Why do trees think it’s autumn already?

Fallen leaves, north London

Early leaf fall in London

From the crunch of leaves underfoot and the fiery foliage adorning the trees, you might think autumn has come early.

But experts say this hint of changing seasons is not genuine. On the contrary, it is the telltale sign of a “false autumn”.

They warn that the heat and drought have pushed the trees into survival mode, with leaves falling or changing color as a result of the stress.

And some may end up dying as a result.

Dying tree in a London park

Dying tree in a London park

Yellow leaves and early leaf fall are both signs that trees are stressed and ‘closing shop’, says Leigh Hunt, senior horticultural adviser at the Royal Horticultural Society.

“It gives the appearance that we’re already in autumn, but the days are too long for those natural autumn processes to begin,” he says.

“Naturally, plants don’t respond to autumn conditions; that’s why we loosely refer to it as ‘false autumn.’

Bodmin, Cornwall

Old tree skeletons exposed due to low water levels in Colliford Lake near Bodmin, Cornwall

He says in all his 45 years, this is one of the most severe years he’s seen in terms of damage to trees in the countryside.

And while established trees can withstand drought through their extensive root network, newer specimens, such as those planted in poor roadside soil, could wither and die.

“Second Spring”

Trees that have lost only a few leaves with some yellowing should recover with enough rainfall, he explains.

There is “a critical point,” however, when the tree cannot replace the water lost through the pores in the leaves and will “literally desiccate,” or dry up.

In conditions like the ones we’ve seen recently, trees may react by producing more seeds—for example, acorns—in an effort to reproduce and survive into the future.

Dry soil

Drought has caused hard, compacted soil, which can affect vegetation

And if there’s a lot of rain, we might even see “a second spring” with trees developing an extra spur, he says.

Early berries

Other signs of unpredictable weather can be seen in berries appearing on plants and bushes.

The Woodland Trust, which records seasonal changes, received its first report of ripe blackberries – from 28 June.

He says fruits and nuts are ripening faster than ever before, which “could spell disaster for the wildlife” that feeds on them.

“The record-breaking heat we’ve just experienced has helped bring about a number of early autumn events,” says Fritha West, from the Woodland Trust.

“We have received some of our first ripe blackberry records from southern England. Hawthorn and rowan are also ripening early in some parts of the country, where early leaf color has also been seen.

“Elder and holly have been recorded as fruiting earlier. Both extreme temperatures and lack of water can cause trees to drop their leaves earlier than we would expect.”

Shrinking rivers

It is difficult to predict the long-term effects of the drought, but ecologists believe weeks of parched grassland and hard ground across much of southern England will have a major impact on wildlife.

American lobster shell

The shell of a crayfish in the dried up bed of the River Thames at Ashton Keyne

In and around the rivers, the drought could be felt for years to come.

During summer droughts, fast-growing algae can smother wetland plants, killing them by blocking light.

Lowering river levels reduce habitat for fish, amphibians and invertebrates, affecting entire ecosystems.

“These plants provide vital habitat for insects and fish, and their loss from the ecosystem causes significant changes in the food chain,” says Dr Mike Bowes of the UK’s Center for Ecology and Hydrology.

“It can take several years before plants can recover or recolonize these drought-affected rivers, and so the impact of severe droughts can be prolonged.”

Follow Helen on Twitter @hbriggs.

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