| Vegetation is a natural“link”that connects soil,atmosphere and water,and acts as an“indicator”in global climate change.The southern slope of Qilian Mountain is an important climate intersection and sensitive area in the northwest region.It is China's ecological security barrier and“natural water tower”.Based on the NDVI data in the MODIS 13A1 remote sensing image from 2001 to 2017,the asymmetric Gaussian function method combined with the dynamic threshold method was used to extract the spatial distribution characteristics and time variation characteristics of the overall phenological period?regreening period,yellowing period and growing season length?of study area and extract the multi-year average characteristics and the temporal variation characteristics of forest land,shrub forest,marsh,high cover grassland,medium coverage grassland and low-coverage grassland?On the basis of this,combined with meteorological factors?monthly temperature,monthly average rainfall?and altitude factors,the relationship between the overall phenological period of the study area and meteorological and altitude factors and the relationship between phenological period of different vegetation types and meteorological and altitude factors are revealed.This revealed relationship provides some enlightenment and reference value for the ecosystem restoration and protection and social and economic development of the southern slope of Qilian Mountain.Through the analysis of the study area,the following conclusions are drawn:?1?The vegetation phenological period in the study area is concentrated in97-129 days,gradually advancing from the southeast low altitude area to the northwest high altitude area;the yellowing period is concentrated in 257-289 days,the spatial variation characteristics are opposite to the greening period;the growth season length is concentrated in Between 113 and 177 days,the spatial variation characteristics are the same as the greening period.The low-altitude and low-latitude regions have the characteristics of early returning to green and late browning.?2?The vegetation phenological period of the study area showed a tendency to rise or fall in the fluctuation.From 2001 to 2017,the overall greening period of the vegetation in the study area was 0.28 d·a-1,and the yellowing period was delayed by0.27 d·a-1.The length of the growing season was extended by 0.57 d·a-1.?3?The phenological periods of different vegetation types in the study area are different.The order of the 6 planting types entering the greening period is:shrub forest>forest land>high coverage grassland>swamp>medium coverage grassland>low coverage grassland;6 planting types enters the yellowing period in the order of low coverage grassland>medium coverage grassland>swamp>High coverage grassland>shrub forest>with forest land;The length of growth season of 6plant types is ranked as forest land>shrub forest>high coverage grassland>swamp>medium coverage grassland>low Coverage grass.?4?Different temporal changes in phenological periods of different vegetation typesIn 2001-2017,the greening period of forest land was generally delayed,and the other five planting types were generally in an early trend.Except for the trend of the greening period of forest land between 2010 and 2012,which is different from the trend of other 5 planting types,the trend of the 6 types of planting in the other years is consistent;the linear fitting results of different vegetation types in the yellowing period are all on the rise.The 6 planting types of the yellow-stained period in the study area has been greatly delayed in 2013-2017,and all of them have turned in 2015,indicating that the response of vegetation to the large climate change is consistent;2001-2017 the linear fitting results of the growth season lengths of different vegetation types showed an upward trend,which was related to the advance of the greening period and the delay of the yellowing period.Under the premise that the change trend of the 6 types of vegetation types is highly consistent,the length of the vegetation growing season is mainly affected by the yellowing period.?5?Correlation between vegetation phenological period and temperature factor in the study areaThere was a negative correlation between the overall greening period of vegetation in the study area and the monthly average temperature from December to May,indicating that the temperature rise would promote the advancement period;the yellowing period of the study area was positively correlated with the average temperature from June to November,indicating the rise of temperature promoted the delay of the yellowing period;the length of the vegetation growing season in the study area was positively correlated with the annual temperature,This shows that the increase in temperature helps the length of the growing season.?6?Correlation between vegetation phenological period and rainfall factors in the study areaThe overall greening period of vegetation in the study area was positively correlated with the monthly average rainfall in December,and negatively correlated with the monthly average rainfall from January to May,the increase of rainfall contributed to the advance of the greening period.The average yellowing period in the study area was positively correlated with the average rainfall from June to November.The increase of rainfall contributes to the delay of the yellowing period;the length of the growing season length in the study area is positively correlated with the monthly average rainfall from January to November?except December?,and the increase of rainfall contributes to the length of the vegetation growing season extend.?7?The overall phenological period of the study area has different responses in different altitude ranges.At altitudes below 2550 m and between 2950-3250 m,the vegetation regreening period tends to increase with elevation.When the altitude is between 2550-2950 m and greater than 3250 m,the greening period is delayed with the elevation.For every 1000 m elevation,the greening period is delayed by 5.3 days;the yellowing period of the vegetation is below 2450 m,2550-2950 m and above 3250m.The elevation trend is in advance with the elevation,When is between at2450-2550 m and 2950-3250 m,the yellowing period is delayed with the elevation.As a whole,the altitude is increased by 1000 m,and the yellowing period is 9.3 days earlier.When the length of the growing season of the study area is below 2550 m sea level and between 2950-3250 m,the length of the growing season is prolonged with the elevation,when the altitude is at When the area is between 2550-2950 m and above 3250 m,the length of the growing season tends to decrease with the elevation. |
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