Author Maria Leunda offers an insight into the research behind her paper Ice cave reveals environmental forcing of long‐term Pyrenean tree line dynamics, which won our 2019 Harper Prize. Maria also further discusses how she ended up doing this exciting research and introduces her study site in the Central Pyrenees.
Alpine ecosystems are particularly sensitive to ongoing global change. Mountain forests are expected to expand upslope due to climate warming. Predicting and anticipating the effect that global change will have on alpine ecosystems requires a long-term perspective of past ecosystem dynamics. In this regard, Paleoecology has made important contributions to the reconstruction of past vegetation changes by studying biotic indicators (e.g. pollen and plant macrofossils) preserved in natural archives, like lakes and mires. However, other archives, such as ice cave deposits, can also contain important information relevant to alpine environment reconstruction.
It was during my Bachelor studies in Environmental Sciences that I first heard about the existence of ice caves; unexplored fossil ice masses lying underground. I was fascinated by them, but I never thought I would have the chance to work within them. 5 years later, in 2015, during my doctoral studies, I met Dr. Carlos Sancho and Dr. Miguel Bartolomé. They kindly asked whether I would be interested in visiting some ice caves to study their pollen and plant macrofossil content in order to assess past vegetation dynamics in the Pyrenees (Spain). At that time, I was conducting my PhD, studying long-term vegetation changes from high altitude Pyrenean lakes, under the supervision of Dr. Penélope González-Sampériz and Dr. Graciela Gil-Romera. They encouraged me to go for it and integrate this new archive into the PhD. So, I did not think twice. I was awarded a mobility grant for 3 months and went to the University of Bern, where I worked with Dr. Christoph Schwörer, Dr. César Morales-Molino and Prof. Willy Tinner in the ice cave project.
The Armeña A294 Ice Cave, located at 2238m a.s.l. within the Armeña cirque in the Central Pyrenees, houses fossil ice dating back between 5700 and 2200 years. It is the oldest known firn ice cave worldwide. Although it is not always easy to find pollen and plant remains in high concentrations within ice deposits, this extraordinary ice archive contains a wide variety of organic material. The identification of these plant remains has allowed us to study the long-term vegetation and tree line dynamics of the Armeña cirque alpine ecosystem.
The paleoecological record shows that altitudinal treeline shifts were closely related to summer temperature changes. 5700 years ago, when summer temperatures were high, mountain pines (Pinus uncinata) were coexisting with birches (Betula spp.) in the surroundings of the ice cave. However, ca. 4600 years ago a progressive descent of the treeline is detected, together with the establishment of alpine meadows, dominated by Dryas octopetala. This is a response to the cooling trend, known as neoglaciation. The treeline shifted upwards ca. 3200 years ago and persisted near the cave until the end of the record 2200 years ago, when the ice sequence ends.
The current landscape is very different to that of 2200 years ago. Only alpine meadows are currently present suggesting that, during the last two millennia, large disturbances, either climatic or human, might have led to a depression of both treeline and timberline, resulting in their current positions in the Armeña cirque at ca. 2000 and 1800 m a.s.l. respectively.
These results uphold the centennial to millennial scale perspective as essential for disclosing long-term processes that might be important for sustainable environmental management, e.g. in forestry and species conservation. Ice archives are however severely endangered by rising temperatures and more scientific attention is needed in order to rescue their untapped scientific information otherwise essential to assess future ecosystem dynamics under global change conditions.
Maria Leunda Institute of Plant Sciences & Oeschger Centre for Climate Change Research and Swiss Federal Research Institute for Forest, Snow and Landscape Research WSL, Switzerland.
You can read all of the Harper Prize 2019 shortlisted papers in our Virtual Issue.
You can also read the Editor’s Choice post on Maria’s paper and the Pyrenean Institute of Ecology (IPE-CSIC) press release for this paper from October 2018.