Ecological dynamics
Parent material is fibric and/or herbaceous organic material more than 40 centimeters thick. These sites usually formed in depressions on glacial moraines and lacustrine and outwash plains, where underlying impermeable layers minimize groundwater movement through the peat. Slopes are less than 2%. The surface water on-site is very acidic (pH <4.2), and mineral concentrations (particularly Ca++) are extremely low. Mean annual precipitation is about 30 inches, and mean annual temperature is about 43 degrees F. The water table is usually at or near the surface but can drop during periods of drought. The main source of water to the site is precipitation, since the site formed through a buildup of peat over thousands of years, causing the soil surface to develop a crest shape with sloping concave sides, and therefore to become isolated from mineral-rich runoff or subsurface flow. Water does pond on these sites, but it is not frequently ponded for very long durations for at least 7 out of 12 months.
For the past 6,000 years the climate of Minnesota has favored the swamping, or paludification, of terrestrial sites (MN DNR 2003). This process, led by stabilization and rising of water tables, peat accumulation, and increased acidification by Sphagnum mosses, elevates the soil surface above groundwater. Precipitation then becomes the primary source of water for plants, and dust (dryfall) the primary source of nutrients. The resulting environment favors self-perpetuation of peat accumulation (rather than nutrient recycling) and further acidification. This process began thousands of years ago in rich forested swamps or poor fens, and typically followed a unidirectional succession over centuries starting with the Young Tamarack phase and succeeding through a Mature Tamarack-Spruce phase to a Spruce Bog (MN DNR 2003).
However, some disturbances can reset this cycle, reverting the acidification process, resulting in richer communities. In Acid Peatlands, fires were not very common historically, perhaps because of the landscape position and wetness of peatland basins. Public Land Survey (PLS) records indicate that catastrophic fires had a recurrence interval somewhere between 500 - 1,000 years, and lighter (superficial) fires about every 90 - 120 years, likely during drought (MNDNR 2003). If fires were intense enough to kill the overstory and burn the peat, releasing nutrients and allowing even a small connection back to the groundwater, the site could be converted to a rich swamp (MNDNR Forestry). Windthrow was also a common, although typically minor, source of disturbance for the site (90 – 120 years), with catastrophic windthrow occurring approximately every 500 – 700 years. Recovery post-disturbance to forested conditions may take decades in these peatlands. Historically, caribou and moose would utilize these sites for forage, thermal regulation, and as migratory paths. These trails are still evident today (due in large part to the damage repeated compaction can do to peat) even though caribou were extirpated from Minnesota in the 1940s (MN DNR Forestry).
State 1
Reference State
Moss cover is very indicative for this site, driving the successional dynamics and species diversity. Sphagnum sp. absorb dissolved mineral cations, and release organic acids, which lowers the pH of stagnant surface water below 5.0 (MN DNR 2003). Sphagnum tends to form carpets of hummocks, which creates an acidic environment cut off from groundwater and on-site ponding. In recently developed bogs, on sites mostly in the western range of the MLRA where soils are less continuously saturated, on sites where water table fluctuations are less variable, or on inclusions of sites where there are upwellings of groundwater, isolated minerotrophic species (i.e. creeping sedge or bluejoint) may be present. Other ground cover includes fine-leaved graminoids, and minimal presence of forbs. Occasionally, seedlings of deciduous tree species associated with adjacent sites (i.e. red maple or paper birch) may become established on Sphagnum hummocks but typically do not survive to become saplings or trees (MN DNR 2003). The overstory usually consists of scattered, stunted (<30ft [10m] tall) black spruce or tamarack (<50% cover) where Sphagum hummocks provide relief from surface flooding (MN DNR Forestry). Most vascular plants associated with Acid Peatlands have a strong association with mycorrhizal fungi, depending on them to obtain minerals and nutrients in this depauperate and harsh environment (MN DNR 2003). These fungi are more diverse on this ecological site than on other sites throughout the region.
