Climate Warning says National Trust
The National Trust is warning that one of the best loved sights of autumn, the changing colour of tree leaves, will be heavily impacted by the warming climate unless urgent action is taken to tackle climate change.
With this year’s record-breaking temperatures[1], prolonged periods of hot, dry weather over the summer months[1] and many areas experiencing drought conditions, trees all around the country are being put under immense stress and some are struggling to survive.
Signs of this have been seen across the UK with many thinking autumn had arrived early with brown leaves carpeting the ground in August – but this phenomenon, known as a ‘false’ autumn, was due to the drought and trees simply not having enough water.
Commenting on what this summer’s weather could mean for this year’s autumn colour, Pamela Smith, Senior National Gardens and Parks Consultant at the National Trust says: “Due to the leaf drop that has already occurred in isolated areas[2] this year’s autumn colour will be reduced due to the simple reason that many trees have already lost a lot of their leaves due to the hot summer. Trees suffering due to drought don’t have the resources to sustain their size, so often the impact is a smaller leaf canopy.
“However, in terms of the typical autumn cycle, it remains to be seen what the drought and high temperatures could mean for this year’s autumn colour, but we may see more golden browns and yellows as a result – and this year could be quite a unique display.
“Biologically, long daylight hours are needed as well as the right mix of sunlight and rain – and hopefully trees were able to build up plenty of sugars in the spring and early summer so that the high temperatures had little impact – and it will only be those trees already under stress that will be impacted.
“It’s likely that well established trees will be more resilient and that we will still see the full colour spectrum, but this year is a warning to us all of how what we’ve previously taken for granted, may be at risk.”
Autumn colour is determined by both what the weather is doing now, but also the weather patterns across the year. While good levels of sunshine, but also rainfall is needed to build up sugars in the leaves, a lack of rain causes stress for the trees with potentially early shows of yellow or brown autumn colour and leaf fall.
Autumn colour also typically only starts to show once temperatures start to get cooler overnight – but remaining above freezing. It’s also important to have a period of calm weather so that leaves aren’t blown off trees before they’ve started to turn.
The shortening of the days and lower light levels in October stops the production of chlorophyl, the green energy creating pigment in leaves. As the green pigment fades (due to the trees withdrawing sugars from the leaves) the underlying colours of reds, oranges, browns and yellows become apparent.
Pamela continued: “Ideally over the next two weeks we need sunshine, rain, no strong winds and to see temperatures starting to dip. We’ll get the first indications of how good this year’s autumn colour will be in the north, as typically temperatures start to drop here first, as the phenomenon spreads north to south across the country.”
So far it does appear to be a good year for nuts and berries – but many trees have fruited earlier than normal. “We’ve also witnessed a glut of fruits and berries out earlier such as acorns, beech masts, rowan berries and elderberries due to these trees and shrubs being under stress.” commented Pam.
“Rowan berries for instance would typically stay on the trees until Christmas, but some are already falling now – which could affect food stocks for wildlife. If acorns fall before they are viable then natural regeneration thanks to burying by jays and squirrels may also be affected. This bountiful period and glut of food may impact our wildlife during the winter months with not so much food available when they need it most – so it is important if you have a garden or green space, to garden with wildlife in mind which will also bring nature closer to home.”
Winter is the perfect time to dig a pond and plant fruit and nut hedges such as blackthorn or hazel to support wildlife in the hungry winter months.
Leaving areas of grass unmown and providing unmanaged wild space around the compost heap can all help wildlife find shelter, food and water in the winter months.
As well as autumn colour, many National Trust gardens have plenty of other sights for people to enjoy. Pam added: “It’s already a great autumn for cyclamen nerines, and we are seeing an extended period of flowering in our borders with verbenas, sedum, Japanese anemones, asters rudbekia, crocosmia heleniums and dahlias – which have never looked so good.”
Pam concluded: “We’re always paying attention to the differences each year brings, but this year particularly will give us the opportunity to watch and learn – to capture information which can better help us to ensure any new planting is more resilient to changes to weather patterns and rising temperatures and aid our planning in years to come.”
To help more people to get outdoors to experience all elements of autumn, the National Trust is offering the public an opportunity to visit a National Trust property for free. For more information visit nationaltrust.org.uk/escape-into-autumn
Longer term impacts of climate change and heatwaves on our native trees:
John Deakin Head of Trees & Woodlands said: “Trees are incredibly resilient – particularly our oldest ones, which have endured centuries of storms, droughts and winter frosts, which is why the conservation of our ancient and veteran trees is so important in the final stages of their long lives as they become more vulnerable to sudden change
“We have witnessed first-hand how they have weathered this year’s summer by drawing back moisture from leaves, which has resulted in some brown leaves desiccating and dropping to the ground already.
“Longer lived tree species are thought to undergo natural cycles of retrenchment – i.e. change in size and form of the tree crown - and new growth to mitigate drought issues, which play out over several years.
“Despite their potential to span millennia, trees will struggle to survive consecutive summers of searingly hot temperatures and not enough rain. The damage to their vascular system and energy reserves is cumulative and may reach crisis point meaning we will unfortunately see more trees starting to decline and die, and they’ll also be more susceptible to pests and disease.
“It’s very likely we won’t understand the full impact of this summer’s temperatures until next spring when their ability to burst into new life may be hampered if they didn’t manage to store enough sugars in their roots over the summer period.
“Bud burst is a very energy sapping process as they move from a period of dormancy to bursting into new life. Some of the impacts of a severe drought like this will only become apparent over many years – even decades. Successive drought events in a changing climate will compound the stresses our trees face, including their susceptibility to tree disease.”
“There seems to be a correlation between drought and acute oak decline, a slow but worrying condition that is affecting many oak trees. We’re working with others to better understand what’s happening, and we already know that improving the health of the soils in which our oaks grow can help improve their vitality.”
John continues: “However, we know that some species will prove more resilient to the increase in extreme weather events and pests and diseases than others. Therefore, species diversification is a part of the answer to climate impacts.”
It’s not just the Trust’s older tree stock that’s been impacted by the dry weather. Luke Barley, Trees & Woodlands Adviser for the National Trust says: “We always aim to plant the right tree in the right place, but in the extreme heatwaves we’ve experienced this year, more trees than normal have died despite our hard work in identifying good locations.
“These young trees haven’t had the chance to fully establish, and therefore unfortunately don’t have the root system or mass to help them survive during periods of drought.
“However, where we have used mulch to help hold in moisture when planting trees, our saplings have fared much better. We will be working over the coming months to work out how we can ensure we incorporate mulch into all our tree planting projects.
“We’re also finding that trees which have developed from natural colonisation, are doing much better. This is because when self-seeding they are establishing good root systems from germination – so have more built-in resilience in times of challenging conditions.
“These are the trees people can help us invest in, confident that they are more likely to thrive and get the care they need.”
The National Trust has big ambitions around tree planting, with a target of planting and establishing 20 million trees by 2030. To make a donation, visit nationaltrust.org.uk/plant-a-tree