Towards a consistent approach for assessing drought risk to terrestrial carbon stores

Background/Question/Methods

Changes in atmospheric concentration of greenhouse gases are driving changes in the distribution of rainfall and temperature regimes.  Warming temperatures and declining rainfall, especially in mid to high latitudes are believed to be important drivers of recent mortality events around the globe.  The impacts of these events on terrestrial carbon stores and other ecosystem services is difficult to predict despite the current surge in research into drought related mortality globally. This difficulty arises for two reasons; (1) it remains difficult to define the climatic conditions/thresholds associated with drought related mortality in a consistent manner and (2) because the physiological mechanisms associated with mortality are poorly resolved.  Together these uncertainties make it difficult to define ecosystems and species vulnerable to future drought events.  Australia has a long history of extremes in climate (‘of droughts and flooding rains’) and provides an excellent case study for responses of terrestrial ecosystem to extreme events.   In this study we are developing approaches for defining drought attributes in a consistent manner across ecosystems and use these to define future trends in these attributes and assess future climatic risks to carbon stocks.  We also develop approaches for linking these drought attributes to species drought tolerance traits, providing a means to predict differential species vulnerability within and across ecosystems.

Results/Conclusions

We used a joint probability distribution relating climatic dryness and temperature to   examine historical patterns in drought related mortality using examples from mortality events around Australia.  We found that historically, mortality events exceed a common threshold across sites and environments.  This finding allowed us to explore trends in event statistics such as the duration, intensity and frequency of droughts capable of causing mortality under a range of future climate scenarios.  Additionally we observed that these climatic thresholds are related to functional traits related to species drought tolerance.  These findings provide a basis to start exploring the risks to ecosystems services such as carbon sequestration and biodiversity posed by future climatic scenarios.  Importantly they may form the foundation for loss functions that would help to quantify risks associated with a range of climate scenarios.

While it is widely recognised that future climates pose significant risks to ecosystem function, approaches for assessing these risks remain limited. Understanding the spatial and temporal distribution of these risks would significantly enhance the deployment of resources aimed at managing these risks to key ecosystem services.