"Prunus fruticosa", a tetraploid with 2n=32 chromosomes, is thought to be one of the parent species of "Prunus cerasus" (the sour cherry) by way of ancient crosses between it and "Prunus avium" (the wild/sweet cherry) in the areas where the two species overlap. Both species can interbreed with each other, as well as with "Prunus cerasus". "Prunus cerasus" is now a species in its own right having developed beyond a hybrid and stabilized. A recent study of native "Prunus fruticosa" stands in northern Poland finds that it is disappearing there by "genetic erosion" or "disappearance of typical morphological characters". It hybridizes naturally with "Prunus cerasus" to form "Prunus × eminens", and with "Prunus avium" to form "Prunus × stacei". These forest plants are brought into closer contact with "Prunus fruticosa" by the modern disappearance of "contemporaneous sites of the steppe relics" once common in northern Poland, due to forest management since the 18th century, and the planting of stands of "Prunus cerasus", which are more prolific in pollen.