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Effects of leaf age and canopy structure on gross ecosystem production in a subtropical evergreen Chinese fir forest
Agricultural and Forest Meteorology  (IF5.734),  Pub Date : 2021-08-26, DOI: 10.1016/j.agrformet.2021.108618
Qiaoli Wu, Conghe Song, Jinling Song, Jindi Wang, Shaoyuan Chen, Lei Yang, Wenhua Xiang, Zhonghui Zhao, Jie Jiang

Despite the advances in technologies to derive the maximum rate of Rubisco carboxylation (${V}_{cmax}$) seasonality at large-scale, factors controlling the temporal dynamics of ${V}_{cmax}$ is largely unknown without extensive field measurements at stand scale. In addition, state-of-the-art process-based terrestrial ecosystem models had not accounted the complex canopy structure for estimating radiation interception by vegetation. Both of which could lead to uncertainties in gross ecosystem production (GEP) estimations. Here, we examined the respective and combined effects of leaf age and canopy structure on GEP by integrating the Farquhar photosynthesis model with a two-leaf (sunlit and shaded) canopy radiation interception model based on the Geometric Optical and Radiative Transfer (GORT) theory. We observed that the ${V}_{cmax}$ of new leaves was approximately 34.4% higher than that of mature leaves. Considering the seasonal dynamics of ${V}_{cmax}$ caused by new leaf expansion, the average bias between the modeled GEP and that derived from the eddy covariance (EC) in the growing season at 8-day temporal scale was reduced from -8% to -1%, with slightly higher correlation (from 0.9 to 0.92) and reduction in root mean squared error (from 6.0 to 3.9 g C m−2 8d−1). Most importantly, the total effect of new leaf expansion and canopy gaps during the growing season was +322 g C m−2 yr−1 and -114 g C m−2 yr−1, which was approximately 22.5% and 8.1% of the total GEP, respectively. Thus, it is important to consider both leaf age and canopy structure when developing a robust terrestrial ecosystem model. We highlighted that ${V}_{cmax}$ seasonality and realistic canopy radiation interception simulation are critical for accurate estimation of canopy GEP for evergreen forests.