Paludifica tion and management of forested peatlands in canada: a litera ture review

Definitions and terminology

The characteristics of forest sites that are prone to paludification are not currently defined in the North American literature. In this review these sites are characterized by: 1) a cold, humid climate, soil physical and chemical properties favouring peat accumulation; 2) the presence ofmosses (mainly feathermoss with an increasing cover of Sphagnum); and 3) an increasing organic matter layer. Upland sites surrounded by lowland sites may also be prone to paludification via the lateral expansion of Sphagnum. Paludification in the boreal forest leads to the formation of fens, swamps, and bogs. Fens are peatlands with a fluctuating water table. The waters in fens are rich in dissolved minerais and are therefore minerotrophic. The dominant peat materials are moderately decomposed Sphagnum in poor fens, and sedges and brown mosses of variable thickness in rich fens (Vitt and Belland 1995; NWWG 1997). Trees occur mostly in the driest fen sites where microtopographic features such as moss hummocks provide habitats as much as 20 cm above the water table (NWWG 1997). Fens also range from acidic to alkaline, and mesotrophic to eutotrophic (Vi tt and Kuhry 1992). Swamp is a term that has been used in Canada and the United States in reference to forested peatlands. According to the North American definition, a swamp can be defined as a wetland dominated by trees or tall shrubs that is influenced by minerotrophic groundwater on either mineral or organic soils.

The essential features of swamps are the dominance ( over 30% of cover) oftall woody vegetation, and the wood-rich peat laid down by this vegetation (NWWG 1997). Bogs are isolated hydrologically from mineral soil-influenced runoff and groundwater from surrounding mineral soils. Generally the water table is at or slightly below the bog surface. Precipitation, fog, snowmelt and nutrient deposition are the primary water sources and mineral element flux; thus, all bogs are ombrogenous (and ombrotrophic) and mostly oligotrophic. Bogs are usually covered with Sphagnum spp. and ericaceous shrubs (Vitt and Belland 1995; NWWG 1997). Three terms have been frequently used in the literature to describe a peatland with a component of woody vegetation: forested, wooded and treed. To our knowledge, there are no clear distinctions made in the literature among these terms, and in discussions with different peatland ecologists, it seems that these three terms may actually have different meanings that may lead to uncertainty and misinterpretation for sorne readers. For sorne, forested contains dense mature trees, while treed is used to indicate a sparser density and generally smaller size trees (Jeglum 1985; Tim Moore, persona/ communication). For others, forested refers to a closed canopy while wooded or treed is limited to open canopy (Jeglum 1991a; Vitt et al. 2003b). Finally, wooded (or woodland in this case) is used for a mix of wood and grass land, mostly savanna (sorne scattered trees in grassland), while forested designates an area covered by trees. The minimum percentage covered by trees might in that case be as low as 25% (Altaf Arain, persona! communication). Consequently, in future research or evaluations, standard definitions for wooded, forested, and treed should be provided. For the purpose of this review, forested peatlands will refer to peatlands that support commercial forest.

Peatland initiation

Peatlands initiate by two main pathways: paludification and terrestrialization (RomeU and Heiberg 1931; Millar 1940; Gorham 1957; Payette 2001; Charman 2002). Terrestrialization is a process by which a shallow water body is gradually infilled with accumulated debris from organic and inorganic sources. This continues to a point where the water table is at or below the surface for at least sorne part of the year, and peat accumulates over the previously deposited limnic sediments (Payette 2001; Charman 2002). Peat accumulation by terrestrialization is not affected directly by forest management and can hardly be reversed by silvicultural treatments. Thus this literature review will concentrate on paludification landscape where forest management can have an impact on timber productivity. Furthemore, a greater proportion of peatlands are created by paludification rather than terrestrialization (Vasander 1996; Payette 2001; Charman 2002). Most paludified sites originate as moist or wet mineral sail sites and tum into peatlands over time, either naturally through successional processes, through extemal factors, or as a result of human activity. In essence, peat accumulation be gins when the production of organic matter exceeds decay (Paavilainen and Paivanen 1995; Charman 2002).

