The precipitation “threshold value” on C_4/C_3 abundance of the Loess Plateau, China

降水“阈值”对黄土高原全新世以来C_4/C_3植物相对丰度的控制(英文)

In this paper, we report organic carbon isotopic characterizations from two loess sequences （JY and GL）, spanning the last 20 ka, from the northwest Chinese Loess Plateau （CLP）. The results indicate that the vegetation type is nearly pure C3 plants in the studied region during the Holocene. In contrast to other reported loess sequences in the central-southeast CLP, the relative abundance of the C4 plants decreases from southeast to northwest, with the vegetation types changing from pure C3 to a C4/C3 mixture should near 36°N in the Holocene. From the perspective of the modem temperature and precipitation distribution, the summer temperature has no obvious change at the same latitude, but there are differences in the summer precipi- tation, which exhibit an obvious increase from west to east. Further analysis indicated that the C4 plant abundance decreases with the decreasing summer season precipitation from the southeast to the northwest CLP during the Holocene. We suggest that with the absence of favorable precipitation condition, increasing temperature and decreasing atmospheric CO2 concentration are insufficient to drive an expansion of the C4 plants on the CLP in the Holocene. According to a Holocene precipitation recon- struction, a ＂threshold value＂ of summer precipitation existed, which mainly controlled the expansion of C4 plants. Compared with the modern δ13Csom and climate data on the CLP, both the Holocene and the present ＂threshold value＂ are near the 360-mm summer precipitation line, although the present precipitation line turned slightly southwestward. Our results provide new insights for further research on the C4/C3 variations with precipi- tation and the relationship to global C4/C3 change.

黄土高原地区C4/C3植物相对丰度变化的主要驱动因素曾被广泛地争论,主要是由于对该区域现代C4/C3植被分布的空间特征及其驱动因素缺乏深入认识.本文总结了前人对黄土高原中部和东南部剖面的δ13Csom研究结果,并增加2个西北部剖面的δ13Csom结果,结合现代气象数据和表土δ13Csom变化特征,从空间变化讨论黄土高原地区C4/C3植被分布特征及驱动因素.末次冰期以来,西北部的古浪和靖远剖面主要发育C3植被,而中部和东南部的剖面则为C3-C4植被混合的特征,且C4植被含量从东南向西北递减.我们认为晚冰期以来,黄土高原地区的温度逐渐升高并达到C4植被生长的温度"阈值",同时大气CO2浓度也处在适宜C4植被生产的低浓度范畴,则夏季降水的梯度变化应该是控制全新世和现代黄土高原东西部植被类型变化的主要因素,并同样存在一个影响C4植被生长的降水"阈值",进一步通过现代夏季降水量对全新世C4植被扩张时期的夏季降水量进行定量恢复,我们发现,无论在全新世还是现代,当夏季降水超过360 mm时,C4植被会发生明显的扩张,而现代的360 mm降水线较全新世向东南方向偏移.