Autumn Leaves and Climate Change

The autumn colours are late this year, but few people are lamenting the delay, especially considering that an extended summer is the cause.

Our unusually  warm and sunny fall has some observers concerned, however, about future fall colours. Some brows are being knitted into knots fretting about global warming and how it might change the annual spectacle of colour.

Research is beginning to show that global warming is causing trees to change colours later in the season. A 23-year observation by Harvard University in Massachusetts concludes that the autumn colour peak arrives there on average three to five days later than in the past.

The later peak correlates with a 1.1-degree Celsius rise in average temperatures in the U.S. northeast.

"Should that pattern continue, by the middle of the century we'd be at well over a week later" for fall colour, says John O’Keefe, an ecologist at Harvard Forest.

Another researcher, Howard Neufeld of Appalachian State University, has written that global warming will move the best autumn leaf displays farther north and upward in elevation.

He adds: “The fall foliage displays that our grandchildren will see at the end of this century will not be the ones we see today.”

Warmer autumns, while delaying the turning of the leaves, might have other effects on trees, such as inability to cope with higher temperatures and invasive species.

The changing leaves provide a huge tourism business in Ontario, and points east. Some estimates put the annual value of fall leaf tourism at $25 billion in the U.S. Northeast. There don’t appear to be any Canadian figures, however, the leaf peeping industry is big here, especially in our own Haliburton County.

What causes leaves to change colour is always a topic for debate. Some people argue that frost makes the leaves change, while others say that it is lots of autumn sunshine.

Daylight and temperatures are two main factors affecting when leaves give up their summer green for brilliant displays of red, orange, yellow, and persimmon.

Think of each leaf as a restaurant. Each has a head chef named Chlorophyll who takes in daylight energy and mixes it with carbon dioxide and water to produce sugars and starches to feed the customer - the tree.

As autumn approaches there is less daylight and temperatures usually are lower so the restaurant begins to shut down. Chlorophyll, which coloured the restaurant’s green for the summer, stops cooking and goes on vacation, taking its colour scheme with it. Without their green, the leaf restaurants are left with wild colours ranging from gold to red to brown.

Chlorophyll has been working longer this year because daylight with sunshine has been abundant, and temperatures above normal. Almost every day this September day and night temperatures have been above average in Haliburton County.

This year’s leaf transformation in this part of the country is one of the latest in several decades. September 27 (Sunday past) is the average date for peak colour in Algonquin Park. Over the past 40 years the fall colour has peaked as early as September 15 and as late of Oct. 9. Last year the peak was judged at Sept. 24.

Ontario Travel reported Sunday that the Algonquin colour change had reached fifty per cent. The Minden area was listed at 10 per cent. Algonquin usually colours up earlier because it is at higher elevation and therefore cooler temperatures.

The guessing is that we will see the colours peak sometime next week, possibly later.

The brilliance of the peak display will be decided by a complicated combination of temperature, light and water supply. For instance lower night temperatures just above freezing help bring out the reds in the maples. Early frost, however, can weaken the reds. The warm September that we have been experiencing likely will  dull the colour intensity.

How long the display lasts depends again on the weather. Wind and heavy rain knock leaves down.

As to what climate change might do to future fall foliage, there’s no use sweating that now. Kick back and enjoy whatever nature is offering at the moment.