干旱对云南蓝果树天然更新的影响机制研究

Nyssa yunnanensis is a critically endangered species (IUCN) with natural regeneration difficulties, which has been listed in China's national grade I protection. Global change and habitat destruction induced drought was one of the vital factors hindering natural regeneration of forests. However, the mechanisms by which drought affect natural regeneration of N. yunnanensis were less well known. Here, we carry out paralell in-situ experiments and controlled experiments to study impacts of water demand constraints and aggravated autotoxicity induced by drought on two key steps in natural generation of N. yunnanensis, i.e., seed gemination and seedling growth. The project contains: (1)Effects of water erosion and the position of seeds in the litter on seed germination, seedling growth and survial rate; (2) Effects of PEG simulated drought stress on seed germination, seedling growth and survial rate; (3) Effects of different concentration gradient of different organ extracts on seed germination, seedling growth and survial rate;(4)Interation effects of water demand constraints and aggravated autotoxicity induced by drought on seed germination, seedling growth and survial rate. The results may illuminate impacts of drought on the natural generation of N. yunnanensis and their mechanisms affecting seed germination and seedling survival. Conclusions made from natural regeneration difficulties may partly help reveal endangered reasons of N. yunnanensis. These would provide scientific and theoretic basis for N. yunnanensis conservation program, through whose good demonstration the effective protection of plant species with extremely small populations is to be promoted nationwide.

云南蓝果树天然更新困难，物种濒临灭绝，是国家I级重点保护的野生植物和IUCN极度濒危种。全球气候变化和生境破坏导致的干旱，是影响植物天然更新的重要因子，但干旱如何阻碍云南蓝果树的天然更新尚不清楚。本项目采用野外原位生态学实验和室内受控实验相结合的方法，研究干旱导致的水分需求限制和自毒效应加剧对云南蓝果树种子萌发和幼苗生长2个关键环节的影响。具体内容包括：（1）水分冲刷和种子散落位置及其交互作用对种子萌发的影响；（2）PEG法模拟的干旱胁迫对种子萌发和幼苗生长及存活的影响；（3）不同浓度梯度的云南蓝果树不同器官浸提液对种子萌发和幼苗生长及存活的影响；（4）干旱导致的水分需求限制和自毒效应加剧对种子萌发和幼苗生长及存活的交互作用。研究结果能够阐明干旱对云南蓝果树天然更新的影响机制，初步揭示云南蓝果树濒危的原因和机理，为云南蓝果树的有效保存提供科学理论依据，促进全国极小种群野生植物的保护与恢复。

气候变化引起的干旱胁迫通过抑制树木的生长，威胁到树木，特别是濒危物种的天然更新。云南蓝果树现仅存2 个天然种群，野外个体数量为8 株，天然更新困难，属典型的极小种群物种。干旱是影响森林植物天然更新的重要因子，但干旱如何阻碍云南蓝果树天然更新的机制尚不清楚。本研究采用野外原位生态学实验和人工控制实验相结合的手段，探究了干旱导致的水分需求限制对极小种群野生植物云南蓝果树天然更新过程中种子萌发和幼苗生长2 个关键阶段的影响。原位实验结果表明，土壤水势低于-5.40 MPa时对幼苗的存活率和幼苗均有显著的抑制作用。幼苗在-5.60 MPa的土壤水势（一月）条件下就会死亡。室内受控实验证明，PEG 6000浓度、浸提液浓度和浸提液来源器官（根、茎和叶）对云南蓝果树种子萌发和幼苗生长均产生极显著的影响，且随着处理水平浓度的提高，云南蓝果树的种子萌发率及幼苗生长指标呈降低趋势，但是12.5 g/L和25 g/L浸提液浓度之间、10% 与15%PEG 6000浓度之间的差异不显著；建立的二元线性回归方程表明，浸提液浓度的标准化影响系数的绝对值都低于PEG 6000浓度的标准化影响系数的绝对值，说明干旱导致的水分缺失对云南蓝果树种子萌发和幼苗生长的影响要强于干旱导致的自毒效应加剧的影响。另外，云南蓝果树幼苗在20% PEG 6000浓度（土壤水势为-5.34 MPa）或1/5的持水（WHC；-5.64 MPa）条件下是无法存活的；野外种子的萌发率从九月（-1.10 MPa）到十一月（-4.30 MPa）呈降低趋势。我们的研究结果表明，-5.34 -5.64 MPa的土壤水势范围是云南蓝果树幼苗不能存活的水势。因此，综合原始生境的改变，气候变化引起的干旱胁迫通过水分缺失和自毒效应加剧两种作用方式，共同影响了云南蓝果树的种子萌发，从而阻碍其天然更新。研究结果阐明了干旱对云南蓝果树种子萌发的影响机制，初步揭示了云南蓝果树濒危的原因和机理，为云南蓝果树的有效保存提供了科学理论依据。