UK phenology effort 'needs more citizen recorders'
Wildlife experts are concerned that a decline in the number of people recording plant behaviour could jeopardise long-running data sets.
Data stretches to 1736 when Robert Marsham, the "father of phenology", began recording seasonal events.
Phenology observes the key moments in plants' annual cycles, such as when they come into leaf or flower, and offer insight into shifts in seasons.
The UK's record, Nature's Calendar, is co-ordinated by the Woodland Trust.
"Our recorder numbers, as a whole, are not bad but what we are finding is that people with really good wildlife skills is the group that is nose-diving at the moment," explained Kate Lewthwaite, Nature's Calendar project manager.
She added that the "expert recorder" group, those who submit 100 or more individual observations, had fallen from the "high hundreds to about 200 people".
"I think one of the reasons is that many of them are quite elderly and we do receive some quite sad letters say that they are too old and they cannot do it anymore," Dr Lewthwaite told BBC News.
"So I would like to issue a challenge to the next generation to see if they can come in and pick up the baton.
"The value of the records is increasing year-on-year, so it would be a real shame if we were unable to continue at that level."
Out of sync
Dr Lewthwaite said the records showed that the UK's growing season was extending, with spring arriving earlier and autumnal events - such as leaf tinting and leaf fall - happening later.
"All of the really early spring events are getting earlier and earlier, such as bud burst on trees, when insects become active. At the other end, generally speaking the leaves are staying on trees longer," she observed.
"You might not think that is a problem if everything is growing longer, but one of the difficulties is that timings may go out of sync.
"Along a food chain, you have all these perfectly adapted timings, but if the leaves are coming out earlier than caterpillars, and earlier than birds can lay their eggs for the chicks to eat the caterpillars, then those natural timings could be threatened."
Dr Lewthwaite said they now had in the region of two million records in total, dating back to the 1700s.
She explained that hundreds of thousands of these records had been recorded since Nature's Calendar, also known as the UK Phenology Network, was established in 2000.
The recorded data was a key resource for academic researchers, she added, with requests for data being received on an almost daily basis.
Simon Toomer, the Forestry Commission's director at Westonbirt, said phenology records at the national arboretum stretched back to the middle of the 19th Century.
"Westonbirt's volunteer phenology group is hugely important to the arboretum and is in fact a group which has been started only in recent years," he told BBC News.
"The new group of volunteers works with the Forestry Commission to record flowering and seasonal colour change in a more systematic approach than ever before.
"The observations of Westonbirt Arboretum's volunteer phenology group show that in many cases, autumn colour is appearing later this year."