Climate change: Old hay reveals how grazing reacts

Creative methods provide new insights into grassland's response to global warming

Old Hay Robe of Park Grass Experiment of the Rothamsted Research Station / Great Britain © Iris Köhler / TUM
Read out

How do plant ecosystems react in the long term to the rising CO2 concentration in the atmosphere? This fundamental question is becoming more and more urgent against the background of climate change. Researchers have now examined them for grassland for the first time worldwide. The scientists found the answer in an unfamiliar place: in ibex horns from Switzerland and in 150-year-old hay from England.

Researchers who want to study the response of trees to the rising CO2 concentration in the air have an easy job: they only need to remove one core from the trunk, because trees store the carbon they absorb in the wood. A hundred-year-old oak tree is thus reflected in its annual rings, how it has dealt with the incipient climate change over a century.

Targeted grassland vegetation

"The grassland vegetation we work on, on the other hand, is quickly consumed or dies within a few months and decomposes, " explains Professor Hans Schnyder, who is researching grassland at the Weihenstephan Science Center of the Technical University of Munich (TUM). Nonetheless, the Swiss scientist wanted to find out how sparingly grassland manages to maintain water, when it gets warmer and the carbon dioxide concentration in the air rises.

You have to know that every plant absorbs CO2 from the air. At the same time, it evaporates water to cool the sunlit leaves. Both happen through the stomata, tiny pores in the leaves whose opening width can regulate the plant. With increasing dryness, it closes its stomata to reduce the loss of water, but also absorbs less CO2. From laboratory experiments it is known that with an artificially increased external concentration of CO2, the absorption capacity for the gas increases with the same opening width of the stomata in the short term.

Old Sample Archive of the Park Grass Experiment of the Rothamsted Research Station / Great Britain Iris K hler / TUM

Water use efficiency studied

However, in order to determine how the water use efficiency of grassland vegetation - that is, the quantity ratio of absorbed CO2 to released water - has really developed over the last century, Schnyder had to find similar time series for grassland as it did for wood, display

Here, the team first came to the aid of the Steinbockh rner trophy collection of the Natural History Museum Bern: The ibex stores in its horns isotopic information about the water use of the vegetation that it has eaten. As a result, the TUM researchers used a carving knife in the museum collection, which spanned the years 1938 to 2006 - and removed a small piece of each horn.

Since ibex horns also possess annual rings, the greenland researchers were able to draw conclusions from these horn samples to the grassland vegetation of the Bernese Alps, where the animals had grazed.

Park Grass Experiment

A unique sample archive at the research station Rothamsted in England finally made the comparison with a second grass landscape possible. In Rothamsted, the oldest cological grassland experiment was started more than 150 years ago, the GrassPark Grass Experiment : since 1857 archived there sample material to later generations of researchers with new measurement methods long-term insight into the Ecosystem on site.

Water-saving potential increased

In fact, the TUM researchers were now able to extract from the up to 150-year-old hay samples - also via an analysis of the respective isotope signature - how the English grassland vegetation there has used the water over the years.

In this way, the Weihenstephan researchers determined the individual isotope signature of the grassland vegetation in the Bernese Alps and the British lowlands over a long period of time: on the basis of the horns over 69 years, on the basis of the hay samples even over 150 years. In a second step, these data were compared with the climate data of the investigated regions, such as air temperature and drought. The result: At both sites, the intrinsic water use efficiency of grassland vegetation has increased over the years.

According to the researchers, this means that the plants have increased their water-saving potential, while it has become warmer and more and more CO2 has been released into the air. For the first time, TUM researchers worldwide have demonstrated the long-term effects of anthropogenic climate change on grassland water use efficiency.

Differences between the locations

However, there were differences between the locations: In Switzerland, the real water use efficiency of the Alpine meadow remained the same, despite the increased intrinsic water use efficiency of the grasslands, because the air has become drier and warmer overall due to climate change.

In England, however, the researchers found this finding only for the autumn confirmed. In the spring, on the other hand, which does not dry out in Rothamsted despite climate change more than 150 years ago, the water-saving potential of the grassland vegetation also prevails in reality.

These results will, according to the scientists, help to further improve climate simulations: Complex calculation patterns that include vegetation have so far only been able to work with the grasslands with estimates. This black box of climate research, the researchers of the TU Munich have now revealed.

(idw - Technische Universität München, 11.12.2009 - DLO)