The Distribution of Some Woodland Herbs in Relation to the Supply of Nitrogen and Phosphorus in the Soil

One of the earliest experimental studies of the ecological requirements of a single species to be published in The Journal of Ecology was contributed by Olsen and is devoted to an investigation of the distribution of nettle (Urtica dioica) in Danish woodlands in relation to soil conditions (Olsen 1921). U. dioica was one of several plants which had been shown by Molisch (1883) to contain free nitrate in its tissues and this was confirmed by Olsen, who also showed that in sites where nettle colonies were adjacent to vegetation dominated by Deschampsia caespitosa, little or no nitrate could be detected in the leaves of the grass. Using an incubation technique, Olsen demonstrated this difference to be related to the intensity of nitrification in the respective soils, but where Urtica grew luxuriantly there was not only high nitrification, but also a high concentration of most plant nutrients including for example, calcium, magnesium, phosphate and in most cases potassium. Olsen also investigated the growth of Urtica in water culture and demonstrated that the species responds over a wide range to increases in nitrogen supply. In these experiments there was no comparison with the response of such species as Deschampsia caespitosa, so that the claim that Urtica dioica has an unusually high nitrogen requirement may seem unjustified. In spite of this criticism, the conclusion that the natural distribution of U. dioica is primarily controlled by nitrogen supply has found wide acceptance. The distribution of U. dioica in Britain has been summarized by Greig-Smith (1948). The species is widespread on disturbed soils around human habitations, farm buildings and at the foot of walls and hedges, especially along roadsides. It is also frequent in many types of woodland and grows luxuriantly in woods of Alnus glutinosa on alluvial soils and in woods of Fraxinus excelsior and Ulmus glabra in the north and west of Britain. In the limestone dales of the Pennines, Urtica dioica is one of several tall perennial herbaceous species which commonly occur together and form the characteristic vegetation along the foot of cliffs and screes, and on moist ungrazed ledges. In many of the wooded Derbyshire dales, this vegetation of tall herbs occurs in a narrow and discontinuous belt on deep mull along the foot of scree slopes, where the whole of the upper part of the scree is occupied by an almost continuous sward of Mercurialis perennis, or a mixture of this species with either Brachypodium silvaticum or Deschampsia caespitosa. This arrangement is illustrated in Fig. 1 by a transect in Monk's Dale, where a colluvial loam fills the bottom of the dale and is occupied by D. caespitosa. The composition of the three types of vegetation in comparable sites in Cressbrook Dale is summarized in Table 1. Both Urtica dioica and Mercurialis perennis are perennials, which die down in winter and produce a fresh crop of photosynthetic shoots each year. Deschampsia caespitosa