A comparison of measured and calculated net community CO<inf>2</inf> exchange: Scaling from leaves to communities

Abstract

Leaf net photosynthesis is crucial for detecting the mechanism of photosynthesis, whereas community net photo- synthesis is useful for understanding the photosynthetic capacity of communities and its relationship with envi- ronmental factors. In particular, we need to scale up eco-physiological models from leaf scale to canopy level to study carbon cycling at regional or global scale. We hypothesized that accumulated leaf net photosynthetic rate (Pc) at community scale, i.e., calculated based on leaf net photosynthetic rate (Pn) and leaf area index (LAI), equals to measured net community CO2 exchange (NCE). The purpose of this study is to verify this hypothesis. Our field study was carried out in Duolun, Nei Mongol, China, where we constructed single-species communities by sowing Medicago sativa 'Aohan' seeds in three plots (3 m × 5 m) on May 30, 2012. On August 16, 2014, Pn of five healthy leaves of M. sativa 'Aohan' in each plot were measured with a LI-6400 portable photosynthesis sys- tem at 10:00, and net ecosystem CO2 exchange (NEE) in each plot was measured simultaneously with a LI-8100 system connected with a assimilation chamber (0.5 m × 0.5 m × 0.5 m). Pc was calculated based on Pn, number of leaves (n), LAI percentage of healthy leaves (r) and percentage of received effective light by leaves (m). NCE was derived from NEE and ecosystem respiration rate (Reco). Pc was 3.52 μmol CO2·m -2 ·s -1 , and very close to NCE (3.56 μmol CO2·m -2 ·s -1 ), suggesting that leaf-scale photosynthesis may accurately predict community-scale pho- tosynthesis. However, our method could not separate community respiration from soil respiration, and future studies, should be designed to counteract this effect. Scaling up from leaf photosynthesis to community photosyn- thesis should also consider vertical structure of communities and nonlinear responses of leaf photosynthesis to changes in light quantum.