Production and microtopography of bog bryophytes: response to warming and water-table manipulations

Boreal peatlands, which contain a large fraction of the world's soil organi c carbon pool, may be significantly affected by changes in climate and land use, with attendant feedback to climate through changes in albedo, fluxes of energy or trace gases, and soil carbon storage. The response of peatland s to changing environmental conditions will probably be dictated in part by scale-dependent topographic heterogeneity, which is known to interact with hydrology, vegetation, nutrients, and emissions of trace gases. Because th e bryophyte community can contribute the majority of aboveground production in bogs, we investigated how microscale topography affects the response of bryophyte species production and cover to warming (using overhead infrared lamps) and manipulations of water-table height within experimental mesocos ms. We removed 27 intact peat monoliths (2.1-m(2) surface area, 0.5-0.7 in depth) from a bog in northern Minnesota, USA, and subjected them to three w arming and three water-table treatments in a fully crossed factorial design . Between 1994 and 1998, we determined annual production of the four domina nt bryophyte taxa within three microtopographic zones (low, medium, and hig h relative to the water table). We also estimated species cover and calcula ted changes in topography and roughness of the bryophyte surface through ti me. Total production of all bryophytes, and production of the individual ta xa Polytrichum strictum, Sphagnum magellanicum, and Sphagnum Section Acutif olia, were about 100% greater in low microtopographic zones than in high zo nes, and about 50% greater in low than in medium zones. Production of bryop hytes increased along the gradient of increasing water-table heights, but i n most years, total production of bryophytes was negatively correlated with height above the set water table only for the wettest water-table treatmen t. Although bryophyte production was unaffected by the warming treatments, the bryophyte surface flattened in proportion to the degree of warming. The se results indicate that production of bryophytes is driven most strongly b y the absolute and relative height of the bryophyte surface above the water table. Predicted changes in water-table height commensurate with changes i n surface temperature may thus affect both production and superficial topog raphy of bryophyte communities.