Are autumn’s icons on the hit list? (Image: Chen Hao/Xinhua/Eyevine)
JOHN KEATS described a “season of mists and mellow fruitfulness”. Nat King Cole sang of “autumn leaves of red and gold”. But what if the leaves cease to turn red and gold, and the mellow fruits don’t cling on until the arrival of autumn mists?
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Howard Neufeld of Appalachian State University in Boone, North Carolina, is one researcher who thinks this may be so. He says the curtain may be descending on one of the season’s defining spectacles, the fiery spread that sweeps through the forests of New England each fall – and climate change is to blame. “Warmer falls reduce the intensity of the red colours – of sugar maples especially, the mainstay of colour in the north-east of the US,” he says. It’s all down to how warmer autumn nights tweak the internal chemistry of leaves: they cause sugar levels to drop, which in turn affects the pigments responsible for those signature reds. Sadly, russet tones are not the only autumn tradition at risk, although amid the losers there could be surprise winners as well.
Autumn is the poor relation of spring in terms of academic interest. Amanda Gallinat of Boston University calls it ““. In the UK, a long amateur tradition of observing the start of spring has produced detailed records that go back to the 18th century. But, according to Kate Lewthwaite, citizen science manager at the UK’s Woodland Trust, the systematic recording of autumn’s timeline only started in 2000.
“Spring captures the imagination of naturalists more than autumn events,” says ecologist Ally Phillimore at the University of Edinburgh in the UK. “And the onset of autumn is less obvious.” The first flower in bloom, the first swallow or cuckoo call, the first insect’s buzz, the first squirrel out of hibernation all grab our attention, whereas the last berry or buzz or birdsong will probably pass unnoticed. Gallinat and others are rapidly filling in our knowledge gaps, but for now we still know a lot more about how climate change is affecting spring.
As winters shorten and the air warms up earlier in the year, spring is starting sooner in the midlatitudes. But whereas some species are shifting their schedules to adapt to changing temperatures, others respond to fixed cues – such as day length – with the result that parts of the ecosystem become out of sync with each other. Plants, for instance, may produce tender shoots before the animals that feast on them arrive.
At the other end of the calendar, you might think that warmer, longer autumns would simply keep the lethal cold of winter at bay. Not so. Researchers are finding that a disrupted autumn is at least as problematic for wildlife as a disrupted spring. Here too, the normal chain of events is being thrown sharply out of sync.
Spectacles at risk
Warmer autumns may disrupt leaf displays in more ways than one. Instead of today’s synchronised cacophony of colour, different tree species could change colour at different times, says Neufeld. And in the case of New England, some trees could disappear altogether. Sugar maples, yellow birch, red spruce and others are likely to shift north into Canada, replaced by less colourful oaks. This is already happening: the US Forest Service reports that .
All of this could be a big problem for a tourist industry worth $25 billion a year. And the spectacle may be at risk elsewhere too. Japanese autumn now comes more than a week later than in the mid-20th century, according to Keiko Masuda of Ryukoku University in Kyoto, Japan. Leaves on maples around the city’s temples often drop to the ground before they turn red, in part because warmer temperatures are drying them out. A similar fate could await the golden hues of aspens in the Rocky mountains and larches in the Alps.
Back in New England, staff at the non-profit conservation body Manomet, based in Plymouth, Massachusetts, have been catching and releasing birds each spring and autumn since 1969, taking note of their size and the timing of their migration. Gallinat is part of the team that has been analysing these records. She says that many birds – particularly those preparing for short migrations, such as the junco sparrow – are taking advantage of the good weather to start their journeys up to a week later than they did a few decades ago. But others, especially those heading for the tropics, leave earlier.
“Earlier ripening poses problems for songbirds that rely on berries late into autumn”
In the UK, Tim Sparks of Coventry University is a leading light in a nationwide project launched by the government’s Centre for Ecology and Hydrology to plug gaps in our knowledge of the changing seasons. He says the same mixed pattern is emerging in Europe as in the US. Many birds that breed in the UK are now arriving earlier in the spring and flying away again sooner: after all, what is there to keep them in the country after their brood has hatched? But the behaviour isn’t universal. “Multi-brood species such as swallows may get in another brood, and end up leaving later,” says Sparks.
