The Response Of Green-up Date To Climate Change In Hulun Buir Grassland

Grassland is a renewable natural resource with the largest coverage,the largest number,and the fastest update among green vegetation in the world.It plays a very important role in the global carbon cycle and climate change.The vegetation green-up date is a sensitive response indicator of grassland ecosystems to climate change,especially in high-latitude and high-altitude areas.In the background of climate change,studying the spatio-temporal characteristics green-up date and its response to climate change at high latitudes grassland will provide important theoretical and practical significance for the sustainable development and adaptive management of grassland ecosystems.This study analyzed the spatio-temporal pattern of the green-up date that extracted by median method in Hulun Buir grassland from 2001 to 2015,as well as the spatio-temporal variation characterwastics of temperature and precipitation.In addition,the effects of temperature and precipitation on green-up date and the relationship between the heat requirement before green-up date with different meteorological factors were explored through partial correlation analysis and sensitivity analysis,and then the response mechanism of different types of grassland to climate change was quantified.The main conclusions were as follows:(1)The green-up date of Hulun Buir grassland occurred from mid-April to late May,and the overall period was 120 ± 11.6 days(mean ± standard deviation).The green-up date was spatially delayed from the southwest and east to the Greater Khingan Mountains.From the perspective of grassland types,the green-up date of temperate steppe appeared first,followed by temperate meadow steppe,and lowland meadow,and the last forupland meadow.The occurrence of green-up date of four grassland types were day 116 ± 14.3,day 121 ± 7.9,day 123 ± 9.2,and day 125 ± 6.7,respectively.The green-up date of study area showed an overall advanced trend,with an average change of-2.0 days/decadeThe proportion of pixels with significant advanced and delayed trends(p <0.05)were 9.3% and 4.4%,respectively.The green-up date temperate grassland had delayed trend,and that of the other three types had advanced trend.(2)During the study period,the inter-annual temperature had no significant change,while the precipitation increased significantly,and the annual distribution was uneven.From 2001 to 2015,the annual average temperature generally showed a nonsignificant decrease trend(-0.70 ?/decade).The average temperature decreased faster in winter(-1.04 ?/decade)than that in spring(-0.20 ?/decade).Among the spring months,the average temperature decreasing trend in March(-1.50 ?/decade)was the largest,while the average temperature change trend in April(-0.02 ?/decade)was the smallest.But the average temperature in May had an rwasing trend(0.33 ?/decade)).In the same period,the annual precipitation of 59.5% of the pixels significantly increased,with an average increase trend of 152.1mm/decade.However,in spring and winter,the precipitation did not show a significant increase trend.In March and April,the precipitation showed an insignificant decrease trend with-1.5 mm/decade and-10.5 mm/decade,respectively.In May,the precipitation showed an increasing trend with 20.9 mm/decade,that 22.7% pixels had a significant increase.It could be concluded that spring precipitation gradually concentrated in May.(3)For temperate steppe,the increase in the daily minimum temperature in winter could promote green-up,with the sensitivity of-2.23 days/?,and the proportion of significant pixels was 38.5%.In addition,precipitation in April could also promote green-up,with the sensitivity of-1.44 days/mm,and the proportion of significant pixels was 32.5%.Except for temperate grasslands,the green-up dates of other typeswere more affected by the temperature in May and less affected by precipitation in the spring months.Specifically,the green-up dateof 21.6% of temperate meadow steppe advanced to 0.33 days/? with the increase of the daily maximum temperature in MayThe greenup date of 15.6% of temperate meadow steppe was delayed to 2.64 days/?,due to the effect of the daily minimum temperature in May,which was greater than the sensitivity of the daily maximum temperature.The green-up dates of lowland meadow and upland meadowswas mainly affected by the daily minimum temperature in May,and the proportion of pixels significantly affected were 37.3% and 36.7%,respectively.The range and extent of significant effects were greater than the daily maximum temperature in May.The sensitivities were 6.19 days/? and 4.24 days/?,respectively.Precipitation in spring months had little effect on temperate meadow grassland,lowland meadow and mountain meadow,within ± 0.4days/mm.(4)Under the premise that the effective temperature was 0 ? and the time interval was 60 days before the green-up date,the heat requirement was most closely related to the meteorological factors.The accumulated growing degree-days(AGDD)was used to express the heat requirement for vegetation green-up date.The trend of AGDD in the whole study area was 2.9 ?-days/year,and only 7.8% showed a significant increase trend.temperate steppe,which had the earliest green-up date,need the largest AGDD(144 ± 130 ?-days,mean ± standard deviation).Among the factors influencing the heat requirement,the surface shortwave radiation had largest effect,followed by the number of chilling days and precipitation.The increase in surface radiation directly led to an increase in AGDD.More than 95% pixels of each grassland type were significantly affected by the surface radiation.The increase in the number of chilling days could reduce the AGDD,and the order of impact on each grassland type was: temperate steppe(81.3%)> lowland meadow(53.7%)> temperate meadow steppe(43.5%)> upland meadow(21.2%).The effect of precipitation on AGDD was the smallest,and only 28.9% of the pixels in the whole area were significantly influenced.