Researchers at the field experiment of the Semi-arid Grasslands Research Center in northern Colorado. Photo: Colorado State University College of Natural Sciences.
Under moderate drought, grasslands stabilise after an initial shock, but under prolonged rainfall deficit their productivity declines structurally.
Ecosystems prove remarkably resilient to a few years of drought – until that drought becomes extreme. At that point, grasslands and shrublands worldwide enter a cumulative downward spiral, in which above-ground primary production can decline by up to 77 per cent after four consecutive dry years. This is the finding of the International Drought Experiment, the results of which were published on 17 October in Science.
The experiment is ambitious in scale: 74 grasslands and shrublands across six continents were artificially deprived of rain for three to four years using so-called rainout shelters – passive structures that divert natural precipitation whilst leaving the microclimate largely intact. Each site received a drought treatment that varied in severity: from moderate, in which rainfall remained slightly below the long-term average, to extreme, comparable to a drought that occurs once every hundred years.
Productivity
Under mild or moderate drought, the ecosystems remained remarkably stable. In the first year of drought, productivity declined by an average of around 29 per cent across all sites. After that, the decline levelled off and productivity remained more or less constant under mild or moderate drought – a sign that many species adapt to the reduced water supply. But once the drought reached extreme proportions, that equilibrium collapsed: the decline grew stronger each year, reaching 2.5 times its initial magnitude after four years. The researchers attribute this to the likely die-off of drought-sensitive species, failed establishment of new plants, and shifts in community structure. ‘The resistance of grasslands and shrublands to prolonged drought erodes rapidly once that drought becomes extreme,’ they write. ‘This threatens long-term stability and the ecosystem services these areas provide.’
Essential
Grasslands and shrublands cover approximately 40 per cent of the Earth’s surface and are essential to the global carbon cycle. The researchers warn that the increase in extreme droughts driven by climate change could put that stability under further pressure: ‘When droughts become longer and more intense, ecosystem functioning shifts from limited losses to increasing and profound damage.’