Oxygen-18 Composition of Sphagnum, and Microenvironmental Water Relations

Abstract

The 180 isotope content of Jive species of Sphagnum from three peatlands in Ontario was analyzed. Sphagnum fuscum and S. capillifolium had significantly more enriched 6'80 values than S. fallax, S. magellanicum, and S. papillosum. The enriched values in the former species indicate that water involved in photosynthesis in hummock species is much more affected by evapotranspiration than in adjacent species situated in depressions. These data suggest that microenvironmental water relations regulate evapotranspiration in Sphagnum species. Environmental isotope analysis (180, 2H) is another technique that may shed light on understanding Sphagnum ecophysiology and water relations along microenvironmental gradients in northern peatlands. The vertical distribution of Sphagnum species along microtopographic gradients has long been noted, indeed, ever since the earliest ecological studies of northern bogs and poor fens (Andrus et al. 1983; Clymo & Hayward 1982; Sj6rs 1948; Vitt et al. 1975). The genus Sphagnum represents a unique group of hydrophytes especially adapted to periodic drought conditions because of large seasonal changes in water supplies reaching peatland surfaces. Ecological factors such as cation exchange capacity, desiccation resistance, water holding capacity, degree of drying, and photosynthetic ability have been found to be important for Sphagnum microenvironmental distribution patterns (Andrus 1986; Clymo & Hayward 1982; Lindholm 1990). Environmental isotopes--oxygen18 (180) and deuterium (2H)-that form part of the water molecules and organic compounds in plant organic matter, have been widely used in ecology and geochemistry (Fritz & Fontes 1980; Rundel et al. 1989). One specific application is in the reconstruction of paleoclimate based on the isotopic composition of fossil wood and peat cellulose (Dupont & Brenninkmeijer 1984; Edwards et al. 1985; Yapp & Epstein 1982). Paleoclimatic inferences are based on the relationship between the 180 and 2H composition of plant cellulose and the isotopic composition of the plant water source and relative humidity (Burk & Stuiver 1981; DeNiro & Epstein 1979; Epstein et al. 1977). We report some results of a project aimed at assessing the relationship between the isotopic composition of bog plant species and environmental factors. The understanding of this relationship is essential if stable isotopes in fossil peat plant cellulose are to be used in paleoclimate studies. This paper proposes that isotopic analysis of Sphagnum mosses may be an important aid in understanding the ecophysiological requirements and microenvironmental distribution of sphagna. MATERIAL AND METHODS Field methods.--Mosses for this study were collected from three peatlands in Ontario. Oliver's Bog is a floating bog and fen complex (sensu Tarnocai et al. 1988) developed in a closed kettle basin 4 km southeast of Cambridge in southwestern Ontario (43019'45N; 80017'15W). An open bog and poor fen mat has a soft bottom closest to the open water area in the centre and is firmly anchored in the treed bog area around the edge of the basin. The mat portion, where mosses were collected for this study, is characterized by oligotrophic lawn and carpet vegetation with some areas periodically in contact with waters of the underlying lake system, and having small ombrotrophic hummocks. Emo Bog is a large ombrotrophic raised bog complex, about 680 ha in size, situated 8 km northeast of Emo in northwestern Ontario, near the Minnesota-Ontario border (48042'N; 93047'W). It is a forested-low shrub bog with Picea mariana, numerous ericaceous shrubs, and welldeveloped hummock microtopography. Rainy River Bog is 40 km west of Emo Bog and 6 km north of the Town of Rainy River (48047'30N; 94034'W). This ombrotrophic bog is developed in a small basin about 10 ha in size with a vegetation cover and microtopography similar to that at Emo Bog. Laboratory procedures. -Representative Sphagnum mosses were collected at each study site for isotope analyses in June or July 1987. Samples that included several moss stems were selected from adjacent paired microsites-hummock top and a position near the bottom of the side of the hummock. Sphagnum nomenclature follows Anderson (1990). Voucher specimens are deposited in the Wetlands Ecology Laboratory at the University of Waterloo. Cellulose was extracted from the living plants 0007-2745/92/445-448$0.55/0 This content downloaded from 157.55.39.54 on Fri, 01 Jul 2016 05:38:50 UTC All use subject to http://about.jstor.org/terms 446 THE BRYOLOGIST [VOL. 95 TABLE 1. Isotopic composition of Sphagnum from three peatlands in Ontario. The sample of S. magellanicum from Emo bog may contain some Polytrichum strictum. Collection sites: OB = Oliver's Bog, a quaking bog, southwestern Ontario; RRB = Rainy River Bog, a raised basin bog, northwestern Ontario; EB = Emo Bog, a raised basin bog, northwestern Ontario; SMOW = standard mean ocean water.