Terrestrial vegetation responds instantly to external forcing

Anthropogenically driven climate change is among the biggest threats to global ecosystem functioning. Increasing our understanding of the timing and rate of natural ecosystem response to external forcing is therefore of key importance to support mitigation and adaptation policies.
Published in Earth & Environment
Terrestrial vegetation responds instantly to external forcing
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Chemical and biological signals within natural archives such as lake sediment sequences allow the reconstruction of environmental dynamics on a range of time-scales, from the present to millions of years ago. Reconstructions from lake sediment sequences often reveal information on both climate change (i.e. driver) and ecosystem change (i.e. response) making it possible to study in detail the response of natural ecosystems to external forcing.

Many available records from northwest Europe detail the Last Glacial-Interglacial Transition (ca. 15-11 kyr ago), a time interval characterised by abrupt and dramatic climate change. One of the most well-studied Quaternary intervals is the Younger Dryas, a 1000-yr long cold period (stadial) that led to abrupt high-amplitude cooling in the North Atlantic region and beyond ca. 12,850 years ago. The onset of the Younger Dryas, represented in many high-resolution archives, provides an ideal test-case for theories on ecosystem response to external forcing. Devising accurate and precise chronologies is hard for any sediment record, where age/depths models are often constructed based on the use of radiocarbon dates, which have uncertainty estimates of up to several hundred years. Compared to the duration of some of the observed climate events – e.g. the ice cores from Greenland suggest that cooling at the onset of the Younger Dryas occurred within a decade – this chronological uncertainty makes it difficult to draw any reliable conclusions about potential spatiotemporal off-sets in both climate dynamics as well as in ecosystem response driven by these changes. Specifically, conflicting theories exist as to when and how fast ecosystems responded to climate change at the onset of the Younger Dryas with some studies suggesting that vegetation and climate change occurred synchronously across large parts of the globe, within centennial-scale dating uncertainties. By contrast, other studies suggest centennial-scale off-sets between vegetation change in Europe and variations in the Greenland oxygen isotope records or show evidence for a time-transgressive palynological shift on a N-S gradient across northwest Europe at the onset of the Younger Dryas cold period.

 

The INTegrating Ice core, MArine and TErrestrial records (INTIMATE) community has focussed on improving the precision of chronologies of independent climate records derived from different natural archives in an attempt to learn more about spatial synchroneity of past abrupt climate change. In 2013 an INTIMATE Example summer school was organised near lake Hämelsee (Germany), aiming to train postgraduate students and ECRs in a range of field, laboratory and numerical techniques used in Palaeoclimatology. Using a so-called piston corer deployed from a floating raft students got to practice recovering 3-m long sediment records from the bottom of the lake. From previous research it was already known that the lake sediments contain certain intervals with annual layers called varves, which are typically formed during periods of summer anoxia. The lake sediments also contain several ash layers that were the result of large volcanic eruptions. Once the sediment cores were brought back from the field, they were opened, their physical properties were described, and they were subjected to initial analyses e.g. under the microscope.

 

The video above shows a mini-documentary filmed during the summer school, including footage of the field- and laboratory work

Following the summer school a number of the participants decided to focus part of their (PhD) research on the materials obtained in the field, and over the years an increasing number of people and labs from across Europe contributed to the research project. For instance, Dr. Gwydion Jones meticulously analysed the cores for their volcanic ash content, and his results formed a chapter in his PhD thesis as well as the basis for a first-author publication. Similarly, Dr. Aritina Haliuc obtained funding to spend part of her PhD time at the GFZ institute in Potsdam, where she analysed the varved interval that occurred ca 13,000 years ago. The data presented in the current paper are the culmination of (mainly early-career) scientists from across Europe who volunteered time, energy or materials to the study of abrupt climate change following the INTIMATE Example summer school.

 

Varved (i.e. annually laminated) sediments
in the lake Hämelsee record

The observation of an extremely well-dated ash layer (the Laacher See tephra, dated to 13,006 ±9yr) within the annually-layered sediment interval of the lake Hämelsee sediment record allowed us to carry out a palaeoecological reconstruction of local and regional ecosystem change across the onset of the Younger Dryas with unprecedented chronological precision. Not only did our results show that climate cooling around 12,820 years ago directly impacted on the lake environment (e.g. through weaker summer stratification), we also show that the regional vegetation as well as the freshwater insect community showed instant changes synchronous to the onset of climate change. Furthermore, we updated the age/depth models for key palaeoecological sites on a north-south transect across Europe (i.e. those with high resolution and a precise chronology), and applied a novel way to identify the first moment that changes in the regional vegetation occurred. In doing so, we observed that vegetation shifts occurred at the same time in a region that spanned from central Germany in the south to Norway in the north. We established that terrestrial ecosystem response to the onset of the Younger Dryas was instantaneous and spatially synchronous, indicating that the environmental impact of climate cooling was more severe than previously thought and illustrating the sensitivity of natural terrestrial ecosystems to external forcing.

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