Components of ecosystem respiration and an estimate of net primary productivity of an intermediate-aged Douglas-fir stand

Continuous half-hourly chamber-measured belowground (R-s) and eddy covariance (EC)-measured total ecosystem (R-e) respiration in a 56-year-old Pacific west coast Douglas-fir stand from 2003 to 2005 were analyzed to study their seasonal and interannual variability. Seasonal variation in both R-s and R-e was better predicted by soil temperature at the 5 cm depth than at any other depth and air temperature. R-e was more responsive than R-s to changes in temperature and soil water content. While nighttime (R-sn) and daytime (R-sd) soil respiration showed no difference in their response to soil temperature, daytime ecosystem respiration (R-ed) was less responsive than its nighttime counterpart (R-en) to changes in soil temperature. Half-hourly R-ed was almost always smaller than R-en possibly due to photoinhibition while daily total R-ed was higher than daily total R-en during summer and spring months but smaller in winter and autumn months with the latter due to fewer daylight hours. A distinct seasonal pattern in the R-s to R-e ratio was observed. The 3-year mean seasonal R-s/R-e was at its minimum of 0.52 in spring followed by 0.63 in summer, 0.81 in autumn and a maximum of 0.86 in winter. Both daily total and half-hourly R-sd/R-ed were larger than R-sn/R-en. It appears that the different responses of R-s and R-e to environmental variables arise as a result of seasonal variations in photosynthesis, mobilization and use of stored carbohydrates, and differences in the phenology of aboveground and belowground plant tissues. On an annual basis, R. accounted for 62% of R-e with the latter accounting for 86% of the carbon (C) assimilated in annual photosynthesis or gross primary productivity (GPP), leaving the net C sequestration efficiency (1 - R-e/GPP) at 14%. In the relatively dry year of 2003, both R-s and R-e as well as GPP were the lowest but with the highest net ecosystem productivity (NEP) of the 3 years. The relatively wetter growing season soil moisture regime in 2005 resulted in the highest R-e and GPP. Partitioning of R, into its autotrophic and heterotrophic components indicated that 54% of GPP was respired back to the atmosphere as autotrophic respiration and an additional 32% was lost in the decomposition of litterfall and soil organic matter. The mean annual estimate of net primary productivity (NPP) at 843 g C m(-2) accounted for 47% of the mean annual GPP of 1815 g C m(-2). (c) 2007 Elsevier B.V All rights reserved.