油松树轮指示的秦岭中部过去194年5—7月NDVI变化

由于归一化差异植被指数(NDVI)观测记录较短,对长时间尺度的NDVI变化研究较少,限制了我们对于全球变暖背景下气候驱动的植被生产力变化及其影响的理解。本研究利用陕西秦岭中部油松树轮样本建立区域树轮宽度指数年表,基于秦岭中部区域年表与5—7月NDVI的较高相关(r=0.624,P<0.01,n=34),利用线性回归模型重建秦岭中部1825—2018年5—7月NDVI变化,方差解释量为38.9%。空间分析表明,重建序列能够较好代表研究区范围内NDVI变化。重建序列表明,秦岭中部过去近194年经历了6个高值期和5个低值期,其中2006—2018年植被生长最好,即在最近的升温停滞期,秦岭中部植被生长呈显著恢复性生长。NDVI低值期与研究区区域干旱事件有着良好的对应关系。小波分析表明,重建序列存在2～4、12～16年准周期。SEA分析表明,重建序列在厄尔尼诺年出现显著下降,而在拉尼娜年事件发生后第1年至第3年出现显著上升。预测油松生长在SSP2-4.5、SSP3-7.0和SSP5-8.5情景下会略微上升。

May-July NDVI variation for the middle Qinling Mountains over the past 194 years indicated by tree rings of Pinus tabuliformis

Due to the short-term observation record of the normalized difference vegetation index (NDVI), the research on long-term NDVI changes is scarce, which limits our understanding of the impacts of NDVI changes in the context of global warming. In this study, a regional tree-ring chronology was developed based on the tree-ring samples of Pinus tabuliformis in the middle Qinling Mountains. The results showed that tree-ring width of P. tabuliformis was significantly positively correlated with May-July NDVI (r=0.624, P<0.01, n=34). The Sig-Free tree-ring width chronology was used to reconstruct May-July NDVI during the period 1825—2018, which explained 38.9% of the total NDVI variance. Results of spatial analysis showed that the reconstructed series could better represent the NDVI changes in the study area. There were six high NDVI periods and five low NDVI periods in the past 194 years. The vegetation grew best in 2006-2018, indicating vegetation cove-rage in the middle of Qinling Mountains had been improved during the warming hiatus. Low NDVI periods in the reconstruction series were consistent with drought over much of study area. Results of wavelet analysis indicated the existence of 2-4 years and 12-16 years cycles in the reconstruction series. SEA analysis showed that the reconstruction series decreased significantly in the El Nino year, while increased significantly in the first to third years after the La Nina event. The growth of P. tabuliformis was predicted to increase slightly under the SSP2-4.5, SSP3-7.0 and SSP5-8.5 scenarios.