Extreme drought or wet conditions have now been found to strongly influence the vegetative development of ecosystems. Semi-arid regions are most affected — raising concerns about their vulnerability to long-term drought in the future.
Extreme climatic conditions such as drought or heatwaves are likely to intensify in the next few decades1. Long-term observations2 of past decades suggest that characteristic recurrence frequencies, intensities and durations of certain extreme events have already increased noticeably. One pressing question is whether key ecosystem services, such as the capacity of ecosystems to accumulate carbon, are affected by extreme events3. The potential of ecosystems to accumulate carbon is intimately related to the phenology of vegetation4 — essentially, all the characteristic periodicities in an organism's life, such as the annual cycles of plant leaf development. Writing in the Journal of Geophysical Research, Ma et al.5 describe how climate extremes modify the seasonal vegetation development of different ecosystems.
Ma and colleagues' study relies mainly on a measure called the enhanced vegetation index. This is used to determine whether a region of Earth contains live green vegetation, and can be derived from spectral data that have been collected by satellites for more than a decade. Vegetation indices of this kind are often used as indicators of vegetation productivity, although they cannot really be directly interpreted as such. Nevertheless, such indices reveal the annual cycle of vegetation-canopy development within and across ecosystems.
Drought stress is also commonly described by an index that allows an intuitive interpretation of an ecosystem's water balance. In combination with temperature data, indices for vegetation and drought allow researchers to describe the complex interplay between vegetation and climate, as Ma and colleagues do in their work.
In contrast to previous studies, which mainly examined the phenology of individual ecosystems on subcontinental scales, Ma et al. revealed the impacts of climatic extremes on the phenology of different ecosystem types on a continental scale. They show that two extreme years, 2002 (extremely dry) and 2010 (extremely wet), had significant effects on vegetation phenology and productivity across southeastern Australia — a region characterized by large rainfall and temperature gradients, and which has vegetation types ranging from arid grasslands to forests. The authors find that 70% of the area had a prolonged growing season during 2010, whereas there was essentially no observable growing season in 2002. These observations indicate the fundamental relevance of fluctuations in water availability for phenology.
A crucial question emerges from this work: which vegetation types and ecosystems are most vulnerable to extreme conditions? By analysing the change in the vegetation index relative to changes in drought conditions for each location, Ma and co-workers suggest that cropland and pastures are most sensitive to fluctuations in climatic conditions. Natural systems, such as the hummock grasslands investigated in the study, adapt to high year-to-year variations in rainfall by increasing growth during wet periods to compensate for poor growth during dry periods (Fig. 1). But agricultural systems cannot do this — they respond only to dry periods by reducing growth, which supports the view that agricultural systems might be strongly affected by extreme climatic events.
The authors show that drought sensitivity peaks in semi-arid regions across different climate regimes. This is of interest, because semi-arid ecosystems cover approximately 40% of the global land surface6. Ecosystems of this kind are often dominated by grasses or shrubs that have developed mechanisms to survive long periods of dryness, and which grow rapidly when rainfall arrives. Ma et al. show that growth was almost completely dormant during 2002, and increased swiftly when the rains arrived during 2010. A remaining question is how long this behaviour can be maintained during severe droughts that last for more than one season. In other words, how resilient are ecosystems to increasing fluctuations in climate? With additional pressure from human land-use activities, the resilience of many of these ecosystems may be on a knife-edge7.
Ma and co-workers note that their findings have serious implications for estimates of the effects of extreme weather events on the global carbon cycle. But whether and how these results can be extrapolated to the global scale is unclear. Semi-arid regions have recently gained attention8,9 for their ability to take up large amounts of carbon dioxide from the atmosphere during favourable conditions, thereby driving the variability in the rate of increase in CO2 levels in the atmosphere. But it is not known whether, or for how long, semi-arid regions and other terrestrial carbon sinks will continue to absorb so much CO2, nor what the magnitude of this carbon sink is — particularly given the uncertainty in predicting changes in the frequency and intensity of extreme events3.
Overall, the authors' results demonstrate the need for a better understanding of plant responses to increased climate variability if regional and global models are to be improved. Obtaining such an understanding will require a multifaceted approach. For instance, differences in the size of responses might be partly attributable to varying rates of biodiversity10, a factor that cannot be considered on the basis of the relatively coarse spectral information contained in conventional vegetation indices. And although Ma and colleagues' study provides much-needed insight into ecosystem behaviour in response to extreme events, we can still only speculate about the exact nature of the underlying ecophysiological mechanisms and the potential legacy of such events.Footnote 1
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Rammig, A., Mahecha, M. Ecosystem responses to climate extremes. Nature 527, 315–316 (2015). https://doi.org/10.1038/527315a
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