Give me a home among the gum trees

But what if those gum trees lose their home to climate change?

Figure 1: Proportional pixel losses and gains by climate regional group (each region includes four climate subgroups), calculated from the 2085 time step for both scenarios (moderate and extreme). The lines with solid circles represent group ranges and means for the extreme scenario; the lines and hollow circles show the group ranges and means for the moderate scenario.

Figure 1: Proportional pixel losses and gains by climate regional group (each region includes four climate subgroups), calculated from the 2085 time step for both scenarios (moderate and extreme). The lines with solid circles represent group ranges and means for the extreme scenario; the lines and hollow circles show the group ranges and means for the moderate scenario.

Eucalypts are iconic trees in Australian landscapes, and given the variety of treed landscapes that are found across Australia, that’s an amazing thing to consider. There are around 800 species (eucalypt taxonomy is a moveable feast) of three genera, Eucalyptus, Corymbia and Angophora that together are known as eucalypts or gum trees, and these trees have dominance or co-dominance in most forest and woodland ecosystems in Australia. You’ll find them in rainforests, up mountains and across the arid zone. And, in all these places, eucalypts are providing a range of essential resources for other animals and plants as well as generating ecosystem services such as climate regulation, water security and carbon sequestration and storage.

How will this important group of trees cope with climate change? Trees are among the first groups to be affected globally by climate change as they are particularly vulnerable due to long generation times and short dispersal distances. In the northern hemisphere there’s been considerable research on the projected impacts of climate change on species and ecosystems but that’s not the case with southern-hemisphere ecosystems.

Working with Laura Pollock (University of Melbourne) and Clive McAlpine (University of Queensland), I recently led the first comprehensive investigation into the vulnerability of eucalypt distributions to climate change across Australia. The findings of our analysis should be out soon but here’s an overview of what we discovered.

While our findings focussed on Australian systems, they have important implications for many other southern hemisphere regions, such as sub-tropical and tropical savannas with seasonally variable rainfall. These occur widely across Africa, South America and the Asian subcontinent.

A continental-scale analysis

As those of us who live here are well aware, Australia’s climate is unique in its high variability, with most of the continent water-limited (apart from during floods and rainstorms!). The additive combination of changes in temperature and rainfall will govern the likely impacts we will experience from climate change.

Climate projections indicate that fluctuations in temperatures and rainfall will increase over time. They will also increase the further  you move away from the coast. This means that the continental interior will become hotter and drier faster than other areas. Available moisture is projected to decrease which will affect evaporation and evapotranspiration. This transition to a hotter and more drought-prone climate represents a major risk for Australia’s ecosystems, particularly for those without the capacity to recover or adapt.

So, how best to carry out a continental-scale analysis, accounting for climate and geographical variation? We used the climate classes from the Bureau of Meteorology to identify four broad bio-geographical regions, each with four climate region sub groups. We selected representative eucalypt species for each climate region, each community role (where the species status is either dominant, typical or endemic) and each type of range breadth (wide/narrow/local). We also included a ‘wide-range’ group of species whose distributions were not closely linked to any of the climate regions (Figure 1).

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One country, many eucalypt-dominated ecosystems. Pictured above are examples of some of the shapes and forms of gum trees across Australia and the ecosystems they influence. Climate change could have profound impacts on many of them.

One country, many eucalypt-dominated ecosystems. Pictured above are examples of some of the shapes and forms of gum trees across Australia and the ecosystems they influence. Climate change could have profound impacts on many of them.

This gave us a total of 108 species, to which we applied a bioclimatic modeling approach using Maxent and two newly available Representative Concentration Pathway scenarios developed for the 5th Assessment Report of the IPCC; a moderate and an extreme scenario. This allowed us to identify which groups of species will be affected by climate change, how the impacts will differentially drive changes in distribution though shifts in suitable climate space, and what are the consequences for biodiversity.

A shrinking climate space

Annual rainfall and two measures of seasonality, dry-quarter precipitation and wet-quarter temperature, were the key bioclimate predictors in the model. We calculated climate space loss and gain per pixel and found that:

  • species ranges in the ‘tropical+equatorial’ group will shift further north and east,
  • species ranges in the ‘subtropical’ group will shift further east and south,
  • species ranges in the ‘desert+openwoodland’ group will shift primarily south and west, and
  • species ranges in the ‘temperate’ group will shift south.

The wide-range species all showed an overall loss of suitable climate space, broadly in the north or west of their range, and some expansion south or southeast of their current range (Figure 2). All species showed some gain and some loss of suitable climate space pixels. However, gain was greater than loss for only ten of the 108 species we modelled. Only four of these – all of them savanna or grassland/open woodland species – showed a greater than 10% gain (Figure 1).

Figure 2: Large scale shifts in climate space under both moderate and extreme scenarios. Tree climate regions were broadly based on the Bureau of Meteorology climate classification, as above. The arrows indicate the overall direction and magnitude of climate space shift for the four general climate regions.

Figure 2: Large scale shifts in climate space under both moderate and extreme scenarios. Tree climate regions were broadly based on the Bureau of Meteorology climate classification, as above. The arrows indicate the overall direction and magnitude of climate space shift for the four general climate regions.

Overall, Eucalyptus species in the central desert and open woodland regions will be the most affected, losing 40% of their climate space under the extreme climate scenario. The least affected species, in eastern Australia, are likely to lose 20% of their climate space under the extreme scenario.

Continental limits

The net losses, and the direction of shifts and contractions in range, suggest that many species in the eastern and southern seaboards will be pushed towards the continental limit. That means that large tracts of currently treed landscapes, especially in the continental interior, will change dramatically in terms of species composition and ecosystem structure. The slow response times of trees means that they are unlikely to move quickly enough to keep up with climate change. There is evidence of this already happening in some eucalypt systems. In the northern savannas and the Murray basin, for example, there has been dieback due to drought.

The projected range shifts of vulnerable eucalypts are more complex than the simple pole-ward adjustment; something assumed by most distribution-shift literature. Interestingly, we found lateral, east-west shifts in suitable climate space were more significant than the north-south shifts for the continent (Figure 2), and this reflects the strong influence of precipitation rather than temperature in central latitudes: arid-zone and open woodland species are especially threatened.

As well as the climate threat faced by the trees themselves, cascading impacts on eucalypt-dependent species and communities will be far-reaching. And that applies to the ecosystem services and landscape-scale bio-climate processes that eucalypt communities contribute to.

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In terms of conservation planning, the results are also significant. Restoration efforts in particular may have to be reframed. Where areas set aside for restoration are currently climatically marginal, and in future will be no longer climatically suitable, the prospects of restoring such areas must be questioned. Establishing trees in such landscapes is extremely difficult and entails financial risk.

‘A home among the gum trees’ characterises much of Australia. Our studies suggest that climate change will have far reaching implications for the condition of that ‘home’ and the gum trees that frame it.

More info: Nathalie Butt n.butt@uq.edu.au

 

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