As the jacaranda trees bloom here in Australia, autumn leaves fall on the northern hemisphere, and partygoers across the United States take to nature for the annual pastime of “leaf-watching.”
- Autumn in the United States usually reaches its peak in late September
- Foresters in the lush forest region of Maine say a third of the trees have not yet changed color
- Warmer, drier conditions cause leaf stress, affecting the color change process
But this culturally significant marker of the passage of time is in grave danger of disappearing.
Recent fall seasons have been disrupted by weather conditions and the trend is likely to continue as the planet warms, much about arborists, conservationists and ecologists.
Typically, in late September, the leaves have fallen in warmer shades throughout the United States.
This year, however, many areas have not even fluctuated from their summer green hues.
In northern Maine, where peak conditions typically arrive in late September, forest rangers have reported that a third of the trees have not yet changed color.
On the other side of the country in Denver, high temperatures have left “dead, dry leaves” early in the season, said Michael Sundberg, a local arborist.
“Instead of trees making this gradual change, they are thrown over these crazy weather events. They change suddenly or they lose leaves early,” Sundberg said.
The reason why climate change can be bad for autumn foliage is linked to basic plant biology.
When autumn comes and day length and temperature drop, the chlorophyll in a leaf breaks down and it causes it to lose its green color.
The green makes room for the yellow, red and oranges that create dramatic autumn shows.
Achieving these peak colors is a delicate balance and one that is at risk for environmental changes, said Paul Schaberg, research plant physiologist at the U.S. Forest Service based in Vermont.
Warm temperatures that extend into the turn of the season can cause leaves to stay green longer and delay the onset of what leaf peers are looking for in terms of fall color, he said.
Worse, dry summers can stress trees and cause their leaves to miss the fall color shift completely.
“Severe drought that really means the tree just can’t work,” he said.
It’s already happening. The summer heat wave in the Northwest Pacific brought temperatures above 43 degrees Celsius to Oregon, leading to a condition called “leaf burning,” in which leaves are browned prematurely, said Chris Still, a professor at the Forest Ecosystems & Society Department at Oregon State University.
The pigment of the leaves was degraded and they fell shortly thereafter, Professor Still said. This will lead to a less scenic fall season in parts of Oregon.
“It’s a really big example of color changes just because of heat wave shock,” Professor Still said.
Climate change also poses long-term threats that can disrupt leaf-looking.
The spread of disease and invasive pests and the northern creep of the tree are all factors linked to warming temperatures that can lead to less vibrant fall colors, says Andrew Richardson, professor of ecosystem science at Northern Arizona University.
The economic impact of bad leaves looking at seasons can also have consequences.
Officials across New England have said that fall tourism brings billions of dollars into these states each year.
Conservationists say it is a good reason to focus on conserving forests and reducing the burning of fossil fuels.
Recent seasons have been less spectacular than typical in Massachusetts, but leaf-watching can remain part of the state’s heritage if forests get the protection 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 have relied on – clean air, clean water, clean forests as well as autumn inspiration,” he said.