Early flowering plants in European forests now start the flowering season on average a week earlier than they did a hundred years ago. This is evidenced by herbarium evidence, such as Dr. Franziska Willems and Professor Oliver Bossdorf from the Institute for Evolution and Ecology at the University of Tübingen, together with Professor JF Scheepens from the Goethe University in Frankfurt. The research team used the collected data from herbarium specimens from more than a century for a newly developed method of geographic-spatial modelling. The team was also able to prove that the earlier flowering time of wild plants is related to global warming. The study has now been published in the journal New Phytologist .
Wood anemone, woodruff, lungwort and spring vetchling bloom early in the year in the undergrowth of the forest. “They use a critical time window for the flowering period, before the deciduous trees sprout their leaves and shade the undergrowth,” explains Franziska Willems. When the temperatures rise, the leaf buds of the trees tend to open earlier, and the early bloomers would also have to adapt to this. “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.”
witnesses from centuries past
Herbaria, as collections of pressed and dried plants, cover long periods of time and large regions. “Many go back 200 years, hundreds of millions of documents are stored worldwide,” says Oliver Bossdorf. “Plants are usually collected when they bloom. The collection date and place are noted on the herbarium sheets. This results in a precise snapshot,” says the researcher.
For the study, the research team examined more than 6,000 herbarium specimens of 20 early flowering species collected across Europe in order to derive shifts in phenology, i.e. the seasonal development rhythms, from the collected data. To properly understand the importance of geographic distribution when studying 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 rhythm of early bloomers and the extent of shifts in response to climate change varies not only between different plant species, but also across different regions,” says Willems.
On average, plants such as hayberries, wild garlic and wood sorrel flowered more than six days earlier than at the beginning of the last century. These changes correlated closely with warmer spring temperatures. “The flowering time shifted forward by 3.6 days for every degree Celsius warming,” says Bossdorf. The spatial modeling showed that in some parts of Europe the plants flowered earlier than expected, but in some also later. “In small-scale studies, the result would have remained unclear. The connection between the postponed flowering period and the rising temperatures only becomes clear when looking at the big picture.”
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