Image caption The team examined 77 sheets of early spider orchids, spanning a 111-year period

Plant cuttings in herbariums around the world could hold vital clues to how the natural world will respond to future climate shifts, say researchers.

Until now, reliable long-term data on plants' natural cycles, such as when they come into flower, had been scarce.

A UK team found that plants pressed 150 years ago offered "virtually identical" data to more recent observations.

The findings appear in the Journal of Ecology, published by the British Ecological Society.

"As far as we know, we are the first people ever to show independently that there is this relationship," said co-author Tony Davy, a professor of ecology at the University of East Anglia's (UEA) School of Biological Sciences.

'Winning the jackpot'

The team compared 77 specimens of early spider orchids (Ophrys sphegodes) from herbariums at the Royal Botanic Gardens, Kew, and London's Natural History Museum, with field observations of the species collected in Sussex between 1975 and 2006.

In detail: Early spider orchid Scientific name: Ophrys sphegodes

Rare in the UK, limited to southern counties

Lip of the flower is said to look like a large spider

Can grow to a height of 30cm, but generally reaches 5-15cm

Flowers in early April to early May - a month before the aptly named late spider orchid

Protected by wildlife legislation

Pollinated by a solitary bee in the genus Andrena

Numbers are declining as a result of agriculture and illegal picking

The study found that both the herbarium data-set, which covered a 111-year period from 1848 to 1958, and the more recent field observations showed that for every 1C (1.8F) increase in the average spring temperature, the orchids flowered six days earlier.

"Even we were surprised in the correspondence between the data sets," Professor Davy told BBC News.

"It felt like winning the jackpot - two independent analyses giving exactly the same result."

He added that phenology - the study of natural seasonal phenomena, such as plants' flowering time in spring - had "come into own" comparitively recently as a result of a growing interest in climate change.

"It has become much more important because, first of all, it enables us to predict what the effects of a change in climate are going to be," Professor Davy said.

"Once we know the likely effects on individual species, we are then in a position to try and predict what the disruptive effect on species' interactions might be.

"For example, if a plant species responds at a different rate from that of its pollinating insect, you could - ultimately - see a disruption of a pollination system.

"We may not be able to do much about it, but it does forewarn us."

Simple choice

Professor Davy explained why the early spider orchid was chosen as the species to be examined by the researchers.

"One of our team, Professor Mike Hutchings of Sussex University, had actually recorded peak flowering time in the field for every year for 30 years.

"Really, it was the only species for which we had independent data that we could use in the validation of the herbarium analysis."

He added that the herbarium analysis was carried out by lead author Karen Robbirt, a PhD student at UEA, and co-author Dr David Roberts from the University of Kent.

This involved selecting specimens that had reliable collection dates. Out of a possible 192 samples, more than half were rejected for a number of reasons, including being undated or imprecisely dated.

Plant collectors generally took plants when the species were at their peak flowering, as this provided the best taxonomic reference for the herbarium. This was another consideration for the team when choosing the specimens for their analysis.

Once they had their final 77 specimens, the team then matched this data against the UK Met Office's historical records for the appropriate years.

"What came out of that was a clear finding that flowering was six days earlier for every degree increase in the average spring temperature," observed Professor Davy.

He went on to say that the close correlation between the two sets of data showed that herbarium collections could be used to examine the relationship between phenology and climate when field-based observations were not available.

Whether or not, this new source of data would become a useful tool for scientists "depends on whether people take up this idea", he suggested.

"There are huge collections in different museums and herbariums around the world; it is estimated that there are 2.5bn specimens stored in these collections."