The Utilization of Bryophytes in Bioclimatic Modeling: Present Distribution of Peatlands in the Mackenzie River Basin, Canada

Abstract

A model was developed that classified and projected the distribution of seven different types of peatlands in the Mackenzie River Basin. The model was based on the relationships between bryophyte indicator species, the types of peatlands they characterize, and regional climate. The model used the presence, absence, and abundance of 15 bryophyte indicator species to classify 81 peatlands in the study area into seven groups. Abundance values were calculated for each of the indicator species along three climatic gradients-Mean Annual Temperature (MAT), Mean Annual Total Precipitation (MATP), and Length of the Growing Season (LGS). The percent cover of all species were then ascribed to appropriate combinations of MAP, MATP, and LGS. The result produced a matrix consisting of 4,560 grid nodes where each node was identified by values for each of the three climatic variables and the types of peatlands that could be found at that climate. An independent data set consisting of climatic and ecological values and vegetation cover for 115 sites was used to test the ability and accuracy of the model to classify and project the climatic distribution of the seven peatland groups. The model correctly classified 106 of the 115 sites and of those, correctly projected the distribution of all but five of the test sites. The model accuracy was 70% for six of the seven groups, and > 90% for three of those. The accuracy for the remaining group was 50% and errors were mostly caused by the failure to project the distribution of three of the test sites. Other errors include: the inability to classify lichen dominated peatlands; the inclusion of wet lawns in bogs into one of the groups which caused a southward extension of that group by approximately 200 km. The overall model accuracy was 88%. Peatlands, primarily bogs and fens, cover extensive areas of the boreal and subarctic zones particularly in the northern hemisphere (Gore 1983). Several studies have demonstrated that peatland types and peatland vegetation are to a large extent controlled by macroclimate, physiology, and local gradients. The macroclimate plays an important role in the regional distribution of the different types of peatlands (Moore & Bellamy 1974). Among climatic variables that have been correlated to the different types are total precipitation (Eurola 1962; Ruuhijiirvi 1960), number of precipitation days (Taylor 1983), and precipitation and temperature together (Ivanov 1981; Moore & Bellamy 1974). The present distribution of peatlands in North America is a function of two climatic variables, precipitation and potential evaporation. Generally peatlands only occur in areas where evaporation does not exceed precipitation (Gignac 1993). Because bryophytes have a high fidelity to such environmental gradients as moisture and pH, they are particularly good indicators of different types of peatlands at the local level (Gignac & Vitt 1990; Horton et al. 1979; Nicholson et al. 1996; Vitt et al. 1975; Vitt & Slack 1975, 1984). The distributions of several of those species are also limited by climate and thus become indicators of climate. For example, in western North America the distribuion several such species as Sphagnum austinii, S. papillosum, S. rubellumn and S. tenellum are limited by climate to areas that have > 1,000 mm total precipitation and are thus indicative of oceanic peatlands (Gignac et al. 1991a). Others, such as Drepanocladus fluitans, Sphagnum lenense and S. riparium, are limited to areas having mean annual temperatres lower than O0C and are indicative of high boreal and subarctic peatlands (Nicholson & Gignac 1995). Also, several geographically widespread species can be indicators of peatlands based not on their presence and absence but on their abundance. The primary division of peatlands into bogs and 0007-2745/98/560-571$1.35/0 This content downloaded from 157.55.39.225 on Tue, 13 Jun 2017 18:01:52 UTC All use subject to http://about.jstor.org/terms 1998] GIGNAC ET AL.: PRESENT DISTRIBUTION OF PEATLANDS 561 fens is based on the source of surface water. The supply of water in ombrotrophic peatlands (bogs) is entirely from precipitation and because of this, the pH of bog surface water is low (Sjirs 1952). In contrast, a portion of the surface water on minerotrophic peatlands (fens) has been in contact with mineral soil. Depending on the nature of the mineral substratum and the quantity of water in the peatland that has flowed over the substratum, the pH of surface water in fens can vary considerably. Bogs are usually classified according to their landform patterns into raised bogs, plateau bogs, eccentric-domed bogs, concentric-domed bogs, and blanket bogs (Glaser & Janssens 1986; Moore & Bellamy 1974). Fens are usually divided according to the species richness of the vegetation into poor and rich (Sjirs 1952). Associated with species richness are differences in pH of the surface water: poor fens have pH values usually below 5.6, while rich fen surface water pH values are usually circumneutral or higher. There are further subdivisions of the rich fen category into moderate and extreme, again depending on the species richness of the vegetation (Sjors 1952, 1963). Other types of fens are aapa and palsa fens, which are classified according to landform and can be either rich or poor. Aapa fens have patterns composed of strings (ridges) and flarks (pools), while palsa fens have mounds (palsas) that contain a core of permafrost (Moore & Bellamy 1974). The distribution of the different types of bogs is related to macroclimate (Moore & Bellamy 1974). Ivanov (1981) for example, determined that the maximum height of a bog is in part a function of the moisture surplus during the driest month. Moisture surplus is the excess of precipitation over evapotranspiration (Damman 1986). The convexity of the dome is also a function of the water balance for the driest month (Ivanov 1981). The surface of bogs in hyperoceanic and oceanic areas may be raised several meters above the water table because there is always a large monthly moisture surplus (Gignac & Vitt 1990). Conversely, continental bogs are only slightly raised above the water table because there are several months during the growing season when there is relatively little precipitation. Although fens can be found in all climatic zones where peatlands occur, some types of fens can be placed into different zones based on climatic factors. Aapa (string) fens are found in continental areas where precipitation is low and peatlands form in basins and are thus more susceptible to influence by mineral soil water. Palsa fens are located in high boreal and subarctic regions that have discontinuous permafrost (Moore & Bellamy 1974). Generally, rich fens, particularly extreme-rich fens, are located in subcontinental and continental areas and are rare or absent in hyperoceanic and oceanic areas (Gignac et al. 1991b; Gignac & Vitt 1990; Vitt et al. 1990). The Mackenzie River Basin provides an excellent geographical area in which to study the effects of cl mate on the distribution of peatlands. It is a large basin that is restricted to continental areas and has wide variations along several climatic gradients, particularly temperature. Also, and most importantly, its southern boundary generally corresponds to the southern limit of peatland development and, with a few exceptions, its northern boundary coincides with the northernmost development of extensive peatlands in western Canada. A previous analysis of the vegetation of peatlands in the Mackenzie River Basin (Nicholson et al. 1996) indicated that the distributions of several peatland types and their bryophyte indicator species were closely related to climate. Those results led to the development of a model that reconstructs the present distribution of peatlands in the Mackenzie River Basin based only on climatic variables. The purpose of this study is to delimit the climatic distribution of each type of peatland that was found in the study area. The following steps were used to model peatland distribution 1) relate bryophyte indicator species to different types of peatlands and the climate in the Mackenzie River Basin; 2) project the present climatic distribution of those types of peatlands; and 3) test the model's validity and projections using an independent data