Botanical Gardens experiment finds leafing in Europe brought forward by three weeks due to climate change

The Botanical Gardens in Glasnevin, Dublin, has been taking part in a Europe-wide experiment since the 1970s recording the time at which leaves and flowers start to appear, wilt and fall, on cloned trees and woody bushes – and have noted that leafing has been brought forward in the continent by approximately three weeks.

Poplars, beech, cherry, confer and flowering currant, woody plants and shrubs are among those that are have been monitored for half a century in a project designed to measure how weather zones are being affected by climate change.

“They all came from German botanic gardens [at] the Humbolt university,” explains Matthew Jebb, director of the gardens in Dublin.

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“They are all clones of one another, identical copies made by cuttings, and the point was to send plants to Finland, Norway, Ireland, Portugal Greece and Spain, all over Europe, and therefore you have the same individual, if you like, and by recording what day of the year the leaves come out, the flowers appeared, you would have a good proxy record for the climate. It was a very elegant idea.”

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Referred to as phenological plants, phenology being the study of timing in nature, the long duration of the ongoing project allows researchers plot changes in the arrival of the seasons even though the arrival of spring can vary significantly year to year.

Data plotted in one location, such as Glasnevin, would be affected by localised weather but having identical plants growing right across Europe is much more effective, says Jebb. Setting up the project was “a remarkable bit of foresight”.

Spring leafing can vary by up to a month depending on the year, but the project shows that climate change has brought leafing forward in Europe by approximately three weeks, a fact that might be “invisible” were the data not being collected.

“When you plot it out and draw that straight line in a graph, you can see that essentially plants are moving four kilometres a year on the landscape,” he says. “The climate is moving by four kilometres a year in a northerly direction.”

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Because of this change, many animals are moving north with the climate, but that is not so easily done by plants, says Jebb.

“Fishermen often come upon leatherback turtles. In the past they used to find them off the cost of Cork and Kerry, but in recent years they have come across them off the coast of Donegal.”

But four kilometres a year is much, much faster than any plant can migrate across a landscape in response to changes in climate zones, he says.

The problem with disrupted seasons is that everything in nature is linked, and earlier leafing and flowering could affect the relationship between plants, pollinators and things that feed on flowers.

“The fear is that there will be a tipping point, where things stop functioning correctly, like seed not being produced, that species will be suddenly be exceedingly limited in their ability to propagate themselves.”

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Spring crosses Ireland at a walking pace, Jebb says, starting in Cork and Kerry, and taking about a week to move gradually northwards to Down and Armagh.

“It’s as if a human set foot in southwest Cork and then walked diagonally up to Down and Armagh. That’s how spring crosses our landscape, like a wave of flowering and greening, and that happens every single year but now it is starting earlier and earlier.”

Mature hardwood trees, he says, capture many kilograms of carbon every year and need to be protected. “Probably the most precious things in the country are the big oak woodlands of Killarney. They need to be protected from fire and damage, they should be top of our list.”