Climate News Network

Too much nitrogen reduces Alpine plant diversity

April 15, 2015, by Tim Radford

As Alpine meadows lose plant diversity, entire ecosystems are damaged
Image: Ximonic (Simo Räsänen) via Wikimedia Commons


Climate change caused by one of the less abundant greenhouse gases is playing havoc with plant life in Switzerland, posing problems for other forms of life and increasing the risk of erosion. LONDON, 15 April, 2015 − Carbon dioxide isn’t the only greenhouse gas to change the world. Swiss scientists have just confirmed that nitrous oxide and other forms of atmospheric nitrogen deposition – from agriculture, from factory chimneys and motor exhausts and so on – is altering the grasses and wildflowers of the Alps and the valleys. Plants cannot live without nitrogen: for most of evolutionary history, it has been available only in limited amounts. With the Industrial Revolution, that began to change. Tobias Roth of the University of Basel and colleagues report in the Royal Society journal Open Science  that the historic rise in the availability of nitrogen has reduced plant diversity everywhere in Switzerland. The finding is not – of itself – new. Researchers tested the hypothesis that increasing levels of nitrogen presented a threat to “hotspots” of global biodiversity almost a decade ago. But in science, one general study is never enough.

More nitrogen

So Dr Roth and his colleagues did something much more detailed and comprehensive. They used six different measures of plant diversity to test what was happening on 381 study plots at a variety of altitudes and in different kinds of ecosystems across just one country. However the scientists measured plant diversity, it had been reduced. That human-triggered changes to the atmosphere have affected Switzerland is not in much doubt: one research team recently established that alpine glaciers were in retreat in response to atmospheric pollution, and Dr Roth – now with the Swiss company Hintermann and Weber AG – last year demonstrated that birds, flowers and butterflies in the country were all heading uphill in response to global warming. The latest research began with a baseline from earlier centuries: data taken from herbarium samples confirmed that available nitrogen had once been much more limited. The scientists then randomly selected 428 study plots – each a kilometre square – as their study samples. Some had to be rejected, because they were entirely water, or in mountainous regions too rugged and dangerous for field research. But that left 381 sample plots, in the Alps and the Jura mountains, between the altitudes of 260 and 3,200 meters (850 and 10,500 feet).

Bad news

The researchers used qualified botanists who had received special training to conduct the surveys, and asked them to conduct, as closely as they could, a diagonal transect examination across each plot, if possible once in spring and again in summer, and to record all vascular plants. Altogether, the surveys delivered 93,621 observations of 1,768 plant species. The scientists found that biodiversity had declined by 19% according to one measure and by 11% in another test. In general, the higher the nitrogen available, the lower the diversity. This is bad news for ecosystems as a whole: diversity means stability. Extra sources of nitrogen fertility benefit some highly competitive groups of plants, at the expense of others. “High plant diversity is important to us humans for many reasons,” said Valentin Amrhein, another of the authors. “For example, in the mountains a large number of plant species with different root depths will stabilise the soil more effectively and prevent erosion.” − Climate News Network

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