Nature 2016 December

An autumn of leaves?

Driving down Hawridge Hill in early November I was surprised to see furious swirls of dead leaves both falling from the trees and being swept along the road by a healthy breeze. The vast majority were beech tree leaves though other leaves such as oak, ash, horse chestnut, sycamore or field maple are likely companions.

Our obsession with the weather derives from living on an island with a temperate maritime climate in uenced by Atlantic weather systems and the Gulf Stream. The English may enjoy the opportunity to converse endlessly about our weather’s unpredictability but this obsession also arms us with the instincts to sense minute progressions in each season. For instance, in autumn the shortening day lengths trigger plant hormone production. This stimulates irreversible chemical reactions in the leaves that progressively export away from the leaves the vital minerals and other components of chlorophyll and related pigments for storage over winter. What remains are the largely toxic products of the breakdown process which the tree needs to dispose of. Compared to other primates we are able to identify many more variations in reds, oranges and browns. Similarly we can discern the earliest signs of spring, with the merest glimpse of vibrant green as leaves begin to appear. By seeing subtle colour changes it also allows us to sense shorter durations of time more accurately.

Know’st thou not at the fall of the leaf
How the heart feels a languid grief
Laid on it for a covering,
And how sleep seems a goodly thing
In autumn at the fall of the leaf?

By the time these bright green tree leaves appear in April or May they are already almost a year old. The leaves form within in embryonic winter buds at the apex of stems and twigs from June onwards the year before. From July and August these winter buds enlarge and by the autumn next year’s leaves are already fully formed and tightly folded beneath the outer scales of the bud. These buds containing new leaves provide an excellent refuse for overwintering insects and trees employ a variety of methods to protect themselves from attack.

The first trees to show signs of the autumn onset are horse chestnuts, famous for their red sticky buds that harden like a resin and provide an almost impregnable barrier to invading bugs. For other trees a chemical in the scales and leaves deters invaders or might be toxic to moth caterpillars which hatch from beneath the scales. Another pest is the larva of gall wasps which can penetrate the outer scales using a hypodermic syringe-like ovipositor to lay eggs. When the larva hatches it produces a hormone which stimulates the tree to produce gall within which the larva can safely develop. In short both tree and pest engage in a sophisticated form of chemical warfare.

And how the swift beat of the brain
Falters because it is in vain,
In autumn at the fall of the leaf
Knowest thou not? and how the chief
Of joys seems not to suffer pain?

As November marches on there are other signs of the looming crossover period for local wildlife, from the period of high activity during the warm summer to the cooler but still productive early autumn and the brisk sprint to avoid demise as single digit temperatures become the norm during late November or early December.

A combination of shortages in certain foodstuffs plus reduced day length and temperatures stimulates a break in the lifecycle of insects. This is called a diapause – the requirement of a defined period of cold to ensure continuation of the lifecycle. During October or early November young queen wasps emerge from the papier mâché nests that once sustained over 6,000 workers. The queens have been reared on high octane food prepared by the last brood of workers. Once they’ve left the nests they can be seen topping-up on high energy drinks from the final crop of now overripe blackberries and the super-rich nectar from autumn flowering plants such as ivy, gorse and hellebore. Then, instead of foraging it seeks out a sheltered location, an abandoned mouse hole, hollowed out tree trunk and, as some of you will have experienced, an insulated loft or cooler room. Once wasps select a suitable location they will hibernate. Some may construct a golf-ball-sized abode within which they rest up. This affords them extra protection from their enemies, be they insectivorous mammals or spiders.

Whereas for wasps the break is at the adult stage, for other insects the break is set to occur at other lifecycle phases. In butterflies, the purple hairstreak overwinters as eggs, meadow browns and common blues as caterpillars, orange tips and green hairstreaks as pupae, and small tortoiseshells and brimstones as adults.

For hibernating mammals (hedgehogs, bats and dormice) it is the extending night time which triggers hormone production. This switches dietary choices to favour increased fat consumption. Reduced food supply affects their body temperature, resulting in the onset of hibernation.

Know’st thou not at the fall of the leaf
How the soul feels like a dried sheaf
Bound up at length for harvesting,
And how death seems a comely thing
In autumn at the fall of the leaf?

In the 18th century Gilbert White, the first naturalist to observe and record wildlife activities in detail, thought swallows and martins, who he noted disappeared in winter, buried themselves in river mud. Until the end of the 19th century their disappearance remained a mystery. For birds, well some at least, we know migration is an option to avoid unsurviveable conditions. How birds choose and undertake migration remains largely unsolved.

One interesting feature that applies to some of our familiar species is that not all birds of a species known to migrate will do so out of a local population. For example, our chaffinches, starlings, jays, robins, blue and great tits are augmented in winter by others migrating from more northerly altitudes. Those arriving are seeking more reliable food supplies and fleeing seasonally unsavoury climatic conditions.

Invertebrates, such as earthworms, slugs and snails have clever strategies to avoid the damaging conditions of winter. In each case their survival is down to seeking out sheltered habitats and a copious quantity of mucus-based slime which is used by snails to seal them inside their shells. Earthworms bury themselves deeper underground and construct a mucus sphere or egg.

And what about all those leaves once they have fallen? If it were not for the aforementioned snails and earthworms we would long since have suffocated under tons of leaves.

‘An autumn of leaves’ is the collective noun to describe a ground-covering carpet of leaves.

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