EFFECTS OF REPEATED NITROGEN-FERTILIZATION ON THE ERICACEOUS SHRUB, SALAL (GAULTHERIA SHALLON), IN 2 COASTAL DOUGLAS-FIR FORESTS

We measured the cover of salal (Gaultheria shallon Pursh) in two fores ts of coastal Douglas-fir (Pseudotsuga menziesii Mirb. France) that ha d received repeated applications of nitrogenous fertilizers. In a tria l at the Pack Forest in Washington, USA, salal was eliminated in a plo t that had been fertilized with nitrogen alone (1540 kg N ha(-1) as am monium nitrate and urea between 1950 and 1982), but was unchanged in a plot that received phosphorus and sulfur in addition to nitrogen (108 2 kg N ha(-1)). In a trial near Parksville, B.C., Canada, salal cover was reduced with increasing amounts of nitrogen, and was eliminated in plots that received 600 kg N ha(-1) as urea in three applications. Re ductions were less pronounced in plots that received sulfur in additio n to nitrogen. In the Pack Forest trial, the cover of snowberry (Symph oricarpos albus (L.) Blake) increased in the plot in which salal was e liminated; in the Parksville trial, no other species became more abund ant in the absence of salal. The stem volume and the stem volume incre ment in each plot, an indirect measure of the amount of shading, was n ot related to salal cover in the plots. In the Parksville trial, conce ntrations of sulfur in salal leaves in plots fertilized with at least 600 kg N ha(-1) were lower than in control plots. No nutrient imbalanc es were apparent in salal leaves in fertilized plots in the Pack Fores t trial. High concentrations of ammonium and nitrate in the forest flo ors in fertilized plots may render salal less competitive, or may inte rfere with ericoid mycorrhizae, contributing to reduced cover of salal in forests receiving repeated applications of nitrogen.