Patterns of Bryophyte Diversity in Peatlands of Continental Western Canada

Abstract

In continental western Canada, 62% of the 110 species found in 96 peatlands occur in Sphagnum-dominated bogs and poor fens, whereas 71% occur in brown moss-dominated rich fens. Alpha (site) diversity is remarkably uniform over the bog-rich fen gradient. Gamma (landscape) diversity is highest in extreme-rich fens, which are the most variable of the five peatland types surveyed. Species richness at the site level is most highly correlated with habitat heterogeneity. Climatic factors are not as important; however, habitat heterogeneity (46%) and temperature (15%) together explain 61% of the variation. When the five peatland types are examined individually, species richness in poor fens increases with pH, in extreme-rich fens decreases with pH, and in peat plateaus, continental bogs, and moderate-rich fens remains relatively constant regardless of pH. Peat plateaus consistently have greater species richness than continental bogs due to limited carpet and pool habitat heterogeneity in the latter. Twenty-five percent of the species were found only once; these species are considered rare in the region. Sixty percent of these occurred only in extreme-rich fens. Habitat heterogeneity, plus pH or temperature in some peatland types, can be effectively used to predict site biodiversity. Patterns of species richness have long been of interest to biogeographers. Attempts to explain these patterns of species occurrence have been varied, ranging from those that are historically oriented (Fischer 1960) to those that are equilibrial and argue that richness patterns are attributable to current conditions (Rosenzweig 1975). Among processes that have been hypothesized to account for richness patterns are productivity, habitat harshness, climatic stability, interspecific competition, and habitat heterogeneity. Peatlands are unique ecosystems that accumulate carbon, as peat, due to unequal rates of plant production and decomposition. Peatlands can be easily separated from non-peat-forming ecosystems based on hydrological and physiographic criteria, and they posess a largely unique bryoflora. Within peatlands, there exist strong gradients in ecosystem age, productivity, stability, and habitat heterogeneity, and peatlands occur under a diversity of regional climates. Peatlands have been considered by many to be stressed ecosystems and in general have relatively low species richness when comparisons that utilize only vascular plants are made to other habitats. Based on hydrology, peatlands can be classified either as bogs ombrotrophic systems, or fens-minerotrophic systems. Wheeler (1993) has compared species richness between these two hydrological types in Britain and found 464 vascular plants and 189 bryophytes in fens, totalling 653 species, whereas in bogs he found only 109 vascular plants and 84 bryophytes with a total of 193 species. Clearly fens have more species than bogs; however, this is an unequal comparison. Fens and bogs are hydrological units, not vegetational units. When peatlands are classified on the basis of vegetation, four types have long been recognized (Sj6rs 1952; Vitt & Chee 1990). In western Canada, where permafrost can have a strong influence, bogs can be divided into two types based on vegetation, namely those largely underlain with permafrost, termed peat plateaus, and those without permafrost called continental bogs (Belland & Vitt 1995). Fens can be divided into three types, namely poor fens, moderate-rich fens (which some incorrectly term intermediate fens), and extreme-rich fens. Thus in continental western Canada, the comparisons of species richness need to be made using five vegetation types, not two peatland hydrological types. Three of the five vegetation types are Sphagnum-dominated (peat plateau, continental bogs, and poor fens) whereas the other two are brown-moss dominated (moderateand extreme-rich fens). When DuReitz (1949) originally proposed that fens can be divided into what he termed poor fens and rich fens, he based his terminology of poor and rich on the number of indicator species present in the fen. Poor fens have fewer indicator species than do rich fens. If this indicator species concept is continued along the bog-fen gradient to bogs, then sup0007-2745/95/218-227$1.15/0 This content downloaded from 157.55.39.100 on Wed, 06 Jul 2016 05:30:35 UTC All use subject to http://about.jstor.org/terms 1995] VITT ET AL.: PEATLAND DIVERSITY 219 posedly bogs have the least number of indicator species. These indicator species concepts have often been misapplied to chemical conditions and to overall species richness. Hence, many researchers have considered rich fens to have greater species richness than poor fens based on an incorrect interpretation of the concepts of rich and poor. Also, these apparent patterns in overall plant species richness are often based only on the vascular plant component, excluding bryophytes. Environmental factors important to bryophytes differ from those important to vascular plants. Vitt and Chee (1990) used multivariate methods to suggest that whereas bryophyte patterns largely follow the gradients of acidity and alkalinity, vascular plant patterns tend to follow nutrient gradients (especially nitrogen and phosphorus). It might be expected that species richness patterns of these two groups of plants also show different patterns. This is evident along gradients of elevation and latitude (Vitt 1991), wherein bryophyte diversity increased along elevation gradients and remained relatively stable along latitudinal ones. Although Glaser (1992) has recently examined vascular plant diversity in bogs at a regional scale in eastern North America, no one has examined the species diversity patterns of bryophytes along the bog-fen gradient using a comprehensive regional data set, and no one has suggested why patterns found in these peatlands might occur as they do. Our objectives here are to answer the following questions: 1) How does bryophyte species richness of a landscape (i.e., gamma diversity) vary along the bog-rich fen gradient? 2) Does bryophyte species richness correlate with environmental factors considered important in peatland classification? 3) How does bryophyte species richness at the site level (i.e., alpha diversity) vary when bogs, poor fens, and rich fens are compared? 4) Can bryophyte species richness be predicted using appropriate factors?