增温背景下低温对温带次生林主要建群种更新初期的影响机制

The frequency of extreme climate and weather events, particularly the extreme low temperature, have been increasing sharply with the intensifying of climate fluctuation under the background of climate warming, which have become one of the obstacles that impact the early regeneration of main tree species in forest ecosystem. In order to study the effects of low temperature climate warming condition on early regeneration, we select three dominant tree species of Fraxinus mandshurica, Quecus mongolica, Juglans mandshurica from the secondary forest ecosystem in Northeast China, to test the responses of seed germination, seedling survival and growth of these tree species to the chilling damage, which will occur under the artificial warming condition in the field. This study will be mainly focused on the impact of low temperature events on the early regeneration of the three main tree species in the field warming stands of secondary forests. The parameters of seed germination, seedling survival and growth, and phenophase will be changed in the field warming stands of secondary forests. The responses of changed parameters to the low temperature events under field warming stands will be investigated to reveal the impact mechanism of low temperature on early regeneration of main tree species in secondary forest ecosystem under field warming conditions. Combined with simulated low temperature experiments and historical meteorological data, it is aimed to present the risk probability of low temperature events occurring in early regeneration of secondary forest under the background of climate warming. The results will provide scientific bases for comprehensive understanding the response of early regeneration of temperate secondary forest to climate warming, and for coping with climate change in the future and solutions for sustainable secondary forest management.

全球变暖背景下，气候波动加剧引发极端天气气候事件频率大幅增加，已成为森林主要树种早期更新的障碍因素之一。为探明增温背景下低温对早期更新的影响机制，本项目以东北次生林生态系统为对象，以大型野外森林增温平台为依托，以增温后低温发生特征与规律及其对主要树种（水曲柳、蒙古栎和核桃楸）早期更新影响为主线，探讨野外增温后种子萌发、幼苗存活、生长及物候变化状态及其对增暖的响应规律；重点研究增温后，低温发生（霜冻害）对由于增温而提早生长和延迟休眠的幼苗存活、生长的影响规律；揭示增温背景下低温对主要树种早期更新的影响机制；结合低温模拟实验和长期历史气象数据，明确气候变暖条件下次生林主要树种早期更新可能遇到低温灾害的风险概率。结果将为全面认识温带次生林早期更新对气候变暖的响应提供科学依据，为未来应对气候变化及早制订次生林经营管理对策提供科学参考依据。

气候变化不仅改变了植物物候，同时也加剧了气候波动，使初终霜日变得非常不稳定，可能会增加低温霜冻害发生的风险，或已成为森林早期更新的障碍因子。那么，气候变暖后林木幼苗的存活生长和物候发生了怎样改变？其内在机理又如何？这些变化是否有可能增加低温霜冻害的风险进而影响早期更新？尚不明确。为此，本项目采用野外增温和室内低温控制的方法，开展增温背景下低温对主要建群种更新初期的影响研究。主要结果如下：.1）明确了增温对林木幼苗存活生长的影响规律。生长前期，增温促进存活生长，后期则产生抑制；增温2℃使林木幼苗光合速率增加，促进生长，增温4℃则使光合系统Ⅱ受损，光合速率降低，抑制生长。.2）揭示了增温后林木幼苗各器官碳的分配策略。增温2℃促进NSC的积累，且生长前期高于后期，而增温4℃对NSC的积累产生抑制。高温逆境时，水曲柳将NSC转移到根部，通过NSC的积累提高存活。增温后胡桃楸幼苗前期生长优于储存，使NSC被利用和传输的少，高温逆境时，叶片向根系运输NSC少，导致根部因碳饥渴而使存活降低。.3）阐明了增温对林木幼苗物候影响的变化特征。增温使各树种的春季物候期均明显提前，以胡桃楸提早最明显。秋季各物候期出现时间在各树种间差异显著，但蒙古栎各物候均延迟。说明气候变暖后，低温霜冻害的风险易发生在春季。.4）明确了林木幼苗对低温胁迫的响应机制。林木幼苗初始生长温度越高，越易遭受低温的危害，说明气候变暖会增加幼苗遭受低温胁迫的风险。各树种耐寒性大小分别为蒙古栎>水曲柳>胡桃楸。.5）建立了低温灾害风险可能发生的概率预测模型。研究区可能发生的概率及分布区以轻度灾害为最大，中度次之，重度最小。低于半致死温度的发生概率，除胡桃楸在高海拔区相对较高（2%-10%）外，其余2个树种可能发生的概率均非常小（<1%）。. 研究结果对于评价和预测温带森林主要建群种对气候变暖的响应、种群变化动态及森林经营管理应对措施的制定都具有重要的科学意义和实践价值。