Questions and Answers: Semi-arid ecosystems emerge as a driver in global carbon cycle dynamics

May 21, 2014

Question and Answers for:

Poulter, B, D Frank, P Ciais, R Myneni, N Andela, J Bi, G Broquet, JG Canadell, F Chevallier, YY Liu, SW Running, S Sitch and GR van der Werf. 2014. Contribution of semi-arid ecosystems to interannual variability of the global carbon cycle. Nature. DOI:10.1038/nature13376

1. What is a carbon “sink”?
A carbon sink refers to an ecosystem state whereby carbon uptake from photosynthesis exceeds carbon losses from ecosystem respiration, including both plant (autotrophic) and soil (heterotrophic) respiration. In contrast, a carbon source is an ecosystem state whereby carbon losses from total ecosystem respiration are greater than the carbon gained from photosynthesis. Losses of carbon from disturbance, such as fire and deforestation, can also be included when calculated ecosystem sink and source status. For more detail on the carbon flux definitions used in our manuscript, please see (Chapin et al., 2006).

2. What is a semi-arid system?
Semi-arid regions are part of the ‘dryland’ biome complex, which includes a range of arid to semi-arid ecosystems. Drylands cover 41-46% of the earth’s surface, and are found worldwide from mid-to-tropical-to-high latitudes. The United Nations Environment Program defines dryland systems as areas with an “aridity index” of less than 0.65, where the aridity index (AI) is a function of annual average precipitation (P) and potential evapotranspiration (PET), AI=P/PET (MEA, 2005)

3. What dryland areas are greening?
Since 1982, there has been on average, a global increase in the normalized difference vegetation index (NDVI) by 0.015 NDVI units (Fensholt et al., 2012). NDVI is an index of vegetation greenness and related to plant vigor, productivity, and a range of ecosystem processes. The increase in NDVI has been observed to be greater in more strongly precipitation limited semi-arid regions than in temperature-limited regions (Fensholt et al., 2012).

4. What is the relationship with desertification?
Desertification has been defined as “land degradation in arid, semiarid and dry subhumid areas resulting from various factors, including climatic variations and human activities” by the United Nations Convention to Combat Desertification. Using ground-based observations of desertification, the Millennium Ecosystem Assessment (2005) summarized the drivers and spatial extent of desertification (MEA, 2005). In comparison to the dryland greening trends identified in our study and others, desertification occurs at smaller spatial scales, with shorter temporal scales associated with drought (Herrmann et al., 2005), and is more likely to be found in sub-tropical latitudes.

5. Are there direct field-based measurements that support the satellite, atmospheric CO2, and modeling findings?
A recent eddy-covariance monitoring study of carbon fluxes at the Ti Tree site in Australia, an Acacia woodland, provides further support for a noticeably large anomaly of net carbon uptake (NEP) of 21.1 mol CO2 m-2 yr-1 in 2010-2011 compared to -0.3 mol CO2 m-2 yr-1 in 2012 (Cleverly et al., 2013). When the observed 2010-2011 average NEP is scaled to the area of Acacia woodlands for Australia, 20% of the continent’s land surface, we estimate a 0.4 Pg carbon sink for this biome alone (compared to our 0.84 PgC anomaly from the LPJ model for all of Australia).

6. What was the role of the Australian Millennium Drought (1997-2009)?
Australia experienced a widespread drought that lasted for almost one decade. This drought however, was superimposed on an increasing precipitation trend, where since 1981, precipitation has increased by 7%, or 34 mm, with particularly strong trends in Western Australia during this time period (Donohue et al., 2009).

7. What are the mechanisms for greening?
As addressed in our publication, global climate-greening studies have led to several hypotheses to explain why dryland systems are greening because precipitation or temperature is unable to fully address the observed trends. In addition to increased precipitation, these hypotheses include i) climate change, ii) vegetation increases in response to fire suppression, iii) a change to shrub vegetation following intensive grazing by livestock, iv) invasion by exotic grasses, and v) increased water-use efficiency. Increased water-use efficiency is related to physiological processes at the leaf level, whereby enhanced water-savings from increasing atmospheric CO2. For example, based on a leaf-level physiological model, an increase of 11% in CO2, from 340-378 ppm over the 1981-2006 period, results in the equivalent to an effective increase in precipitation of 15% in arid regions (Donohue et al., 2009).

8. Will greening semi-arid systems mitigate climate change?
Dryland systems store carbon in plant organs that have short lifespans, and consequently, the carbon residence time is brief (Haverd et al., 2013). This means that carbon in leaves and small diameter stems is either quickly respired following senescence-related processes or when wildfires directly consume and emit carbon back to the atmosphere. There are also feedbacks between productive years and fires, where, for example, the wildfires of 2012-2013 in Australia were representative of above average conditions and directly related to increased fuel amounts from the highly productive La Nina year of 2011 (Bushfire CRC, 2012).

9. How does El Nino affect Australia?
As equatorial Pacific Sea Surface Temperatures continue to increase, there is an increasing chance that 2014 will be an El Nino year. In general, El Nino brings decreased precipitation to Australia and other Southern Hemisphere semi-arid regions leading to negative NDVI anomalies and decreased productivity.

Bushfire CRC: Southern Australian Seasonal Bushfire Outlook 2012–13, 2012.
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Cleverly, J., Boulain, N., Villalobos-Vega, R., Grant, N., Faux, R., Wood, C., Cook, P. G., Yu, Q., Leigh, A., and Eamus, D.: Dynamics of component carbon fluxes in a semi-arid Acacia woodland, central Australia, Journal of Geophysical Research, 118, 1-18, 2013.
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