Energy Perspective For Fluxes Measurement In Tropical Seasonal Rainforests In Complex Environments

Tropical rainforest has tall and dense canopy,abundant and diverse species,complex biotopes,and strong environmental impact,coupled with its prominent role in mitigating global warming,the tropical rainforest has become a hotspots for climate change research.It is crucial for us to clarify the processes and quantify the fluxes of mass and energy exchange between these forests and the atmosphere.The eddy covariance technique is currently the most reliable method for directly measure flux and energy at the ecosystem scale.Nevertheless,this method is susceptible to be interfered by external environmental factors,which in turn affect the observation results.The energy balance closure,based on the first law of thermodynamics,can reflect the accuracy of flux monitoring results to some extent.And these results are eventually further used to validate models as well as other analyses of large-scale data.Xishuangbanna tropical seasonal rainforest is located at the northern edge of Malaysian tropical rainforest region,which is a transitional forest type between tropical rainforest and subtropical forest.The mosaic type of the vegetation distribution,the fragmentation and undulating topography in the distribution area of these forests,coupled with the low turbulence exchange intensity due to the calm air,poses a great challenge for conducting flux monitoring in this region.Based on the measured data in this region,we viewed the eddy flux results from the perspective of energy budget and closure.Based on a deeply understanding of the energy budget and closure patterns,we simultaneously attempt to analyze and figure out which factors play an important role in the formation of these patterns,and how these factors act on them through micrometeorological and biological processes.Finally,we explored the level of energy closure in terms of annual total amount and seasonal pattern,and how it affects evapotranspiration estimates based on eddy fluxes.The data used for the analysis in this study are obtained from the eddy covariance system,which was installed on forest canopy crane in the Parashorea chinensis forest of Bubang Village,Mengla County,Xishuangbanna,Yunnan province.The main results are as follows:(1)Based on both the energy closure ratio and the linear regression method,the average energy closure of the tropical seasonal rainforest is around 65%.This value is significantly lower than the average level of energy balance closure in other terrestrial ecosystems around the world.According to the information we have so far,the energy balance closure in this region is extremely low.Even compared with eddy flux sites in mountainous terrain,this closure level is significantly lower.(2)The energy balance closure has obvious seasonal variations.Overall,the data show that the level of energy closure in rainy season is significantly higher than that in dry season.In some periods of rainy season,the energy closure level can approach 80%,reaching the average level of global forest eddy flux sites.In dry season,especially in foggy-cool season,the energy closure is only about 55%.The seasonal dynamics of energy closure are most similar to the seasonal pattern of air temperature.(3)The turbulence intensity is the most important factor affecting the energy closure.The energy closure regularly changes with the level of atmospheric stability and the friction velocity.With the increase of friction velocity,the energy closure increases continuously and finally stabilizes at about 80%.In other words,even turbulence is well developed,there is still about 20% of unclosed energy fraction that cannot be explained.Considering the energy storage in canopy air and soil,the energy closure level cannot be significantly changed,and its contribution does not exceed 2%.(4)The low energy closure not only significantly affects the estimated values of evapotranspiration,but also affects its seasonal pattern.On the whole,the underestimated evapotranspiration is small in rainy season,while in hot-dry and foggy-cool seasons,a large amount of evapotranspiration is underestimated.In addition,the seasonal variation of the evapotranspiration values is minimized by the energy closure correction,while it shows strong seasonal variation dynamics when absence energy closure correction.