The role of soil-water depletion and plant-moisture stress in soil classification and cambial activity of Douglas-fir Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/7s75dg626

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  • Relationships between soil-water stress and plant-moisture stress were studied with respect to soil classification and cambial activity in Douglas-fir. The study sites were forested with Douglas-fir and located in McDonald Forest and near Marys Peak in the Oregon Coast Range. Soil-water depletion was followed by the use of gypsum blocks on sites representing seven soil series and one soil phase. Plant-moisture stress was determined by a pressure bomb and recorded in conjunction with the soil-water depletion throughout the growing season. Cambial activity of a Douglas-fir tree growing on each study site was also recorded throughout the growing season. Summer soil temperatures were measured at a depth of 50 cm. Moisture tensions and bulk density for each horizon of each soil were determined in the laboratory. Dixonville (shallow variant), Jory (west slope), Nekia, and Jory (north slope) soils were located in McDonald Forest and had a soil-water stress of 15 bars for at least 60 consecutive days within the soil-moisture control section. Current and past weather records show that this stress will occur in at least seven out of ten years. These soils are classified as xeric-mesic. Blachly, Kilchis, Klickitat, and Hembre soils were located in the vicinity of Marys Peak and had soil-water stresses of 15 bars ranging from 21 to 47 consecutive days within the soil-moisture control section. Current and past weather records show that these soils will not develop a 15 bar stress for 60 consecutive days in seven out of ten years. Therefore, they are not xeric. The term moist-mesic is used to define such soils. Difference in elevation within a local area is not a sufficient indicator by itself to separate the xeric and non-xeric zones. Plant-moisture stress cannot be used to indicate soil-water stress for the purpose of soil classification. Some trees evidently obtain water from spaces within the fractured bedrock. This water source is not defined in the soil classification scheme. There was a highly significant difference in plant-moisture stress between the study trees growing in the xeric-mesic zone and moist-mesic zone at the time of the springwood-summerwood transition. Trees in the two zones showed large differences in plant-moisture stress at the time of dormancy. In both cases, trees growing in the xeric-mesic zone had higher stresses than trees growing in the moist-mesic zone. There were very large differences between the xeric-mesic and moist-mesic zones in the percent of available water used from the upper one-third of the soil profile at the time the trees stopped producing springwood. Large differences in the percent of available water used from the entire profile occurred between soils in the two zones at the time the trees became dormant. At both transition and dormancy, more available water was depleted from the soils in the xeric-mesic zone than the moist-mesic zone. Trees in the xeric-mesic zone produced springwood a greater number of days than the trees growing in the moist-mesic zone without necessarily producing more springwood. Trees in the xeric-mesic zone grew an average of 27 more days than trees in the moist-mesic zone without necessarily producing more total wood.
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