Multiple drivers of seasonal and interannual variation in Pmax: Implications for leaf photosynthesis of Artemisia ordosica

Revealing the seasonal and interannual variations in leaf-level photosynthesis is a critical issue in understanding the ecological mechanisms underlying the dynamics of carbon dioxide exchange between the atmosphere and shrub ecosystem. Artemisia ordosica is a dominant shrub species in semi-arid area of northwest China. Photosynthetic gas exchange, leaf nitrogen content(LN), specific leaf area (SLA) and some environmental factors were measured simultaneously on clear days (rotated every 10 days) of the growing season from 2011 to 2018, to quantify the temporal variations and environmental controls of photosynthetic parameters. Our results demonstrated that mean value of light-response curve parameters, the maximum photosynthetic capacity (P_max), appear quality efficiency (AQE), respiration in dark (R_d), light saturated point (LSP) and light compensated point (LCP) had a gradual decline with the growth (spring> summer>autumn). Structural equation modeling (SEM) was used to elucidate the direct and indirect effects of biophysical factors on P_max. The driven factors of P_max in growing season changed, but stomatal conductance (g_s) was the dominant factor in all stages. The g_s was influenced by SLA and LN，and the soil water content at a depth of 10cm (SWC₁₀) affected the P_max in spring. In summer, P_max was significantly positively related with g_s and transpiration rate (T_r), and g_s was influenced by SLA, LN and soil water content at a depth of 30cm (SWC₃₀). In autumn, P_max was significantly positively correlated with g_s, while was significantly negatively correlated with air temperature (T_a). This simulation based on situ ecophysiological research suggest that P_max of A. ordosica responded to the environment factors of seasonal and interannual variations, which is not the inherent genetic characteristics. Soil water content is the major environmental factor influencing P_max in spring and summer, while T_a is the major one in autumn. Knowledge of how environmental change will affect the photosynthesis of A. ordosica in the future is essential for their protection, adaptation strategies and carbon fixation prediction in shrub ecosystems.