How plants will cope in a changing climate makes for disturbing reading. The answer, in short, is not very well at all. While plant breeders and government scientists have been selecting plants with traits that favour a shifting climate for years (eg. new succulent hybrids and drought-resilient wheat varieties), our local, indigenous flora and revegetated areas don’t have the same luxury. The idea that local plants suit the local environment has been central to bushland management and indigenous gardening for decades, but in the face of a changing climate, holding onto this idea could do far more harm than good to our beloved local plants.
With average global surface temperatures looking likely to exceed 1.5-2.0 oC by the end of the century, increasing heat is not even half the problem for our wild plants. A bigger challenge still is the predicted shift in the global water cycle, which will increase the disparity between wet and dry regions as well as wet and dry seasons. More heat, less rain with increasing intensity in extreme rainfall events. Rainfall is one of the biggest, if not the biggest predictive factor of vegetation type – what plant communities you find growing together and where. When you consider that Sydney’s climate is now that of 250 kms north (halfway to Coff Harbour) since records began, that’s an astonishingly fast change in a short space of time. Trends like these are being observed all around Australia and the world, and they’re set to continue and accelerate.
Where does this leave our wild plants? It is logically true that local plants are adapted to the local environment, but the pace of change is happening so fast that, on an evolutionary level, they’re going to struggle to keep up. A lot of evidence suggests they already are. Add to this the increasing fragmentation of habitat and the problem is compounded further still. Conservation geneticists are worried about all of the above. A perfect storm is brewing that could see biodiversity losses on a massive scale as huge tracts of bushland change or disappear completely within the next couple of hundred years.
Bushland managers have for years been carefully selecting seed for revegetation projects. They select for locality as well as good genetic material. Good genetics is imperative, as sourcing seed from small, fragmented populations can lead to a reduction in the quality of genes due to inbreeding. When habitat was continuous this was seldom an issue and genes ‘flowed freely’ between genetic individuals over greater distance than today. Local provenance is currently the name of the game, where seed that is collected for revegetation comes from very close by (tens of kms) for high quality bush with little disturbance, up to a couple of hundred kilometres for more degraded or totally cleared sites. The reasons for this are various, but the ‘local plants suit local climate’ idea underpins it.
While all plants are under threat, the threat is greater for some. Trees, the longest lived of our floral friends, are under the darkest cloud. Genetic individuals that are hundreds of years old are from genetic stock that self selected over millennia. Thousands of current generations of genera can be seen as the genetic culmination of an historical adaptation to a climate they will never experience again. Trees do not adapt as fast as grasses, herbs, forbes and a myriad of perennial shrubs, but this does not mean these types of plants don’t face the same uphill battle. The threat to all of them is terrifyingly real.
One of the solutions to this problem of local provenance seed saving in a climate change context is composite provenance seed saving (CPSS). It is a method that hopes to inject greater flow of genes between fragmented populations by sourcing seed from a greater area. The idea is to introduce new genes from different regions to make current populations more genetically diverse, hopefully equipping them with the bits and pieces they need to keep pace with the abrupt changes ahead. Sourcing seed of like species in regions where the future local climate is predicted to shift to is the ideal, however crossing the same species from many hundreds of kilometres apart poses its own challenges.
There are two main issues faced by CPSS, those of inbreeding depression and outbreeding depression. Each presents different but similar issues. Inbreeding depression is where the biological fitness of plant populations that are fragmented or isolated is reduced due to inbreeding. Think of two of the same species of tree growing side by side. They are far more likely to be closely related that another genetic individual 500m, 10 km or 100 km away. Collecting seed from the two trees side by side is potentially problematic because they’re close relatives, the risk of collecting seed too closely related, inbred, is high. Collecting seed from farther afield is an obvious solution, but it, too, is not without its problems.
Outbreeding depression is one of those problems. The concept states that offspring from crosses of genetically distant individuals can result in lower biological fitness than crosses of two closely related individuals. New genes introduced into a distant ecosystem may also ‘swamp’ the locally adapted species, resulting in potential weediness and and an outcompeting of local genetic stock. So our side by side trees above, although potentially inbred to some extent, may produce better quality genetic offspring than a cross with the same spaces from much further away.
It’s a bit of genetic paradox, but there’s been research to suggest the old adage of ‘local is best’ is no longer the way to think. Sgro et al (2011) suggested that building evolutionary resilience into local revegetation by introducing novel genotypes from greater distance apart (and hence genetically distant), could be a useful tool to encourage better adaptation to fast changing local environments. It’s an idea that flies in the face of a ‘local is best’ mentality, but the reality of a rapidly changing climate means that locally adapted species are going to face an increasingly uphill battle. Might the introduction of distant genetic stock be the key to ensure their survival in a climatically unstable future? Sgro et al certainly thinks so. The idea is to still use mainly local seed, with seed from farther afield comprising a small portion of the mix.
I think the argument is a compelling one. However, having it work in reality means combining complex climate modelling with seed saving practices, running experiments to ensure outbreeding depression is minimised and then, somehow, making sure this information is available to bushland managers on a national scale. This could take decades, but we might not have decades to wait until we are certain it will work. The small, dedicated and passionate teams of bushland managers nation-wide would need to upskill on a massive scale to ensure the idea is implemented effectively.
The concept also only holds for species that have a nationally cosmopolitan distribution. That is, species that inhabit large areas that cross state boundaries. We have thousands of species that have only local populations. One of my favourite spots here in my home state of Victoria is Gariwerd, otherwise known as the Grampians. Much of the flora there is found nowhere else in the country. What is to happen to them in the context of a changing climate?
In the end, I’m glad there are boffins looking into it in the hope that our local bushland and revegetated areas don’t become genetic cul de sacs and turn up their toes as the march of global warming continues unabated. With the pace of climate change only set to ramp up into the future I still can’t shake a sense of doom hanging over the whole issue. I think it would be easier to try and stop the warming of the planet, rather than try to breed resilience into revegetated and local plant populations. In the meantime I’ll just keep gardening…
[Ed: And here’s more fascinating reading on this topic at ‘Assisted colonization as a climate change adaptation tool‘ by RV Gallagher et al and ‘Building evolutionary resilience for conserving biodiversity under climate change‘ by Sgro, Lowe and Hoffman, 2011 ]