In his final days, Prof. Yasuyuki Aono, who was affiliated with Osaka Metropolitan University, eagerly anticipated the arrival of the cherry blossoms. Throughout his career, he meticulously documented the spring blooming dates of cherry trees across Japan, contributing to one of the longest climate records monitoring this seasonal event worldwide.
Drawing from historical data as far back as the 9th century, Aono demonstrated that the blooming of cherry trees has increasingly occurred earlier in recent decades, highlighting a significant indicator of climate change.
Last April, he shared a spreadsheet on social media, marking April 4th as the peak flowering date for the mountain cherry, or Prunus jamasakura, the specific species he studied. Below this entry, he had already noted 2026, but he passed away on August 5th, as confirmed by colleagues who spoke to the Guardian.
Tuna Acisu, a data scientist at Our World in Data, remarked on Aono’s intent to continue his work, saying, “You can very much see that he planned to continue. That made me a little bit emotional.”
Following concerns about the potential discontinuation of this 1,200-year cherry blossom record, Acisu initiated a search last week. This effort bore fruit when a researcher in Japan expressed willingness to formally observe the spring flowering of the mountain cherry.
Acisu stated, “He is consulting the same sources as Prof. Aono to determine this year’s cherry blossom peak bloom and will confirm the date shortly.” The researcher prefers to remain anonymous until the arrangement is finalized.
Acisu and her team first suspected something was amiss when Aono’s university webpage became inactive in January. Discovering his passing and the absence of any successors to continue his observations left a gap in the data just as spring approached.
After Acisu’s outreach to identify a new observer for the cherry blossoms, she received numerous responses. “It’s truly reassuring to know that the dataset will continue,” she expressed, feeling grateful to the new researcher. “I feel very relieved.”
Importantly, Acisu sought a contact in Japan capable of tracking not only the same cherry species but also in the same geographic area: Arashiyama, Kyoto.
While there are other initiatives monitoring cherry tree blooming across Japan, particularly related to its cultural and tourism significance, none focus specifically on this species. For instance, the Japan Weather Association tracks the Somei-yoshino cherry (Prunus x yedoensis), a cultivar introduced in the 19th century.
Aono’s extensive collection of mountain cherry flowering data, spanning over 1,200 years, has lent exceptional value to his research, according to Acisu.
Scientists have identified indicators of climate change through various other sources, including tree rings, plant pigments in seabed sediments, and even temperature records noted by organ tuners in churches in Britain.
Among Aono’s significant findings were the peak flowering dates for 2021 and 2023, which marked the earliest occurrences in the mountain cherry record, falling on the 85th and 84th days of those respective years.
Toshio Katsuki, a dendrologist at the Forestry and Forest Products Research Institute in Ibaraki, emphasized the importance of Aono’s work, noting that continuing to record the spring flowering dates of the same species would yield valuable academic insights.
Richard Primack, a biology professor at Boston University, recounted his meeting with Aono during a trip to Japan in 2006. He learned that Aono had mastered reading archaic Japanese to compile his dataset of mountain cherry blooming dates. In historical archives, Aono uncovered references to cherry blossom festivals in Kyoto, which helped him deduce specific blooming dates for various years.
Despite some gaps in the data, Aono’s earliest record dates back to 812. “It was truly an extraordinary experience,” reflected Primack. “You realize just how dedicated he was.”
In a recent paper, Primack and Katsuki discussed the impact of climate change on the flowering of the Somei-yoshino cherry in southern Japan. Their analysis from 1965 to 2024 indicated that milder winters were causing spring-flowering cherries to exhibit a “bedraggled look” instead of a vibrant display, with many flower buds falling off before they could bloom.
