- Top of page
The main goal of most projects in ecosystem restoration is to shorten the successional pathways towards plant communities that are functionally similar, if not structurally similar, to the pre-disturbance state (Lockwood & Pimm 1999). One approach taken to achieve this goal is similar to that proposed for determining assembly rules of plant communities (Keddy 1992, 1999). Managers must first define the species pool available for recolonization, secondly determine the main abiotic and biotic filters operating during the recolonization process, thirdly determine how species respond to these filters, and finally manipulate these filters in an efficient manner until the community is restored. The initial information required is the composition of the post-disturbance species pool available for recolonization. This species pool consists of adult plants and propagules that persist through the disturbance and propagules that immigrate from surrounding sites (Noble & Slatyer 1980). Following a severe disturbance, the species pool is entirely dependent on the immigration of propagules and recolonization, in the absence of introductions, is propagule-limited (del Moral & Wood 1993; Ash, Gemmell & Bradshaw 1994).
Milled peatlands are examples of severely disturbed environments that rely entirely on immigration or active introductions for recolonization. Ombrotrophic peatlands in eastern Canada are exploited for their peat over several decades using the method of milling and vacuum collection (Crum 1988). Once abandoned, milled surfaces are flat and large (up to 5 km2) with a dense network of ditches. Fibric to sapric, Sphagnum peat substrates usually remain (> 1 m thick) that are acid (pH 3–5) and nutrient-poor (Wind-Mulder, Rochefort & Vitt 1996). There is no residual plant cover nor a seed bank (Salonen 1987). Subsequent recolonization is slow and many typical ombrotrophic peatland species are absent, notably Sphagnum species (Curran & MacNaeidhe 1986; Salonen 1990; Salonen & Setälä 1992; Salonen, Penttinen & Särkkä 1992; Desrochers, Rochefort & Savard 1998). The poor recolonization has been attributed to the paucity of immigrant propagules and harsh edaphic conditions for plant establishment (Salonen & Setälä 1992) but the relative importance of each is poorly understood.
The immigration of plants into disturbed ecosystems can be determined in a straightforward manner by trapping the diaspore rain. However, such determinations are labour-intensive and forcibly site-specific (Salonen 1987; Poschlod 1995). Furthermore, traps in milled peatlands are often filled by wind-blown peat (D.R. Campbell, personal observation). Immigration may also be quantified using physical models such as those developed for determining wind dispersal of tree seeds from forest edges to clearings (Greene & Johnson 1996) or from isolated stands (Nathan, Safriel & Noy-Meir 2001). But, if the vegetation structure surrounding the disturbance is variable, as is the case for milled peatlands, the consequent wind environments will be as well, making predictions difficult (Nathan, Safriel & Noy-Meir 2001). A more general approach has been proposed to define local and regional species pools of target communities, which in effect defines immigration potential (Zobel, van der Maarel & Dupré 1998). A series of key autecological factors for immigration are considered for each species; a factor with a low value reduces the overall probability of a species of belonging to a species pool of a particular target community. The approach cannot provide quantitative information on dispersal distances, but it allows for a general determination of probable immigrants to a community from a large suite of potential species.
In this study, a method was developed to assess the relative immigration potential of plants colonizing milled peatlands based on the approach proposed for determining local species pools (Zobel, van der Maarel & Dupré 1998). Immigration is considered strictly as the arrival of potential colonists to a milled surface; establishment and subsequent community dynamics affected by restoration conditions are not considered here. A comparative approach is used, and an index of immigration potential is determined for each species relative to other species considered. This method allows for an evaluation of the role of immigration in determining recolonization. Poor recolonization success despite good immigration potential would suggest that other steps after immigration control recolonization. Moreover, it allows for the identification of species that must be introduced prior to restoration. Although this approach can easily be applied to individual sites, it is applied here to the general case of milled peatlands in south-eastern Quebec, Canada.
- Top of page
The immigration of potential colonists to milled peatlands depends on the occurrence of residual populations in edges, their fecundity and their ability to disperse by an available vector. The disparity between species is evident just from their residual populations in milled edges. Many typical peatland species are rare (e.g. herbs) while other species that are absent from natural peatlands have become frequent in edges (e.g. birches). A pronounced edge effect is also evident where many species vary in abundance as a function of distance from the edge. The dissimilarity of the vegetation between milled edges and natural peatlands in part reflects decades of disturbance from drainage and extraction activities. Direct drainage effects appear to have an impact because drainage age predicts the occurrence of many species. Drainage in peatlands affects species composition as a result of altered hydrologic conditions and consequent effects on soil aeration and nutrient dynamics (Laine, Vasander & Laiho 1995; Poulin, Rochefort & Desrochers 1999). However, it is unclear to what extent drainage vs. other disturbances associated with peat extraction are responsible for this edge effect. For instance, the decline of Picea mariana near edges may be caused by the practice of clearing trees prior to exploitation.
