Heat characteristics and its variation with growing stages in non-irrigated apple orchard system of the Loess Plateau

Abstract: 【Objective】Heat resources in the orchard system have important effects on the development of roots of fruit trees, the ground evapotranspiration, and orchard’s water and heat cycle. The Loess Pla-teau has sufficient sunlight and big temperature differences between day and night, which is appropriatefor apple cultivation. However, orchards in different regions of the Loess Plateau are affected by diverse topography and regional location so that their heat conditions are different from each other. Clarifying the heat characteristics and differences in different growth stages of the non-irrigated apple orchard in the Loess Platea of Western Shanxi province, China, can provide scientific bases for orchard accurate management.【Methods】The study area was selected in an organic apple orchard (110° 35.655′ E, 36°04.739′ N) with the area of 1.67 hm² in Ji County of Shanxi province, the loess gully region where annu-al precipitation was about 522.8 mm. The apple trees in this orchard were about 2.5 m of height and ar-ranged with distance of 6 m within lines and 4 m between lines. For measuring meteorological ele-ments, the total solar radiation sensor CMP11 (Kipp & Zonen, NL), the net solar radiation sensor NR-LITE2 (Kipp & Zonen, NL) and the air temperature sensor AV-10TH (AVALON, USA) were installed on about 3 m above the ground in the center of the orchard. The air temperature sensors were also in-stalled on 20 cm above the ground. The soil heat flux sensor HFP01 (Huk Sefflux, NL) and temperature sensor ECH2O (Decagon, USA) were buried under 10 cm below the soil surface. The total solar radia-tion (Rs), net radiation (Rn) and air temperature (Tair) above the canopy of the apple trees, the air tempera-ture close to the ground layer (Tground), the soil temperature (Tsoil) and the soil heat flux (G) in the upper layer of the soil were observed for one year in the orchard in 2018. Statistical analysis of the daily changes and seasonal dynamics of the above-mentioned variables were used to further clarify the heat characteristics.【Results】Generally, along with stages of the flower stage, fruit formation, fruit expan-sion and fruit ripening stage during the growing season, the variables of the Tair, Tground, Tsoil, Rs and Rn dis-played a clear seasonal pattern with an increase followed by an decrease. However, the G showed a de-clining seasonal change as a whole. During the experimental period, the daily means of the Tair, Tground,and Tsoil were 18.1, 19.1, and 18.1 ℃, and the highest values of them were 35.1, 37.5, and 27.3 ℃, and occurred on June 6, June 6 and July 29 respectively. The highest values of the Rs, Rn, and G were 3.74,2.80, and 0.38 MJ·m-2·h-1, and occurred on June 9, August 23 and April 7, respectively. From the flow- ering stage to the fruit expansion stage (April-August), the monthly means of the Tground were higher than those of the Tair and Tsoil, and the means of the G were positive, indicating that soil absorbed heat from air. However, in the fruit ripening stage (September-October), the average of Tsoil was higher than those of Tground and Tair, and the means of the G were negative, demonstrating that soil released heat. The dura-tion of high temperature (Tground >35 ℃) were 58 hours, and that of the Tground >30 ℃ and with Tground