If you think the leaves are taking their time turning colour this year, scientists think you might be right.

There's growing evidence that the timing of fall colours is linked to climate change. And there are signs that the leaves are turning colour later because of warmer temperatures.

"As the climate continues to warm the autumn is getting later every year, by as much of a week or more, says Richard Primack, a biology professor at Boston University. "We have variations from year to year but there is a long-term trend to later autumns."

Primack says scientists in Japan and Europe have been monitoring fall leaf colouring and timing for years but there hasn't been much done in North America.

But he says there's now a growing interest in tracking the fall of leaves as indication of a changing climate for both environmental and economic reasons.

"If the weather isn't cold, crops will take longer to mature and harvest times will change." 

Leaves turn colour because of shorter days but they are also affected by temperature and the amount of moisture in the soil. Warmer temperatures trick the trees into thinking it's still summer and they hang on to their leaves longer.

But fewer cold autumn nights mean less colour in the leaves. That could have big economic impacts on fall tourism says Primack, who is an expert on how climate change is affecting New England.

"There is a possibility that the fall foliage will be less dramatic ... the trees will just turn brown but not turn bright colours... that is something people are concerned about."

'Citizen scientists' help track changes

Now a new U.S.-based network is recruiting volunteers to track the changing leaves.

"The ultimate goal is to see whether the timing of the fall is changing and how to use that information and how to adapt to that change," says Jake Weltzin, executive director of the National Phenology Network and an ecologist with the U.S. Geological Survey. "We are enlisting citizen scientists to team up and help us."

Phenology is the study of nature's timing.

The network did a trial run last year and now has 4,000 volunteers in the U.S. and a handful in Canada. They're looking for more Canadians to help, says Weltzin. "Fall crosses the border."

Basically volunteers can keep an eye on whatever trees they like, the ones in their backyard or on their favourite walk. They answer a set of specific questions about what the leaves look like and then send in the information online.

"We need to know what is happening on the ground in the fall," says Weltzin. "If you look at satellite pictures you only get the big picture, we need to know the details."

The network's fall leaf watch is based on a system designed by a Canadian group called PlantWatch. That group's science advisor, Elisabeth Beaubien, who is based in Edmonton, worked with the National Phenology Network to set up a way  to track the subtle change of fall colours and figure out the best trees to watch.

Beaubien says PlantWatch doesn't track fall leaves in Canada right now but is hoping that will soon change.

"We need to think about climate change and how it's affecting the plant's survival."

In the meantime, Beaubien has been keeping her own personal notes on fall foliage in Edmonton every year since 1987, and she has no doubt the season is getting later.

"We still have aspen poplar with leaves, and even about 10 per cent green leaves in some parts of the city. This fall is shaping up in central Alberta to be one of the later years for fall colouring in the last 25."