Ecosystem carbon measurement, modeling, and management in a young ponderosa pine plantation in northern California

Based on field measurements from 1998 to 2000, I developed an ecosystem carbon management model to examine the effects of different ecosystem management practices on ecosystem carbon dynamics in a young ponderosa pine plantation in the Sierra Nevada Mountains, California. I developed a new technique to measure stem and branch respiration easily and accurately. Soil surface CO 2 efflux was measured using an LI-6400 Soil CO2 Flux System. Microbial biomass, fine root (<5 mm) biomass, and physical and chemical properties of the soil were also measured at each sampling location.; I found that stem + branch respiration was about 0.4, 0.7, and 0.25 mumolm -2 ground area s-1 in June, August, and October, respectively. I also found that the mean soil CO2 efflux in the plantation was 4.43 mumolm-2s-1 in the growing season and 3.12 mumolm-2s -1 in the nongrowing season. The annual maximum and minimum CO2 efflux were 5.87 and 1.67 mumolm-2s -1, respectively. The maximum occurred at the end of May and early June and the minimum occurred in December. The Q10 values, calculated from soil temperature at 5 cm depth (Q10(T5)), ranged from 1.2 to 2.47 among the 18 sampling locations with an average of 1.61. Q10 also showed a strong seasonal variation with the annual minimum occurring in mid summer and maximum in winter. Soil moisture affects both soil respiration rate and temperature sensitivity (Q10). Soil surface CO2 efflux was almost independent of temperature during midsummer when soil was extremely dry.; From June to October 1998, the total ecosystem respiration (ER) was 5.7 +/- 1.3 mumolm-2s-1 (based on daily average). About 67% of ER was from soil surface CO2 efflux, 10% was from stem and branch (stem + branch) respiration, and 23% was from foliage respiration. Shrub leaves contributed about 24% to the total foliage respiration, and the current year needles (1998 age class) accounted for 40% of the total tree needle respiration. Root respiration comprised about 47% of the soil surface CO2 efflux.; I found that the ecosystem was a significant net carbon sink, with 268gCm -2 absorbed between June 1 and October 31, 1998, based on daytime eddy covariance measurements and nighttime modeled respiration. The net primary productivity (NPP) was 578gCm-2 during the same time. The modeled and measured daytime mean net ecosystem productivity (NEP) match well with the coefficient of determination (R2) of 0.72--0.73 depending on the stomatal conductance models used. Seasonal mean GPP, NEP, NPP, and ecosystem respiration were 7.2, 1.7, 3.7, and 5.5 mumolm -2s-1, respectively.; Model results suggest that shrub cutting will change the ecosystem from a net carbon sink of 268gCm-2 to a net carbon source of 148gCm-2 in the same period. A thinning intensity of 20% and 40% (by DBH) will reduce the net ecosystem carbon absorption by about 55% and 92%, respectively. A thinning intensity of 60% will turn the ecosystem to a substantial net carbon source of 71gCm-2.