Effect of extreme heat on winter wheat canopy carbon assimilation

Canopy carbon flux was continuously measured in 2003–2013 using the eddy covariance system in a winter wheat field in Yucheng station, Shandong Province, to explore the effect of extreme heat on winter wheat canopy carbon assimilation. The extreme heat (EH) threshold was determined as the 95th percentile of daily maximum temperature (T_{a_max}), which was 30.58 ℃ during grain-filling stage in 2003–2013 in the study area. Two typical couples of EH and non-EH days in 2004 and 2012 were selected and compared to determine the characteristics and mechanism of the effects of EH on net ecosystem productivity (NEP, daytime), which was used to denote winter wheat canopy carbon assimilation rate. The results showed that T_{a_max} in the EH days were 3.10 ℃ and 3.17 ℃ higher than those in the non-EH days respectively in 2004 and 2012. Then total amounts of daytime NEP in EH days decreased by 3.25 mg(CO₂)m^-2s^-12)m^-2s^-1 (with decreasing rate of 19.17%), respectively, compared with that in non-EH days in 2004 and 2012. NEP had a significant quadratic curve correlation with PAR, but no significant when PAR was greater than 1 000 μmolm-2s^-1, at that time NEP accounted for over 52.31% of total daytime NEP for both EH and non-EH days. The difference in NEP between EH and non-EH days increased with increasing PAR, especially when PAR was greater than 1 000 μmolm-2s^-1. There were no significant correlations between relative humidity of atmosphere (RH) and NEP, whether during whole day or when PAR was more than 1 000 μmolm-2s^-1. In the four observed days, 020 cm soil water content (SWC) was about 80% of field capacity. This was the appropriate soil water content at grain-filling stage of winter wheat, which had no negative effect on NEP. For the whole days, NEP had no significant correlation with air temperature (T_a) in both EH and non-EH days. However, during the daytime with PAR > 1 000 μmolm-2s^-1, NEP had significant negative correlation with Ta. NEP decreased by 7.28%9.53% in 2004 and by 6.94%10.42% in 2012 with every 1 ℃ increase in T_a. In conclusion, NEP was largely restrained by Ta when T_a was above 30.58 ℃ at the grain-filling process of winter wheat in the North China Plain. While the contribution of T_a to daytime NEP inhibition was 59%83%, total NEP decreased by 6.05%6.37% with 1 ℃ increase in T_a for EH days.