Response of Hieracium pilosella in Breckland grass-heath to inorganic nutrients

(1) Effects of the addition of inorganic nutrients to Breckland grass heath were investigated in the presence and absence of rabbits. Five treatments (no addition, + N, + NK, + NP, and + NPK) were arranged in two identical latin squares, one grazed and the other ungrazed. Salts were added on eleven occasions over 3.5 years. (2) The density of Hieracium pilosella rosettes was recorded each spring from 1975 to 1979. The corresponding density of flowering and stoloniferous rosettes was recorded each summer. At the end of the experiment, the above-ground standing crop of each species and the litter was determined. (3) The untreated quadrats showed little change. Populations in quadrats that received nutrients declined over the 3 years; the severity of treatment in inducing decline could be ranked: + N < NK < + NP < + NPK. Rabbit grazing ameliorated the effects of nutrient addition and was almost able to offset the effect of nitrogen alone. (4) The proportion of rosettes initiating capitula and stolons was greatly increased by nutrient addition; this was maintained for all four summers under grazing. In the ungrazed plot flowering was stimulated only in the first summer, before the response of competitor species diminished the vigour of H. pilosella. (5) Nutrient addition resulted in large accumulations in the biomass of Festuca ovina, Koeleria macrantha and litter in the ungrazed plot. Most other species were affected adversely. Rabbit grazing prevented biomass accumulation, but the changes in species representation were similar if less marked. Only Astragalus danicus appeared to be unaffected by nutrient addition when grazed. (6) The major influence of inorganic nutrients on populations of Hieracium pilosella was indirect, through stimulation of the growth of its competitors, Festuca ovina and Koeleria macrantha. However, the concomitant direct stimulation of flowering increased the turnover of rosettes and accelerated the population decline. It also supports the proposition that the negatively density-dependent flower initiation response is mediated by the availability of nutrients and therefore competition for them.