Toward an integrated model for raised-bog development: theory and field evidence

Ecology, Dec, 1995 by H. Almquist-Jacobson, D.R. Foster

INTRODUCTION

Background

Assessing ecosystem responses to climate change requires understanding not only modern climate-ecosystem relationships, but also the developmental processes inherent in those ecosystems and their sensitivity and response to past changes in climate. Geographic trends in mire types indicate that climate influences the development and differentiation of peatlands. Across boreal and subboreal regions the zonation of raised bogs and minerotrophic fens corresponds to isopleths of temperature and precipitation (Granlund 1932, Eurola and Vorren 1980). Among raised bogs in particular, geographic gradients in bog height and shape, surficial landforms, and gross stratigraphy across Scandinavia (Granlund 1932) and eastern North America (Damman 1977, Glaser and Janssens 1986) have been attributed primarily to moisture regime. These geographic trends and the reliance of raised bogs on atmospheric inputs for moisture and nutrients have led researchers to interpret peat stratigraphy from bogs as sensitive records of past climate. Consequently, stratigraphic changes in peat composition, humification, and rates of vertical accretion have been interpreted climatically for over a century (Blytt 1876, Godwin 1952, Barber 1981; see Frenzel 1983).

However, in recent decades scientists have debated the degree to which raised bogs respond to allogenic (external) influences such as climate and local hydrology, as opposed to autogenic processes associated with their own composition or hydrology. Theoretical models describing hydrology and growth of raised bogs (Clymo 1978, 1984, Ivanov 1981, Ingram 1982, Winston 1994) have suggested that autogenic mechanisms are responsible for some changes in vertical accretion, lateral expansion, and consequent shape of these peat-lands. As a result, studies in which raised-bog development is interpreted solely in terms of climatic change have been replaced by others in which field evidence is used to test developmental models (Foster and Wright 1990, Korhola 1992, Warner et al. 1993). Globally, boreal mires represent an important sink for carbon (see Malmer 1992). However, at a particular time they may act as either a source or a sink of carbon, depending on their developmental stage and prevailing climatic conditions (Warner et al. 1993).

Teasing apart allogenic and autogenic influences on ecosystem development can be approached by paleoecological and paleogeographical studies. Here we consider an integrated model of mire development, together with independent evidence of Holocene climate inferred from pollen and lake-level studies, to infer the processes affecting the initiation and development of seven raised bogs within a 6500 [km.sup.2] area of central Sweden (Fig. 1; Table 1). Specifically, we evaluate spatial and temporal patterns in (1) mire initiation, (2) conversion of fen to bog, (3) fen and bog expansion, and (4) modern bog geometry.

The integrated model

Raised bogs are domed mires that can be up to 10 m in height, and often circular or elliptic in plan view. The peat mass consists predominantly of partially decomposed Sphagnum mosses that function in two layers, the uppermost "acrotelm," which includes a few decimetres of undecomposed to slightly decomposed peat lying above the drought stage of the bog's water table, and the lower "catotelm," which includes permanently saturated peat (Clymo 1984). Although water-saturated almost to the surface, these ombrotrophic mires receive virtually all water and nutrients from the atmosphere. In some cases the base of the mire lies several metres above the local groundwater aquifer (J. E. Almendinger et al., unpublished data). Many bogs [TABULAR DATA FOR TABLE 1 OMITTED] have concentric or eccentric patterns of surficial features, including hummocks and hollows or arcuate pools along the elevational contours.

The "integrated model" of raised-bog development presented here combines theories of (1) bog hydrology (Ivanov 1981, Ingram 1982), (2) peat growth (Clymo 1978, 1984), and (3) the differentiation of surface features resulting from hydrology and patterns of peat accumulation (Foster and Wright 1990). Part of the model involves equations that describe hydrological and biological constraints on the growth and geometry of an idealized concentric raised bog initiating from a single locus and expanding across (paludifying) an unobstructed horizontal impermeable plane under steady-state conditions. However, in its entirety, the model does not assume that bogs initiate from a single locus or that they are unaffected by allogenic factors. Rather, it describes conceptually the hydrodynamic consequences of bog coalescence and hydrologic mechanisms through which climate and local hydro-geology together influence bog development and geometry. Although fen development is also examined in this paper, the integrated model, itself, applies only to the raised bogs that succeed the fen phase.

Mathematical relationships. - In the most basic sense, concentric raised bogs behave (in cross section) as hemi-elliptical groundwater mounds with vertical equipotentials, as described by the equation,