Modeling CO₂ exchange and meteorological factors of an apple orchard using partial least square regression

The eddy covariance (EC) technique was used to measure variations of orchard-atmosphere CO₂ exchange, as a function of meteorological variables in an apple orchard in 2016-2017. The annual average CO₂ exchange rate was 2.295 kg m^-2. Excavations and biomass assessments demonstrated that the orchard stored close to 20.6 tC ha^-1 as plant C over a 15-year period. Seasonally, high rates of CO₂ uptake and low CO₂ emissions occurred between May and August and December and March, respectively. The maximum rates of monthly CO₂ exchange were 144.44 and 153.61 gC m^-2 month^-1 in August 2016 and June 2017, respectively. Partial least squares (PLS) regressions were used to analyze the influence of meteorological factors to on CO₂ exchange rates. Temperature and photosynthetic active radiation (PAR) were observed to exert the largest influence on driving variation in CO₂ exchange. The main meteorological factors affecting CO₂ exchange on daily and monthly time scales were soil temperature (T_soil), air temperature (T_a), PAR, below canopy CO₂ concentration (BCC), vapor pressure deficit (VPD), and soil water content at 50 cm (SWC_50cm). The regression model equation describing CO₂ exchange included T_a, VPD, precipitation (PPT), and sunshine duration (SD), as significant variables. This model curve fitting explains over 80% of the variation in CO₂ exchange. This study provides CO₂ exchange characteristics and a model equation capable of predicting CO₂ exchange of an apple orchard. Graphical Abstract.