Plants are blooming earlier than ever, and here’s how they sense the seasons

Plants are blooming earlier than ever, and here’s how they sense the seasons

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Credit: Pixabay/CC0 Public Domain

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Credit: Pixabay/CC0 Public Domain

Hedges in mid-February may traditionally have appeared white with snow; This year the white was the work of the blackthorn blossoms – a harbinger of spring. Although it is a welcome sign after a rainy and dreary winter, early flowering brings concern to experienced season observers. I wondered does this plant always bloom in mid-February or is something changing?

Fortunately, the science of recording and understanding seasonal events, or phenology, has a long history in Britain. Robert Marsham, the 18th-century naturalist, kept records of the appearance of flowers, birds and insects in his village in Norfolk as early as 1736. Marsham’s descendants continued recording until 1958. The Woodland Trust maintains the tradition through the Nature’s Calendar, a planned program of During which members of the public are invited to record various seasonal events.

Detailed analysis of nearly half a million plant records conducted by scientists in 2022 showed that when all species are considered together, the average flowering time in the UK has advanced by a month over the past 40 years. There was a difference between species. Hawthorn, a common hedgerow plant, generally blooms 13 days earlier than it did in the early 1980s while the horse chestnut tree’s flowers appear ten days earlier.

The climate has warmed rapidly since the 1980s. By flowering earlier, plants realize that winters are becoming shorter and milder. They sense that the days are getting warmer and change their spring development in a way similar to how humans feel warmth on their skin and therefore go out with fewer layers of clothing. The exact mechanisms for detecting these signals vary between plants and animals, but both respond to the climate as it changes.

Detect light and heat without eyes and skin

Plants detect shortening autumn days using a pigment called phytochrome that is particularly sensitive to wavelengths in the red region of the electromagnetic spectrum. The long autumn nights change the quality of this red light. While this subtle shift eludes humans (our eyes are not sensitive to this part of the spectrum), a plant can detect this shift and begin to change.

Just as autumn can cause a decrease in the level of serotonin in our blood, a plant that has sensed the approach of winter will increase the production of a hormone called abscisic acid. This has multiple effects. In deciduous trees, branches stop growing and grow winter-hardy buds that are able to survive frost, snow, and leaf fall.

Growth in spring is determined by similar stimuli as light length and temperature, but temperature usually has the most important role. If plants care only about light, they run the risk of starting to grow when killing frost is still a threat or losing good growing time on mild early spring days. Temperature detection determines when spring flowers will appear. This is why the phenomenon of global warming shows the early appearance of these flowers.

It is not fully understood how plants detect temperature. Some of them may be due to the breakdown of growth-inhibiting hormone in its cells when the air drops below a certain temperature, which in turn allows growth hormone to increase.

While humans have nerves in their skin to detect temperature, plants likely rely on pigments, although the mechanism is not fully understood. Heat is part of the same electromagnetic spectrum as phytochrome, so this pigment is likely involved. Whatever the mechanisms responsible for initiating growth, temperature also determines how quickly plants grow.

Flowers and pollinators are out of sync

Insect pollinators, such as bees, must synchronize their life cycles so that they are on the wing when the flowers they feed on appear. The timing of their emergence in winter is also determined by the effects of temperature, day length, and hormones.

Evolution over many generations of pollinators has established a close link between the appearance of flowers and the emergence of their pollinators. If the emergence of flowers and pollinators is not synchronized, insects will not have nectar and plants will not be fertilized.

A similar link exists between the appearance of leaves and the herbivorous insects that graze on them. The speed of climate change and subtle differences in how the two groups respond threaten to break this synchronization, with dire consequences for both sides.

A large study by German scientists, looking at when flowers and their pollinators appeared between 1980 and 2020, found a complex picture. Both responded to climate change by flowering and emerging earlier, but plants made a greater shift.

There was variation among insect groups, and bees and butterflies metamorphosed in synchrony with plants, but this was not observed in hoverflies. There was also a difference between the types of these insects.

Even when plants and their insects change timing synchronously, the next stage of the food chain may not be so flexible. Oak moth larvae feed on oak leaves. This in turn is the primary food for chicks of birds such as blue tits and older flycatchers. The chicks hatched at about the same time, while the acorn leaves and caterpillars appeared early and remain in sync until now. But for how long?

Blackthorn flowers remain a welcome relief from winter and a sign that spring is on its way. But it is also a sign of climate change: an unfolding experiment around the timing and synchronization of plants and animals – and the complex food chains of which they are part.

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