Trees and climate change

After what felt like weeks of incessant rain in Sydney – because it was, the deluge just kept coming – I was unable to get much field work done in the Royal Botanic Garden Sydney as stairways became waterfalls, flocks of gulls and ducks hunkered down around newly manifested lakes, at least two established trees collapsed and plants suffocated due to roots too long in saturated soil.

I think all this water caused the death of my new Correa cultivars in the Trial Garden. This really brought home to me the challenge we gardeners face in this period of extreme weather events. Our cities, homes and lives have been turned into an extensive experiment due to the choices made that destabilise our planetary climate systems with fossil fuels.

Sydney’s future climate

CSIRO’s online climate change projection tools show that Sydney’s climate will become more like that of Brisbane, with less rainfall, hotter days and higher instances of extreme heat, drought and wet. An increase of 3–5C° by 2090 is on track to take place within the lives of children born today; these too are the conditions saplings planted now will face as trees. This may well be the death knell for some of the trees we grow in Sydney that are already at the top end of their range such as acer, betula, prunus, platanus and pyrus.

And I only read with sadness on social media some people suggesting somewhat triumphally that perhaps now we will all start planting endemic species. They gravely misunderstand the problem. Being native is not a protective factor if plant communities cannot adapt to this sudden environmental change. It is not only weather patterns that plants will need to adapt to, but also increased disease burden as novel pathogens and pests extend their range as conditions favour them.

The challenge is to keep our cities green

This rather bleak situation does not mean we should lose all hope. The challenge is to keep our cities green, which fortuitously is also one means of ameliorating both the immediate and long-term effects of climate change. Trees do not just make us feel good psychologically, there are also well-researched material benefits they provide us, which are captured in the unfortunately ugly phrase ‘environmental services’.

Well-chosen and maintained trees reduce the impact of extreme heat in cities, primarily through shading and secondarily through evapotranspiration. This in turn reduces the amount of energy required to power air-conditioning as our neighbourhoods are cooler. Well selected trees protect soils from erosion and can assist in the management of rainwater runoff and drainage, as well as reducing pollution and assisting in carbon sequestration.

‘Well-chosen’ is the catch though. Plants that require greater amounts of inputs, such as water, synthetic fertilisers and petrochemically driven management practices, reduce their environmental credentials. Dead or perpetually stressed trees are not an asset!

Selecting trees for a changing climate

So how do we choose the best trees for Sydney and its changing climate? As with all tree selection we primarily want to be looking and asking questions of the site, such as what is the soil type? How does water move on the site? What space is available above and below ground for growth? And what functions do you want your tree to serve? With this information we can overlay the projected
weather changes for our region and begin exploring which tree species might be adapted to our new climate and weather context. Do not forget your local context as part of the site analysis. For instance, I am very conscious that both my home and workplace are around one metre above sea level. Trees and plants that can survive inundation and are salt tolerant will be the order of the day here.

This is just one thing that gardeners can actively do now, particularly when planting long term prospects like trees. And yes, it is exploratory and speculative, and will continue to be until we build up more data and experience as we try to manage whatever this new world is that we are bringing into existence. There is still a world of trees out there to select from. Perhaps a look at Brisbane’s street trees might help fill in some possibilities? Or some online research into hardy trees from across the globe at similar latitude and altitude as Brisbane. Creating, sharing and trialling these lists will be one way to prevent canopy loss in Sydney. What we plant now will provide time for trees to establish while others already established but less well suited to our new climate decline.
I would love to see what innovations people come up with on their lists!

Written by Bryn Hutchinson, Horticulturalist at the Royal Botanic Garden Sydney trial garden.