|The Term disturbed ground|
|By Esther Hegt and Dr.Hans Kruijer (National Herbarium Netherlands, Leiden)
A plant growing in its natural habitat has to deal with a large number of biotic and non-biotic factors.
In a forest it’s a natural occurrence in say, once every twenty or thirty years, to have a big storm which blows down many trees. That’s all part of the game and helps the natural rejuvenation of the forest, especially in an old and large forest, where this does not even count as a disturbance. For the overblown trees it is, and a rather disastrous one at that. For the few trees that remain standing it is a disturbance as well, and because his neighbours are no longer there he will get much more light. For plants like foxglove and fern it is a disruption as well, one that offers the opportunity for massive growth.
Ragwort needs an open piece of soil to germinate. A hole made by a worm can’t really be considered a disturbance of the soil; a worm-hill is too small. Whereas a molehill forms a favourable disturbance of the soil. From the perspective of the ragwort, people are nothing but large moles who create disturbances in the vegetation (ploughing, moving of soil when building homes and roads) but also in a more indirect way by letting too many cattle graze in fields and nature preserves. When you look at a field as a whole, at a large scale, molehills are not a real disturbance, because moles are part of the fauna and are essential for forming the soil like worms. But for ragwort molehills are ideal germination grounds.
Ragwort is a native species in Holland, but until a few decades ago the plant was quite rare in parts of Holland (like Drenthe and neighbouring Groningen). There the species were sown and made use of the many recent man-made disturbances. Whether ragwort favors poor soil is doubtful. Drenthe’s soil was very poor and the species was rare there until recently. This does confirm that people disturb the ground and that ragwort can also grow on more fertile soil, provided that the vegetation is open enough and the ground has been burrowed. And again, multiple factors are at work, as plants do not obey the rules that we humans have in mind for them. When it is written in the text books that a plant usually grows on poor soil, that doesn’t mean that a plant will never grow on rich soil. The occurrence of a plant in a certain habitat is determined by many biotic and non-biotic factors. It might be that one or more of these factors are not optimal for the plant (i.e. a soil that is richer than would be ideal for ragwort) but this is compensated for by other factors that are optimal (i.e. soil that has been disturbed).