Soil carbon measurement and modeling in forest and savanna ecosystems of the Sierra Nevada: Temporal and spatial patterns and management impact

Soil respiration and its variation are influenced by soil temperature, moisture, and root density, and also affected by management activities. By conducting multivariate regression I analyzed the impact of a forest thinning in May 2000 on soil respiration in a young ponderosa pine plantation in the Sierra Nevada. The thinning decreased the spatial heterogeneity of soil respiration, but did not change the sensitivity to temperature and moisture. Although the thinning would theoretically decrease soil respiration, the actual change of soil respiration was not significant due to the varied temperature and moisture with the thinning. The impact of thinning on soil respiration was explained by the change of root density, soil temperature and moisture.; I partitioned soil respiration into autotrophic and heterotrophic respiration by conducting trenching experiments to exclude roots in the pine plantation, and separately modeled these three components between October 2001 and 2002. In addition to environmental variables, root respiration was affected by plant physiology and phenology. The ratio of autotrophic respiration to total soil respiration was not a constant seasonally with an average of 0.33. The spatial variation of soil respiration was mainly explained by root density.; I compared the soil respiration in a young and mature plantation between October 2001 and 2002. The difference of soil respiration was not significant, but soil respiration in the mature plantation would be 1.2 times greater than that in the young plantation if the difference of soil temperature and moisture between two sites is removed. A model that I developed incorporated soil temperature, moisture, stand density, and tree size, and well explained the spatial variation of soil respiration and soil carbon dynamics.; I developed an automated flux measurement system by burying small CO ₂ sensors and continuously measuring CO₂ concentration gradients in a savanna ecosystem in California in the summer, 2002. I calculated diffusion coefficient, and then estimated CO₂ efflux. The diurnal variation of CO₂ concentration and efflux was more significant than day-to-day variation. The temperature sensitivity (Q₁₀) was 1.27 in the dry season. The high correlation between CO₂ efflux and temperature explained the diurnal pattern of CO₂ efflux, but moisture may become another factor driving the seasonal pattern when moisture changes over seasons.