Two alternate regimes in peatland systems are described in this document: bogs, sphagnum-dominated peatlands with long-term carbon storage in peat, and fens, peatland in which vascular plants have a more dominant role, leading to higher-productivity but reduced peat long-term accumulation. The most important variables and mechanisms considered are peat accumulation and height of the surface above the water table, nutrient flux, and competition between plant functional groups. The key drivers of the shift are related with changes in climate (precipitation and temperature) and in nutrient input. The relevance of this shift in terms of ecosystem services and human well-being is the tradeoff between potential gains of nutrient-rich soils for agricultural activities on drained peatlands, versus a loss of long-term carbon accumulation with potentially great implications for global climate change.
Key direct drivers
- External inputs (eg fertilizers)
- Environmental shocks (eg floods)
- Global climate change
- Small-scale subsistence crop cultivation
- Large-scale commercial crop cultivation
- Extensive livestock production (rangelands)
- Land use impacts are primarily off-site (e.g. dead zones)
- Tropical forests
Key Ecosystem Processes
- Soil formation
- Primary production
- Nutrient cycling
- Food crops
- Fuel and fiber crops
- Climate regulation
- Livelihoods and economic activity
Typical spatial scale
Typical time scale
- Contemporary observations
Confidence: Existence of RS
- Speculative – Regime shift has been proposed, but little evidence as yet
Confidence: Mechanism underlying RS
- Well established – Wide agreement on the underlying mechanism
Peatlands are characterized by often deep accumulations of incompletely decomposed organic material (i.e. peat). The accumulation of peat occurs when carbon sequestration exceeds the long-term loss through decomposition or export by hydrological flow. Such rates of decomposition can be very low in acidic and anaerobic conditions. Globally, peatlands occupy about four million km2, of which boreal and subarctic peatlands constitute about 87%, mainly in Russia, Canada, United States, Finland and Sweden. Tropical peatlands also exist, a big percentage of which is in Indonesia (Vitt, 2008). A major distinction within peatlands is between fens (minerotrophic peatlands: plants have access to geogenous water) and bogs (ombrotrophic peatlands: plants are dependent on precipitation). These and other differences in hydrology, acidity and climate produce a great variety of peatlands worldwide. Such variety in genesis and characteristics makes any generalization of peatlands functional values very difficult. A fen to bog transition is often seen as the natural course of peatland succession (Hughes & Barber 2004), whereas a change from bog to fen usually requires external changes in conditions or strong perturbations. Both bogs and fens exhibit internal feedbacks that provide them with some stability, and threshold conditions associated with the transition between each other have been suggested in the literature (Belyea & Malmer 2004).
Bogs (Sphagnum-dominated peatland, with long-term carbon storage)
This regime is characterized by a landscape dominated by sphagnum-mosses, covering low hummocks and lawns, usually exhibiting no conspicuous spatial pattern in vegetation. Under the surface a thick layer of slow-decaying peat is mostly kept in waterlogged and acidic conditions. The concept of ecosystem engineers (Jones et al. 1994) has been used to describe Sphagnum mosses, given their functional capacity to create and maintain an acidic and nutrient-poor environment, harsh for most other plants (references in Ohlson et al. 2001). Additionally, mosses decay at extremely low rates due to their unique tissue chemistry, what strongly limits nutrient fluxes to other types of plants (Pastor et al. 2002). In the absence large external climatic changes or direct human impacts on hydrology or nutrient input, the species composition of these type of peatlands has proven to be very stable (references on Limpens et al. 2008).
Fens (Moss and vascular plant coexistence, with reduced peat accumulation)
Vascular plants also alter the environment in ways that negatively affect Sphagnum mosses. So an increase in vascular plant cover beyond critical thresholds inevitably leads to a decrease in Sphagnum (Berendse et al. 2001). Some of the mechanisms involved in this process act by preventing or even reversing ombrotrophication, in favor of minerotropic conditions. While other mechanisms might operate in bogs prompting the development of particular microtopography and associated vegetation spatial patterns. Such microtopography is also believed to be very stable (references in Eppinga et al. 2007 and in Limpens et al. 2008). This regime is characterized by an increasingly dense and connected vascular plants cover, exhibiting spatial patterns (strings-flarks on slopes and maze on flat landscapes) consisting of densely vegetated bands (hummocks forming ridges), alternating with wetter zones that are more sparsely vegetated (hollows forming pools) (Rietkerk et al. 2004).
