 1988, 10pairs of circular 1/10th of an acre plots (11.35-m radius) were located in a 4 by 5 grid with plot
centers approximately 40 m apart (Fig. 1). All trees on the plots were marked with metal tags. On average, treated plots
experiment near Sun River
on the Deschutes National
Figure l.--Schematic Forest. F = fertilized layout of and C
= control.
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