Hanami, the Japanese tradition of contemplating the impermanence of life by gazing at the fleeting beauty of cherry blossoms, has never been more relevant than in the past year. Ironically, it is a custom that contradicts its own meaning. Even in these turbulent times, people all around the world still gather in Hirosaki Park in Kyoto, the Tidal Basin in Washington DC and Regent’s Park in London year after year to gaze once more at the flowering trees. But until how long can this tradition go on?
Hanami dates back to the “Tale of Genji” from the 10th centaury, rumored to be the world’s first novel. The blossoms are so beloved that diarists have chronicled its flowering dates over the past 1,200 years. Recently, a disturbing pattern has emerged. A graph from the Economist shows the full-flowering date dipping earlier towards the year. From 1970, the average date has been April 7th, weeks earlier than centuries ago. This is largely due to the peculiar biology of cherry trees.
Every day, as the sun sweeps the tips of the cherry trees, an invisible clock starts ticking. After a specific number of sunlight hours, plant cells within the tree produce high levels of a FT protein. The protein is responsible for initiating process that help growth. However, it has a crucial property: in the absence of sunlight, it curls up and dies, degrading quickly after the sun has set.
In the long, warm summer days, the molecule is abundant in the last few hours of sunlight, enough to kick-start vital growth processes. Flower initiation takes place in the summer, and they develop through the waning early autumn days. The FT protein produced in darkness in late autumn degrades. It acts as a signal for the tree to shed its leaves and halt production of new buds.
In the winter, sunlight hours and temperatures reach the annual nadir. The cherry blossom trees undergo vernalisation, a dormancy period crucial in ensuring their survival. During the prolonged cold of winter, a floral repressor gene is deactivated by epigenetic silencing. The gene acts as a barrier to flowering and its inactivation enables flowering in the spring.
Cherry trees detect the arrival of spring by changing temperatures. They use a method called the temperature sum: after a certain number of warm days in a row, the blossoms bloom. The precise temperatures and durations vary by plant.
However, as our planet’s temperature becomes warmer and more unstable due to climate change, appropriate vernalisation could be prevented. Due to warmer winters, plants may never go dormant. They could interpret spring as unexpected winter warmth and refuse to open. This is known as the ‘extreme vernalisation response’. Even if the Earth never reaches these temperatures, Cherry trees’ pollinators could still be affected by global warming. The calendars of plans and its pollinators could go out of sync, preventing the existence of the next generation of cherry trees.
The custom of Hanami may become unironic in the near future, as gazing at pastel-pink cherry blossoms in springtime becomes a thing of the past.
