By Richard Black
Environment correspondent, BBC News
Flowers are now emerging about five days earlier than 30-40 years ago
British plants are flowering earlier now than at any time in the last 250 years, according to new analysis.
Researchers stitched together nearly 400,000 first flowering records covering 405 species across the nation.
Writing in the journal Proceedings B, they show that the average first flowering date has been earlier in the last 25 years than in any other period.
Flowering dates are closely linked to temperatures recorded in the Central England Temperature Record.
This is the longest continuous instrumental record of temperatures anywhere in the world, dating back to measurements made in 1659.
During the 1980s and 1990s, the temperatures it registered rose by about 1C, although there is large variability from year to year.
"There is a strong correlation between the flowering index and temperature, so what you see is in large part a reflection of the CET (Central England Temperature Record)," said Richard Smithers, UK conservation adviser at The Woodland Trust and one of the researchers on this project.
"There have been other periods [in the record] when temperatures were warm, but the last 25 years is certainly the period when the index has been earliest," he told BBC News.
Across the plant record, researchers found that a temperature difference between two years of 1C equates to a difference in flowering time of about five days, with some species responding much more than others.
In general, spring-flowering species respond more to temperature changes than those blossoming later.
This is far from being the first study to look at how climatic changes are affecting timings of events in the natural world, the discipline known as phenology.
Researchers have investigated such phenomena as the appearance of blossom, the emergence of insects, and the hatching of birds.
Birds such as chaffinches are also responding to higher temperatures
Earlier this year, another study showed that on average, spring in the UK now begins 11 days earlier in the year than 30 years ago.
However, many sets of records are short-term or fragmented, and many focus on just one species.
The world's longest sequence plots the flowering dates of cherry trees in Kyoto back to the 9th Century, pieced together from diaries and chronicles.
Mr Smithers' team developed a technique for blending fragmented records in a way that takes account of where in the UK the records came from, what length of time they cover, and the differences between the flowering times of different species, from the snowdrops of early Spring to the ivy of Autumn.
It is a kind of nationwide, year-long, species-wide average.
Systematic recording of flowering times began in the UK in 1875 when the Royal Meteorological Society established a national network of observers.
But before and after that date, sightings have also been made both by full-time biologists and part-time enthusiasts, supplemented in recent years by mass-participation projects such as BBC Springwatch.
"What this does is to take all of the data that exists, and meld it all together into one index," said Mr Smithers.
"That means we've been able to use data from all sites from anyone who's ever recorded [a first flowering date], whether that's one person who happened to make a recording in one place or someone who's spent 50 years diligently working on a site."
The UK has a better record than the vast majority of countries, owing largely to the long tradition of amateur naturalism.
This has culminated in the UK Phenology Network, which allows anyone to send in sightings using the web.
But the same research team is aiming to see whether the same techniques can be employed on a larger scale, to give a regional or global picture of nature's response to temperature change.