Wood anemone, woodruff, lungwort and spring vetch: Early flowering plants in European forests start their season on average a week earlier today than they did a hundred years ago. This is evidenced by herbarium specimens, as German scientists discovered. They used data collected from over a century ago for a newly developed method of geospatial modeling. Franziska Willems and Oliver Bossdorf of the Institute for Evolution and Ecology at the University of Tübingen and JF Scheepens of the Goethe University in Frankfurt were also able to prove that the early flowering of wild plants is linked to global warming. The study has now been published in the journal New Phytologist.
Cold start with hazards
The first plants bloom at the beginning of the year in the undergrowth of the forest. “They use a critical time window for the flowering period, before the deciduous trees bud and shade the undergrowth,” explains Willems. When temperatures rise, tree leaf buds tend to open earlier, and early blooms would also have to adapt to that. “However, they run the risk that their open flowers will be damaged by late frosts. In addition, they cannot do without pollinating insects, which must already be active at the time of flowering.”
Dried flowers are accurate snapshots
Herbariums, as collections of pressed and dried plants, span long periods of time and large regions. “Many date back 200 years, hundreds of millions of documents are stored all over the world,” says Bossdorf. “Plants are usually collected when they bloom. The date and place of collection are noted on the herbarium sheets. This results in an accurate snapshot,” says the researcher.
More than 6,000 herbarium specimens evaluated in the model
For the study, the research team examined more than 6,000 herbarium specimens of twenty early flower species collected across Europe to derive changes in phenology, i.e. seasonal developmental rhythms, from the data collected. To properly understand the importance of geographic distribution in the study of phenology, the team built models of flowering times that included geographic information and compared them to models without spatial data.
The result was clear: “The annual rate of early flowering and the extent of change in response to climate change varies not only between different plant species, but also between different regions,” says Willems. “Robust studies of climate change-related phenological changes require a large-scale, long-term perspective.” Until now, these studies have often only been carried out geographically.
Early flowering by more than six days
On average, plants like hay, wild garlic and sorrel flowered more than six days earlier than at the beginning of the last century. These changes correlated closely with warmer spring temperatures. “Flowering time has advanced by 3.6 days for every degree Celsius of warming,” says Bossdorf.
Spatial modeling showed that in some parts of Europe plants flowered earlier than expected, but in some also later. “In small-scale studies, the outcome would have remained uncertain. The connection between the delayed flowering period and rising temperatures is only clear when looking at the big picture.