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Expansion of Sphagnum fallax in bogs: striking the balance between N and P availability

Abstract Nitrogen deposition may cause shifts in the Sphagnum species composition of bogs, ultimately affecting the conservation value of these systems. We studied the effects of N and P on the expansion of S. fallax and S. flexuosum in bogs. We related historical census data of S. fallax, S. flexuosum, and four of their accompanying species to changes in N deposition. In addition, we conducted two fertilization experiments with N and P; one at a low deposition site with S. flexuosum and one at two high deposition sites with S. fallax. Finally, we related existing data on capitulum N and P concentrations of S. fallax to its abundance in the field. A relative increase in observed frequency of S. fallax coincided with an historical increase in N deposition in the Netherlands. There was no indication that S. fallax consistently outcompeted one of the other five Sphagnum species; the observed frequency of the Sphagnum species analysed was rather stable with time. The census data on S. flexuosum did not indicate a response to N deposition, but the species expanded at the low N deposition site when extra N was applied. In contrast, the expansion of S. fallax at the high deposition sites was limited by P. Organic nutrient concentrations suggested that when S. fallax can maintain a capitulum N concentration of 7 mg g-1 or higher and a P concentration of 0.7 mg g-1 or higher the species can grow to dominate. We conclude that S. fallax will gradually colonize an increasing number of new habitats in areas with a low, albeit increasing, N deposition, but will only grow to dominate when P supply is adequate. Then, the expansion of S. fallax may lead to ousting of the other Sphagnum species present.