A distinct seasonal pattern of the ratio of soil respiration to total ecosystem respiration in a spruce-dominated forest

Annual budgets and fitted temperature response curves for soil respiration and ecosystem respiration provide useful information for partitioning annual carbon budgets of ecosystems, but they may not adequately reveal seasonal variation in the ratios of these two fluxes. Soil respiration (R-s) typically contributes 30-80% of annual total ecosystem respiration (R-eco) in forests, but the temporal variation of these ratios across seasons has not been investigated. The objective of this study was to investigate seasonal variation in the R-s/R-eco ratio in a mature forest dominated by conifers at Howland, ME, USA. We used chamber measurements of R-s and tower-based eddy covariance measurements of R-eco. The R-s/R-eco ratio reached a minimum of about 0.45 in the early spring, gradually increased through the late spring and early summer, leveled off at about 0.65 for the summer, and then increased again to about 0.8 in the autumn. A spring pulse of aboveground respiration presumably causes the springtime minimum in this ratio. Soil respiration 'catches up' as the soils warm and as root growth presumably accelerates in the late spring, causing the R-s/R-eco ratios to increase. The summertime plateau of R-s/R-eco ratios is consistent with summer drought suppressing R-s that would otherwise be increasing, based on increasing soil temperature alone, thus causing the R-s/R-eco ratios to not increase as soils continue to warm. Declining air temperatures and litter fall apparently contribute to increased R-s/R-eco ratios in the autumn. Differences in phenology of growth of aboveground and belowground plant tissues, mobilization and use of stored substrates within woody plants, seasonal variation in photosynthate and litter substrates, and lags between temperature changes of air and soil contribute to a distinct seasonal pattern of R-s/R-eco ratios.