Effect Of Incident Radiation Under Forest On Frozen Soil

Northeast China is the second largest permafrost region in my country,and it is also the only high-latitude permafrost region in China.Forest vegetation is a decisive factor affecting the existence of permafrost in the Northeast Xing'an Mountains.It affects snow and frozen soil by changing understory radiation and soil thermodynamic properties.In order to study the influence of understory radiation on frozen soil temperature,and to understand the mechanism of understory incident radiation in deciduous forest on frozen soil.This paper selects three main types of forests in the Daxinganling Forest Ecological Station as the research objects,namely,Larix gmelinii,Larix tatau and Larix Betula.Boreholes,automatic meteorological observation stations and radiation sensors have been set up in the selected woodland experimental area to observe and record the temperature,climate and radiation of the frozen ground in the experimental area for a long time.In 2018,the canopy photos of different forest types were taken by fish eye camera.After a series of processing,the annual changes of the canopy structure of different forest types were obtained.Based on fisheye photos and hemispherical images,a method of calculating understory radiation by using sky visible factor and direct solar radiation shielding factor was proposed.Comparing the observed radiation with the calculated radiation,the method is proved to be true and available,and the influence of different forest types on solar radiation was studied at the same time.Finally,Based on the proposed method,the original SHAW model was improved.The radiation module under the model forest was improved.The response of understory radiation to permafrost under three different canopy structures was studied by using the improved model.After data collection and analysis,the following conclusions are drawn:1.In the Daxinganling Ecological Station,the canopy structures of the three forest types are quite different.The canopy of Larix Betula is larger and there are more branches and trunks.In summer,the canopy gap is small.When the summer turns to winter,the canopy is more deciduous.The height of the canopy is generally 10?20m.The understory species are relatively small.Abundant;The canopy of Larix tatau is small,and the gap between canopies is relatively large.In summer,the canopy has a poorer shielding effect on solar radiation than other forest types.The height of the canopy is 7?12m,and the under-shrub layer is not well developed.,Mainly herbaceous layer.The canopy structure of Larix gmelinii is between the two,with a canopy height of 15-25 m.The forest type is mainly composed of arbor,shrub,and needle moss layers.2.The canopy can absorb and reflect a large amount of solar radiation.The three forest types have different canopy structures and have different shielding effects on solar radiation.In the selected study area,the annual average shielding effect on solar radiation is from large to small,in order of Larix Betula,Larix gmelinii,and Larix tatau.Among them,the solar radiation shielding of Larix Betula is the largest in summer,reaching 71%,the radiation shielding of Tatou larch is 11%,and the solar radiation shielding rate of Larix gmelinii in June is 52%.3.Due to the effect of the canopy on the solar radiation,the temperature distribution in the forest area changes,resulting in the spatial difference in the distribution of frozen soil.The canopy structure is complex,the lower the transmittance of solar radiation,the lower the temperature of the forest area,which slows down the demise of permafrost.4.The model simulates the change of frozen soil temperature under different canopy structures,and it is found that the active layer is more sensitive than the permafrost layer.The canopy changes,and the active layer changes significantly.As the soil depth increases,the sensitivity is lower.The short-term impact on the permafrost is small.The canopy structure varies greatly,and the thickness of the active layer will also change.The more complex the canopy,the smaller the understory radiation and the smaller the thickness of the active layer.The overall study shows that in the Greater Xing'an Mountains,understory incident radiation has an impact on the evolution of frozen soil,and its specific impact mechanism is more complicated.The change of frozen soil is caused by many factors,and understory radiation is one of the more important ones.This relatively concise and clear calculation method for understory radiation proposed in this paper proves the difference in the shielding effect of the canopy on solar radiation,also explains the mechanism of solar radiation absorption and reflection by the canopy.Finally,through improved model simulation,the qualitative study of the impact of incident radiation under the forest on the frozen soil,and the explanation of the influence of canopy structure changes on the frozen soil temperature,will lay the foundation for a more systematic and quantitative study of frozen soil changes in the cold area in the future.