Tour of Flanders video footage shows climate change impact on trees

Predicting how the timing of cyclic life history events, such as leafing and flowering, respond to climate change is of paramount importance due to the cascading impacts of vegetation phenology on species and ecosystem fitness. However, progress of this field is hampered by the relative scarcity, and geographic and phylogenetic bias, of longterm phenology datasets.

By analyzing nearly four decades of archive footage from the cycling Tour of Flanders, researchers from Ghent University have been able to detect climate change impacts on trees. Focusing on trees and shrubs growing around recognisable climbs and other landmarks along the route of this major annual road cycling race in Belgium, the colleagues looked at video footage from 1981 to 2016 obtained by a Flemish broadcaster. They visually estimated how many leaves and flowers were present on the day of the course (usually in early April) and linked their scores to climate data. 

They found that the trees had advanced the timing of leafing and flowering in response to recent temperature changes. Before 1990, almost no trees had grown leaves at the time of the spring race. After that year, more and more trees visible in the television footage, in particular magnolia, hawthorn, hornbeam and birch trees, were already in full leaf. These shifts were most strongly related to warmer average temperatures in the area, which have increased by 1.5°C since 1980.

Early-leafing trees can be good news for some species as they grow faster and produce more wood. However, their leaves also cast shadows. When trees flush earlier in the year, they shadow for a longer period of time, affecting other animals and plants, and even whole ecosystems. Some of the flowers growing under these trees may not be able to receive enough sunlight to bloom. As a result, insects can go without nectar and may struggle to find enough spots to sunbathe.

Phenology (the study of natural phenomena that recur periodically such as leafing and flowering) is mostly based on long-term observations and repeat photography, with data often being biased towards common species or geographical regions. In this study, archive footage allowed the researchers to use previously unexploited records of twelve tree species in the Flanders region in order to build long-term datasets of phenological responses.

Our method could also be used to collect data on other aspects important for ecological or evolutionary research, such as tree health, water levels in rivers and lakes, and the spread of invasive species. Only by compiling data from the past will we be able to predict the future effects of climate change on species and ecosystems.