Bright days and low winds make golden days for leaves

A combination of bright days and little wind have conspired to give Ireland one of the best displays of autumn leaf colour for several years.

These same colourful leaves painted brilliant yellows and reds in reality are actually “zombies”, already dead leaves awaiting their ultimate fate – leaf drop.

A tree coming into the autumn is actually like "decommissioning a light factory", said Dr Matthew Jebb, director of the National Botanic Gardens of Ireland. "When you decommission you take out everything you can recover and plants are very efficient at that."

Leaves go green in spring because they are full of the green pigment chlorophyll, which supports the production of sugars that in turn feed the plant.

“In the autumn the plants break down this pigmentation and anything moveable like glucose is shifted into the bark for winter storage.”

Complex sugars

The tree abandons the otherwise unusable red and yellow pigments and complex sugars that it can’t digest. This sugar combines with the pigments to bring on the autumn colour, even though the leaves are now biologically cut off from the tree and are effectively dead, Dr Jebb explained.

Whether we get to see bright autumn colour now depends on what the weather does. Most often it gets cold and Atlantic storms blow in with plenty of wind that strips away the leaves before they can develop the red and yellow pigments.

This did not happen this year because stable high pressure settled down over Ireland for weeks and we are still under its influence, said Evelyn Cusack, deputy head of forecasting with Met Éireann.

We are seeing more autumn colour because we have had an anticyclone over us for the past few weeks bringing dry, settled and mild weather, she said.

Deciduous forests

“This was the high pressure we were hoping for but didn’t arrive during the summer,” she said. “We can get high pressure at any time of the year, we just happened to get our high now and that is why we are enjoying the leaf colour now.”

An attractive "fall" leaf display isn't just about colour in the US northeast, where bussing people about to see the deciduous forests in all their glory is a billion dollar industry. But climate change may threaten this, with leaf-break now coming a few weeks early than in the past, Dr Jebb said.

“A lot of plants have a pre-programmed life expectancy for their leaves. They can’t keep them longer in the autumn if they start earlier because of climate change.”

The pleasant autumn has kept the leaves off the streets but Dublin City Council’s cleansing units are already in action. A spokeswoman said that there is “considerable leaf fall in the city at present”.