Response And Tradeoff Analysis Of Major Functional Indexes In Black Locust Plantations To Density In Loess Hilly Region

The loess hilly region is located in the middle of the Loess Plateau and is one of the most serious areas of soil erosion.Afforestation is an important measure to control soil erosion and improve the ecological environment.black locust is the largest plantation tree species in the Loess Plateau.It has many characteristics such as rapid growth and strong resistance.It plays an important role in erosion control,windbreak and sand fixation,carbon fixation and oxygen release.In order to ensure the survival rate of afforestation and play the function of soil and water conservation as soon as possible,most of the artificial forests in the loess hilly area have high initial planting density and lack of intensive tending management measures.With the increase of forest age,the sustainability of ecological service functions has become increasingly prominent.In this paper,five middle-aged（19-20 years old）stands with different densities（I:2130 plants/hm²,II:2577 plants/hm²,III:2825 plants/hm²,IV:3133 plants/hm²,V:3558 plants/hm²）were selected since the implementation of the project of returning farmland to forest in the loess hilly region.Through field sample survey and collection and analysis of plant and soil samples,the ecological service functions such as wood production,carbon fixation and oxygen release and soil conservation were measured and evaluated,and the response of each service function to density was revealed.The trade-off relationship between functions was analyzed to provide theoretical support for the optimal management of plantation ecosystems in the region.The main conclusions are as follows:（1）The DBH and tree height of black locust plantation in the loess hilly region showed an approximate normal distribution as a whole.The DBH of 19-20-year-old middle-aged forest was mainly distributed in the range of 6-10 cm diameter class,and the tree height was mainly distributed in the range of 6-10 m.The low-density forest accounts for a large proportion in the 14-20 cm diameter class,and the high-density forest is mainly distributed in the low diameter class.The tree height distribution of low-density stands also showed a right deviation,with more trees at the 10-14 m high level,while the high density was opposite.Within the density range of 2130-3558 plants/hm2,there was a negative correlation between stand density and DBH,tree height and individual volume.The stand volume increased first and then stabilized with the increase of locust density.The yield of black locust with different densities was density I（49.85%）>density II（44.25%）>density III（43.00%）>density IV（38.87%）>density V（30.29%）,and the growth and wood production function of low- density trees were relatively better.（2）The biomass of each organ in the tree layer of black locust plantation was trunk>root>branch>leaf,and the density IV was trunk>branch>root>leaf.The biomass of trunk was significantly higher than that of other organs（p<0.05）.The total biomass of tree layer increased first and then tended to be flat with the increase of density,and decreased slightly at density V.The biomass of litter and fine roots increased with the increase of black locust density,while the biomass of herb layer did not change significantly with density.The carbon density of tree trunk was significantly higher than that of other organs（p<0.05）.The carbon density of tree layer of black locust density I was significantly higher than that of other densities（p<0.05）.The soil carbon density decreased with the increase of soil depth,and decreased first and then tended to be gentle with the increase of stand density.The soil carbon density of density I was significantly higher than that of other densities（p<0.05）.The carbon density of black locust plantation ecosystem decreased first and then leveled off with the increase of stand density.The carbon density of density I was significantly higher than that of other densities（p<0.05）,which was 188.65t C/hm2.The annual carbon sequestration and oxygen release of vegetation increased first and then decreased with the increase of density,which was similar to the normal distribution,reaching the maximum at density III,and the carbon sequestration and oxygen release of density III and density IV were significantly higher than other densities（p<0.05）.（3）The contents of soil organic matter（SOM）,total nitrogen（TN）,nitrate nitrogen（NO₃^--N）and available phosphorus（AP）decreased with the increase of soil depth,showing a’surface accumulation phenomenon’.At different stand density levels,the contents of SOM,TN,NO₃^--N and AP decreased gradually with the increase of stand density,and the content of density I was significantly higher than that of other densities（p<0.05）.Soil bulk density（SBD）was significantly negatively correlated with soil TN,NO₃^--N and AP contents（p<0.001）.There was a significant positive correlation between soil nutrient indexes（p<0.05）,and there was a good coupling relationship between each index.The soil nutrient indexes of different densities of black locust plantation were comprehensively evaluated by principal component analysis-correlation method.The results showed that density II（0.400）>density I（0.307）>density III（0.298）>density IV（0.287）>density V（0.275）.The relationship between stand density and soil nutrients is not clear in this middle-aged forest.（4）The relative yield,soil organic matter storage,soil total nitrogen storage and soil total phosphorus storage of black locust density I were higher.The trade-off relationship of high density black locust plantation is mostly above the medium level,while the trade-off relationship between the ecosystem service functions of density I and density II black locust plantation is mostly below the medium level,and some of them are low trade-offs.Within the density range of this study,the trade-off between the functions of low-density black locust plantation is relatively weak,which is conducive to the exertion of ecological functions.The study revealed the response law of the comprehensive ecological function of middle-aged black locust forest to density in this area,which provided theoretical support for the development of reasonable management measures.The relevant analysis methods can also be popularized and applied in the evaluation and analysis of other stands.