Response Of Vegetation Phenology To Hydrothermal Conditions On The Mongolian Plateau

The Mongolian Plateau is located in the middle latitudes of the Northern Hemisphere.Under the combined influence of geographical location and climatic characteristics,the Mongolian Plateau has become one of the regions most sensitive to global climate change.Vegetation plays a key role in the energy exchange and hydrological cycle on Earth’s surface.However,the vegetation monitored from remote sensing is affected not only by climate change but also by other disturbances;such effects severely limit our understanding of the change characteristics of vegetation and reflect regional and even global climate change trends.Taking the vegetation phenology of the Mongolian Plateau as an entry point,this paper studies the spatiotemporal characteristics of vegetation phenology;the response of vegetation phenology to hydrothermal conditions;and the main driving factors affecting vegetation phenology at different preseason time scales.By studying the response of vegetation phenology to hydrothermal conditions on the Mongolian Plateau from1982 to 2015,the main conclusions were drawn:(1)During 1982-2015,the start of the growing season(SOS)on the Mongolian Plateau ranged from 115-140 day of the year(DOY);the end of the growing season(EOS)ranged from 270-295 DOY.During the study period,the change trend of vegetation phenological parameters was that the SOS was advanced and the EOS was delayed,and the change trend was quite different among different climate regions and different vegetation types.The characteristics of the temporal standard deviation of vegetation phenology were smaller in the northeast and north,and larger in the south and southwest;it increased from the northeast to the southwest.(2)The inhibitory or promoting effect of hydrothermal conditions on vegetation phenological parameters gradually weakened with increasing of preseason length.The responses of the phenology of different vegetation types to hydrothermal conditions showed that in the Mongolian Plateau region,whether in the forest or grassland areas,there was an advancing trend under normal hydrothermal conditions.However,in a year with meteorological drought,the SOS in forest areas was advanced,and the SOS in grassland areas was delayed.The response of the EOS of different vegetation types to the hydrothermal conditions showed that the EOS on the Mongolian Plateau was delayed under normal hydrothermal conditions.In meteorological drought years,the EOS in the whole study area and grassland area was advanced,and the EOS in forest areas was delayed.(3)Wildfire disturbances could advance the SOS or delay the EOS.The vegetation phenological parameters of typical burnt sites showed a response pattern of first increasing and then decreasing with increasing distance from wildfires.The response of vegetation phenological parameters to wildfire of different intensities showed that the SOS was most sensitive to medium fire burns.However,the EOS was most sensitive to high intensity wildfires.(4)The dominant driving factors affecting vegetation phenological parameters at different preseason time scales had obvious differences in different climate regions and different vegetation types.On the whole Mongolian Plateau,the significant area of the SOS response to temperature in the first month of the growing season accounted for the largest proportion,and then gradually decreased.However,the EOS had the largest proportion of significant areas in response to the meteorological drought index in the three months before the end of the growing season.The analysis results of the SOS dominant factors in different climate regions showed that in humid and semihumid regions,the dominant driving factor affecting the SOS was temperature.In semiarid and arid regions,the dominant factor affecting the SOS was precipitation.The analysis of the dominant factors of the EOS in different climatic regions showed that temperature played a dominant role in the EOS in humid and semihumid regions.In semiarid and arid regions,the influence of the three meteorological factors was less different.The analysis results of the dominant driving factors of vegetation phenological parameters in different vegetation types showed that the vegetation phenological parameters in broad-leaved forest and coniferous forest were most sensitive to temperature.The dominant factor affecting the SOS in the desert steppe and meadow steppe was precipitation,and the dominant factor affecting the EOS was temperature.The dominant driver of the SOS in a typical steppe was precipitation,and the dominant driver of the EOS was the meteorological drought index.