Walnut, a tasty and popular species, more and more often escapes from parks and gardens to natural and semi-natural environments, mainly to abandoned fields, according to observations carried out in Poland. In these escapes, walnut is aided by diligent rooks.
Walnuts, whose numbers in run-down fields are growing, drew the attention of Magdalena Lenda from the Institute of Environmental Sciences, Jagiellonian University and the Institute of Nature Conservation PAS in Kraków. "While conducted unrelated field research, I noticed that there are plenty of walnut trees among wild goldenrod. But how did walnut trees appear in the fields, if their mother trees grow in cities or on properties? Walnut seeds are so large that they could not get there by chance" - described Lenda in an interview with PAP .
Scientific investigation has shown that walnuts are brought to the fields by rooks, and to a slightly smaller extent by crows and jackdaws. Rooks and crows tend to hide and stack their food. And they really like walnuts. In one field rooks can bury hundreds of walnuts. And, although they are famous for their memory, they sometimes forget about some hiding places. As a result, some walnuts can later grow in peace.
"Research has shown that to hide the nuts gathered in the gardens, birds fly a few hundred meters, sometimes even a mile, to the surrounding farmland. Here, however, was a surprise. It turned out that birds did not hide walnuts in abandoned fields, but in the still used fields, in ploughed soil or on the balk! So, how do you explain the fact that wild walnuts grow mainly on abandoned agricultural land?" - noted another study participant, Dr. Piotr Skórka from the Institute of Zoology, University of Life Sciences in Poznań.
During their research, the scientists also noticed after the political system transformation, in the early 1990s, a lot of arable land in Poland was abandoned. "There are places where the share of such abandoned fields in the landscape is almost 50 percent. Everything points to the fact that wild walnuts could grow on abandoned fields because they had been buried there by corvids" - said Magdalena Lenda.
"A large number of walnuts growing in abandoned fields in Poland is thus the result of the behaviour of our native birds corvids and ... a consequence of political changes" - emphasized the researcher. Walnuts hidden by birds could easily germinate only when the land ceased to be used by people with heavy equipment such as harrows. Observations confirmed that on abandoned agricultural land walnut is a really frequent visitor, and within a few - dozen years, an abandoned field can turn into a young walnut forest. Age of trees quite accurately corresponds to the time during which the studied fields were abandoned. Seedlings from the 1990s is are now almost 10 m high trees.
Magdalena Lenda and her team checked how frequent is the expansion of walnuts. Tracking down these trees all over Poland, they found them on cropland, abandoned fields, wastelands and in small woods. In some parts of the south have found even small wild walnut coppices. "The farther north you go, the less walnuts you find, because fewer seedlings survive in more severe climates" - said the ecologist from Kraków.
According to the researchers, the walnut has a strong impact on the environment, because it secretes a number of substances into the soil and can inhibit the growth of plants. The multitude of walnuts in the wild suggests that they are becoming a new invasive species in Europe. "What has grown on abandoned land, however, is not worthless" - said Lenda. Wild walnut trees can be a source of wood, and even though they bear fruit a little later than the trees growing in the gardens, the fruits are equally tasty.
PAP - Science and Scholarship in Poland, Anna Ślązak
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