Liam Creedon: Dry spring, cold summer and vagaries of a strange moth

This summer has been one of the coldest for nearly 20 years, with the Met Office recording the average temperature from early June to mid-August in central England at 15C. That makes 2011 the coolest summer since 1993 and in the North, it’s been even colder.

Normally, we can at least console ourselves by glancing out upon Britain’s green and pleasant landscape for solace, but this year, gardeners up and down the UK have noticed something different.

Instead of an endless palette of green, the leaves on many of our trees have prematurely turned a dingy brown. And it seems that this colour transformation is taking place far earlier than would normally be expected.

The apparent early approach of autumn was first noticed by experts at the Royal Horticultural Society (RHS). As early as mid-August, gardeners in the south of the country were reporting leaves on maples, hazels and laburnum developing yellow, red and brown tints.

There were also reports of fruits ripening several weeks early on apple and pear trees, while autumn raspberries and wild fruits such as hawthorn were also ahead of schedule.

Autumn is triggered by changes in day length and temperature – but was there something else at play that resulted in this early explosion of colour?

If you trawl back just a couple of months, farmers, landowners and the media were bemoaning the fact that parts of the UK were in the grip of one of the driest springs for almost a century.

Drought conditions were declared in South East England as rain failed to fall for weeks on end.Fears of ruined crops and threats of the dreaded hosepipe ban were temporarily mooted until the weather reverted to type.

Could this unprecedented dry spell have contributed to the early autumn colours? The RHS certainly seems to think so, with experts declaring that the lack of rainfall since the drought had in turn left trees desperately short of water.

Guy Barter, chief horticultural adviser at the RHS, said there had been a five-inch soil moisture deficit during the summer.

He explained: “The autumnal leaf colouring and leaf loss on trees is due to dry soils.

“Trees and shrubs are under a lot of water stress. It’s not fatal because they are well adapted, but it makes them get rid of their leaves.”

However, it may not be all bad news. Evidence from other parts of the natural world show that the premature leaf browning does not necessarily point to an unseasonably early autumn.

The British Trust for Ornithology (BTO) runs BirdTrack, a system which monitors the arrival and departure of migrating birds in the UK and its data shows that swifts, yellow wagtails, common terns, nightingales and various warbler species all left or were leaving our shores on schedule.

Also, there is no evidence that the dry spring had any measurable effect on the breeding success of our migratory bird species, so could something else be behind Britain’s early autumnal colours?

Prior to the 1980s, nobody had heard of the Horse Chestnut Leaf Miner moth, but this tiny and determined invader is increasingly making its presence felt in Britain’s woodlands.

The insect had for time immemorial been confined to an isolated stretch of the Balkans, where it happily munched away on horse chestnut leaves.

But, suddenly, it started to spread rapidly westwards and in a matter of years it established itself as one of the major pest species in Europe.

Its caterpillars feed inside the leaves of the white flowering horse chestnut tree, producing telltale “mines” between the leaves’ veins. The moths do not kill the tree but their work can devastate leaves and result in them prematurely turning brown.

So, rather than witnessing the dramatic shifting of normal seasonal patterns, the early browning of at least some of our trees can be put down to the vagaries of the Great British weather and the ravenous endeavours of a troublesome insect.

But the importance of these events cannot be underestimated. We have just experienced one of the driest springs in a century coupled with a record-breaking cold summer.

These extreme weather events are becoming increasingly common and their long-term toll on our wildlife is yet to be revealed.