To untangle all of this, ecologists must determine what cues different species use to time their migration, hibernation or leaf fall. The patterns may be complicated. Studies at the Harvard Forest in Petersham, Massachusetts, which has been under continuous observation for more than a century, suggest that some tree species have a fixed amount of time “in leaf”. “An early spring budding is correlated with an earlier leaf fall, irrespective of the weather,” says ecologist John O’Keefe of Harvard University.
Whether because of their biological clock or summer warmth, fruit trees and bushes are also changing habits. In many parts of the northern hemisphere, fruits are ripening earlier than before. Humans may enjoy bumper harvests, but the shift is a growing problem for birds and mammals that rely on berries for food late into the autumn. Gallinat says that this is particularly true for many New England songbirds, which have been delaying their migrations in warmer years but are finding that the native blueberries and huckleberries on which they normally feed don’t last to the end of the longer season. There is no luxury in a long Indian summer if your larder is bare. Vulnerable birds include the hermit thrush and the blackpoll warbler, a tiny songbird with a particular need to be well fed because .
Alien feast
Luckily, there is another option for late departers. Gallinat says they are seeking out other sources of food, including berries from invasive trees and bushes that are thriving in New England’s warming climate. These aliens almost all . “When walking in the forests of New England, you see the invasive oriental bittersweet and others with still-green leaves when native species have [already] lost theirs,” says her colleague Richard Primack. “They fruit later than native species, and warblers and thrushes are feeding on them.” Alien species generally get a bad press from ecologists, but in this instance appear to be more lifesavers than ecological terrorists.
Sad but true: jack-o’-lanterns can’t take the heat (Image: Jonathan Birch/Narratives/Plainpicture)
Warmer autumns probably help the invaders to prosper, and in more ways than one. Gallinat notes that by filling a new ecological niche and sustaining late-migrating birds, invasive plants gain an advantage: the grateful birds preferentially excrete their seeds across the landscape. Could the invasive fruit trees be nature’s way of adapting to climate change? “That is an interesting question,” says Gallinat. “It is certainly possible they will help bird species adapt by providing a late-autumn food source as native fruit availability declines.”
There are other signs of a new order settling in. In the Arctic, the spring snowmelt is coming ever earlier, but instead of breeding earlier, Arctic ground squirrels are taking a breather. Because global warming is also delaying the autumn chill, it makes sense for them to wait for warm conditions to settle in properly before they breed, safe in the knowledge that there will be enough time for their young to fatten up for the winter. Michael Sheriff of Pennsylvania State University in University Park has . Longer springs, summers and autumns mean that conditions are much less harsh for the young than they once were, and many more survive.
This again suggests that the potential for some species to adapt to a changing climate is greater than was thought. The Intergovernmental Panel on Climate Change has predicted that up to one-third of species could die out in a warmer world as climate zones shift. If species are more adaptable, that could be an overestimate. Delayed autumns ought also to mean longer growing seasons, more photosynthesis and so more carbon dioxide sucked from the atmosphere and locked away into plants. That could be a welcome negative feedback on climate change.
Not so fast, says Shilong Piao of the Laboratory of Climate and Environmental Sciences in Gif-sur-Yvette, France. Warmer autumns will also boost soil respiration, which releases CO2 to the atmosphere as all that extra plant matter decays. So both photosynthesis and respiration should get a boost in warmer autumns.
For now, respiration has the edge. Piao reached this conclusion after examining records of the annual cycle of CO2 levels in the atmosphere, sometimes known as the Keeling curve after Charles Keeling, who began logging CO2 in 1958. The curve shows more CO2 in the air during the northern hemisphere’s winter as dying vegetation switches to respiration, and less in summer after photosynthesis resumes. Over the past 25 years, that the winter CO2 build-up has been happening earlier. Luckily, says Piao, for now the autumn trend is more than made up for by a more vigorous photosynthesis during warmer springs. We must hope that continues. “If future autumn warming occurs at a faster rate than in the spring, the ability of northern ecosystems to sequester carbon may be diminished,” warns Piao.
