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Harvard Forest Symposium Abstract 2005

• Title: A Distinct Seasonal Pattern of the Ratio of Soil Respiration to Total Ecosystem Respiration in a Spruce-Dominated Forest
• Primary Author: Eric Davidson (University of Maryland - Center for Environmental Science)
• Additional Authors: Bryan Dail (University of Maine); David Hollinger (USDA Forest Service); Andrew Richardson (Northern Arizona University); Kathleen Savage (Woods Hole Research Center)
• Abstract:

  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. Many Fluxnet sites produce measurements of both soil respiration and ecosystem respiration, but most comparisons are made only as part of annual carbon budgets. Soil respiration (Rs) typically contributes 30-80% of annual total ecosystem respiration (Reco) in forests, but the temporal variation of these ratios across seasons has not been investigated. Potential mismatches of spatial footprints, time of day of measurement, and sampling frequencies of chamber-based soil respiration measurements and eddy covariance based ecosystem fluxes have made comparisons difficult. We have analyzed these sources of potential errors and biases at the Howland Forest in central Maine in order to provide confidence in seasonal estimates of Rs/Reco ratios. The objective of this study was to investigate seasonal variation in the Rs/Reco ratio in a mature forest dominated by conifers at Howland, Maine, USA, using chamber measurements of Rs and tower-based eddy covariance measurements of Reco.
  While the annual mean Rs/Reco ratio at the conifer forest of Howland is about 0.6, a consistent seasonal pattern in this ratio observed across seven years demonstrates seasonal differences in aboveground and belowground C cycling processes (Fig 1). Each spring, the Rs/Reco ratios typically fell to about 0.4. The ratios then increased during the spring, summer, and autumn, peaking near 1.0 by the late autumn and winter. The ratios occasionally exceeded 1.0 in the late autumn and winter, when the fluxes are small and the measurement error is probably large relative to the mean flux. Combining the Rs/Reco ratios for all years, a second order Fourier function describes a general seasonal trend (Figure 2). The Rs/Reco 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 Rs/Reco ratios to increase. The summertime plateau of Rs/Reco ratios is consistent with summer drought suppressing Rs that would otherwise be increasing, based on increasing soil temperature alone, thus causing the Rs/Reco ratios to not increase as soils continue to warm. Declining air temperatures and litter fall apparently contribute to increased Rs/Reco ratios in the autumn. These explanations are largely speculative, but the observed patterns demonstrate that partitioning Reco into its components reveals differential responses to seasonal climatic patterns. The observed distinct seasonal pattern of Rs/Reco ratios points to a need for improved understanding of differences in the 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.
  

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