Will Climate Change Be Good For Plants?

If the climate changes as predicted in this Department of Environment, Climate Change and Water model, what impact will this have on Sydney region plants?

This is my third posting from the G2G meeting. Belinda Medlyn from Macquarie University (with Melanie Zepel) has been working on what she calls the vulnerability of Australian forests to climate change.

According to Belinda the general prediction for Australia as a whole is warming by 2-3 degrees by 2070 and a reduction in rainfall over much of country during this time (but rainfall is more difficult to predict).

Belinda said we can test the effect of these changes on forests by 'modelling' current distributions under new conditions (bioclimatic envelope modelling) or using ecophysiology (looking an mechanistic approach – how will plants or communities respond)?

Bioclimatic Modelling has received most but still not enough attention. In one of the first studies, Lesley Hughes and colleagues used herbarium records to model 800 eucalypt species. More than half the species found in temperature ranges of less than 3 degrees so if temperature change by more than 3 degrees, many species may be in trouble. These species occur around the country.

A similar study with banksias in south-west Australia (by Fitzpatrick and others in 2008) showed that limited climatic tolerance would mean many species are threatened with extinction or at least stress. Hilbert and others in 2001 looked at rainforests generally and found that climatic changes predicted would result in rainforests being subject to climate more suitable to another forest type. Work on eucalypt plantations found that suitable areas for, e.g., Tasmanian Blue Gum, would shift dramatically.

The conclusion from most studies is that climate change will have a big impact on Australian forest species, or at least the individual species in them... But, there are lots of caveats. These are correlations only, so not necessarily what we call 'causation' – it may be something else that causes the change (we criticise pseudoscience for this so have to be careful!). Rising carbon dioxide is not accounted for. And average climate values are used but extremes might be more important (good data doesn’t exist for modelling with climate extremes). There are other caveats and Belinda says to look up an article by Yates and others in one of the 2010 issues of Austral Ecology.

So what about this ecophysiology stuff. We can look at the direct impact of carbon dioxide, temp and rainfall on processes such as photosynthesis, transpiration, respiration rate and son on, and estimate how this will effect plant vigour and survival.

Early experiments show that carbon dioxide doesn't enhance growth as may be expected. However nutrient limited plants may respond differently and local experiments (by University of Western Sydney) were using eucalypts in nutrient-limited soils. However even in this study, water was used more efficiently so there was some positive effect of rising carbon dioxide. Belinda said it depends too on where in the ‘optimum curve’ your plant sits – most Australian plants probably at bottom part of this optimum curve so increased temperatures may not change survival. Rainfall likely to have bigger impact.

This kind of ecophysiology modelling can predict likely changes in productivity. For plantation eucalypts, productivity is likely to go up with climate change models. In south-western Australia, modelling suggests that plantations will be little effected (in terms of wood production) even sizable changes in temperature and rainfall – they have deep roots!

The conclusion from most ecophysiology modelling is that Australian forest plants will increase in productivity with climate change. But again there are caveats. Drought mortality hasn't been included (i.e. they might die under stress of particular years even if over a long period they would in theory do better). Establishment and reproduction have not been considered, nor other biotic interactions. It may be that new seedlings don't survive so even if mature trees thrive a natural forest may suffer.

So how do you reconcile the two scenarios? Can we integrate the approaches? It may be that both are correct, and looking at different aspects of a forest or plant community. Of course some species will suffer but overall a forest will adapt in some way.

Belinda suggested using more directed ecophysiology studies to test particular climatic boundaries. Transplant experiments could be used more to test ecophysiology models. She suggested we also look at ‘absence’ data – i.e. where plantations fail would such models have predicted this? We could also incorporate CO2 data in bioclimatic models.

There was a lot of interesting discussion after the talk about how different studies told us different things. WE all concluded that plants will survive, we just don't know which ones and in what quantity.

Anyway, there were more fascinating G2G talks but that's probably enough high powered science for my blog. Remember though, our scientists need ideas and then testable hypotheses.