Ancient oak trees – individuals more than 400 years old – extend back into the deep and dark recesses of history.

During that time, these senior trees will have been surrounded by huge fluctuations in climate; to say nothing of the monumental changes in human history and how we have modified landscapes.

For those that can read them, the patterns of tree rings can provide a hint of past seasons, since the growing trees lock in valuable data about the climate for every year of their life.

However, that is not the only assistance that oak trees can provide to those researchers looking to understand more about our climate and how it is changing. By observing and charting the changes when individual trees come into leaf or shed their leaves, scientists and climatologists can glean valuable data about fundamental shifts in climate since individual plants respond to the shifting waves of weather and climate patterns from one year to the next.

Meteorologists have often been interested in how the weather and climate have profound impacts on phenology: the study of recurring events in nature and their relationships with climate. And indeed in 1875 the (Royal) Meteorological Society embraced the timing of natural events within the growing interest of phenology.

Dr Debbie Hemming is a Met Office scientist conducting research to improve understanding and modelling of the interactions between vegetation and climate. She said: “Trees and other species are natural data stores. They can be used in so many ways to inform us about the past and present of our changing world.”

With climate change already affecting the world, including the UK, there is a rapidly expanding interest in phenology and how the timing of natural events are responding to climate shifts, like rising temperatures and fluctuations in rainfall patterns.

For 31 years the BAMS Global State of the Climate report has been monitoring climate trends to provide a snapshot of the shifts in global and regional climate. The latest report is published today [25 August 2021] covers 2020.

Using results from phenology studies across the globe, the report states the seasonal cycle of vegetation across the Northern Hemisphere showed a generally earlier spring and later autumn (fall) in 2020 compared to recent decades. These differences were larger across Eurasia than North America.

Debbie Hemming added: “Satellite observations analysed by colleagues at the NASA Ames Research Centre, in the United States, showed that about 55 per cent of the Northern Hemisphere experienced an earlier spring and about 65 per cent a later autumn (fall) in 2020 compared to recent decades.”

One UK study – involving citizen scientists from the Woodland Trust’s Nature Calendar project – has been charting the duration of leaves on the pedunculate oak from first leaf to bare tree. The pedunculate oak – colloquially known as the English oak – actually has a range across much of temperate Europe and Asia Minor, and it is the national tree of several European nations, not solely the UK.

Oak trees came into leaf across the UK earlier in 2020 than in any of the previous 20 years.

The average first ‘leaf date’ for the 2000–09 baseline was 26 April (day 116), and average ‘bare tree’ date was 30 November (day 334), giving a 218-day growing season.

Both events are strongly influenced by temperature; the first leaf date advances by approximately six days for every 1°C increase in mean February–April temperature; and the ‘bare tree’ date is  delayed by approximately three days for every 1°C increase in October temperature.

The year 2020, like 2019, had a very warm spring, and this resulted in the earliest United Kingdom first leaf date in the 20-year series (10 days earlier than the in-situ baseline).

October temperature was similar to recent years, and the bare tree date (note these were predicted from the temperature relationship due to COVID-19 monitoring restrictions) was approximately two days later than the baseline.

Professor Tim Sparks, speaking on behalf of the Woodland Trust, said: “The net result was a United Kingdom “oak season” 12 days longer than the baseline. The earlier spring and later autumn (fall) vegetation was associated with warmer than average temperatures during these seasons. Nature is having to keep pace with rapid changes – perhaps never more rapid.

“However, not all species are changing at the same rate. Whilst some species can take advantage of a warmer climate, for others climate change will be a further threat to their existence.”