Forest plants now bloom a week earlier than a century ago

Early-flowering plants in European forests now begin their flowering season on average a week earlier than a hundred years ago. This is reflected in the herbarium specimens, as discovered by Dr. Franziska Willems and Professor Oliver Bossdorf of the Institute of Evolution and Ecology at the University of Tübingen, as well as Professor JF Scheepens of the Goethe University Frankfurt. The research team used herbarium specimen collection dates over a century old for a new method of geospatial modeling. It also allowed the team to prove that the early flowering of wild plants is linked to global warming. The study has just been published in the journal New Phytologist.

Wood anemones, woodruff, lungwort, and spring peas bloom early in the year in the forest understory. “They use a critical window of opportunity to flower before the leaves of deciduous trees grow and shade the understory,” says Franziska Willems. As temperatures rise, leaf buds tend to open earlier, and early bloomers have also had to adapt to this, she says. “However, they run the risk of their flowers being damaged by late frost. They also cannot do without pollinating insects, which must be active at the time of flowering.

Witnesses of previous centuries

Herbaria – collections of pressed and dried plants – cover large areas and especially long periods of time. “Many date back 200 years, and hundreds of millions of specimens are stored worldwide,” says Oliver Bossdorf. “Plants are usually collected when they are in flower, and the date and place of collection are noted on the sheets of the herbarium. This provides an accurate snapshot,” says Bossdorf.

For the study, the research team examined more than 6,000 herbarium specimens of 20 early-flowering species collected across Europe to map changes in phenology, or seasonal developmental rhythms, from these data. To better understand the importance of geographic distribution in studying phenology, the team created models of flowering times that included geographic information and compared them to models without that spatial data. The results were clear: “The annual rhythms of early blooms and the magnitude of their changes in response to climate change vary not only between different plant species, but also between different regions,” says Willems. “For a robust study of phenological changes associated with climate change, you need a large-scale, long-term perspective.” Until now, she says, these studies have often been geographically limited.

The study showed that, on average, plants such as bilberry, wild garlic and wood sorrel were flowering more than six days earlier than at the turn of the last century. These changes were closely correlated with warmer spring temperatures. “Flowering time advanced by 3.6 days per degree Celsius warming,” says Bossdorf. Spatial modeling showed that plants were flowering earlier than expected in some parts of Europe, but later in others. “With small-scale studies, the result would have remained unclear. The relationship between the forward shift in the flowering period and rising temperatures only becomes clear in the big picture.