Drivers and causes of the regime shift
A Sphagnum dominated peatland becomes more susceptible to invasion by vascular plants as the nutrient input increases (Pastors et al. 2002; Eppinga et al. 2007, 2009; Limpens et al. 2008). Ombrotrophic systems are particularly sensitive to nitrogen enrichment, which can prompt the invasion by graminoid species (e.g. Molinia caerulea) and woody species (e.g. Betula pubescens) with a parallel decline of ombrotrophic species (references in Tomassen et al. 2003). Sphagna are negatively affected by high nutrient input due to ammonium toxicity (Fritz et al. 2012). Additionally, related C:N ratio changes could enhance decomposition, further amplifying this nutrient positive relation with vascular plants.
Decreasing wetness during the growing season promotes vascular plant growth and hampers moss development (Eppinga et al. 2007; Limpens et al. 2008), and perhaps more importantly, it weakens the nutrient flux delay of sphagnum peat by increasing aerobic decomposition (Hilbert et al. 2000). Reduced precipitation and droughts in general directly decrease Sphagnum productivity, as photosynthetic rates are strongly dependent on water saturation levels. Additionally, longer dry seasons can potentially increase wild fire frequency, with negative effects on peat depth (references in Limpens et al. 2008). In high latitude peatlands an increase in temperature implies a longer growing season for vascular plants and increases mineralization rates. As vascular plants abundance increases and Sphagnum cover reduces, the temperature feedback in regime 1 weakens, allowing for even warmer conditions in the rooting zone further increasing the growing season of vascular plants (references in Eppinga et al. 2007).
How the regime shift works
Shift from Bogs to Fens
Different mechanisms are responsible for maintaining Bogs, mostly ombrotrophication, which is the ability of Sphagnum mosses to effectively decouple the rooting zone of vascular plants from geogenous mineral-rich water, thereby depriving those plants from key nutrients. This process occurs as a result of peat accumulation, which increases with the slow rates of decay of mosses (Pastor et al. 2002). Once the ombrotrophic conditions are reached, the competitive ability of sphagnum boosts, increasing their dominance. This in turn contributes to more peat accumulation (Granath et al. 2010). Sphagnum out-competes vascular plants, by getting inputs from the atmosphere and releasing decay-resistant and low-nutrient litter, creating a slow nutrient flux. Particularly relevant is nitrogen immobilization, making vascular plants entirely dependent on the slow mineralization of peat (Pastor et al. 2002; Malmer et al. 2003; Tomassen et al. 2003). Additionally, the low-porosity character of sphagnum peat promotes waterlogged conditions that further reduce mineralization rates (references in Eppinga et al. 2007). Other mechanisms include acidification of the environment, competition for light and space, and belowground temperature lowering. Through the organic acids resulting from its humification, bog sphagnum actively promotes acidification of the environment, which limits vascular plant growth (references in Eppinga et al. 2007). In such sphagnum-dominated sites, the establishment and growth of vascular plants is limited by the thick moss carpet that overgrows seedlings (Ohlson et al. 2001; Malmer et al.2003), and its vertical growth engulfing meristemic tissues (references in Pastor et al 2002). Also, the growing season for vascular plants is shortened, given the poor heat conductivity of sphagnum peat, which hampers the functioning of vascular plant roots (references in Eppinga et al. 2007).
Drought or drainage can trigger a shift from Bogs to Fens. A prolonged drought might lower the water table long enough to force moss desiccation and allow for increased mineralization (Tomassen et al. 2003; Eppinga et al. 2009; Granath et al. 2010). This reduces the competitive pressures on vascular plants, increasing the opportunities for colonization. Drainage is a primary shock altering peatland dynamics by lowering the water table (Limpens et al. 2008), and it is done for agricultural and forestry purposes. Also, an influx of water high in nutrients or minerals (e.g. Ca) from surrounding land or from atmosphere could also damage Sphagna and cause vascular plant invasion. Similarly, liming (i.e. spreading Ca- or Mg-rich minerals) is another direct human intervention to eliminate Sphagnum. Basically, as soon as the inorganic nutrient pool that vascular plants require ceases to limited by slow sphagnum peat mineralization rates, this regime shift could occur.
