Drylands are defined as regions that have an index of aridity (ratio of mean annual precipitation to mean annual potential evapotranspiration) of 0.05 to 0.65 (Middleton & Thomas 1997). These ecosystems are a key terrestrial biome, covering 41% of Earth’s land surface and supporting over 38% of the total global population of 6.5 billion (Reynolds et al. 2007), and are highly vulnerable to climate change and desertification (Körner 2000; Reynolds et al. 2007), two of the most important and pressing environmental and socio-economical issues currently faced by mankind. The sensitivity of drylands to these problems derives from the fact that their primary productivity is strongly limited by precipitation and soil nutrient availability, and both of these factors will change as a consequence of ongoing global environmental change. Climate models predict increased frequency and duration of summer droughts in many dryland areas worldwide (Solomon et al. 2007). However, and despite the global extent of drylands, the consequences of predicted changes in climate on the plant communities inhabiting these ecosystems are poorly known.
Overcoming limitations: testing the effects of drought on slow-growing species
Experiments manipulating rainfall provide a powerful tool to understand and predict how plant communities will change under future climatic conditions. However, most climate change experiments carried out to date have been conducted over short periods (usually 1-4 years), a time scale that is not long enough to appropriately test vegetation changes in drylands, which contain many slow-growing species that are also resistant to drought conditions (Whitford 2002). The study by Evans et al. (2011: Deﬁning the limit to resistance in a drought-tolerant grassland: long-term severe drought signiﬁcantly reduces the dominant species and increases ruderals) overcomes this limitation, as the authors present data from an 11-year drought experiment carried out in a shortgrass steppe in Colorado. This unique experiment constitutes one of the longest-running precipitation manipulation experiments published to date.
The authors aimed to quantify the response of shortgrass steppe vegetation to two levels of experimental drought severity. They evaluated changes in total vegetation cover, dominant plant (Bouteloua gracilis) cover and ruderal cover in plots receiving 25% and 50% of ambient precipitation during the growing season. To manipulate rainfall conditions, the authors used rainout shelters that automatically covered drought plots during rain events, and re-added a proportion of the ambient water excluded from the plots to achieve the two levels of drought used in the experiment.
Evans et al. (2011) found that 11 years of drought treatment resulted in large reductions in plant cover, but signiﬁcant differences did not emerge until late in the experiment (the 4th and 7th year of drought for the dominant and ruderal species, respectively). The cover of B. gracilis was consistently lower in the drought treatments than in the control treatment throughout the experiment. However, a statistically signiﬁcant decrease in B. gracilis under a 25% drought reduction did not emerge until the 8th year of the experiment. At the same time, ruderal cover increased in both drought treatments over time. Important changes in species density also occurred, as drought reduced the average number of species per plot during the ﬁrst years of the experiment. The number of species compared to the control began to increase in the 50% drought treatment in the 5th year, whereas this did not occur in the 25% drought treatment until the 9th year. Interestingly, reductions in cover of B. gracilis corresponded strongly with increases in the cover and richness of ruderal species, suggesting that the dominant species plays a strong role in maintaining the structure of the whole community.
The results obtained by Evans et al. (2011) provide strong evidence that semi-arid plant communities, even if they are resistant to water limitation, can be signiﬁcantly disturbed by long-term droughts. Since such droughts are likely to be more common in many dryland areas in the future, the findings reported provide important insights on how semi-arid communities will respond to climate change, information interesting for managers and scientists alike. This study also supports the idea that the response of dominant species can be an important, or perhaps the only needed, indicator for response of the entire community. This important result has also implications for the establishment of monitoring programs aiming to detect early warning signals of irreversible ecosystem change associated to global environmental change and desertification (Kéfi et al. 2007; Maestre & Escudero 2009). The fact that many of the drought effects were not found for several years also highlights the importance of long-term ecological research to understand the behaviour and dynamics of semi-arid ecosystems, and caution about the interpretation of short-term climate change studies.
Fernando T. Maestre
Associate Editor, Journal of Ecology
- Evans , S.E., Byrne, K.M., Lauenroth, W.K. & Roth, I.C. (2011) Defining the limit to resistance in a drought-tolerant grassland: long-term severe drought significantly reduces the dominant species and increases ruderals. Journal of Ecology, 99, 1500-1507
- Kéfi, S., Rietkerk, M., Alados, C. L., Pueyo, Y., ElAich, A., Papanastasis, V. & P. C. de Ruiter (2007) Spatial vegetation patterns and imminent desertification in Mediterranean arid ecosystems. Nature, 449, 213-217.
- Körner, C. (2000) Biosphere response to CO2 enrichment. Ecological Applications, 10, 1590-1619.
- Maestre, F. T. & Escudero, A. (2009) Is the patch-size distribution of vegetation a suitable indicator of desertification processes? Ecology, 90, 1729-1735.
- Middleton, N. J. & Thomas, D. S. G., eds. (1997) World Atlas of Desertification. Edward Arnold, London.
- Reynolds, J.F., Stafford Smith, D.M., Lambin, E.F., Turner II, B.L., Mortimore, M., Batterbury, S.P.J., Downing, T.E., Dowlatabadi, H., Fernández, R.J., Herrick, J.E., Huber-Sannvald, E., Leemans, R., Lynam, T., Maestre, F.T., Ayarza, M. & Walker, B. (2007) Global desertification: Building a science for dryland development. Science, 316, 847-851.
- Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K. B., Tignor, M. & Miller, H. L., eds. (2007) Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge.
- Whitford, W. G. (2002) Ecology of Desert Systems. Academic Press, London.
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