However, this process is not simple as there are five factors influencing peatland vegetation (mainly Sphagnum masses) establishment, which triggers peat accumulation.: clirnate, geomorphology, geology and soils, biogeography, and human activities (Gorham 1957; Heinselman 1975; Liu 1990; Kuhry et al. 1993; Halsey et al. 1997, 1998; Charman 2002; Crawford et al. 2003): 1) Clirnate is probably the most important factor in deterrnining whether there will be a surplus of water available for peatland initiation. The precipitation-evaporation balance is cri ti cal and a surplus can result from low precipitation – low temperature or high precipitation- high temperature regimes, as weil as from high precipitation – low temperature conditions (Charman 2002); 2) Geomorphology involves topography, which creates spatial diversity in the hydrological characteristics of the lands cape. The magnitude of the effect of geomorphology is seen in the situation of the two major peatlands in the world (Gorham 1991). The largest lies on the vast and nearly level West Siberian Plain between the Ob and Yennisey rivers in the former Soviet Union. Slopes may vary from 0.1 to 0.8 in 1000 in wet peatlands to as muchas 4 in 1000 in less wet sites.

The next largest peatland occupies the Hudson Bay- James Bay Lowlands of Canada, another region of flat topography where the slope is commonly less than 1 in 1000, and where rather impermeable marine silt/clays and other deposits favour waterlogging (Riley 1982); 3) Geology and soils (bedrock and texture) are also very important in peatland initiation (Halsey et al. 1997), as mineral soil composed ofheavy clay acts as an impermeable substrate that facilitates water accumulation. However, peatlands are also reported to form on glacial till, sand, gravelly outwash, soil formed in situ and rock (Rigg 1925; Gorham 1957; Hulme 1994); 4) Particular areas of the world may be more susceptible to peat accumulation because of the presence of the beaver (Castor canadensis), and particular plants or group of plants; 5) Lastly, human activities such as agriculture and forestry may modify the hydrological balance of the peatland and it may also change peatland development (Franzen 1983). Consequently, these five main factors can create conditions promoting excess water and the establishment ofpeat-forming vegetation, especially Sphagnum spp.

Development offorested peatlands

Paludification often occurs in wet basins and depressions (Payette 2001 ). As the thickness of the organic matter increases, the peat surface is isolated from the mineral soil and the conversion to bog occurs. As the peat accumulates, it may eventually expand beyond the deepest parts of the depression onto the surrounding landscape (Korhola 1994, 1996). Paludification also occurs outside of basins on flat or even sloping (up to 20% (Gorham 1957; Payette 2001; Charman 2002)) terrain. In such situations, as the peat deposit grows, it must either develop its own internai drainage system or it will eventually become so big that the accumulation of water during heavy rain may exceed the holding capacity of the . peat, and lead to a flow of the whole mass downslope (Gorham 1957). Acid peat may form on upland soils even when they are relatively rich. Moreover, if the atmosphere is sufficiently humid and precipitation exceeds evaporation at all seasons, sorne species of the Sphagnum moss may grow where the ground is saturated. Also, if rainfall is high, even fairly rich soils may be leached of nutrients at the surface, as a result of creating favourab1e conditions for the establishment of Sphagnum mosses and unsuitable conditions for the rapid breakdown of their dead residues (Gorham 1957).

