Extract from ABC News
As the jacaranda trees bloom here in Australia, the autumn leaves fall in the northern hemisphere and revellers across the US take to nature for the annual pastime of "leaf peeping".
Key points:
- Autumn in the US usually reaches its peak at the end of September
- Forest rangers in lush forest region Maine say a third of trees are yet to change colour
- Hotter, drier conditions are causing leaf stress, which impacts the colour-changing process
But this culturally significant marker of the passage of time is in serious danger of disappearing.
Recent autumn seasons have been disrupted by weather conditions and the trend is likely to continue as the planet warms, widely concerning arborists, conservationists and ecologists.
Typically, by the end of September, leaves have cascaded into warmer hues throughout the US.
This year, however, many areas have yet to even pivot from their summer green shades.
In northern Maine, where peak conditions typically arrive in late September, forest rangers have reported a third of trees are yet to change colour.
On the other side of the country, in Denver, high temperatures have left "dead, dry edges of leaves" early in the season, said Michael Sundberg, a local arborist.
"Instead of trees doing this gradual change, they get thrown these wacky weather events. They change all of a sudden, or they drop leaves early," Mr Sundberg said.
The reason climate change can be bad for autumn foliage is linked to basic plant biology.
When autumn arrives, and day length and temperature drop, the chlorophyll in a leaf breaks down and that causes it to lose its green colour.
The green gives way to the yellows, reds and oranges that make for dramatic autumn displays.
Achieving those peak colours is a delicate balance and one jeopardised by changes in the environment, said Paul Schaberg, a research plant physiologist with the US Forest Service based in Vermont.
Warm temperatures stretching into the change of season can cause leaves to remain green longer and delay the onset of what leaf peepers look for in terms of fall colour, he said.
Worse, dry summers can stress trees and cause their leaves to miss the fall colour turn altogether.
"Severe droughts that really mean that the tree just can't function," he said.
It's happening already. This summer's heatwave in the Pacific Northwest brought temperatures of over 43 degrees Celsius to Oregon and that led to a condition called "foliage scorch," in which leaves prematurely browned, said Chris Still, a professor at the Forest Ecosystems & Society department at Oregon State University.
The leaves' pigment was degraded and they fell shortly thereafter, Professor Still said. That will lead to a less scenic fall season in parts of Oregon.
"That's a really big example of colour change just due to heatwave shock," Professor Still said.
Climate change also poses longer-term threats that could disrupt leaf peeping.
The spread of diseases and invasive pests and the northward creep of tree species are all factors tied to warming temperatures that could make for less vibrant autumn colours, said Andrew Richardson, a professor of ecosystem science at Northern Arizona University.
The economic impact of poor leaf peeping seasons could also be consequential.
Officials throughout New England have said autumn tourism brings billions of dollars into those states every year.
Conservationists say that's a good reason to focus on preserving forests and reducing burning fossil fuels.
Recent seasons have been less spectacular than typical in Massachusetts, but leaf peeping can stay a part of the state's heritage if forests are given the protections they need, said Andy Finton, landscape conservation director and forest ecologist for The Nature Conservancy.
"If we can keep the big, important forests intact, they will provide what we've depended on — clean air, clean water, clean forests, as well as fall inspiration," he said.
AP
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