EXPERIMENTS ON THE EFFECTS OF WATER AVAILABILITY AND EXCLUSION OF FUNGAL HYPHAE ON NUTRIENT-UPTAKE AND ESTABLISHMENT OF PINUS-SYLVESTRIS SEEDLINGS IN CARPETS OF THE MOSS PLEUROZIUM-SCHREBERI

The feathermoss Pleurozium schreberi has the capacity to suppress tree seedling regeneration in late successional northern boreal forests. W e investigated, through a field experiment in a P. schreberi-dominated old-growth Norway spruce forest in northern Sweden, the effects of th ree precipitation regimes representing the range of natural July preci pitation amounts (28, 112 and 256 mm), and extra influences of tempora ry exclusion of fungal hyphae in senescent shoots of P. schreberi, on seedling establishment, growth and nutrient acquisition of Pinus sylve stris. Seedling establishment of Pinus sylvestris was enhanced by incr eased amounts of artificial precipitation. Seedlings in plots with the lowest precipitation had a much lower mortality and significantly hig her total nutrient content, higher specific absorption rate (SAR) and lower nutrient use efficiency (NUE) compared to seedlings grown in plo ts with the highest precipitation. Seedling nutrient (especially N and P) content was also significantly higher when fungal hyphae were excl uded, and this was most evident in treatments with the intermediate pr ecipitation level. Seedlings in plots without hyphal isolation but wit h intermediate precipitation levels had much lower total contents of N , P and Mg after one growing season than were initially present in the seeds, indicative of a net nutrient loss during the early establishme nt phase. Possible rhizosphere interactions, causing immobilisation of nutrients and seedling nutrient losses are discussed in relation to r esults from greenhouse experiments where no such nutrient losses were found in seedlings. We hypothesize that P. schreberi, in association w ith ericaceous plants and fungal hyphae, is a powerful controller of t ree regeneration from seeds over wide precipitation gradients, due to its inhibition of seedling establishment and strong interference with nutrient acquisition of new seedling cohorts.