On Competition Between Galium Saxatile L. (G. Hercynicum Weig.) and Galium Sylvestre Poll. (G. Asperum Schreb.) On Different Types of Soil

Abstract

The publication, so long ago as 1865, of Nageli's paper on the conditions of occurrence of species and. varieties within their areas of distribution1 brought into prominence an ecological problem of considerable interest and importance, namely the relation of competition between species, particularly closely related species, to their soil preferences. The striking case of Achillea moschata and A. atrata, particularly their distribution in the Heuthal, a high alpine valley in the Bernina region of the Upper Engadine, at once suggests the investigation of this relation, but definite attempts to elucidate the problem appear to have been lacking. Achillea moschata is a widely distributed species of siliceous soils in the Alps and is reputed calcifuge: A. atrata, on the other hand, is calcicole; and each, on its own soil, is said to suppress the other. But where either of the two is absent altogether from a region the other grows on calcareous and non-calcareous soils alike, though A. atrata is said to colonise non-calcareous soils with less difficulty than that experienced by its congener in colonising calcareous soils. In the Heuthal Nageli found a large block of limestone, barely covered with a thin layer of soil, which had fallen on to an area of non-calcareous schist. On this block a colony of A. moschata had established itself, competition with the other species being excluded. A number of pairs of closely allied species or varieties, both in the Alps and in other parts of Europe, show similar sVrongly contrasted soil preferences, and the relation described by Nageli, each form being bodenstet where both occur and bodenvag where its congener is absent, appears to be common2. In order to elucidate this interesting phenomenon it is evidently first necessary to grow the two species of such a pair in competition on different types of soil and to see exactlv what happens. Of a number of pairs of closelv allied