Graduate Thesis Or Dissertation
 

Understory competition for resources in Pinus ponderosa forests of northeastern Oregon

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/2801pk584

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  • The objective of this research was to determine which environmental resources, light, water, and nutrients, control understory plant production and composition in a Pinus ponderosa forest in northeastern Oregon. A split-plot experimental design, with three blocks, four treatments, and 44 plots, was established in the summer of 1985. Twenty plots (4 x 4 m) were trenched approximately one meter in depth, and 24 non-trenched plots were used to assess the effects of root competition of overstory trees on understory plants. Trees were commercially thinned in the winter and spring 1986 from a density of 345 to 148 trees/ha⁻² to increase light levels to the understory. Thinning significantly increased light, decreased midday relative humidity and increased midday air temperatures and soil temperatures. Xylem potential of the dominant graminoid, Carex qeyeri, soil water potential, mineralizable nitrogen, and pH were significantly increased within the trenched treatment in comparison to the nontrenched plots. Micro and macro nutrients in C. qeyeri and Symphoricarpos albus, the dominant shrub, significantly increased in both treatments Controlling root competition for soil water and nutrients did significantly increase understory aboveground biomass, whereas increasing light had no effect. A supplemental experiment during the third year of this study indicated that water and nitrogen had a synergistic effect in improving production. Species composition, cover and density, however, were significantly effected by light, water, and nutrients. This research demonstrated that belowground resources were the primary controlling factors of understory production in P. ponderosa forests in northeastern Oregon. However, belowground and aboveground resources influenced species composition.
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