Effect of western juniper on understory herbage production in Central Oregon

Graduation date: 1986Evidence suggests that western juniper\ud (Juniperus occidentalis) in Central Oregon affects\ud understory production and composition. As trees\ud increase in size and density, understory production is\ud reduced and composition changes. This study was\ud designed to identify the relationship between\ud production and composition of understory vegetation\ud and various tree canopy sizes of western juniper, and\ud to describe the responses of understory vegetation to\ud removal of western juniper. Understory production was\ud sampled in 1983 and 1984 by clipping plots on an\ud individual trees basis from areas with trees present and areas from which trees had been removed in 1982.\ud Biomass production was determined from trees within\ud three canopy diameter size classes and from two zones,\ud beneath the canopy and in the interspace. Production\ud was examined on two sites, a lower slope, shallow soil\ud site and an upper slope, moderately deep soil site.\ud Some year-to-year differences in production\ud of individual species and specific tree sizes may be\ud explained by variation in precipitation during\ud critical growth periods the two years of the study.\ud Sandberg bluegrass and, possibly, Idaho fescue\ud production may have been greater in 1983 than 1984 due\ud to variation in late spring precipitation, especially\ud on shallow soils associated with small trees.\ud Differences in patterns of production of\ud individual species were most clearly developed\ud associated with large, rather than small and\ud intermediate trees. Cheatgrass, squirreltail,\ud bluebunch wheatgrass, and Idaho fescue were common\ud beneath the canopy, while Sandberg bluegrass was\ud common in the interspaces. Cheatgrass and perennial\ud forb production beneath the canopy increased with tree\ud size. Production of other species, such as Sandberg\ud bluegrass, was apparently not affected by tree size. Canopy removal resulted in species-specific\ud increases in biomass production both years. Production\ud increases were greater beneath the canopy than in the\ud interspaces. Perennial grasses provided small variable\ud production increases beneath the canopy of large\ud trees. Annual grasses, primarily cheatgrass, and\ud annual forbs, primarily Epilobium paniculatum,\ud contributed most to elevated productivity the first\ud two years following juniper removal. Cheatgrass\ud response was mainly associated with large trees, while\ud annual forb response was independent of tree size.\ud Sandberg bluegrass production seemed more closely tied\ud to growing season precipitation than to canopy\ud removal, regardless of tree size