Diverse responses of vegetation phenology to changes in temperature and precipitation in Northern China

The start and end of the vegetation growing season (SOS and EOS) determine the length of the vegetation growing season and are necessary parameters to assess the primary productivity and carbon stocks of vegetation as well as the key dates to determine the start and end of soil wind erosion in arid zones. However, how SOS and EOS of different types of vegetation respond to changes in temperature and precipitation is still unclear. In this study, Moderate-resolution imaging spectroradiometer (MODIS)-normalized difference vegetation index data from 2001 to 2015 were used to estimate SOS and EOS for each vegetation type, the temperature and precipitation data recorded by the meteorological stations in the study area were used to calculate the average temperature and precipitation in 19 periods from June of the previous year to November of the following year, Spearman's rank correlation analysis was used to analyze the response relationships of SOS and EOS to average temperature and precipitation in each period. The results showed that SOS, EOS, the average timing of SOS (AvSOS) and the average timing of EOS (AvEOS) all had strong spatial heterogeneity. Precipitation is the key factor controlling SOS and EOS, an increase causing SOS to advance and EOS to be delayed, while higher temperature has the opposite effects. Overall, the average advancement rate of SOS was 0.88 day/year, the average delay rate of EOS was 0.33 day/year. From southeast to northwest, AvSOS gradually changed from 121st day to 141st day, whereas AvEOS was the opposite, advancing from 271st day to 282nd day. We divided the vegetation into 12 types in the soil wind erosion area of northern China according to latitudinal zonation and dominant vegetation species, the results avoided t a shortcoming of previous studies in which SOS and EOS were identified without distinguishing vegetation types.

植被生长季的起始（Start of Season, SOS）与终止（End of Season, EOS）决定了植被生长季的时长，是评估植被初级生产力与碳储量的核心参数，同时也是确定干旱区土壤风蚀起止节点的关键依据。然而，不同类型植被的SOS与EOS如何响应气温与降水变化，目前仍未明确。本研究采用2001—2015年的中分辨率成像光谱仪（Moderate-resolution imaging spectroradiometer, MODIS）归一化差值植被指数（Normalized Difference Vegetation Index, NDVI）数据，估算各类植被的SOS与EOS；同时利用研究区内气象站点观测的气温与降水资料，计算前一年6月至次年11月共19个时段的平均气温与平均降水量，并采用斯皮尔曼秩相关分析（Spearman's rank correlation analysis），解析各时段内SOS、EOS与平均气温、平均降水量之间的响应关系。研究结果表明：SOS、EOS、平均生长季起始时间（Average Start of Season, AvSOS）与平均生长季终止时间（Average End of Season, AvEOS）均存在显著的空间异质性。降水是调控SOS与EOS的关键因子：降水增加会使SOS提前、EOS延后，而气温升高则会产生相反的调控效应。整体而言，SOS的平均提前速率为0.88天/年，EOS的平均延后速率为0.33天/年。从东南向西北方向，AvSOS逐渐从第121日推移至第141日，而AvEOS则呈现相反的变化趋势，从第271日提前至第282日。本研究依据纬度地带性分异与优势植被种类，将中国北方土壤风蚀区的植被划分为12类，规避了既往研究未区分植被类型便直接识别SOS与EOS的研究缺陷。