Concurrent Measurements of Soil and Ecosystem Respiration in a Mature Eucalypt Woodland: Advantages, Lessons, and Questions

초록

Understanding seasonal and diurnal dynamics of ecosystem respiration (R-eco) in forests is challenging, because R-eco can only be measured directly during night-time by eddy-covariance flux towers. R-eco is the sum of soil respiration (R-soil) and above-ground respiration (in theory, R-AG = R-eco - R-soil). R-soil can be measured day and night and can provide a check of consistency on R-eco, as the difference in magnitude and time dynamic between R-eco and R-soil should be explained by R-AG. We assessed the temporal patterns and climatic drivers of R-soil and R-eco in a mature eucalypt woodland, using continuous measurements (only at night for R-eco) at half-hourly resolution over 4 years (2014-2017). Our data showed large seasonal and diurnal (overnight) variation of R-eco, while R-soil had a low diurnal amplitude and their difference (R-eco - R-soil,R- or R-AG) had a low seasonal amplitude. This result implies at first glance that seasonal variation of R-eco was mainly influenced by R-soil while its diurnal variation was mainly influenced by R-AG. However, our analysis suggests that the night-time R-eco decline cannot realistically be explained by a decline of R-AG. Chamber measurements of autotrophic components at half-hourly time resolution are needed to quantify how much of the R-eco decline overnight is due to declines in leaf or stem respiration, and how much is due to missing storage or advection, which may create a systematic bias in R-eco measurements. Our findings emphasize the need for reconciling bottom-up (via components measured with chambers) and direct estimates of R-eco (via eddy-covariance method).

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