Key mechanisms behind the maintenance of Fens are related to peat accumulation and vascular plants. While vascular plants, which can become highly productive under the influence of geogenous water or some other source of important nutrients, minerotrophic conditions seriously prevent bog sphagnum from settling given its high sensitivity to calcareous water (Granath et al. 2010). Advective transport of nutrients has been proposed by Rietkerk et al. (2004) as a scale-dependent mechanism, having a locally positive and long-range negative effect on nutrient concentration, to explain the spatial patterning of peatlands. This mechanism would be driven by transpiration of vascular plants, enhancing drier microforms (hummocks, ridges) with increased nutrient concentration that vascular plants require to establish and further reinforce their presence. Additionally, transpiration rates are higher in the presence of vascular plants, and this stimulates a lowering of the water table (Rietkerk et al. 2004). While this reduction of water in the surface negatively affects sphagnum productivity by desiccation (references in Tomassen et al 2003), it also leads to an increase of mineralization rates, which reinforces the presence of vascular plants (Tomassen et al. 2003). Another competitive mechanism operates as vascular plants reach above the moss carpet, limiting its development through shading and burial by litter (Berendse et al. 2001; Pastor et al. 2002; Malmer et al. 2003). Under these conditions the limited growth of Sphagnum prevents peat accumulation (Malmer et al. 2003; Limpens et al. 2008).
Shift from Fens to Bogs
Increases in precipitation could lead to a higher water table, potentially causing substantial dieback of vascular plants (references in Eppinga et al. 2007). Moreover, the minerotrophic conditions that are detrimental for bog sphagnum growth are reduced as the relative importance of precipitation water increases over that of geogenous water (Pastor et al. 2002). The height of the peatland surface above the water table is a key determinant of plant species distribution and primary productivity, and consequently the rates of litter production and litter decay losses (Hilbert et al. 2000; Belyea & Malmer 2004). Given the explained mechanism of delayed nutrient flux, it is clear that at a certain height of the surface above the water table, bog sphagnum becomes a superior competitor (Granath et al. 2010), and the reinforcing feedbacks of Bogs are activated.
It is worth mentioning that even though the literature cited here in reference to these alternate stable states relies deeply on modeling (e.g. Hilbert et al 2000; Pastor et a 2002; Rietkerk et al 2004; Eppinga et al 2007, 2009), qualitatively similar shifts have been reported in paleoecological studies as well as present day observations: dominance of deciduous shrubs and graminoids during the early stages of peatland development with little or no presence of moss, followed by landscapes dominated by Sphagnum lawns after the ombrotrophication process, which are later subjected to invasion by shrubs and spruce (references in Pastor et al 2002). The fact that such abrupt shifts between a near-monoculture moss state and a co-existence with vascular plants state have been reported to occur sometimes but not always, suggests that these systems do exhibit alternate stable states.
Impacts on ecosystem services and human well-being
Shift from Bogs to Fens
Since sphagnum litter enhances the formation of collapsible peat (i.e. with low porosity), which easily becomes waterlogged, the conditions for low rates of decomposition, and an overall net carbon sequestration effect are maintained in Bogs. Rates of carbon sequestration and methane emission depend strongly on height of the peatland surface above the water table (references in Belyea & Malmer 2004). Even though the same processes of anaerobic decomposition that increases carbon accumulation, also increases methane production, the overall desirable effect in relation to climate change gets lost in this shift, since the effect of removing long-lived atmospheric carbon dioxide ultimately surpasses that of releasing short-lived methane (Limpens et al. 2008).
Because of the complex relations of acrotelm thickness with vegetation and microtopography, these two are considered to primarily determine carbon sequestration/emission rates (Belyea & Malmer 2004; Belyea & Baird 2006). Peat formation rate is greatest for 'intermediate microforms' (lawns, low hummocks) and lowest for microforms at the extremes of the water table gradient (high hummocks and pools) (Belyea & Malmer 2004). Given this trade-off of decomposability and productivity between elevated and low microforms (reference in Limpens et al 2008), a landscape will exhibit higher short-term sequestration of carbon or long-term storage depending o the microforms by which it is dominated.
Shift from Fens to Bogs
A reduction in species richness and the loss of agriculturally suitable lands are potential impacts of the shift to sphagnum-dominated peatlands.
Damming or blocking ditches are common management practices aimed at raising the water table. More direct practices to recover or sustain sphagnum vegetation is to establish rafts of floating brush or clumps of peat for floating mosses settle, as well as the creation of suitable microclimates through techniques such as applying a layer of straw mulch, and carving depressions that promote carpet or lawn level Sphagnum (references in Limpens et al. 2008 and in Rydin & Jeglum 2006).
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