Comparison of the eddy covariance and automated closed chamber methods for evaluating nocturnal CO 2 exchange in a Japanese cypress forest

Abstract

Underestimation of nocturnal CO 2 respiration using the eddy covariance method under calm conditions remains an unsolved problem at many flux observation sites in forests. To evaluate nocturnal CO 2 exchange in a Japanese cypress forest, we observed CO 2 flux above the canopy ( F c), changes in CO 2 storage in the canopy ( S t) and soil, and trunk and foliar respiration for 2 years (2003–2004). We scaled these chamber data to the soil, trunk, and foliar respiration per unit of ground area ( F s, F t, F f, respectively) and used the relationships of F s, F t, and F f with air or soil temperature for comparison with canopy-scale CO 2 exchange measurements (= F c + S t). The annual average F s, F t, and F f were 714 g C m −2 year −1, 170 g C m −2 year −1, and 575 g C m −2 year −1, respectively. At small friction velocity ( u *), nocturnal F c + S t was smaller than F s + F t + F f estimated using the chamber method, whereas the two values were almost the same at large u *. We replaced F c + S t measured during calm nocturnal periods with a value simulated using a temperature response function derived during well-mixed nocturnal periods. With this correction, the estimated net ecosystem exchange (NEE) from F c + S t data ranged from −713 g C m −2 year −1 to −412 g C m −2 year −1 in 2003 and from −883 g C m −2 year −1 to −603 g C m −2 year −1 in 2004, depending on the u * threshold. When we replaced all nocturnal F c + S t data with F s + F t + F f estimated using the chamber method, NEE was −506 g C m −2 year −1 and −682 g C m −2 year −1 for 2003 and 2004, respectively.