Dominant plant species
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black spruce (Picea mariana), tree
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tamarack (Larix laricina), tree
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bog willow (Salix pedicellaris), shrub
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creeping snowberry (Gaultheria hispidula), shrub
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creeping sedge (Carex chordorrhiza), grass
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bluejoint (Calamagrostis canadensis), grass
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sphagnum (Sphagnum fuscum), other herbaceous
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sphagnum (Sphagnum angustifolium), other herbaceous
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papillose sphagnum (Sphagnum papillosum), other herbaceous
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sphagnum (Sphagnum subsecundum), other herbaceous
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sphagnum (Sphagnum majus), other herbaceous
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polytrichum moss (Polytrichum strictum), other herbaceous
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aulacomnium moss (Aulacomnium palustre), other herbaceous
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Schreber's big red stem moss (Pleurozium schreberi), other herbaceous
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(Cladopodiella fluitans), other herbaceous
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Fern or Fern Ally (Fern or Fern Ally), other herbaceous
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clubmoss (Lycopodiella), other herbaceous
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Liverwort (Liverwort), other herbaceous
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bunchberry dogwood (Cornus canadensis), other herbaceous
Community 1.1
Mature Tamarack-Spruce
Overstory occasionally has tamarack but is mostly dominated by black spruce, most likely due to the longer lifespan of the spruce trees. In cases where tamarack is monotypic, it is usually due to a lack of seed source for black spruce, or mortality of black spruce caused by species-specific insects or disease (MN DNR Forestry). Sometimes, paper birch can be present in the canopy. In this phase, tree growth is fairly stagnant, although stocking and tree height are higher than in the Young Tamarack phase (1.4). The moss layer is dominated by Sphagnum sp., between which in the hollows can be found brown mosses. In cases where tree growth exceeds 50% canopy cover, more shade-tolerant species can be present in the understory, including creeping snowberry, soft-leaved sedge, ferns, clubmosses, groundpines, and bunchberry (MN DNR 2003).
Community 1.2
Spruce Bog
Some scattered tamarack may still remain in the overstory, but mostly the sites are dominated by black spruce. On well-developed crests of older sites, trees can reach up to 30 ft tall, making them more desirable for timber harvesting. While the few graminoids and forbs may remain the same as in 1.1, fewseed sedge and boreal bog sedge are more indicative of this phase. In this phase, the Sphagnum has accumulated and typically forms large carpets with greater hummocks, along with Pleurozium. In this plant community phase acidity is the lowest of all of the plant community phase. Only true bog species remain, with no minerotrophic indicator species left.
Community 1.3
Poor Fen
In this phase, the overstory is often stunted (<30 ft [10m]). This phase is characterized by the presence of light-loving species that can tolerate more water-logged conditions, where the water source may contain more nutrients. In general, the elevated water table inhibits the establishment or growth of tree species, although with enough time, Sphagnum hummocks can provide a refuge for further tree growth or recruitment (MN DNR 2003). Shrub cover can vary greatly, but the presence of a few minerotrophic species (bog willow) can help distinguish this phase from the Spruce Bog (1.2). Moss and liverwort cover is almost always near 100%, and can include oligotrophic Sphagnum angustifolium and S. papillosum in hollows, as well as minertrophic S. subsecundum, S. majus forming carpets. Polytrichum strictum, Aulacomnium palustre, and Pleurozium schreberi are also common, as well as the liverwort Cladopodiella fluitans, which is an indicator for this phase.
Community 1.4
Young Tamarack
Pathway 1.1A
Community 1.1 to 1.2
Lack of fire, accumulation of peat, lowering of pH<5.0
Pathway 1.1B
Community 1.1 to 1.4
Surface fires or windthrow or higher that average precipitation
Pathway 1.2B
Community 1.2 to 1.1
Neutralization of acids on-site raising of pH
Context dependence. Alterations in hydrology which result in the alkalization of bogs, or fire disturbances that can result in mineral inputs from outside sources that offset depletion of cations (especially Ca++) by Sphagnum sp, (even groundwater inputs of <5% of the total water budget relative to precipitation) can revert the process of peat accumulation and acidification, and result in a community type with more minerotrophic
Pathway 1.2A
Community 1.2 to 1.3
Fire, if peat depth is lowered and thereby allowing roots of plants to access groundwater or higher than average precipitation.
