Research On The Relationship Between Phenological Changes Of Steppe Vegetation And Net Primary Productivity In Inner Mongolia,China

Inner Mongolia steppes are highly representative of temperate steppes and are also one of the most sensitive regions to global change.In the context of climate change,exploring the impact of phenological changes in vegetation on the net primary productivity（NPP）and carbon cycle of steppe ecosystems is of positive significance for the rational use of steppe resources,tapping the value of steppe carbon sinks,and achieving the goal of‘carbon neutrality’.In this study,we used AVHRR NDVI remote sensing data from 1982 to 2019,global daily carbon flux simulation data,GLASS NPP products from 1982 to 2018,data from meteorological observation stations,and field-observed data to investigate the phenological changes in the Inner Mongolia steppe and their impacts on NPP.Based on a series of mathematical and statistical methods,the temporal and spatial variations in phenology in the Inner Mongolia steppes over the past 40 years were investigated,and the NPP and its temporal and spatial shifts were simulated and analyzed.Then,the relationships between vegetation phenology and NPP were quantitatively revealed to provide a theoretical basis for future ecosystem management and ecological construction in this region.The main findings of the study are as follows:First,during 1982-2019,the steppes in Inner Mongolia showed an overall pattern of changes at the start of the growing season（SOS）and the end of the growing season（EOS）,both being insignificantly advanced,and the length of the growing season（LOS）was insignificantly shorten.The rates of advanced SOS and EOS were 0.08 d/a and 0.17 d/a,respectively,and the shortening rate of LOS was 0.09 d/a.SOS was significantly advanced in16.42%of the study area,mainly in the eastern meadow steppe.The EOS occurred earlier in the eastern part of the study area than in the western and central parts,with the EOS mainly concentrated on the 260th to 280th day.The area with significantly advanced EOS accounted for 9.28%,mainly in the desert steppe.The growing season was longer in the western and central parts of the study area and shorter in the eastern meadow steppe.Only 3.54%of the entire steppe had a significantly shorter LOS and was located in the transition zones between desert steppe and typical steppe.In addition,the overall NPP of steppe vegetation in Inner Mongolia showed an increasing trend from 1982 to 2019,with rates of approximately 0.55 g C/（m²·a²）（calculated based on the BEPS model products）and 0.25 g C/（m²·a²）（results of the CASA model simulation）.The peak period of NPP accumulation was from April to October each year.The annual mean NPP of the meadow steppe was the highest and that of the desert steppe was the lowest.The NPP accumulation of different steppe types mainly occurred in summer.Spatially,NPP in the study area was higher in the east and lower in the west.For the BEPS model products and the results of the CASA model simulation,the NPP in Inner Mongolia mainly tended to increase,with average growth rates of 0.88 g C/（m²·a²）and 0.60 g C/（m²·a²）,respectively,with significant increases in NPP in approximately 32%and 19%of the study area,which were distributed in the meadow steppe.However,analysis based on GLASS NPP products showed a predominantly decreasing trend in NPP in the study area from 1982 to2018,with an average rate of decrease of 0.99 g C/（m²·a²）and significant decreases in approximately 13%of the study area,mainly in the typical steppe.Last but not least,the regions in the study area where the SOS was significantly and negatively correlated with spring NPP were mainly located in the meadow steppe and typical steppe.In these regions,SOS advanced,and spring NPP increased.In contrast to the CASA model simulation results,for the GLASS NPP products and the BEPS model products,the areas where SOS was significantly positively correlated with annual NPP were distributed in the typical steppe and part of the desert steppe.In the entire steppe,annual NPP showed an increase followed by a decrease with the delay of SOS.A similar situation existed in the fitting results of NPP products to the typical steppe.The calculation results of 3 kinds of NPP data showed that the annual NPP of the meadow steppe gradually decreased with the delay of SOS,while the annual NPP gradually increased in the desert steppe.Fitting results for both NPP products showed that autumn NPP decreased with a delay in EOS for the entire steppe,typical and desert steppes,and vice versa for the meadow steppe.For the fitting results of the CASA model-simulated NPP,autumn NPP in the desert steppe also gradually decreased with the delay of EOS,and conversely,autumn NPP in the entire steppe,meadow and typical steppes gradually increased with the delay of EOS.In contrast to the CASA model simulations,the annual NPP for both kinds of products were significantly negatively correlated with the EOS in the typical and desert steppes.The fitting results of the quantitative relationships also indicated that annual NPP gradually decreased with the delayed EOS in the entire steppe and the typical and desert steppes.Conversely,when the EOS was delayed,the annual NPP increased in the meadow steppe.Analyzed by using the NPP products simulated by the BEPS model,the meadow steppe had a shorter LOS but more accumulation of NPP,and NPP increased with a longer LOS.The longer growing season gave plants much more time for photosynthesis and reduced atmospheric CO₂concentrations,which in turn increased NPP.The typical steppe and desert steppe had longer LOS but relatively less NPP,and in most areas,LOS was shortened,although NPP still increased overall.In addition,NPP decreased significantly in the typical steppe and desert steppe at the time of spring and autumn phenology.In these areas,the warming effects of earlier phenology enhanced ecosystem respiration（ER）,limiting the accumulation of NPP to some extent.This study is an important reference for deepening the understanding of the relationship between phenology and productivity in steppe ecosystems.