European forests have been long regarded as invasion-resistant; however, recent findings suggest that invasive alien plant species increasingly colonise this ecosystem. In a new study published by Journal of Ecology, researchers from the Polish Academy of Sciences, Nebraska-Lincoln University and Jagiellonian University have analysed the mechanisms of invasion of alien walnut Juglans regia in over hundred forests across southern Poland.

A past study (Lenda et al., 2012) showed that invasion of J. regia (walnut) into Central European farmland is a complex interaction between humans planting trees, land abandonment and bird dispersal. Walnut was originally introduced to Europe from Persia by monks and has been planted around human settlements in Central Europe for more than 700 years. However, due to the political transformation in 1989 when large areas of agricultural land became abandoned in Central Europe, only in the last 30 years have we seen walnut invade areas outside of gardens and plantations. The study demonstrated that a native corvid, the rook Corvus frugilegus, cached walnut seeds in agricultural fields which if abandoned, enabled the seeds to grow freely. It takes several years for these walnuts to become mature seed-producing trees.

Fig 1: Currently, alien walnuts are abundant in abandoned farmland (Photo: Piotr Skórka).

In this new study published in Journal of Ecology, we show that once mature, these trees are now a source for seed dispersing animals. Among several animal species collecting and dispersing walnut seeds there is the jay Garrulus glandarius. This is another native corvid that harvest seeds both from walnuts growing in human settlements and abandoned fields, and transport them into forests. Of course, there are other animals dispersing walnut seeds as well, such as the red squirrel Sciurus vulgaris, but they are less abundant.

Fig 2: Jay Garrulus glandarius is major disperser of walnut seeds into forests (Photo: Rafał Mencel).

Our research confirmed that walnuts growing in human settlements and abandoned fields are seed sources for animals transporting them to different habitats. However, we also performed experiments which showed there is another dispersal pathway – seeds hidden by rooks in sloping agricultural fields can be passively transported to forest edges during rainy weather conditions. See figure 3 for more information.

Fig 3: The invasion of Juglans regia Central European forests

Results of our research have important consequences for understanding invasion of alien trees. Walnut invasion is a multifaceted process with unexpected linkages among the invasive plant, native dispersers, land management and topography that together can cause cascading changes in ecosystems. These changes may occur at different times and spaces, indicating that political transformations can have long-lasting ecological aftermaths that instead of fading, increase over years and threaten native ecosystems.

Magdalena Lenda, Polish Academy of Sciences

Read the full paper: Cascading effects of changes in land use on the invasion of the walnut Juglans regia in forest ecosystems