Context dependence. Typically, the interval of surface fires are approximately 120 years. Pockets of poor fen vegetation can develop in a spruce bog where alkalization of the bog occurs on the lower flanks of the crest, where the water source can receive nutrients from surface runoff, or where the runoff is channeled into drains or water tracks. The transition of the entire site to poor fen can occur on a broader scale when fire release nutrients and burn peat, reducing tree cover, reducing evapotranspiration and increasing water-logged conditions, resulting in even minimal groundwater and nutrient inputs to the site.
Pathway 1.3A
Community 1.3 to 1.4
Time, accumulation of peat.
Context dependence. This pathway usually occurs naturally with enough time for peat to accumulate and elevate the growing surface out of water-logged conditions, so that tree growth and recruitment can be more successful.
Pathway 1.4A
Community 1.4 to 1.1
Time, accumulation of peat, acidification, establishment and growth of older trees.
Pathway 1.4B
Community 1.4 to 1.3
Severe catastrophic fire or windthrow or higher that average precipitation
State 2
Logged State
In this state, the vegetation usually exhibits a structural pattern of “hard” edges which does not mimic the patch-scale distribution of canopy gaps we see from natural disturbances. Compositional changes in this state, as a diversion from reference, can be concerning, most notably the reversal of abundance of tamarack and black spruce (MN DNR Forestry). Tamarack regeneration has proven problematic for management, and can be susceptible to devastating outbreaks of larch sawfly and larch beetles. Tamarack regeneration in this state could be accomplished by leaving some tamarack seedtrees rather than always clear-cutting and seeding just black spruce (MN DNR Forestry). Harvesting should always be done in this state when the entire peat surface is frozen, although even then just a few passes with heavy equipment can damage the structural integrity of the site, or damage standing trees, thus prohibiting tree regeneration or maintenance on-site. In the field, areas identified by the presence of broad-leaved sedges and rough alder have far less structural integrity than the Sphagnum mat and should be avoided at all costs by heavy equipment. When dwarf mistletoe is present, control of the disease through broadcase burning, or by use of the “5 foot cutting rule”, can eradicate the disease, but success is dependent on total elimination of all living black spruce, and treatments (hand cutting, winter shearing, herbicides, combination treatments) need to be continued for 10 years after the initial harvest (MN DNR Forestry).
State 3
Impounded State
In this state, the excess of water on-site for longer than normal duration typically results in the killing off of tamarack and spruce trees, and the establishment of invasive species such as Typha sp. or reed canary grass (Phalaris arundinacea). Sometimes, other graminoids (Carex sp.) can remain on site and form floating mats. Sometimes alder and willow shrubs will remain or establish on the edges.
State 4
Open Bog State
Possible open water. Develops where the peat becomes isolated from mineral rich runoff or groundwater. Mineral and nutrient inputs come from precipitation and deposition. The saturated conditions and quick accumulation of Sphagnum prevent or inhibit establishment/growth of black spruce and tamarack. Variation in species composition in the community occur.
Transition T1A
State 1 to 2
This transition involves logging, usually clearcutting, but with reserves for seed sources, site preparation, control for disease and invasive species, and seeding, in order to maintain the site as a viable commercial timber harvesting state. This transition is only possible, or desirable, when the management is applied to phases in which radial growth has been prolific and trees have reached maturity, exhibiting taller, more commercially viable trees.
Transition T1B
State 1 to 3
Impoundment or maintenance of water on-site, and/or establishment of invasive species. Beaver activity, roads, blocked drainages, and other alterations in hydrology can transition the Acid Peatlands out of Reference to an Impounded State, where water is ponded on site for longer durations and receives excessive nutrients from overland surface flow.
Transition T1C
State 1 to 4
Impoundment or maintenance of water on-site. Beaver activity, roads, drainage, and other alterations in hydrology can transition the Acid Peatlands out of Reference to an Open State, where water is on site for longer durations and receives excessive nutrients from overland surface flow causing stunted tree growth.
Transition T2A
State 2 to 3
Poorly logged peatlands that are excessively clearcut combined with severe rutting or road building can result in impoundment.
Restoration pathway R3A
State 3 to 1
Unblocking road culverts that cause ponding, filling in drainage ditches that are perennially blocked and impounded by beaver activity, etc.
Restoration pathway R4A
State 4 to 1
Draining or maintenance of water on-site causing alterations in hydrology that can transition the Open state back to the Reference State, where water is on site for shorter durations and receives less nutrients from overland surface flow causing increased tree growth
