Oecologia (Berlin) (1987) 74:310-315 Oecologia 9 Springer-Verlag1987 The role of mosses in the phosphorus cycling of an Alaskan black spruce forest F.S. Chapin, III 1, W.C. Oechel 2, K. Van Cleve 1, and W. Lawrence 2' * Institute of Arctic Biology, University of Alaska, Fairbanks, AK 99775-0180, USA 2 Systems Ecology Research Group, San Diego State University, San Diego, CA 92115, USA Summary. Mosses account for 75% of the annual phospho- rus accumulation in aboveground parts of an Alaskan black spruce forest, although they comprise only 17% of the phosphorus pool in aboveground vegetation. Sphagnum subsecundum and feathermosses (Hylocomium splendens and Pleurozium schreberi) have a higher capacity to absorb phosphate than do the fine roots of black spruce (Picea mariana) that are situated beneath the moss layer. In three of the four moss species studied, phosphate absorption ca- pacity increases with increasing age of green tissue and de- creases with increasing age of brown tissue. In the two feathermosses, which acquire moisture primarily from the air, and in Sphagnum, phosphate absorption is more rapid in green than in brown tissue. In contrast, the endohydric moss Polytrichum commune, which transports water through stem tissue from soil, absorbs phosphate most rap- idly from stems in mineral soil. Two treatments designed to reduce activity of mycorrhizae (cutting of roots extending beneath the moss carpet or application to the moss surface of a fungicide that kills mycorrhizal hyphae) tended to in- crease phosphate retention by mosses and reduce phosphate transfer out of the experimental plots. This suggests that mycorrhizae are an important avenue of phosphorus move- ment out of the moss carpet and a means by which the black spruce competes with the overlying mosses for nu- trients. Key words: Mosses - Mycorrhizae - Phosphorus cycling - Picea mariana In the boreal forest tree growth is strongly limited by nu- trient availability (Weetman 1971; Van Cleve and Zasada 1976; Chapin et al. 1986). Black spruce forests appear ex- treme in this respect (Weetman 1971). For example, in inte- rior Alaska, tree productivity and phosphorus availability are lower in black spruce than in any other forest type, because low soil temperature restricts rates of decomposi- tion and nutrient cycling (Van Cleve et al. 1983). However, these black spruce forests are characterized by a dense car- pet of mosses 10-20 cm thick (Weetman and Timmer 1967; Busby et al. 1978; Viereck et al. 1983) that has an annual production equal to that of the black spruce overstory (Skre and Oechel 1979). In contrast to the trees (Weetman 1971), * Current address: Center for Energy and Environment Research, GPO Box 3682, San Juan, Puerto Rico 00936, USA Offprint requests to: F.S. Chapin III growth of the dominant moss species is not nutrient-limited (Skre and Oechel 1979). This raises questions about how mosses are so effective in acquiring nutrients and what im- pact they may have upon nutrient supply to the forest over- story. Mosses readily accumulate cations by cation exchange (Clymo /963; Ruhling and Tyler 1970), but we are aware of only one study (Wells and Richardson 1985) on absorp- tion of phosphate or other anions by mosses in controlled experiments (see Proctor 1981; Brown 1982). The study by Wells and Richardson (1985) demonstrated that Hyloco- mium actively absorbs phosphate by the same carriers that accumulate the pollutants, arsenate and selenite. Mosses play an important role in nutrient cycling in many forest ecosystems, because they intercept nutrients from litter and throughfall before nutrients return to the rooting zone (Tamm 1953, 1964; Weetman and Timmer 1967; Rieley et al. 1979; Binkley and Graham 1981). How- ever, their importance in nutrient uptake, accumulation, and cycling relative to the vascular vegetation has not been documented. In Alaskan black spruce forests, trees are par- ticularly dependent upon recycling of nutrients within the ecosystem, because most mineral soil is permanently frozen, and there are no spruce roots in the shallow layer of mineral soil that thaws briefly during summer (Tryon and Chapin 1983). Because all spruce roots are situated in the organic mat between the mosses and mineral soil (Tryon and Cha- pin 1983), it is important to know the extent to which nu- trients move through the moss carpet to the spruce rooting zone. Here we report on the phosphate uptake potential of mosses in an Alaskan black spruce forest in comparison with the fine roots of black spruce, evaluate the capacity of mosses to retain the phosphate they have absorbed, and document their role in the cycling of phosphorus in a black spruce forest. Study site and methods All field work and sample collections for this study were conducted in a 135-year-old black spruce (Picea mariana (Mill.) B.S.P.) forest at Washington Creek (65 ~ 10'N, 147 ~ 50'W, 300 m elevation), 45 km north of Fairbanks in interi- or Alaska. Black spruce is the only major overstory species, and beneath a scattered shrub understory is a thick carpet of the feathermosses Hylocomium splendens (Hedw.) Brid. and Pleurozium schreberi (Brid.) Mitt (Dyrness and Grigal 1979; Viereck et al. 1983). The soil is a poorly developed silt loam overlain by a 2(~30-cm thick organic mat. Perma- frost is present, and the maximum summer thaw is 50 cm.