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CO2 exchange was measured on the forest floor of a coastal temperate Douglas-fir forest located near Campbell River, British Columbia, Canada. Continuous measurements were obtained at six locations using an automated chamber system between April and December, 2000. Fluxes were measured every half hour by circulating chamber headspace air through a sampling manifold assembly and a closed-path infrared gas analyzer. Maximum CO2 fluxes measured varied by a factor of almost 3 between the chamber locations, while the highest daily average fluxes observed at two chamber locations occasionally reached values near 15 μmol C m−2 s−1. Generally, fluxes ranged between 2 and 10 μmol C m−2 s−1 during the measurement period. CO2 flux from the forest floor was strongly related to soil temperature with the highest correlation found with 5 cm depth temperature. A simple temperature dependent exponential model fit to the nighttime fluxes revealed Q10 values in the normal range of 2–3 during the warmer parts of the year, but values of 4–5 during cooler periods. Moss photosynthesis was negligible in four of the six chambers, while at the other locations, it reduced daytime half-hourly net CO2 flux by about 25%. Soil moisture had very little effect on forest floor CO2 flux. Hysteresis in the annual relationship between chamber fluxes and soil temperatures was observed. Net exchange from the six chambers was estimated to be 1920±530 g C m−2 per year, the higher estimates exceeding measurement of ecosystem respiration using year-round eddy correlation above the canopy at this site. This discrepancy is attributed to the inadequate number of chambers to obtain a reliable estimate of the spatial average soil CO2 flux at the site and uncertainty in the eddy covariance respiration measurements. [Copyright &y& Elsevier]

A process-based model of the transport of heat and water in the soil and surface energy balance components is extended to include production of CO2 from heterotrophic (microbial decomposition) and autotrophic (root) respiration, and transport of the resulting CO2 in both the gaseous and liquid phases. The production of CO2 is determined by the amount and type as well as distribution in the soil profile of organic matter and roots, and their respective first-order rate constants. Influences of soil water and temperature are considered through their effects on respiration, CO2 diffusivities and chemical equilibria among various C species in the soil–solution–air continuum. Also included is CO2 uptake by plant roots associated with root water uptake.Model calculations, using independently determined parameters, are compared with measurements of soil CO2 concentrations and chamber-measured forest floor fluxes in a second-growth 54-year-old Douglas-fir forest located on the east coast of Vancouver Island, Canada. Measured and modeled forest floor CO2 effluxes and soil CO2 concentration profiles, as well as soil water contents and temperatures showed good agreement. The efflux was most sensitive to soil temperature, and the influence of soil water content was relatively small. In this rapidly draining soil, the CO2 efflux, at time scales as small as half-hour, was very well approximated by the total production of CO2 in the soil profile, even during and after rainfall, which significantly increases soil water content. This is because CO2 diffusion in this forest soil is relatively rapid compared to changes in the rates of CO2 production. In this podzolic soil ecosystem with low soil pH, liquid-phase diffusion appears to play a minor role in CO2 transport. High soil CO2 concentrations, up to 10,000 μmol mol-1 in summer and 6000 μmol mol-1 in winter, and a positive downward gradient at the 1 m depth indicate some CO2 sources associated with very low CO2 diffusivity at deeper depths. [Copyright &y& Elsevier]

Continuous automatic chamber and eddy covariance (EC) measurements were made at an old aspen forest (SOA) located at southern boreal treeline in Saskatchewan, Canada to examine the temporal variability in soil (Rs), tree bole (Rb), and ecosystem respiration (RE) during 2001. Climatic conditions were significantly warmer and drier than the 30-year climate normal, resulting in lower RE and an unprecedented increase in net ecosystem productivity (NEP). In the 7-year record (1994, and 1996–2001) of CO2 exchange at SOA, the year 2001 showed the greatest carbon gain (300 g C m-2 year). Scaled chamber measurements (1315 g C m-2 per year) were 37% larger than the EC estimate of RE (961 g C m-2 per year). The difference between the scaled chambers and the EC estimate was reduced to 20% after correcting for lack of energy balance closure. Annual RE was approximately 170 g C m-2 per year lower than the average of the previous 6 years. Annual estimates of microbial-heterotrophic (Rh) (510 g C m-2 per year) and autotrophic respiration (Ra) (805 g C m-2 per year), based on chamber measurements, were used to help validate the EC estimate of RE. Ra represented 61% of the total chamber respiration. This fraction was used to partition RE into Ra and Rh to calculate net primary production (NPP). The values of NEP (300 g C m-2 per year) and NPP (675 g C m-2 per year) were more characteristic of temperate forests. The NPP/Pg ratio of 0.54 was within the range of recently published values using biometric techniques and supports that the annual ecosystem respiration budget and its partitioning was well constrained. We recognize, however, that this ratio will vary interannually depending on climatic conditions. In 7 years of annual EC CO2 flux measurements at SOA, this study provides the first evidence that drought can lead to a transient increase in CO2 sequestration resulting from a reduction of RE. [Copyright &y& Elsevier]

