Comparative study of some soil characteristics of grassland and forest/grassland transition in western Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/p26770050

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  • Vegetation as a soil-forming factor was studied under forest transition and grass while attempting to keep other soil forming factors constant in the western part of the Willamette Valley, Oregon. The physiographic units consist chiefly of the interior foothills and slopes formed from an old basaltic flow with some interspersed sedimentary formations. The soils and vegetation of these physiographic units are developing under a temperate humid climate characterized by dry summers and mild, rainy winters., The foothills and slopes support stands of Douglas-fir (Pseudotsuga menziesii) and Oregon white oak (Quercus garryana). Scattered through the area are non-forested openings. Many ecologists have looked into the successional relationships of the communities within the Willamette Valley. One school believes that the open conditions became established during a time of warmer and perhaps dried hypsithermal climate that occurred near the middle of the postglacial time. Fire control activities motivated by the settlers are, however, thought to be responsible for the successional change--replacement of a prairie by Quercus savanna, then by Quercus /Pseudotsuga, then by Pseudotsuga, etc. The soils under both vegetation types were similar (classified as series) and they ranged from Mollisols to Ultisols. To gain insight into the effects of vegetation on soil development, physical and chemical analyses were carried out and values were obtained for the following: particle sizes, pH, organic matter and organic carbon, total N, exchangeable cations and free Fe oxides. The establishment of two different vegetational assemblages has had a direct impact on the soils. The soil parameters that are affected by the plant cover are directly or indirectly associated with the organic matter with the exception of exchangeable sodium. Generally, there are some differences in both color and structure of the soils under these two vegetation types, but they are not adequately quantified to show statistical differences. The profiles range from very strongly acid in soils derived from sedimentary rock, to neutral in the soils derived from basaltic rock. The pH of soils under grass is significantly higher than that of the forest transition soils. There is subsurface accumulation of silicate clays but there is not much sign of the accumulation of oxides. The exchangeable potassium contents of forest transition soils are significantly higher than those of the grassland soils. This is attributed to greater recycling of potassium by tree species. The percentages of total nitrogen in the A horizons of the grassland soils are significantly higher than those of the A horizons under forest transition. The carbon/nitrogen ratios of forest transition soils are significantly higher than those of grassland soils. All these are attributable to the qualities of the organic matter in the soils of the two vegetation types. The overall exchangeable sodium in the forest transition soils is significantly higher than that of the grassland soils. The reason for this might be that the area under study is very close to the ocean and that the trees are able to trap wind-borne salt spray from the ocean, which is eventually washed down the profile Research is needed to prove this hypothesis. The soils of the study area do not possess features of podzolic soils even though they occur in a latitude and under a forest vegetation where podzolic soils are commonly formed. This can be attributed to the climate of this area and the nature of the parent material of the soils under study.
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