Lichens more tolerant against winter warming stress than vascular and non-vascular plants: insights from an alpine field experiment

1. Extreme weather events influence carbon cycling and lead to pervasive changes in ecosystem structure and function. Vegetation at high latitudes and in alpine bioclimatic zones can be particularly sensitive to winter warming events, which are short-lived climatic events where temperatures are unusually high and often include rainfall. With climate change the frequency and severity of winter warming events are increasing.
2. We report here from a field experiment on a lichen-dominated ridge at a high mountain plateau in central Norway. This is a common vegetation type at high latitudes and altitudes, yet little is known about ecophysiological responses to winter warming events in lichens, and how it may differ from responses in bryophytes and vascular plants. We ran a week-long simulation of vegetation stress from winter warming events through thaw-freeze and ice encasement, during late winter in 2021 and 2022.
3. The thaw-freeze treatment had minor effects on summer ecophysiology in lichens (Cladonia mitis, Cetraria islandica and Nephromopsis nivalis), while the species N. nivalis and to a lesser extent, C. mitis had reduced vitality after the ice encasement treatment. Contrastingly, the bryophyte Polytrichum juniperinum, and vascular plant Empetrum nigrum had reduced photosynthetic efficiency and seasonal growth in both thaw-freeze and ice encasement treatments. The ice encasement treatment was overall more lethal and led to reduced NDVI (Normalised Difference Vegetation Index). However, reduction in vitality of vascular and non-vascular plants was not enough to impact overall ecosystem CO2-flux.
4. Synthesis: The lichen’s stronger tolerance against thaw-freeze and ice encasement than co-existing plants oppose the general effects of summer climate warming, where lichens may succumb under greater plant-growth and warmer soils. This mechanism is likely to be more important in ridge ecosystems than in snow-rich leesides. This study advocates for the importance of year-round ecology to understand vegetation change under climate change.

1. 极端天气事件会影响碳循环（carbon cycling），并引发生态系统结构与功能的广泛变化。高纬度与高山生物气候带（alpine bioclimatic zones）的植被对冬季增温事件（winter warming events）尤为敏感——这类短期气候事件的气温异常偏高，且常伴随降雨。随着气候变化，冬季增温事件的发生频率与强度正不断提升。
2. 本研究基于挪威中部高山高原上一处以地衣（lichen）为优势种的山脊开展的野外实验展开报道。这类植被在高纬度与高海拔区域广泛分布，但学界对地衣应对冬季增温事件的生理生态响应，以及其与苔藓植物（bryophytes）、维管植物（vascular plants）响应的差异尚不清楚。本研究于2021年与2022年冬末期间，通过融冻（thaw-freeze）与冰包裹（ice encasement）两种处理，模拟了为期一周的冬季增温事件引发的植被胁迫过程。
3. 融冻处理对地衣（Cladonia mitis、Cetraria islandica及Nephromopsis nivalis）的夏季生理生态特性影响微弱。其中，N. nivalis在冰包裹处理后活力显著下降，C. mitis的活力下降程度相对较轻。与之形成鲜明对比的是，苔藓植物Polytrichum juniperinum与维管植物Empetrum nigrum在融冻与冰包裹两种处理下，光合效率与季节生长量均出现下降。整体而言，冰包裹处理的致死性更强，会导致归一化差异植被指数（Normalised Difference Vegetation Index，NDVI）降低。不过，维管植物与非维管植物（non-vascular plants）的活力下降，并未对生态系统整体的CO₂通量（CO₂-flux）产生显著影响。
4. 综合分析：地衣对融冻与冰包裹胁迫的耐受性显著强于伴生植物，这与夏季气候变暖的普遍效应相悖——夏季气候变暖情境下，地衣可能因植物生长加速与土壤温度升高而逐渐衰退消亡。相较于积雪富集的背风坡，该机制在山脊生态系统中可能发挥更为关键的作用。本研究强调了全年生态学（year-round ecology）对于理解气候变化下植被动态变化的重要性。