Amazing Nature

Warm Winters Bring Early Blossoming of Spring Flowers

April 2020
Author: Green Power
Blossom of Prunus campanulate
Appreciating cherry blossom is a popular tourist activity. In Japan, every year there is an official forecast of blossoming time. (The photograph shows Prunus campanulate in Hong Kong.)

The arrival of spring usually means the nature thrives and flowers blossom. However, in recent years, there have been worldwide records of early flowering of spring plants - species that generally begin flowering in March and April are in full bloom as early as January, when winter is still in season. When the weather is warmer, the flowers are observed to bloom earlier. The exact relationship between flowering time and the weather, and how the weather affects the flowering mechanism, has long been on the research agenda of scientists and botanists.

Early in the 20th century, scientists knew that plants can sense the weather and the length of daylight, and in turn decide the most appropriate flowering time, with an unknown mechanism. In 2010, a research team at the Trinity College, Dublin, Ireland, discovered that the key to the timing of flowering is a gene named "Apetala1 (AP1)".

The flowering "trigger"

The research team took Thale Cress (Arabidopsis thaliana) of the Brassicaceae family as a model organism to study the plant genome. They found that blossoming Thale Cress had an active AP1 gene. But if the AP1 gene is inactive, the Thale Cress plant will only thrive with leaves but not flowers, or only a few flowers bloom. The AP1 gene is like a switch that triggers flowering.

Different species have different flowering periods. The timing of flowering depends on a series of environmental factors including weather, temperature, sunshine, length of daylight, rainfall and humidity. The plant will transmit environmental information to the AP1 gene. When the environmental factors are favourable, the AP1 gene will be activated. It will release various proteins that in turn activate over 1,000 growth genes that control physiological changes to the plant, which include sending signals to the meristem at the growing part to stop producing leaves and start to develop flowers instead. However, there is still no definitive conclusion on how the plant transmits environmental information to the AP1 gene.

“Double mechanisms” of flowering

Botanists have observed and investigated how the flowering time changes with weather and temperature, as the blossoming of flowers can directly affect agriculture, horticulture and even tourism. Take the renowned sakura, i.e. cherry blossom in Japan, as an example: botanists found that the past flowering dates were associated with weather changes. By studying the relationship, botanists can forecast the flowering time, as a convenient reference for visitors.

Academics have pointed out that plants have a “double mechanism” to “calculate” the flowering time: the chilling requirement and the "Growing Degree Hour" requirement. The former refers to the accumulated cold hours, while the latter refers to the accumulated warm hours. Every species has its own flowering criteria. When both the numbers of cold and warm hours have reached the requirement, it indicates that winter has gone and spring has come. The plant will break its dormancy and start to develop flowers. This clever double mechanism ensures that the plant is protected from temporary weather changes. For instance, it helps safeguard against the plant blooming during a brief warm period in winter, when the flowers will be damaged by the cold when the temperature drops again.

A threat to biodiversity

Climate change has been shown to impact flowering time. The UK Centre for Ecology and Hydrology collected flowering data of 405 species over the past 250 years, to study the relationship between flowering dates and temperature. It turns out that flowering is 5 days earlier for every 1 degree Celsius increase in temperature.

Red Azalea Rhododendron simsii
Red Azalea (Rhododendron simsii) (above) and African Tulip Tree (Spathodea campanulata) (below) are common spring flowering plants in urban areas of Hong Kong.
African Tulip Tree Spathodea campanulata

In recent years, cherry blossoms in New York and Washington began appearing in January because of the unusually warm winter weather. In Hong Kong, there are records of Azaleas (Rhododendron spp.), Tree Cotton (Bombax ceiba), African Tulip Tree (Spathodea campanulata) and Camel's Foot Tree (Bauhinia variegate) flowering as early as December and January in recent years, while their usual flowering periods are from March to May. This phenomenon aligns with the climate data from the Hong Kong Observatory: 2019 was the warmest year on record since 1884. The annual average temperature was 24.5˚C, which is 1.2˚C higher than normal. January 2020 was also the warmest January on record, with an average temperature of 18.6˚C, which is 2.3˚C higher than normal. With the influence of the warm winters, Hong Kong may see frequent early blossoming of spring flowers in the years to come.

Trees with flowers
Flowers are important food sources for various animals. Disorderly flowering times will bring ecological chaos.

The disorderly flowering time will upset the ecological interactions between different species. It could threaten organisms that rely on each other, and potentially threaten biodiversity on a large scale. If the flowering time shifts, a plant’s pollinators (such as butterflies and bees) may miss the feeding time and thus their reproduction and survival will be affected. On the other hand, when pollinators’ populations decrease, propagation of plants is also affected. The impact may be profound, as the whole food chain and ecosystem could be affected eventually. In the long run, climate change will increase the risk of species extinction, reduce crop yields, and threaten economic activities and even survival of the worldwide population.

Flowering Peach Prunus persica ready for sale
Flowering Peach (Prunus persica) is a popular Chinese New Year flower in Hong Kong. The early blossoming in warm winters has caused economic losses for farmers.