Graduate Thesis Or Dissertation

Response of Soil Temperature, Moisture, and Respiration Two Years Following Intensive Organic Matter and Compaction Manipulations in Oregon Cascade Forests

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  • Forest soils contain a substantial portion of global terrestrial carbon stores. Forest management can influence the soil carbon pool and how soil organic matter functions. The long-term productivity of forests is an ongoing goal where land managers utilize biomass and timber. A site-specific understanding of intensively managed forests can ensure achievements of this goal. Within a managed forest in the western Oregon Cascades, treatments were installed to harvest three levels of biomass, with and without compaction, to monitor impacts to growing season characteristics of Douglas-fir roots. Soil temperature and moisture conditions were continuously monitored from 10 to 100cm depth, and three sources of soil respiration were measured monthly for two years immediately following treatments. Negligible differences in the length of growing season were detected, however the daily-10cm average, maximum, and diel flux of soil temperatures significantly increased by 1.5, 2.7, and 2.5°C, respectively, with increasing biomass harvesting. Organic matter removal strongly influenced growing season soil characteristics down to a 100cm depth. Diel temperature flux at 100cm for the least and most impacted treatments were 5.7 and 7.8°C, respectively, a magnitude equivalent to seasonal shifts in soil temperature at the same depth. In spite of favorable temperature and moisture conditions with less organic matter left on the surface, soil respiration was moderately higher on bole only harvests. A priming effect may explain why these sites with more surface biomass, although significantly cooler, had the highest rates of soil respiration. The combination of increased temperatures throughout the soil profile after forest harvesting, and higher additions of dissolved organic matter from forest residuals, could have an impact on deep soil carbon. These responses have implications for long-term nutrient cycling that have yet to be elucidated for deeper soils; but this should be considered when land managers are planning forest fertilization and rotation lengths.
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