Why autumn leaves are staying green for longer

By Michael Lavelle, Anglia Ruskin University

Every year people ask me, “why haven’t the leaves fallen yet?” If autumn starts on September 1 then by early November winter should be well on its way. And yet, an amateur photographer who has captured the season’s colours for several years in southern England recently told the BBC that 2024 appears to be running behind schedule.

You may have noticed it too: it is becoming more common for leaves to drop later in the season. A 2015 review of 64 studies, with observations ranging from 1931 to 2010 across Asia, Europe and North America, found that rising global temperatures had delayed leaves falling in the northern hemisphere.

To understand what is happening, we need to remember that trees perceive the world differently to us. They follow a combination of environmental cues to time the shedding of their leaves. Foliage tends to fall gradually, starting in October and sometimes taking until early December. Most trees native to the UK hold onto their leaves until it gets cold, which usually happens by November. In essence, it either gets too cold, or too dark, and eventually, the plant becomes fully dormant.

Blackberries in June

While the mechanisms involved are complex, deciduous trees bed down for winter in three stages. The first of these starts in early autumn and is linked to a chemical produced by the buds called auxin, which slows the tree’s growth. This is produced in growing tissue and suppresses bud growth further down the branch. As the days shorten and temperatures drop, the tree produces abscisic acid. Throughout the autumn both these chemicals build up in living tissue and eventually arrests cell division and expansion in the resting buds.

The tree is no longer growing by mid-autumn but it may still be clothed in foliage if conditions stay mild. This allows time for the tree to reabsorb nutrients from its leaves (that’s what causes the colour change). Eventually, the branch forms a protective layer of cork at the base of each leaf which causes them to fall.

Winter dormancy descends once all leaves have fallen and it is maintained by low light levels and cold temperatures. Exposure to the cold for around two to three months slowly removes chemically induced dormancy. In spring, the return of warmth and lengthening days prompts the plant to recommence growth.

The changing climate means that autumn may not always progress as expected. In 2022, trees that normally flower in spring had an autumnal flowering. This was due to the unusual summer, which was the UK’s joint hottest (with the highest ever recorded temperature) and the fifth driest since 1890.

By August, leaves were already turning brown and falling. Heat and drought had slowed growth and caused levels of abscisic acid to spike. Fruit had also appeared early: blackberries normally only seen from August onwards were reported by the charity Woodland Trust as early as June 28. It seemed that autumn had arrived at the height of summer.

The situation changed once meteorological autumn arrived. Conditions remained warm, (the third warmest autumn since 1884) but the drought broke. The fifth driest summer since 1890 gave way to the 13th wettest autumn since 1836. The trees started to grow again.

The cost of confusion

This growth was different, though. While trees commonly display several flushes of growth and intermittent resting in a season, they only tend to grow leaves. The initial phase of spring growth is what usually includes flowers. Flowering parts are only included in the resting buds that form in the late summer and early autumn, just prior to leaves dropping.

All buds resting over winter require two to three months of chilling to break their inbuilt dormancy and prepare for flowering. Yet in 2022, the mild and wet autumn prompted a flush of new growth in many tree species, and a number of spring-flowering types, including magnolia, cherry, apple, blackthorn, dogwood, hawthorn, horse and sweet chestnut, produced unseasonal blooms, particularly in southern England. While this isn’t the only time this has ever happened, for the horse chestnut, it was the first time this had been recorded since 1944.

The explanation for this lies in the combination of record high temperatures and drought, which caused extreme stress for trees and resulted in a buildup of abscisic acid. Also, growth had stopped suddenly and very early; well before normal autumn bud dormancy had a chance to set in. The sudden mildness of the autumn then was akin to the conditions a bud would experience at winter’s end, and they sprang into growth.

Sadly, this “false spring” was to end abruptly in the first half of December, as a weather system that moved south from Norway, the so-called “Troll of Trondheim”, produced a prolonged spell of cold weather and snowfall. One of the most significant cold snaps since December 2010, its effects on the young, unprepared autumnal growth of 2022 was devastating. At Anglia Ruskin University’s Writtle campus, some species in our collection have only recently started to show signs of recovery. Somewhere in the region of 30% of our (usually) cold-hardy plants were either lost or damaged in that cold, dark end to 2022.

Could this then be the shape of things to come? Summer drought is predicted to become a regular feature in the UK as the century progresses. If this splits the growing season for deciduous trees, their future within the UK may be threatened in all but the most favourable locations.

So should we be worried about leaves dropping late? I don’t think so. The plants are behaving as they should do and getting properly prepared for anything the winter throws at them. They just happen to be taking their time over it this year.

Michael Lavelle, Senior Lecturer in Landscape Management, Anglia Ruskin University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

The opinions expressed in VIEWPOINT articles are those of the author(s) and do not necessarily reflect the views of ARU.

If you wish to republish this article, please follow these guidelines: https://theconversation.com/uk/republishing-guidelines