Differences in immigration ability between study species become even more evident when edge populations are coupled with their maximum fecundity and dispersal ability by wind, water and animals. Species differ dramatically in their ability to disperse by a particular dispersal agent, and several are not limited to a single agent. However, wind is the key dispersal agent in all milled peatlands because of their large, open and aerodynamically smooth substrates (Campbell, Lavoie & Rochefort 2002). Several studies have determined the prevalence of wind-dispersed diaspores in the seed rain (Curran & MacNaeidhe 1986; Salonen 1987; Salonen, Penttinen & Särkkä 1992; Poschlod 1995). For instance, Betula species are known to be entrained for long distances over smooth surfaces (Matlack 1989; Greene & Johnson 1997). In Europe, Betula pubescens dispersed up to 1061 viable seeds m−2 year−1 in milled peatlands from a distance of 250 m from the closest vegetated edges (Salonen 1987).
Surface water is present in milled peatlands during snow melt and for longer periods in parts of the drainage ditch network. Water can therefore act as a seasonal dispersal agent for diaspores. However, in comparison with wind dispersal, water dispersal is more site-specific and depends on the duration, location and flow direction of surface water in the drainage network with respect to the edges of the milled zones.
Remnant peatland fragments in milled peatlands have more abundant and diverse assemblages of songbirds than natural peatlands (Delage, Fortin & Desrochers 2000). Small mammals are also more diverse in these fragments than in natural peatlands (Mazerolle, Drolet & Desrochers 2001). Their effectiveness as seed dispersal agents, however, is unknown. The lack of vegetation on milled peatlands restricts the use of milled surfaces by animals and any consequent seed dispersal. However, the prevalence of Vaccinium angustifolium in abandoned milled surfaces appears to indicate that animals do disperse seeds on milled peatlands. In addition, occasional tracks of large mammals in milled bogs indicate that epizoochory of propagules may occur.
This study provides evidence that a potential exists for moss spores to immigrate to milled surfaces. Moss species are only moderately common and fertile in edges of milled peatlands but are very fecund and have great wind-dispersal abilities. Previous studies have shown that dispersal of moss spores is concentrated within short distances of adult plants (1–2 m; McQueen 1985; Kimmerer 1991; Miles & Longton 1992). However, only one study compared spore release vs. observed deposition patterns (Miles & Longton 1992). Less than 13% of released spores were deposited within 2 m in forested and less in open habitats. Mosses appear to have highly leptokurtic dispersal distributions with long, fat tails (sensuClark et al. 1998) that favours long-distance dispersal.
The discrepancies between immigration potential and actual recolonization for several species suggest that other factors control their colonization success after immigration. The recolonization failure of mosses must primarily be a result of problems during establishment on milled peat surfaces. Sphagnum species are especially vulnerable to drought and substrate instability during the establishment phase (Rochefort 2000). Picea mariana is also frequent in edges, relatively fecund and has high wind-dispersal ability, but is a rare component of the vegetation in abandoned milled peatlands. Harsh substrate conditions, possibly summer drought or needle ice, could prevent its establishment and survival. Conversely, Eriophorum vaginatum is infrequent in edges but relatively fecund with high wind-dispersal ability, and frequently colonizes milled peatlands. Similarly, Rubus chamaemorus is infrequent in edges, has low fecundity and only disperses by animals, yet is more frequent in milled peatlands than many species. Its presence in abandoned milled peatlands infers that the remainder of its life cycle, namely their germination, establishment, clonal growth or sexual reproduction, is favoured on milled peatlands relative to other species.
The colonization of disturbed environments is often limited by the availability of immigrant propagules (del Moral & Wood 1993; Ash, Gemmell & Bradshaw 1994; Pywell et al. 2002). Knowledge of the immigration potential of species should be useful for planning the restoration of these environments. Decisions could then be made on the need for introducing species.
Determining the relative immigration potential of species is relatively straightforward given a list of colonists with suitable habitat requirements, data on source populations of propagules, and autecological information on fecundity and dispersal characteristics of species. It is also possible to compare species with a wide range of life forms, from mosses to trees. Bottlenecks during the immigration sequence can be pinpointed. It should be especially useful where the relative immigration of desirable vs. undesirable species to restoration sites is unknown. If desirable species can be shown to immigrate, restoration efforts can concentrate on encouraging their establishment. The main disadvantage of this approach is the lack of quantitative determination of dispersal distance. The probable concentrations of diaspores on disturbed sites at different distances from edges cannot be determined. However, actual immigration varies greatly between sites and years as a result of many factors. Any quantitative determinations of immigration would vary likewise. A qualitative determination of immigration potential using a relative index does not suffer from this problem, yet provides useful determinations of the likely initial species pool following disturbance.
For milled peatlands in southern Quebec, immigration per se appears to be a recolonization constraint for the herbaceous species studied, but does not appear to be as great a constraint for most mosses, ericaceous shrubs or trees. The poor recolonization of many of the latter species is probably a result of establishment failure, especially in the case of mosses. Restoration efforts in milled bogs should therefore concentrate on introducing species with low immigration potential (e.g. herbaceous species) while promoting microenvironments suitable for the establishment of other species. However, the introduction of species with good immigration potential may still be required as a result of the large size of milled peatlands.