Every year, the media reports that the transition between winter and spring is arriving earlier, and that plants are no longer blooming when they used to. But accurate records of the effects of changing temperatures on plant flowering times are patchy at best.
The shortage of available data may not be a problem for much longer. A study published in the Journal of Ecology has shown that the vast collections of dried plant specimens held in museum and botanic garden herbaria around the world can be used to track the relationship between changing temperatures and plant flowering times reliably, even though many of the specimens are more than a hundred years old.
Previous studies have used information from herbarium specimens in combination with meteorological data to investigate the effects of changing temperatures on plant flowering times, and to help predict the effects anthropogenic climate change may have on plant species. However, it was never clear whether the information gathered from dried plant collections was reliable.
Now, a team led by PhD student Karen Robbirt of the Royal Botanic Gardens, Kew has found that herbarium sources are as reliable a guide to the effects of changing temperatures on plant flowering times as more modern datasets, suggesting the 2.5bn specimens held in various institutions around the world could help scientists understand what might happen to plant species’ flowering behaviour as global temperatures rise.
Robbirt and her colleagues had access to a modern data set spanning more than 30 years between 1975 and 2006. This was an accurate yearly record of peak flowering times – when the plants are in full flower – for a particular population of early spider orchids (Ophrys sphegodes).
The team also selected 77 herbarium specimens of O. sphegodes which appeared to have been in full flower when collected and were accurately dated. These specimens had been collected by botanists between 1848 and 1958.
They then gathered data on average spring temperatures (from March to May) between 1848 and 2006 from the UK Meteorological Office, and created two graphs of flowering time against temperature, one using the field-collected data and the other using the collection dates on the herbarium specimens. The patterns seen in the graphs were then compared to see if the effect of temperature on flowering time was consistent between the herbarium samples and the more recent records.
A surprisingly high level of correlation was seen between old and new datasets – both suggested that for every one degree centigrade rise in average spring temperature, flowering occurred six days earlier.
Tony Davy, an ecologist at the University of East Anglia and co-author of the paper, says this is the first time the relationship between herbarium data and more recently collected information has been tested. “It exceeded our best expectations, it was pretty nearly exact,” he says.
Interest in phenology – seasonal phenomena such as flowering time – has increased in recent years in response to worries about climate change, and the effects it may have on ecosystems. Using herbarium records should help scientists predict what the effects of a change in climate could be.
One possibility is that plant flowering times change while the behaviour of their insect pollinators remains the same, or undergoes different changes. For species with highly specific pollinators, such as orchids, this could be disastrous.
Robbirt will now go on to investigate other orchid species and the insects which pollinate them. Davy says this should indicate whether there is a developing asynchrony between these plant species and the pollinators they rely on.
Image: Ophrys sphegodes