Satellite Observed Land Surface Greening in Summer Controlled by the Precipitation Frequency Rather Than Its Total Over Tibetan Plateau

Abstract

Land surface greening has substantially changed the carbon sequestration and hydrological processes over Tibetan Plateau (TP). Previous studies have revealed that the increased total precipitation (Ptotal) is the main driver of the enhanced peak growth in TP. However, the role of precipitation pattern, especially the frequency of precipitation (Pfreq, number of rainy days [>0.1 mm]), has not been well analyzed. We used time series of satellite‐derived Normalized Difference Vegetation Index (NDVI) to investigate the effect of Pfreq in controlling the peak growth (the maximum of the NDVI [NDVImax]) of different vegetation types in TP for the period of 1982–2015. We found that the widespread greening trend with Ptotal disappeared when Pfreq was introduced as a controlling variable, and that Pfreq alone contributed more than Ptotal to the increase of NDVImax. The underlying mechanism for the higher contribution of Pfreq than Ptotal to NDVImax is that increased Pfreq significantly improved soil moisture, reduced daytime temperature while increased nighttime temperature, thereby alleviating summer drought. Our results highlight the importance of Pfreq in interpreting the variation of peak growth, and these effects might be better represented in ecosystem models by considering Pfreq rather than Ptotal alone with future climate change.

Can the Accuracy of Fine-Resolution Precipitation Products Be Assessed from the Surrounding Water Balance and Drought Chain (WBDC) in the Qinghai–Tibetan Plateau?

“…This study further validated that there is a close relationship between EDI and SMAP soil moisture in the QTP (Figure 9). Precipitation frequency contributed more than total precipitation to peak vegetation growth in the Tibetan Plateau, especially in arid areas [53]. Our drought chain method reveals monthly precipitation changes.…”

Effects of air pollution on summer peak vegetation growth in the Beijing-Tianjin-Hebei region

“…条件而定 [68][69] 显著扩大， 集中在冀东北和南部地区 [70] ； 在较干旱地 区， 水为植被生长的限制因素， 降水能增加土壤水分 供给， 提高土壤中营养物质的可用性， 增加植被的养 分含量 [71] ， 促进植被生长； 并且降水频率高会使晴空 少有， 一定程度上降低白天的太阳辐射、 增加夜间的 长波辐射， 进而使白天温度更低、 夜间温度更高， 夜 间温度增加对夏季植被绿化具有更强的作用 [68] 。 燕山南麓降水增加， 可能会使土壤水分过多， 导致 内部通风不畅， 植被根系无法呼吸 [72] ， 影响正常生 长。张家口北部地区增加的降水可能仍无法缓解 干旱 [73] ， 降水增加， 植被仍表现为生长缓慢。 此外， 本文发现季前温度使 POS 提前和延后的 比例(分别为 49.5%和 50.5%)相差 1%， 与 NDVImax的 相关关系中负相关占主导地位(占比 91.8%)。足够 的积温是触发植被发生物候事件的必要因素 [74][75][76] ， 温暖的环境会促进植被的光合作用， 局地或全球气 候变暖使得大多数的植被开始提前生长 [77][78] 。季前 温度与 POS 呈负相关区域是因为大多数植被在温 度升高后， 植被中酶活性提高， 表现为提前生长， 很 可能提前到达光合作用最大值； 并且温度升高可能 会造成夏季干旱， 植被为适应环境， POS 提前； 正相 关区域基本分布在坝上地区， 春季寒冷， 夏季具有 较高温度有可能会导致干旱 [70] ， 从而使得 POS 延 后。温度升高， 导致植被蒸腾作用加强， 土壤中的 可利用水分减少 [79][80] 和草原的总初级生产力降低 [20] 。北京南部、 廊坊中 北部和石家庄东部较为湿润 [82][83] ， 在去除 PM2.5的影 [21] 。中等水平的气溶胶负 荷下， 太阳下的叶片的总入射光合有效辐射降低， 相对湿度增加， 促进气孔导度的提高， 增加光合作 用 [84][85] ； 气溶胶引起的漫反射增大了植物叶片的光 合面积， 从而使长期缺少光照的遮荫叶片的光合作 用增加 [21] ， 因此， 一定浓度范围内 PM2.5 的增加会使 NDVImax增大， 并且北方地区 NDVImax的增大一般对 应 POS 的提前 [16][17] 。…”