What Happens to Trees in Autumn: The Hidden Changes You Might Not Notice

Autumnal beech trees in a Gloucestershire woodland displaying golden and copper leaves.

Autumn is a beautiful time of year. As temperatures start to drop, trees begin the process of drawing inwards and winding down for winter, providing us with a marvellous display of colour and life. Here’s a behind-the-scenes look at what goes on as they prepare for their winter state.

Field maple and blackthorn showing orange and gold autumn colour along a field boundary in a natural setting.

#1 Leaf colour

As the days shorten and temperatures start to drop, trees begin to wind down for winter. Photosynthesis slows, and chlorophyll, the green pigment in leaves, fades. As it breaks down, other pigments present all along become visible, giving us the beautiful autumn colours: carotenoids for oranges, yellows, and golds, and tannins for warm browns. Anthocyanins — responsible for reds, crimsons, and purples — are newly produced in autumn from sugars trapped in the leaf when nights are cool and days are sunny. This year, the UK’s record-breaking hot summer followed by mild autumn days and chilly nights has created ideal conditions for anthocyanin production, resulting in a particularly vivid display of autumn colour across our hedgerows and woodlands, from fiery reds and purples to golden yellows and rich browns.

#2 Leaf drop

During the growing season, the hormone auxin keeps leaves attached. As autumn approaches and light levels and temperatures fall, ethylene is released and triggers abscission – the process where the leaf detaches from the tree. As auxin levels decrease and ethylene increases, the leaves gradually let go.

Close-up of Deadman’s Fingers fungus (Xylaria polymorpha) growing from decaying wood on a woodland floor.

#3 Fungi sporulating

Some fungi are perennial (their fruiting bodies appear all year round), while others are annual (visible only at certain times). In the UK, many are annuals that respond to damp, mild conditions — which is why autumn is one of the best times of year to see such a variety. What we see as a toadstool, bracket or puffball is just the fruiting body; the main organism lies hidden underground. Autumn’s ideal conditions allow fungi to sporulate — releasing spores to reproduce and support both the trees and the wider woodland ecosystem.

#4 Preparing for winter

As daylight shortens and temperatures fall, broadleaf trees slow their growth and enter a resting state — often called dormancy. But they’re not truly asleep: underground, roots stay quietly active while the soil remains warm, anchoring the tree, absorbing water, and storing energy for the seasons ahead.

#5 And what about evergreens?

Unlike broadleaf trees, evergreen trees keep their leaves all year — but they, too, enter a quiet resting state. Their waxy, needle-like leaves help conserve water and withstand cold winds, while their internal processes slow right down to save energy. They even produce natural sugars and proteins that act like antifreeze, protecting their cells from freezing damage. On mild, sunny days they can still photosynthesise though, giving them a head start when spring returns.

#6 Bark thickening and protective compounds

In autumn, many trees strengthen their outer defences. Bark tissues thicken, resins harden, and protective compounds form — helping to guard against frost, pests, and disease.

Grey squirrel on an oak tree in autumn, gathering food for winter.

#7 Wildlife partnerships

Squirrels storing acorns, birds fattening up on berries, fungi trading nutrients with tree roots — autumn is a season of exchange. Trees and wildlife work in quiet partnership, each preparing for the colder months ahead.

#8 Seed dispersal and natural regeneration

From jays burying (and forgetting) acorns, to an abundance of birds who eat and disperse the seeds, and wind-scattered ash keys and beech mast underfoot — autumn’s abundance ensures the next generation of woodland life.

#9 The forest floor coming alive

As leaves decompose, mycorrhizal fungi and worms and invertebrates get to work turning leaf litter into rich soil. These are the hidden workers and recyclers of our woodlands.

While the woods may appear to quieten, autumn is anything but still. Every leaf, spore, and root has a role to play in nature’s quiet process of renewal. Perhaps there’s something we can take from this: to notice the smallest steps in each process, or to remember that we, too, deserve rest — just like nature. What will you take away from the trees?