Graduate Thesis Or Dissertation
 

Effect of western juniper removal on ponderosa pine and associated understory vegetation

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/dr26z1132

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  • This research assessed the effect of western juniper (Junlperus occldentalls) removal on understory plant production and cover and ponderosa pine (Pinus ponderosa) growth and plant water status. A complete randomized block design, with four blocks and four treatments was established in the summer of 1984 near Prineville, Oregon. The four treatments were: Control - all trees left at original densities, Pine Thinned - ponderosa pine was thinned to 5 m x 5 m spacing, Juniper Removed - all western juniper were removed, and Pine Thinned/Juniper Removed - ponderosa pine was thinned to 5 m x 5 m spacing and all western juniper were removed. Understory plant production was obtained one year after tree removal, and cover two years after tree removal, to determine response to treatment. Plants were separated into species or groups in the field, then dried and weighed. Dominant grasses on the sites were, bluebunch wheatgrass (Agropyron spleaturn), Idaho fescue (Festuca idahoensis), bottlebrush squirreltail (Sitanion hystrex), Sandberg bluegrass (Poa sandbergii), and cheatgrass (Bromus tectorum). Mountain big sagebrush (Artemisia trldentata ssp. vayseyana) and Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) were the principal shrub species. Perennial grasses, total shrub, and total production and cover showed a significant response to western juniper removal. Wyoming big sagebrush, mountain big sagebrush and forbs showed no response to western juniper removal. Thinning ponderosa pine appeared to reduce production and cover of most groups. Ponderosa pine basal area and height growth were examined for response to tree removal. Basal area growth was determined by measuring diameter at breast height (1.4 m) immediately after, one year after and two years after tree removal. Height growth was sampled in a similar manner using a metal tape on trees under 2 m and a Relaskop on trees over 2 meters. Response of ponderosa pine was similar to the understory vegetation. In both years growth of ponderosa pine was greater in treatments where western juniper was removed, although only the height response was significant. Small trees, under 5 cm in diameter, had significantly greater percent growth than the other trees. Predawn needle xylem pressure potential and soil moisture were also observed. Measurements were taken five times at three week intervals throughout the summer using a pressure chamber. In general early season predawn xylem pressure potentials of ponderosa pine in treatments with western juniper appeared higher than pine in treatments with western juniper removed. This trend reversed midway through July. Soil moisture was sampled gravimetrically on the same date as the pressure chamber measurements. Soil moisture response was similar to predawn xylem pressure potential response. Treatments with western juniper present started the season with higher soil moisture, but fell below the treatments without western juniper later.
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