| As the main environmental factor of vegetation growth and development,the temperature directly affects the growth of forests,the decomposition rate of litters,the activity of soil microorganisms and the soil respiration.The research on changes of vertical gradient temperature of secondary oak forests in Kongqing Mountain is of great significance to carry out the scientific management and improve the productivity of the oak forest,providing the basic data for the national ecological security.In this paper,with the secondary oak forest in northern subtropical region as the research object,the continuous monitoring was carried out from January 2015 to January 2017 to systematically analyze the change law of solar radiation,air temperature and soil temperature gradient.Also,the mathematical statistics were carried out to explore the hysteresis effect of the gradient air temperature and the soil temperature as well as the correlation between the soil temperature and the canopy temperature,so the following conclusions were reached:(1)The leaf area index of oak forest was increased from February to August and decreased from September to January,which had a negative correlation with the ratio inside forest and the ratio outside forest,and there was a positive correlation between the leaf area index and the ability of the canopy to intercept solar radiation.The regression equation of the radiation ratio inside and outside the forest and the leaf area index was: y = 0.0426x2-0.3108 x + 0.6962(y was the ratio of the monthly mean of the radiation inside and outside the forest,x was Leaf area index,R2 = 0.91).(2)After analyzing the changing law of the gradient temperature and soil temperature of the sunny day in different seasons,the daily change of temperature showed a single peak curve,and the daily change of soil temperature was a sinusoidal curve.Mainly due to the reducing effect of canopy layer on the solar radiation,the difference of the vertical gradient between the temperature and the soil temperature in the oak forest appeared,and the temperature inside forest of the daytime was: the forest canopy top layer of 25m>(the forest canopy bottom layer of 10 m,the forest canopy middle layer of 20m)> the under-forest layer of 5m,and the temperature difference was not significant between 10 m and 20 m in the same forest cannon;and the temperature inside forest of the night was: the forest canopy top layer of 25m> the forest canopy bottom layer of 10m> the forest canopy middle layer of 20m> the under-forest layer of 5m,showing a more obvious inversion phenomenon.In spring and autumn,except the soil layer of 50 cm,the temperature of other soil layers of the daytime decreased with the increase of soil depth and increased with the increase of soil depth.In summer,the soil temperature of the whole day decreased with the increase of soil depth;while in winter,the soil temperature of the whole day increased with the increase of soil depth.(3)There was a more obvious hysteresis phenomenon in the gradient soil temperature and air temperature within the oak forest.The highest temperature appeared at 5m and 10 m on the ground(14:30 in spring,14:00 in summer,14:30 in winter,14:30 in winter),and the highest temperature appeared for about 60 min late at 20 m,25m and 30 m on the ground.The highest temperature appeared on the surface layer at around 15:00 in every season,and hysteresis time increased with the deepening of the soil(about 180 min late at 5cm,about 300 min late at 10 cm,about 540 min late at 20cm);for the ground gradient temperature of the day,the lowest temperature difference was not significant appearing at about 6:00(about 60 min late at 0cm,about 120 min late at 5cm,about 350 min late at 20cm)(4)This paper also studied the correlation between cumulative temperature and cumulative radiation inside and outside the oak forest,so the regression model of accumulated temperature and cumulative radiation at 30 m and 10 m in each season was obtained,which reflected the relationship between radiation and temperature rise inside and outside the oak forest.The cumulative temperature-rise model at 10 m in spring: y =-4.4078x2 + 13.475x-0.2011,R2 = 0.903;the cumulative temperature-rise model at 30m: y =-0.0766x2 + 1.5273 x +0.0491,R2 = 0.858.The cumulative temperature-rise model at 10 m in summer: y =-7.6536x2 + 13.101x-0.635,R2 = 0.812;cumulative temperature-rise model at 30m: y =-0.0941x2 + 1.4852x-0.6128,R2 = 0.87.The cumulative temperature-rise model at 10 m in the autumn: y =-4.8624x2 + 9.8624x-0.5699,R2 = 0.743;cumulative temperature-rise model at 30m: y =-0.1517x2 + 1.7438x-0.8175,R2 = 0.679.The cumulative temperature-rise model at 30 m in winter: y =-1.0832x2 + 5.2618x-0.3567,R2 = 0.725;cumulative temperature-rise model at 30 m: y =-0.3853x2 + 2.838x-0.1692,R2 = 0.647.(5)In the paper,the regression analysis of canopy temperature and gradient soil temperature was carried out.Results show that with the increase of soil depth,the correlation coefficient between canopy temperature and soil temperature decreased from 0.976 to 0.897,which decreased with the increase of leaf area index.By establishing the regression equation of canopy temperature and soil temperature in 2015 and using 2016 data to test the prediction accuracy,satisfactory results were obtained. |
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