It is often assumed that the initiation ofpeat accumulation requires an extemal factor su ch as climate change, but it may also be just a result of natural succession (Rigg 1925; Zack 1950; Heilman 1966, 1968; Viereck 1970; Engstrom and Hansen 1985; Makila et al. 2001). In the absence of fire, there is an accumulation of organic matter and an increase in Sphagnum cover in a black spruce-feathermoss community (i.e., the main forest community in the eastern boreal forest in Canada (Bergeron et al. 1999)). By promoting cold soil temperatures and, in sorne areas, permafrost, Sphagnum spp. reduces 1) organic matter decomposition rates, 2) rnicrobial activity, and 3) nutrient availability (Taylor et al. 1987; Bonan and Shugart 1989; Payette 2001). The reduction in mineralization may be due to immobilization but also to the chernical composition of Sphagnum litter that contains refractory cell-wall material, or secondary metabolites (e.g. sphagnol) produced by living Sphagnum plants (Weber and Van Cleve 1984; Verhoeven et aL 1990; Johnson and Damman 1991). Thus, with time, a black spruce-feathermoss community may develop into a peatland black spruce forest. Once Sphagnum is established, it is unlikely to disappear as long as climate and hydrological conditions remain stable (van Breemen 1995; Charman 2002). However, in the absence of a severe fire, if it does not reach astate where it is very thick, it can be reversed if it is bumt to the mineral soil. Ultimately, it remains very difficult to determine if paludification of these black spruce stands originated from a chronosequence (i.e., accumulation due to time since the last frre) or a toposequence (i.e., accumulation due to a depression or basin). Thus, it is not always possible to distinguish paludification by natural succession produced by lateral expansion from surrounding peatlands or depressions. Peat stratigraphy and an understanding of local and surrounding topography are essential to discriminate between these two causes (Korhola 1994). Because appropriate forest management strategies differ between the two situations, understanding the origin of peat formation is important.

Disturbances: wildfire and logging Although wildfires have played a major role in boreal forest dynamics since the last glaciation, logging has become one of the main disturbances in the eastern Canadian boreal forest in the last century. Consequently, in the past five decades, wildfires and logging have affected equivalent areas (Schroeder and Perera 2002). Foresters have historically believed that from a stand perspective, clearcuts have similar effects as severe wildfires, as both initiate stands and modify the soil microclimate (temperature and moisture) (Keenan and Kimmins 1993; McRae et al. 2001; Simard et al. 2001). Currently, it is accepted that, while similarities exist, fires and clearcuts have distinct suites of effects on the forest floor of the boreal forest. First, the level of soil disturbance differs between fires and clearcuts. While soil disturbance is relatively even across the stand in clearcuts, after a fire soil disturbance, severity is heterogeneous. Second, logging may also increase soil compaction and rutting (Brais and Camiré 1998; Harvey and Brais 2002) and, as a consequence, reduce soil hydraulic conductivity. Third, depending on intensity and frequency, fire can partially or entirely remove the soil organic layer, the feathermoss and the Sphagnum layers (Lutz 1960; Carleton and MacLellan 1994).

Moderate and severe fires have a variety of impacts on forest soils. These include an increase in soil fertility via an increase in soil temperature and organic matter decomposition rates (Lutz 1960), an increase in soil pH and exchangeable cation availability (Lutz 1960; Dymess and Norum 1983; Van Cleve and Dymess 1983; McRae et al. 2001; Simard et al. 2001), mineralization ofimmobilized nutrients (tuming organic matter into ash), and reduction of the organic C and microbial biomass (Dymess and Norum 1983; Fritze et al. 1993; Pietikainen and Fritze 1993; Simard et al. 2001). Harvesting also has a significant influence on nutrient cycling, but unlike wildfire, logging also removes the large amounts ofP, K, Ca and Mg contained in the tree biomass. These materials are mostly conserved in situ during a wildfire (McRae et al. 2001). Lastly, several studies suggest that the deposition of charcoal after a fire may also be favourable to tree growth. Charcoal has the ability to absorb phenolic compounds and Zackrisson et al. (1996) and Wardle et al. (1998) concluded that charcoal might catalyze important ecological soil processes in early successional boreal forests. This effect diminishes as succession proceeds, and may ultimately have important long-term consequences for stand productivity and ecosystem function, especially in forests dorninated by ericaceous shrubs that are under strict fire control (Zackrisson et al. 1996; Wardle et al. 1998; DeLuca et al. 2002).