Survival and growth of Douglas-fir and ponderosa pine during eight years of whiteleaf manzanita and herb competition in southwest Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/0k225d67c

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  • This thesis examines the 8-year growth trend of Douglas-fir (Pseudotsuga menziesii (mirb.)Franco), ponderosa pine (Pinus ponderosa Dougl. ex laws.), and whiteleaf manzanita (Arctostaphylos viscida Parry). Manzanita stands ranging from 1700/ha to 27000/ha density were grown among mixed conifers of the same age, which were planted in 1981 at 250x250 cm spacing as 2-0 bareroot seedlings. Four years of herb control was provided by herbicides or paper mulch in all spacings. At a density of 13500/ha of manzanita, plots were grown with no control of herbs, and others were grown with no conifers. Mixed conifers also were grown with no competitors, and some plots, initially free of all vegetation, later supported only annual herbs. Three sites were used in a complete randomized block design with nine treatments as above. All were located on south slopes at low elevation in southwest Oregon, representing the warmest and driest area of the Douglas-fir region. Conifer stem volume and biomass yield were inhibited quantitatively by manzanita competition. As manzanita density increased the yield of conifers decreased in an increasing trend through time. Significant biomass yield decrease (14 percent) was associated with 17 percent brush cover in ponderosa pine. Douglas-fir had high mortality in manzanita at less than 27000/ha density, while ponderosa pine mortality was minor. Herb competition severely inhibited survival and growth of both manzanita and conifers. Shrub canopy cover reached a maximum by age seven at 27000 manzanita/ha density, but continued to expand at lower densities. Shrub yield was not affected by conifer growth, but intraspecific competition at the high densities of manzanita decreased yield and caused high mortality by the seventh year. Soil moisture was depleted most rapidly in the top 30 cm of soil. When manzanita competition occurred, soil water was depleted more than in pure conifer or pure herb stands. Manzanita used most of the available soil water to below 60 cm early in the season (May-June). Trees in pure conifer plots grew at the fastest rate, and ended the summer with the highest soil moisture content. Conifers can survive and grow satisfactorily under harsh environments if brush cover is reduced to low densities, and herbs are sprayed in the first two to three years. Ponderosa pine is better suited for reforestation of hot south aspects in the foothill region. Douglas-fir needs higher levels of brush and herb control, and remain vulnerable to intense heat and drought through age eight.
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