Drought vs Deluge – How Will Grasslands Cope with Climate Change?

Climate change due to human activities is predicted to change many aspects of the environment, from atmospheric carbon dioxide to temperature and rainfall1. Modellers are confident in the projected temperature increases, but the predictions about rainfall are much less certain. Changes in rainfall patterns will impact on many aspects of ecosystems, including how nutrients move.

Associate Professor Sally Power studies how these nutrient cycles are being affected by human-induced changes in the environment. She took up a position two years ago at the Hawkesbury Institute of the Environment, University of Western Sydney after completing her studies and working at the Imperial College in London. She previously completed a post-doctoral position at La Trobe University, Melbourne and loved Australia, so now she’s here permanently. Associate Professor Power is passionate about the understanding the interactive impact of multiple climate drivers on ecosystems.

At a recent seminar at Macquarie University Associate Professor Power spoke about three projects she is involved with at the moment:

  1. Drought and diversity in the UK (DIRECT)
  2. Rainfall extremes (DRI-grass)
  3. Elevated CO2 impacts on forest nutrient cycling (EucFACE)

The DIRECT project (Diversity, Rainfall and Elemental Cycling in a Terrestrial Ecosystem) aims to answer questions about how grassland ecosystems will respond to predicted rainfall changes and whether biodiversity will buffer these effects of a rainfall pattern change2. To test these ideas the research team constructed an array of grassland plots with a range of plants functional groups – perennials, caespitose grasses and annual plants (Figure 1)3.

Figure 1. Plant traits selected for the DIRECT experiment Image: Grantham Institute, Imperial College London (4).

Figure 1. Plant traits selection for the DIRECT experiment
Image: Grantham Institute, Imperial College London (4).

Rainfall predicted for the year 2100 (down 30% in summer, up 15% in winter) was applied to these plots to see how different vegetation communities might respond to rainfall changes2. Key ecosystem processes (such as respiration rate and nutrient cycling) were faster when there were a range of perennial plants present. Process rates in vegetation plots dominated by annual plants or caespitose grasses were not strongly affected by changes in rainfall2. This research showed that plant functional groups are important for maintaining grassland ecosystem function and they need to be considered in future management plans2.

In addition, the researchers used different plots in the same area and changed the rainfall pattern to see if drought and deluge impact differently on the grassland ecosystem. The rainfall treatments used were5:

  • Current levels;
  • Prolonged drought – 30% drop in rainfall; and
  • Reduced frequency – same amount of rain, concentrated into heavier falls less frequently.

The key findings were that changing the frequency of rainfall affected the number of species, especially the perennial species5. Surprisingly the number of species was not affected by the change in the total amount of rain (prolonged drought). The reduced rainfall frequency also lead to an increase in respiration and the grassland ecosystem switched from being a net carbon sink to net carbon source (from overall absorbing carbon to overall emitting carbon; Figure 2)5. The results of this experiment suggest that grassland ecosystems are relatively resistant to predicted rainfall changes5.

Figure 2. Change from carbon sink to carbon source for each of the  rainfall treatments (A = ambient; PD = prolonged drought; RF = reduced frequency). (Adapted from image presented by Associate Professor Sally Power)

Figure 2. Change from carbon sink to carbon source for each rainfall treatment (A = ambient; PD = prolonged drought; RF = reduced frequency; adapted from image presented by Associate Professor Power)

Associate Professor Power is also in the preliminary stages of some large scale experiments in western Sydney. The first of these experiments is DRI-grass (Drought & Root Herbivore Interactions in a Grassland Ecosystem). This study asks whether Australian grassland ecosystems have stronger responses to the amount or frequency of rain and whether these responses are affected by root herbivores6. Associate Professor Power emphasised that root herbivores are very abundant and their weight can exceed the weight of the sheep in a hectare7. Root herbivores can respond directly and indirectly to changes in rainfall patterns and can make it harder for plants to cope with climate change impacts8.

The research team has set up five different rainfall treatments: +50% rain; -50% rain; 3 week rainfall cycle with the same total amount of rain; summer drought; and the ambient conditions (Figure 3). The rainfall treatments only began in June 2013 and the root herbivores are not yet in place. So far the researchers have observed there are lower species abundances under drought conditions and an increase in summer rain has led to the dominance of African lovegrass.

Figure 3. Rainfall shelters for the DRI-grass experiment in the foothills  of the Blue Mountains (Image: The Hermon Slade Foundation; 6)

Figure 3. Rainfall shelters for the DRI-grass experiment in the foothills
of the Blue Mountains (Image: The Hermon Slade Foundation; 6)

The second project in western Sydney is being conducted in the EucFACE facility (Eucalyptus Free Air CO2 Enrichment)9 located in an intact Cumberland Plain Woodland ecosystem. Associate Professor Power and her team are looking at how elevated CO2 increases rates of nutrient cycling in the ecosystem. So far they have noticed there is an increase in available phosphorus, but no change in the amount of available nitrogen in elevated CO2 conditions.

Once the data is collected from these long term experiments, Associate Professor Power aims to understand some of the impacts of climate change on grassland ecosystems and make recommendations about how these systems should be managed to mitigate these impacts.

 

Learn more:

  1. IPCC (2013). Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Working Group I Contribution to the IPCC Fifth Assessment Report. Cambridge University Press, Cambridge.
  2. Fry EL, Manning P, Allen DGP, Hurst A, Everwand G, Rimmler M & Power SA (2013). Plant Functional Group Composition Modifies the Effects of Precipitation Change on Grassland Ecosystem Function. PLoS ONE, 8(2): e57027. doi: 10.1371/journal.pone.0057027.
  3. Fry EL, Power SA & Manning P (2014b). Trait-based classification and manipulation of plant functional groups for biodiversity-ecosystem function experiments. Journal of Vegetation Science, 25, 248–261. doi: 10.1111/jvs.12068.
  4. Fry E, Hurst A, Everwand G, Rimmler M, Manning P & Power S (2009). Poster: “Diversity, Rainfall and Elemental Cycling in a Terrestrial ecosystem, (DIRECT)” presented at Committee for Atmospheric Pollution Effects Research AGM. https://workspace.imperial.ac.uk/climatechange/public/pdfs/CAPER_poster.pdf, accessed 25 May 2014.
  5. Fry EL, Manning P & Power SA (2014a). Ecosystem functions are resistant to extreme changes to rainfall regimes in a mesotrophic grassland. Plant Soil, doi: 10.1007/s11104-014-2137-2.
  6. The Hermon Slade Foundation (2014). Drought, deluge and diversity decline – How do root herbivores affect grassland resilience to predicted changes in rainfall patterns? http://www.hermonslade.org.au/projects/HSF_13_12/hsf_13_12.html, accessed 25 May 2014.
  7. Britton E (1978). A revision of the Australian chafers (Coleoptera: Scarabaeidae: Melolonthinae) Vol. 2. Tribe Melolonthini. Australian Journal of Zoology, 26, 1–150, Supplementary Series.
  8. Bardgett RD & Wardle DA (2003). Herbivore-mediated linkages between aboveground and belowground communities. Ecology, 84, 2258-2268. doi: 10.1890/02-0274.
  9. Hawkesbury Institute of the Environment (2014). EucFACE, http://www.uws.edu.au/hie/facilities/face, accessed 25 May 2014.
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