Effect of snowbrush on the establishment and growth of Douglas-fir seedlings Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/td96k513f

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  • Six, recent clear cut areas were selected in the Douglas-fir zone of the Western Cascade Range of Oregon to determine the environmental and nutritional effects that snowbrush has on the establishment and growth of Douglas-fir seedlings. One thousand eighty milacre sample plots on the six clear cuts were used to determine the survival, total height growth and browsing damage of Douglas-fir seedlings. The nutrient status of the seedlings growing in association with snowbrush and in the open was also determined on the six clear cuts. Needles on the current year's growth were analyzed at three times during the year, for nitrogen, calcium, magnesium, potassium and phosphorus. The nutrient levels of the seedlings were then related to their position relative to snowbrush. The percent total nitrogen and the exchangeable calcium, magnesium and potassium in the soil under snowbrush and in the open were analyzed. Three of the six clear cuts were selected and soil moisture in the open, along the edge and under snowbrush was measured with Colman fiberglas moisture blocks and gravimetrically, In all, 33 moisture stations were used. Moisture was measured at 2, 6 and 24 inches. At each moisture station, the maximum soil surface temperature was determined with a maximum reading mercury thermometer. In addition to the mercury thermometers, six continuous recording thermometers were used to determine the number of consecutive days on which high soil temperature occurred. Precipitation through the growing season, both in the open and under snowbrush, was determined with home-made rain gauges. Survival and growth of planted Douglas-fir seedlings were better under snowbrush than in the open. The milacre stocking under snowbrush was 48% greater than in the open. The total height was 2.5 feet greater on nine-year-old seedlings, whereas, the yearly height growth was as much as 5.1 inches greater on seedlings associated with snowbrush, than those in the open. Browsing damage was greatly reduced by brush cover. Only 28.3% of the seedlings under snowbrush were damaged, compared to 69.0% and 73.5% damaged in the open and along the edge, respectively. Throughout the growing season, soil moisture in the top six inches contained more moisture under snowbrush than in the open. At 24 inches, less water was available under the brush than in the open. The soil moisture consumption suggests that a newly planted or germinating seedling will have more moisture available to it under brush cover, than in the open, however, the seedling will eventually have to compete with the brush for moisture as its roots penetrate the soil to about 24 inches. Lethal soil surface temperatures were encountered in the open. Soil surface temperatures in the open reached levels high enough to cause death to first year seedlings and at least some damage to planted stock, whereas, under the brush, soil surface temperatures never reached the lethal limit, even during the extremely hot, dry summer of 1967, Lower soil surface temperatures under snowbrush are probably one of the most important factors in maintaining higher soil surface moisture levels under the brush. The concentration of nitrogen, calcium, magnesium and potassium in current year's Douglas-fir needles is greater in seedlings growing in association with snowbrush than in those growing in the open. The concentration of phosphorus in needles, however, did not appear to be related to snowbrush cover. The concentration of all nutrients tested, except phosphorus, declined as the growth rate increased. Seedlings growing in association with snowbrush did not have as marked a decrease, particularly in the case of nitrogen,as did seedlings in the open. The effect of snowbrush on the declining nutrient content was more marked on poorer sites, than on better sites. The effect of snowbrush appeared most pronounced during periods of high nutrient stress. The nutrient content of the soil under a stand of snowbrush was higher in terms of total nitrogen and exchangeable calcium, magnesium and potassium than the soil in the adjacent opening. It would appear that snowbrush is not only capable of increasing nitrogen through fixation, but is also instrumental in maintaining more cations in the surface soil, through re-cycling in the litter. Snowbrush in the Western Cascades appears to be instrumental in creating a more favorable microenvironment as well as a more favorable nutrient regime for newly establishing Douglas-fir seedlings.
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