1963—2018年中国垂柳和榆树开花始期积温需求的时空变化

积温需求是决定北半球温带地区木本植物开花时间的主要因子。全球变暖使植物在冬季受到的冷激量减少,可能会改变植物开花的积温需求。气候变化导致的中国木本植物开花始期积温需求的时空变化仍不清楚。有鉴于此,本文基于“中国物候观测网”1963—2018年垂柳（Salix babylonica）和榆树（Ulmus pumila）开花始期数据,利用3种积温算法（GDD、GDDS和GDH）系统分析了两种植物开花积温需求的空间格局和在代表性站点的年际变化,构建了基于冷激日数模拟积温需求的模型。主要结论为：垂柳和榆树开花始期的积温需求在低纬度地区大于中纬度地区。站点多年平均积温需求与冷激日数呈显著的负指数关系,即随冷激日数增加,积温需求降低。时间上,3个典型站点（贵阳、西安和牡丹江）垂柳开花积温需求的变化趋势分别达到1.28~1.41 °C·d/a（P < 0.01）、1.63~1.89 °C·d/a（P < 0.01）和0.12~0.58 °C·d/a（仅GDD算法P < 0.05）,榆树开花的积温需求在贵阳和西安同样显著增加,但在牡丹江变化不显著。冷激日数随时间减少是两个站点积温需求显著增加的主要原因。因牡丹江冬季气温低,冷激日数多且年际变化小,冷激对积温需求变化的影响不显著。基于时空耦合样本建立的冷激日数—积温需求模型对垂柳开花积温需求的模拟效果较好,R²达0.54~0.66。对榆树开花积温需求的模拟效果稍差（R²为0.33~0.64）。就不同算法而言,冷激日数对GDD算法得到的积温需求模拟效果更好。本文为量化植物开花积温需求的时空变化及在气候变化背景下的花期预测提供了重要的科学依据。

Spatio-temporal variations in the thermal requirement of the first flowering dates of Salix babylonica and Ulmus pumila in China during 1963-2018

The forcing temperature in spring is the main factor that determines the flowering time of woody plants in the Northern Hemisphere. Global warming has reduced the number of chilling days in winter, which probably alters the thermal requirement of flowering. In the past 50 years, the spatio-temporal changes in the thermal requirement of spring phenology in China remain unclear. Based on the first flowering date (FFD) data of Salix babylonica and Ulmus pumila derived from China Phenological Observation Network during 1963-2018, we used three methods to calculate the thermal requirements of FFD and systematically analyzed their spatial and temporal patterns at representative sites. We also developed chilling days-thermal requirement models to quantitatively simulate the thermal requirement at each site in different years. The results showed that the thermal requirement of FFD exhibited a large spatial difference, with a relatively high value at low latitudes than that at middle latitudes. There was a significant negative exponential relationship between the average thermal requirement and chilling days across sites. The thermal requirements of FFD also changed over time. The trend in thermal requirement of Salix babylonica FFD in Guiyang, Xi'an and Mudanjiang reached 1.28-1.41 °C·d/a (P<0.01), 1.63-1.89 °C·d/a (P<0.01) and 0.12-0.58 °C·d/a (P<0.05) for the growing degree day method, respectively. The thermal requirement of Ulmus pumila FFD also increased significantly in Guiyang and Xi'an, but the trend in Mudanjiang was not significant. The decrease in the number of chilling days was the main reason for the increase in the thermal requirements. Due to the low winter temperature in Mudanjiang, the number of chilling days was large and had a small interannual variation, thus chilling days exerted no significant impact on the thermal requirement. The chilling days-thermal requirement model performed better in simulating the thermal requirement of Salix babylonica FFD, with R ² of 0.54-0.66. In comparison with Salix babylonica, the model showed a relatively low precision in simulating the thermal requirement of Ulmus pumila FFD, with R ² ranging from 0.33 to 0.64. Among the three methods, the thermal requirement could be better simulated by the growing degree days method compared with the growing degree days-sigmoid and growing degree hours methods. This study provides an important scientific basis for quantifying the spatio-temporal variation of the thermal requirement of flowering and for predicting the future flowering date of woody plants under the background of climate change.