1. The field- (vascular plants) and ground- (epigeic bryophytes and lichens) layer vegetation was investigated in four old fertilization experiments in coniferous forest. The experimental sites differed in site quality from poor to intermediate and were situated in the boreal zone in central-northern Sweden. Experimental plots had been fertilized with 120 to 600 kg N ha-1, repeated three to five times at 5-7-year intervals between fertilizations. In normal forestry practice. c. 150 kg N ha-1 is applied two to five times at 5-10-year intervals. 2. Most vascular plants had responded to nitrogen fertilization so as to make the vegetation more similar to that of naturally more fertile sites. Species with their ecological optima on poorer sites than the fertilized site had decreased, and species with their ecological optima on richer sites had increased. Most epigeic bryophytes, on the other hand, had responded negatively to fertilization irrespective of the site quality of the fertilized site. Some 'pioneer' species of vascular plants and bryophytes had responded positively to nitrogen fertilization irrespective of site quality. 3. Evidently most epigeic bryophytes and lichens are susceptible to temporary toxic levels of the fertilizer when applied, but the degree of susceptibility to fertilizers varies among species. Most vascular plants, on the other hand, respond only to the changed competitive relationships associated with the increase in nutrient availability. 4. The number of species was not changed by the treatments, but at one site there was a positive relationship between the degree of species replacement and fertilizer dose.
