| The thermodynamic process of canopy is very important for the carbon and water metabolism and the energy balance of vegetation.Canopy temperature is one of the main key parameters reflecting the thermodynamic characteristics of the canopy,and the monitored canopy temperature can be used to characterize the degree of water stress of plants,so it has always been a hot topic in biometeorological functional ecology.In this paper,in order to explore the change rules and mechanism of canopy temperature of tropical forest in different time scales,based on the fact that the micro meteorological observation tower was used to monitor the canopy temperature of rubber plantation in the western part of Hainan Island continuously for three years,from 2017 to 2019,combined with the meteorological and soil environmental data of synchronous monitoring in the studied area,supplemented by the calculated saturated water vapor pressure difference,we analyzed the dynamic,hydraulic and thermal characteristics of the studied area were analyzed,and the relationship between environmental factors and canopy temperature was discussed by partial correlation and complex correlation analysis.Finally,we also try to calculate and analyze the canopy impedance based on the canopy temperature data in the framework of Penman-Monteith hydrothermal equation.The main conclusions of the study can be summarized as follows:(1)Spatial and temporal characteristics of a single environmental factor in rubber forest:the wind speed,relative humidity and VPD in wet season increase with height on vertical gradient;the correlation between water pressure,air temperature and VPD in dry season and height is not obvious;the fluctuation of soil temperature and water content is negatively correlated with depth;all environmental factors have obvious time change characteristics throughout the year,mainly divided in growth season and non-growth season.The relative humidity in the whole year is more than 80% in general.The main wind direction in the growing season is the east southeast wind,with low wind speed.At this stage,the soil moisture content continues to rise.The water pressure is mainly affected by solar radiation,plant transpiration,rubber crown structure and crown height.In the non-growth season,the main wind direction is the easterly wind,with high wind speed,and the soil moisture content continues to decline.The water pressure is quite slightly affected by solar radiation and temperature.On the daily average change level,the relative humidity presents a single valley curve,the radiation,atmospheric temperature and canopy temperature all present a single peak curve,the extreme points appear around noon,among which in the night of March,June and September,the atmospheric temperature also presents a significant inversion phenomenon,the VPD changes more intensely in the daytime than at night.(2)Correlation between canopy temperature and environmental factors at different heights: the atmospheric temperature,relative humidity and VPD of 1.5m have the strongest correlation with the canopy temperature;the measurement data of the soil temperature of 5cm and the soil water content of 50 cm have the strongest correlation with the canopy temperature.At the same time,considering the combined effects of all the observed environmental factors,the annual canopy temperature has a very significant correlation with it,in which the correlation between wind speed and humidity is not obvious,specifically in its positive and negative correlation is not stable.(3)Change characteristics of rubber canopy resistance:it has obvious time variation characteristics and is strongly influenced by canopy temperature and atmospheric temperature.In May,the change of canopy resistance value was the most intense,and in July,it was the most gentle.The change of canopy resistance value in growing season was "U" type,which was consistent with the growth stages of rubber trees.In this paper,the temporal and spatial variation characteristics of single environmental factor and its correlation with canopy temperature of rubber forest are revealed,and the variation characteristics of canopy resistance are calculated and analyzed preliminarily,which will provide supportive theoretical basis and data for future analysis of forest environmental characteristics,further analysis of the mechanism of canopy temperature variation,revision or exploration of a new calculation model of canopy resistance applicable to rubber forest and even the whole tropical forest. |
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