Effects of repeated nitrogen fertilization on the ericaceous shrub, salal (Gaultheria shallon), in two coastal Douglas-fir forests

Abstract We measured the cover of salal (Gaultheria shallon Pursh) in two forests of coastal Douglas-fir (Pseudotsuga menziesii Mirb. Franco) that had received repeated applications of nitrogenous fertilizers. In a trial at the Pack Forest in Washington, USA, salal was eliminated in a plot that had been fertilized with nitrogen alone (1540 kg N ha−1 as ammonium nitrate and urea between 1950 and 1982), but was unchanged in a plot that received phosphorus and sulfur in addition to nitrogen (1082 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. Reductions were less pronounced in plots that received sulfur in addition to nitrogen. In the Pack Forest trial, the cover of snowberry (Symphoricarpos albus (L.) Blake) increased in the plot in which salal was eliminated; in the Parksville trial, no other species became more abundant in the absence of salal. The stem volume and the stem volume increment in each plot, an indirect measure of the amount of shading, was not related to salal cover in the plots. In the Parksville trial, concentrations of sulfur in salal leaves in plots fertilized with at least 600 kg N ha−1 were lower than in control plots. No nutrient imbalances were apparent in salal leaves in fertilized plots in the Pack Forest trial. High concentrations of ammonium and nitrate in the forest floors in fertilized plots may render salal less competitive, or may interfere with ericoid mycorrhizae, contributing to reduced cover of salal in forests receiving repeated applications of nitrogen.

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