Carbon dioxide exchange of a Sitka spruce plantation in Scotland over five years

Scotland's 17% forested land cover sequesters 10% of Scotland's emissions of greenhouse gases. The expected rise of forested land cover to 25% by the middle of this century make forests an important component of the national carbon budget. This paper presents the carbon exchange measurement methodology, results, and modeling associated with a five-year experiment at a plantation forest in Scotland, UK, as part of a European initiative to develop and verify vegetation carbon-exchange models.We have developed a site specific correction for advective flux loses. While based on ustar corrections, it differs from current approaches in that it includes site topographic effects, corrects existing data instead of replacing it, and applies over the entire diel cycle instead of only during nocturnal periods.Models of soil respiration are compared and reveal the inadequacy of soil state parameterization. A non-rectangular hyperbola model of assimilation is selected to examine the maximum assimilation rate and quantum use efficiency parameters’ responses to environmental temperature, vapour pressure, and sky condition. Intra-annual variability is found to be highest in early autumn, linked to a combination of seasonal changes in radiation and phenological changes in canopy quantum yield. Inter-annual variability was low, with less than %5 variation from year to year, and factors driving this variability were not clearly apparent.Over the 5 years of reported measurements, this forest sequestered about 6 tonnes of C per hectare per annum. Photosynthetically active radiation use efficiency of the forest is about 4.2%. Constancy of radiation use efficiency was observed at both diel and annual scales. This constancy depended on the ecosystem's thermal inertia and assimilation/radiation response curve characteristics at diel time scales but depended on thermal inertia and phenological changes in quantum yield at annual time scales.The net carbon uptake rates are consistent with two other flux sites in the British Isles and suggest that the relatively maritime climatology of these areas is highly suitable for the species of trees routinely planted here.

► Five years of eddy covariance CO2 fluxes were measured for a Scottish spruce forest.
► Intra-annual variability was shown to be associated link to increases in quantum use efficiency and lower radiation levels during Autumn.
► Inter-annual variability was small and appeared only as a significant dependence of net CO2 exchange on inter-annual variability of radiation.
► Corrections were developed for advective flux losses over the diel period, which had the effect of partially negating advective nocturnal flux losses.
► Radiation use efficiency found to because by different factors associated with assimilation at diel and annual time scales.