| Due to its wide spatial distribution and high resolution,tree-ring data can accurately record the change process of natural environment and be used to reconstruct historical climate changes.It plays an important role in the interdisciplinary research of climatology,forestry,ecology and other disciplines.In recent years,with the vigorous development of dendrochronology,shrubs have received increasing attention as new tree-ring research objects.The woody plants are mostly shrubs and semi-shrubs in arid and semi-arid desert regions or high-altitude and high-latitude mountainous areas in China.The distribution characteristics of shrubs and high sensitivity to climate change determine the irreplaceable role in revealing regional climate change,environmental protection and ecological restoration.However,due to the special shape of shrub tree-ring,radial allometric growth and high measurement difficulty,there are few reports on the radial growth of shrub tree-ring and its correlation with environmental factors.Therefore,it is of great significance to explore the measurement methods and measurement index suitable for shrub rings in cold and arid areas,and analyze the correlation between shrub tree-ring and environmental factors,which is of great significance for studying regional climate and environmental changes and reconstructing historical NDVI sequence.In this study,Haloxylon ammodendron and Tamarix ramosissima,which are typical desert plants in cold and arid regions,were selected as the research objects.Firstly,the tree-ring parameters of Haloxylon ammodendron were extracted and measured based on ENVI and GIS software,and the accuracy was verified by Win DENDRO professional tree-ring analysis software,aiming to propose a measurement method suitable for shrub dendrochronology.Secondly,the tree-ring characteristics of Tamarix ramosissima were extracted through U-Net image semantic segmentation and GIS measurement,and the tree-ring growth rate index(TRGR)was proposed,which was compared with tree-ring width(TRW)and basal area increment(BAI),in order to provide a new substitute index for the study of shrub dendrochronology.In addition,the seasonal curve parameters of NDVI are reconstructed by using TIMESAT time series analysis software,and the interannual variation trends of NDVI under different scales were analyzed.Finally,the early-wood radial growth of Haloxylon ammodendron and the TRGR of Tamarix ramosissima were correlated with the key parameters of NDVI seasonal curve,respectively,to explore the response of their radial growth to environmental changes.The main findings of the study are as follows:(1)The study combined with ENVI image classification and GIS measurement and analysis,a method was proposed for measuring the tree-ring parameters of Haloxylon ammodendron,which can automatically extract the tree-ring boundary line,effectively improve the measurement efficiency and reduce human interference.This method lays a foundation for the automatic measurement of tree-ring parameters such as perimeter and area.Professional ring analysis system Win DENDRO was used to test the measurement results.The results were proved that the accuracy of the method meets the measurement requirements.In addition,the method constructed in this study is universal and suitable for any tree ring analysis.(2)The TRGR index calculates the annual activity of cambium per unit length,which was measured by using U-Net image semantic segmentation and GIS.This index is not affected by the DBH trend of trees,and can accurately reflect the overall radial growth of tree-rings.(3)In the Gurbantunggut desert,there was a positive correlation between the growth of Haloxylon ammodendron and the key parameters of the NDVI time series at the sampling points in the east-west and north-south directions.Among them,in the north-south direction,the early-wood radial growth of Haloxylon ammodendron and NDVI were generally significantly positively correlation due to little human influence.In the east-west direction,the positive correlation between the two is weaker than that in the north-south direction due to human activities.(4)There was a significant positive correlation between the TRGR of Tamarix ramosissima and NDVI key seasonal parameters,the correlation coefficients were all above 0.6 and the highest was 0.97.The results have important reference value for exploring the response of shrub tree-ring radial growth to climate change and reconstructing the historical NDVI of no arbor distribution area.(5)The results showed that tree-ring index of Tamarix ramosissima had more significant positive correlation with NDVI parameters than index of Haloxylon ammodendron in the same habitat,because the tree-ring characteristics of Tamarix ramosissima was obvious and there was only one ring every year.Therefore,Tamarix ramosissima is more suitable for the analysis of shrub dendrochronology.In comparison,the tree-ring index of Haloxylon ammodendron was highly correlated with NDVI in only part of the sampling points because of the fragmented growth of cambium.Therefore,the measurement and analysis of radial growth of Haloxylon ammodendron should be further discussed in the future.(6)The study found Haloxylon ammodendron no aestivation,through continuous observation in the whole growing season.The radial growth of Haloxylon ammodendron stopped in a certain direction between interannual,and the numbers of tree-ring at the maximum radius were less than or equal to the actual age,and there is no false ring phenomenon.In conclusion,the tree-ring measurement method proposed in the study has the advantages of low cost and high precision,and has highly economic and social value.Compared with the traditional tree-ring research indexes,such as TRW and BAI,TRGR index can more completely reflect the radial growth information of trees,so it can replace TRW and BAI as a new index for shrub dendrochronology in cold and arid areas.Tamarix ramosissima can be used as a tree species for shrub dendrochronlogy in arid area.The results provide reference value for reconstruction of regional historical NDVI or climate change. |
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