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