Flowers picked up to 150 years ago in Victorian England show that old collections of pressed plants around the world can help the study of climate change, scientists said.
Ecologists compared samples of early spider orchids, held in collections with notes showing the exact day in spring when they were picked in southern England from 1848-1958, and dates when the same flower blossomed in the wild from 1975-2006.
"Warmer years were associated with earlier flowering. In both cases flowering was advanced by about six days per 1 degree Celsius (1.8 Fahrenheit) rise in average spring temperature," they wrote in the Journal of Ecology after cross-checking with local temperature records.
The match between higher temperatures and quicker flowering for both old and modern orchids showed for the first time that botanical collections could be a reliable source to study climate, even if temperature records were lacking, they said.
Vast numbers of specimens of plants and animals are in collections around the world, some of them dating back 250 years and long before there were reliable temperature records in many nations.
"It potentially opens up new uses for ... specimens - this could provide us with long-term data about climate," said Anthony Davy, a professor at the University of East Anglia who was a co-author of the study led by colleague Karen Robbirt.
The UN panel of climate scientists said in a 2007 report that average world temperatures rose 0.7 degree Celsius (1.3 F) over the 19th century, mainly in recent decades due to a build-up of greenhouse gases from burning fossil fuels.
Tree rings are among biological indicators of climate in past centuries, caused by natural swings. Manmade global warming is very likely to be the dominant cause of warming in the past half-century, according to the UN panel.
The 77 pressed orchids, picked when in full bloom, had meticulous records of dates and sites. Early spider orchids have greenish petals and a purple-brown part which looks like the back of a spider.
Davy told Reuters that spring temperatures were the main factor determining flowering times for orchids - rather than the length of daylight or the changing availability of nutrients.
He said one issue for future study was whether climate change might bring a mismatch between the appearance of flowers and insects vital for their pollination. Bees, for instance, might not be around when fruit trees are in bloom.