Spatiotemporal Distribution Of Snow Cover And Plant Phenology And Its Influence Relationship Over Three-river Headwaters Region

The fragile ecosystem of the Three-River Headwaters Region(TRHR)is highly vulnerable to global change and human activities,which in turn lead to changes in snow and plant phenology in the region.Snow is an important influence on plant growth;snow prevents frostbite in plant during cold season conditions,and snow generates hydrothermal conditions that directly or indirectly affect plant rejuvenation,wilting and growing season length.In recent decades,significant climate change in TRHR has led to significant changes in both alpine snow and alpine plant.Therefore,it is important to study the influence relationship between snow and plant phenology and the spatial and temporal characteristics of snow and plant phenology in TRHR to reveal the ecological environment,disaster analysis and healthy development of water resources management in the region.This thesis uses MODIS snow cover products,snow depth data and MOD13A1 data from 2001 to 2018 to extract snow parameters and plant phenology parameters;on this basis,the spatial and temporal distribution characteristics of snow and plant phenology are explored;finally,Pearson correlation and grey correlation analyses were used to reveal the effects of snowpack changes on plant phenology.The main contents and results of the research are as follows:(1)Based on snow remote sensing data,the spatial analysis method was used to extract Snow Cover Fraction(SCF),Snow Cover Days(SCD)and Snow Depth(SD)and analyse the spatial and temporal distribution characteristics of snow in TRHR.The results show that SCF in TRHR shows a slightly increasing trend year by year with obvious seasonal variations;SCD is widely distributed and extremely heterogeneous;SD has a large difference in distribution in all seasons and is highly spatially heterogeneous.In addition,SCF increases with elevation and slope,and SCF on the northern slope is significantly higher than that on the southern slope;the correlation between SCF and different climatic factors varies significantly,and SCF is significantly influenced by temperature and precipitation.(2)Based on MOD13A1 data,the spatial and temporal distribution characteristics of plant phenology in TRHR were analysed using S-G filtering,six-term fitting and derivative methods to extract Start of Growing Season(SOS),End of Growing Season(EOS)and Length of Growing Season(LOS).The results showed that: spatially,with increasing altitude,SOS of plants in TRHR was delayed,EOS was advanced and LOS was shortened;with increasing slope,SOS in TRHR was advanced,EOS was advanced and LOS was delayed;SOS and EOS were earlier on the northern slope than on the southern slope,and LOS was greater on the northern than on the southern slope.Temporally,SOS and EOS of TRHR alpine meadows and the whole plant ecosystem showed an early trend and LOS a lengthening trend;while alpine grassland EOS showed a delayed trend.(3)Snowcover variability has an important influence on plant phenology in TRHR.With the increase of SCF,SCD and SD,the SOS of alpine meadows,alpine grasslands and vegetation ecosystems in TRHR showed an early trend;the EOS of alpine meadows was early;the LOS of alpine meadows was shortened,and the LOS of alpine grasslands and vegetation ecosystems was extended.The correlation between snow accumulation parameters and plant phenology parameters in alpine meadows and alpine grasslands were all ranked from largest to smallest: SCF>SD>SCD.The research on the response of plant phenology to snow cover provides an important reference for water resources management and ecological environment protection in the TRHR Region.