Data for: Nitrogen addition increases freeze resistance in black mangrove (Avicennia germinans) shrubs in a temperate-tropical ecotone

Low temperature stress is the primary factor determining the latitudinal limits of tropical plants. As the climate warms, tropical species are migrating poleward, displacing native species and modifying ecosystem structure and function. Changes are particularly evident along latitudinal gradients with the highest velocity of change occurring in wetlands. In coastal wetlands, saltmarshes dominate at latitudes above 30°, whereas mangroves occur mostly in the tropics because most species are intolerant of freezing temperatures, but others, like Avicennia germinans (black mangrove), do tolerate freezing temperatures. In response to a warmer climate and fewer killing freezes, mangroves are currently expanding into saltmarshes. However, the speed of the transition from saltmarsh to mangrove can also be modified by extreme events and nutrient subsidies. In a fertilization experiment along the Atlantic coast of North America, we found that nitrogen addition altered plant traits in Avicennia, which increased their resistance to freezing temperatures. This trait shift resulted in negligible freeze effects during a January 2018 extreme freeze event compared to unfertilized plants, which lost >80% of the leaves and >40% of the wood in their canopies. The freeze-killed litter from unfertilized plants provided a nutrient pulse that influenced recovery, growth and mangrove cover for three years following the freeze. Nutrient enrichment and recovery from the freeze effects led to increased growth and structural complexity of the mangrove canopy, which further enhanced freeze tolerance, shrub growth form, and the ability of Avicennia to displace the saltmarsh in the temperate-tropical ecotone.

低温胁迫是决定热带植物纬度分布范围的主要因素。随着气候变暖，热带物种正逐渐向两极迁移，取代本地物种，并改变生态系统的结构与功能。这种变化在纬度梯度上尤为显著，其中湿地地区的变化速度最快。在沿海湿地中，盐沼在北纬30°以上占据主导地位，而红树林则主要分布于热带地区，因为大多数物种对冰冻温度的耐受性较差，但如银叶树（Avicennia germinans，又称黑 mangrove）等少数物种则能够耐受冰冻温度。针对气候变暖和减少的致命霜冻，红树林目前正逐渐向盐沼扩展。然而，从盐沼向红树林的过渡速度也可能受到极端事件和营养补充的影响。在北美大西洋沿岸的一项施肥实验中，我们发现氮的添加改变了银叶树的植物性状，提高了其对冰冻温度的抵抗力。这种性状的转变使得在2018年1月的一次极端冰冻事件中，与未施肥的植物相比，银叶树几乎不受冻害影响，而未施肥的植物则失去了超过80%的叶片和超过40%的树冠木质部分。未施肥植物所释放的冻死落叶为营养脉冲，影响了随后三年的恢复、生长和红树林的覆盖面积。营养的富集和冻害后的恢复导致了红树林树冠生长和结构复杂性的增加，进而提高了其耐寒性、灌木生长形态，以及银叶树在温带-热带过渡生态带中取代盐沼的能力。