观察春季植物展叶是科学家判断气候变暖的一项指标。日前，北京大学城市与环境学院生态学系教授朴世龙研究发现，随着全球温度的上升，春天新叶的萌发对于温度越来越不敏感，导致春季新叶展叶的时间越来越早。该项研究成果发表在9月24日出版的《自然》杂志上。

朴世龙及其研究团队使用跨欧洲物候学项目中的数据，分析了跨越中欧1245个观测点中的7个欧洲树种在30年间展叶情况，其中树种包括了普通赤杨、垂枝桦、欧洲七叶树、欧洲山毛榉、小叶椴、夏栎、欧洲白蜡树等。研究人员发现面对气候变暖，在所有观测点的每个树种都表现出了展叶减少的现象，而这一现象占所有树木总量的40%左右。同时研究人员根据数据模型证明，这种现象部分可以归因于冬季低温期的减少，树木没有获得足够的低温效应从而触发展叶。

研究人员表示，植物展叶同样影响着区域和南北半球的碳平衡、树种的分布以及植物与动物相互作用的物候。根据春季物候实质性的变化可以证明北半球大多数地区正在经历气候变暖，并且气候变暖还将进一步加强。在温带和亚寒带气候下，树木需要经历一段时间的低温，才能结束休眠并开始展叶。这项研究表明，全球气候变暖会减少低温效应，降低树叶对气温的敏感性，从而抵消了气候变暖导致提前展叶的效果。

澳大利亚麦考瑞大学的特雷弗·基南教授对于此项研究成果发表了一篇观点认为，还需要更多的研究，尤其是分析世界许多其他地区长期的记录，结合植被的卫星观测、实验数据和理论认识，来评估是否其他的物种和地区也表现出类似的温度敏感性的下降。（来源：科技日报）

Declining global warming effects on the phenology of spring leaf unfolding

Abstract  Earlier spring leaf unfolding is a frequently observed response of plants to climate warming. Many deciduous tree species require chilling for dormancy release, and warming-related reductions in chilling may counteract the advance of leaf unfolding in response to warming. Empirical evidence for this, however, is limited to saplings or twigs in climate-controlled chambers. Using long-term in situ observations of leaf unfolding for seven dominant European tree species at 1,245 sites, here we show that the apparent response of leaf unfolding to climate warming (S_T, expressed in days advance of leaf unfolding per °C warming) has significantly decreased from 1980 to 2013 in all monitored tree species. Averaged across all species and sites, S_T decreased by 40% from 4.0 ± 1.8 days °C⁻¹ during 1980–1994 to 2.3 ± 1.6 days °C⁻¹ during 1999–2013. The declining S_T was also simulated by chilling-based phenology models, albeit with a weaker decline (24–30%) than observed in situ. The reduction in S_T is likely to be partly attributable to reduced chilling. Nonetheless, other mechanisms may also have a role, such as ‘photoperiod limitation’ mechanisms that may become ultimately limiting when leaf unfolding dates occur too early in the season. Our results provide empirical evidence for a declining S_T, but also suggest that the predicted strong winter warming in the future may further reduce S_T and therefore result in a slowdown in the advance of tree spring phenology.

原文链接：http://www.nature.com/nature/journal/vaop/ncurrent/pdf/nature15402.pdf