A shifting autumn will, of course, change agriculture round the world. Much of the time, farmers will simply change their practices in response. But along the way, some of the flavours and traditions of autumn could be lost: pumpkins, for instance. Warm temperatures are a boon to pumpkin crops but hot temperatures make them go bust as the plants shut their flowers for the much of the day, keeping pollinators out. High temperatures in the late US summer and early autumn have already been partly blamed for a ruined pumpkin crop in 2013. , with temperatures on the last day of October exceeding 20 °C. As trick-or-treaters sweltered, the country suffered a dreadful pumpkin harvest. This most seasonal of crops could fall foul of climate change.
Even Bonfire Night, which heralds lengthening nights in the UK, is changing. Sparks remembers 5 November as the day British gardeners would pack up for the year, content that their gardens were dead for the winter. “In my childhood in the 1960s, my father burned all the leaves and garden rubbish on Bonfire Night. There was nothing left to fall from the trees.” But nowadays, horse chestnut leaves around his home obscure the view of the firework displays.
Bear in mind, however, that although change may unsettle human traditions, and perhaps provide new annoyances (see “A season of sniffs“), it is not necessarily bad for nature. Species find ways to adapt. Maybe there will be new leaf colours in the fall. Maybe weeds and trees moving north will keep hard-pressed birds and insects in food where the natives falter. Maybe invasive species will continue to find new roles, feeding migratory birds. The old mellifluous synchronies may be dead. But we must hope there will be new ones.
A season of sniffs
The nose knows (Image: James Ross/Millennium Images, UK)
Never mind the demise of red leaves and pumpkins (see main story), some autumnal changes could literally get up your nose.
A study earlier this year by Lynda Hamaoui-Laguel at the Laboratory of Climate and Environmental Sciences in Gif-sur-Yvette, France, forecasts a dramatic lengthening of the European hay-fever season. By mid-century, she found, it could stretch long into the autumn owing to the spread of ragweed – an invasive species from North America to which many people are allergic.
Just as troubling is the prediction that many disease-carrying insects will buzz on for longer, breeding several more generations as the year progresses, says plant ecologist Amanda Gallinat of Boston University. The results are likely to range from plagues of Scottish midges biting their way into October and November to a growing prevalence across the northern hemisphere of mosquito and tick-borne diseases, such as dengue fever, Lyme disease and West Nile virus.
The timing of bird migrations could be an aggravating factor. Birds are a prime target for bloodsucking insects such as mosquitoes. If birds head out on their migrations while it is still warm enough for mosquitoes to flourish, the insects will need other sources of blood.
Mammals, including humans, already get bitten more in late autumn after migrating birds depart, says Gallinat. That is why human infections of West Nile virus are much more prevalent in the autumn. A longer, warmer season will only worsen the situation.
The Monarchs’ reprieve
Migration on hold? (Image: Frederic Larson/San Francisco Chronicle/Polaris/Eyevine)
Autumn may be changing (see main story), but we should not be too nostalgic. Take heart in the unexpected winners – such as the monarch butterflies that famously migrate from the US Midwest to winter roosts in central Mexico. The sight of millions of migrating butterflies arriving to drape themselves over trees is a true wonder of nature.
But the migration has declined by 90 per cent since 1990. Last year, alarmed ecological institutions petitioned the US government to give the monarchs new protection under the US Endangered Species Act. , a herbicide they say is wiping out milkweed plants across the Midwest. Monarchs lay their .
However, David Wagner of the University of Connecticut in Storrs thinks warmer autumns may come to the plant’s rescue by nudging it north into non-agricultural lands in Canada and the northern US states. “This will almost certainly increase the habitat and breeding success of monarch butterflies,” he says.
