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

Abstract

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 s 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 s 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.