| The Populus simonii Carr in Bashang Plateau belongs to the north area of Sanbei Shelterbelt Project which has been constructed for more than 50 years.During the construction of the project,it has achieved remarkable ecological,social and economic benefits,and has become an important ecological barrier in Beijing and Tianjin district.However,in recent years,shelterbelt has shown a declining trend,its ecological function has weakened,and a large area of P.simonii Carr has degraded or even died.In our paper,the P.simonii Carr plantation in Zhangbei area of Bashang area was taken as the main research object,using remote sensing data to analyze the regional distribution of plant degradation in the study area.Combined with field investigation and stand monitoring data,the degradation process of tree structure,growth and function was analyzed.The degradation causes of plantation were quantitatively analyzed.The mechanism of tree degradation due to water was studied by using stand monitoring and control experiment,so as to reveal the degradation mechanisms of P.simonii Carr shelterbelt in Bashang plateau.The main conclusions are as follows:(1)In this study,a degradation evaluation system of Populus simonii Carr plantation was established.A model based on stability and degradation index was proposed as:D=?W1B1-?W2B2.The forest was classified into four degradation degrees,among which the degradation index was greater than 0.25 for non-degraded forest,and the degradation index was light-degraded forest between-0.5 and 0.25,and the degradation index was medium-degradation forest between-1.25 and-0.5,the index of less than-1.25 was severedegenerated forest.(2)The distribution of vegetation degradation areas in Bashang area gradually deteriorated from east to west.The vegetation coverage of Weichang County and Fengning County in the east was higher,Zhangbei County and Shangyi County in the west was worse,and Guyuan County in the middle was at average level.LAI distribution and vegetation coverage distribution were similar,with an average of 2.51.Fengning County and Weichang County are the main areas with high NPP distribution,while Zhangbei County and Shangyi County in central and western China are the representatives with low NPP.Zhanhai Township in the eastern part of Zhangbei County is represented by good vegetation growth,while the vegetation in the northwestern part of Zhangbei County is poor,mainly including Hailiu Tu,Liangjing and other towns.(3)The study on the degradation process of Populus simonii Carr plantation showed that the growth process of Populus simonii Carr plantation could be divided into three periods:rapid growth period(1976-1996),slow growth period(1997-2008)and degraded growth period(2009-2018).With the aggravation of degradation,the width of tree ring decreases gradually.The index values of stand structure(diameter,tree height,canopy density,density and LAI)decreased gradually with the aggravation of degradation degree.LAI of non-degraded and light degraded stands in 15-20 years was mostly above 2,while LAI of medium and severe degraded stands was mostly below 2.The stand densities with different degradation levels eventually remained between 420-800 trees/hm2 with the increase of stand age.The root degradation process of P.simonii Carr showed that the distribution range of tree roots decreased gradually with the aggravation of the degradation degree,and the root surface area density,root biomass and root length density decreased with the increase of soil depth.The functions of windbreak,sand fixation and soil and water conservation of P.simonii Carr plantation were obviously weakened with the aggravation of degradation degree,and the biodiversity of undergrowth herbaceous forest was increased with the aggravation of degradation degree.(4)In this study,the relative contribution of structure,water,site and other influencing factors leading to the degradation of P.simonii Carr plantation were obtained.The analysis showed that natural factors were the dominant factors leading to tree degradation,and water factor was the main factor leading to tree degradation,which could explain 38.9%of stand degradation alone.The thresholds of the main indexes were obtained.The tree degradation increased significantly when the soil water content was less than 12%,the soil density was more than 600 trees/hm2,and the soil minimum water holding capacity was less than 11%.The growth of trees will change significantly after 15 years of age.The degradation of available K was obvious at about 115 mg/kg,and the threshold of available P was around 2.4 mg/kg.(5)The soil water sources of P.simonii Carr gradually moved from the deep layer to the surface layer with the aggravation of degradation,and the utilization of soil water and groundwater in the lower layer decreased gradually.The non-degraded poplar mainly used soil water with a depth of 280-400 cm,the light degraded poplar mainly used soil water with a depth of 200-280 cm,the medium degraded poplar mainly used soil water with a depth of 20-80 cm,and the severe degraded poplar mainly used surface soil water with a depth of 020 cm.(6)Analysis of different degree of degradation of P.simonii Carr water transmission rule discovery:the growing season is non degraded trees sap flow(0.685 g/h)>light degradation(0.462 g/h)>medium degradation(0.323 g/h)>severe degradation(0.239 g/h),non-degradation and light degradation did not flow down to 0 or so compared with medium and severe degradation back about 1 months or so,shows good P.simonii Carr physiological activity time is longer.The sap flow of P.simonii Carr was positively correlated with PAR and air temperature.The sap flow was sensitive to the radiation range of 0-500 UE.When the air temperature was 0-17?,the sap flow increased slowly,and when the air temperature was above 17?,the sap flow remained stable.There was a negative linear correlation between sap flow and relative humidity.In the face of mild drought,P.simonii Carr will improve the root water conductivity efficiency to make up for the lack of water,but the continuous drought will destroy the root tissue,make its water conductivity rate drop,weaken the water transport capacity.(7)According to the analysis of water use rules of P.simonii Carr with different degrees of degradation,the water consumption of leaf photosynthesis and transpiration in growing season decreased rapidly with the intensification of degradation.The water consumption of non-degraded and light degraded stands was significantly higher than that of medium and severe degraded stands.In the growing season,the water consumption increased first and then decreased,and reached the maximum in July.The non-degraded and light degraded stands of WUE were significantly higher than those of medium and severe degraded stands.The maximum daily variation was usually from 6 to 8 am,and the maximum monthly variation was in June.Mild drought can increase the maximum photochemical efficiency of leaves and improve the water use efficiency,but long-term drought risk can aggravate the possibility of mesophyll stomatal cell structure damage,so that the photosynthetic system of leaves is damaged.According to the water balance equation,the carrying capacity of soil water and vegetation in four degraded stands was 486,601,649 and 964 trees/hm2,respectively.(8)It was found that soil bulk density and soil minimum water holding capacity were positively and negatively correlated with the degradation degree,soil total P,total N,available K and available P were negatively correlated with the degradation process,and elevation,slope and stand age also had significant effects on degradation.Based on the study of P.simonii Carr plantation,the restoration countermeasures of degraded P.simonii Carr plantation in Bashang area were proposed. |
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