Continuous measurements of carbon exchange using the eddy covariance (EC) technique were made at three boreal forest mature sites including Southern Old Aspen (SOA), Southern Old Black Spruce (SOBS) and Southern Old Jack Pine (SOJP) in 2000. Climatic conditions were slightly warmer than normal with precipitation exceeding evapotranspiration at each site. Annual ecosystem respiration (R) derived from daytime analyses of EC data was 1141, 815 and 521 g C m−2 per year and was consistently lower than nighttime EC estimates of 1193, 897 and 578 g C m−2 per year for SOA, SOBS and SOJP, respectively. The differences, however, were not statistically significant given the large uncertainty associated with each analytical technique. The uncertainty in annual net ecosystem productivity (NEP) was assessed by randomly simulating missing data and gap filling using simple biophysical algorithms. The uncertainty analysis supports the finding that each site was a net sink, and that differences in NEP were only significant between SOA and SOBS. The annual NEP and uncertainty for SOA, SOBS and SOJP was 122 (64–142), 35 (18–53) and 78 (61–91) g C m−2 per year, respectively. These relatively old growth forests represent a weak to moderate carbon sink. Despite having the shortest growing period, carbon sequestration was greatest at SOA because of its relatively large photosynthetic capacity (Amax). At the evergreen sites, Amax was marginally larger at SOBS; however, annual carbon sequestration was smaller as a result of greater R. The evergreen sites exhibited a pronounced mid-season reduction in NEP, which was attributed to a large increase in R while Amax had not reached its full capacity. Non-growing season R resulted in a carbon loss of 285, 120 and 64 g C m−2 and accounted for 70, 80 and 46% of the summertime NEP at SOA, SOBS and SOJP, respectively. Six years of EC data at SOA indicate that carbon sequestration at boreal aspen sites may benefit from warmer climatic conditions because R is relatively conservative and photosynthesis increases in response to a longer growing period. [Copyright &y& Elsevier]

Two years of continuous eddy covariance measurements were used to characterize the seasonal and annual variability of the latent and sensible heat fluxes above a 50-year-old, 33 m tall coastal Douglas-fir forest on the east coast of Vancouver Island, Canada. The total annual evaporation was found to be very conservative for this temperate coniferous rainforest despite variability in weather between the years (432 mm in 1998 and 435 mm in 1999). Winter evaporation was a significant component of the annual total, on average 27% of the mean 434 mm per year. Seasonal variations in the magnitude and direction of the sensible heat flux above the canopy were linked to changes in the surface conductance to water vapour transfer. The wet canopy tended to act as a sink for sensible heat, especially throughout the winter months, resulting in an average daily 24 h Bowen ratio of −1.7. This contrasted dramatically with summer daytime turbulent exchange, which was usually dominated by upward sensible heat flux during the summer months (April–September, inclusive). The total 24 h Bowen ratio for the summer was 1.1, with daily 24 h values reaching a maximum of 3.1 for dry-canopy conditions. The average rate of transpiration was 1.7 mm per day reaching a maximum of 3.7 mm day, while canopy conductance ranged from 1 to 30 mm s−1.Although the total winter and summer evaporation was similar between years, differences in the timing of the maximum evaporation rates and flux partitioning patterns resulted in considerable variability within the seasons. The cooler, cloudier and wetter weather of 1999 maintained relatively low evaporation rates and low Bowen ratios through the entire summer, while clear skies and hot and dry conditions in 1998 resulted in greater evaporation until mid-July. After this time, drought and a decrease in canopy conductance reduced evaporation and led to a daily daytime Bowen ratio as high as 3.8. [Copyright &y& Elsevier]