Ecological dynamics

Mucks form where net primary productivity greatly exceeds the rate of decomposition due to the lack of oxygen under seasonally or semipermanently saturated conditions. Fens occur under semipermanently saturated conditions, whereas wet meadows prevail under seasonally saturated conditions. Peat accumulation in fens is mostly from dead sedges (Cyperaceae) and brown mosses (Scorpidium scorpioides), while in acid bog sites it is due to peat mosses (Sphagnum spp.). More seasonal hydrology allows more decomposition of brown peat into black muck, and a greater production of plant cover. The thickness of peat and muck is directly related to the duration of non-saturated conditions, whereas longer periods of dryness would tend to result in more thorough decay of organic matter, and a thinner veneer of muck over mineral soil. However, hydrology may change over time, and current muck thickness is as much a legacy of historic hydrology as it is an indicator of current hydrology. Thus, the relationship to present vegetation is only loosely related to muck thickness.

In submergent areas high in dissolved calcium, the green algal genus Chara precipitates calcium carbonate within its cell walls. Over time, dead Chara contributes to the accumulation of marl, which may later become buried in peat and muck.

Michigan Natural Features Inventory (Kost et al., 2010) describes a range of communities that occupy Mucky Depressions, which are arrayed along hydrologic, nutrient, and structural development gradients. Succession from open communities to shrub-dominated through forested communities is kept in check or reversed by fire, beaver activity, and windthrow. However, the rate of succession is slow in the wettest areas, and in areas with more extreme alkalinities or acidities that inhibit nutrient availability.

Fire frequency generally is greatest where adjacent upland sites are oak savanna and prairie rather than closed forest, which was historically the case in the southern half of the MRLA, where prairie fens and wet meadows were most common. Fire has the effect of killing aboveground biomass, and thus conferring an advantage of herbaceous plants over woody species. Graminoids in these open communities provide the main fuels, presuming that standing dead biomass is dry enough to sustain fire. Once a site becomes forested, low understory graminoid cover and thin leaf litter that quickly integrates into the substrate tend to reduce the potential for fire to carry.
The lack of fire adaptation among the dominant tree species is evident from the generally thin bark among the maples.

Beavers (Castor canadensis) affect succession in two distinct ways. First, beavers directly remove trees and shrubs both as a food source and as building materials. Second, beavers may inundate an area via dam building, causing mortality in species of trees and shrubs that are less flood-tolerant. Mucks that occur adjacent to flowing water have the greatest opportunity for beavers to modify water levels.

Windthrow mostly creates single tree gaps. However, swamp forests are more susceptible to large blowdowns than upland sites, because high water tables limit rooting depth and organic substrates are structurally less stable.

The interaction among all these disturbance regimes, including fire, is summarized in the narratives for the broadly defined Central Interior and Appalachian Swamp Systems, and Central Interior and Appalachian Shrub-Herbaceous Wetland Systems Landfire models (USGS, 2008). The ecosystems assignable to swamps were modeled to have a mean fire return interval (probability) of 1,000 years, a beaver flooding interval of 300 years, a patch windthrow interval of 100 years, and a catastrophic windthrow of 600 years. The systems assignable to fens and wet meadows, were modeled to have a fire return interval of 10 years, and a beaver flooding interval of 200 years. A 1,000 year return interval isn’t intended to imply an event taking place regularly every 1,000 years, but rather the probability of 1 out of 1,000 (0.1%) for the event to take place in any given year. As such different patches of the site may not experience the same event simultaneously such that across a large enough area in can be expected that 0.1% of the area could experience the event during an average year. But processes of species dispersal, minimum viable population size, and the species composition of the surrounding landscape (e.g., Driscoll, et al., 2013) must be considered in context of fragmented habitat.

Zonation along a hydrologic gradient is responsible for the maintenance of contrasting plant communities in relative proximity. This also allows for vegetation to respond to short-term changes in climate given the immediate seed source for species in adjacent communities. Several years of increased potential evapotranspiration and lower precipitation can lower local water tables and thereby move the boundaries between zones. These drier conditions are represented by community pathways toward less hydric communities such as from marsh to wet meadows. Likewise, several years with decreased potential evapotranspiration and higher precipitation can raise local water tables, resulting in a shift toward more hydric communities.

The wettest sites are represented by submergent marsh (Nuphar advena - Nymphaea odorata Aquatic Vegetation), but aquatic vegetation such as this are more representative of subaqueous ecological sites rather than Mucky Depressions. Nevertheless, the elements of aquatic vegetation may occur locally among emergent communities.

The emergent marsh (Schoenoplectus tabernaemontani - Typha spp. - (Sparganium spp., Juncus spp.) Marsh, Typha spp. Midwest Marsh) community represents the wettest sites normally found in Mucky Depressions, which are dominated by cattails and bulrushes. Marshes are typically ponded to at least 15 cm during part of the growing season. At higher macronutrient (nitrogen and phosphorous) loadings, cattail productivity may overwhelm the dominance of all other species. Native cattail (Typha latifolia) is outcompeted by non-native cattail (Typha angustifolia) and its hybrid (Typha ×glauca), particularly in higher nutrient or saline conditions. Factors that increase nutrient runoff include agriculture, while road salt is the main cause of saline conditions. The green alga, Chara, is an important submergent component of calcareous marshes. Chara is intolerant of high phosphate or nitrate concentrations, and may disappear when water becomes polluted (Lambert and Davy, 2011).

In areas that are seasonally saturated, southern wet meadow (Carex stricta - Carex spp. Wet Meadow) may form. Wet meadows are dominated by grasses and sedges like Carex stricta, Carex lacustris, Calamagrostis canadensis, but also have a diversity of forbs, including joe pye weed(Eutrochium maculatum), swamp aster (Symphyotrichum puniceum), rough-leaf goldenrod (Solidago patula), and mountain-mint (Pycnanthemum virginianum).

Without beaver activity and occasional fire spilling over from the uplands maintaining the open conditions, trees and shrubs come to dominate the site within 10 to 60 years (USGS, 2011). Marshy sites within Mucky Depression may succeed to a buttonbush (Cephalanthus occidentalis) dominated inundated shrub swamp (Cephalanthus occidentalis / Carex spp. Northern Shrub Swamp) without sufficient disturbance. Wet meadows succeed to shrub-carr (Cornus sericea - Salix spp. - (Rosa palustris) Shrub Swamp).

In the absence of beaver flooding, trees become established and the community succeeds to hardwood swamp (Acer (rubrum, saccharinum) - Fraxinus spp. - Ulmus americana Swamp Forest) after about 80 years. Fluctuating water table favors flood-tolerant silver maple (Acer saccharinum) and green ash (Fraxinus pennsylvanica), and is convergent with floodplain forest in species composition. This community has relatively high fertility in terms of macronutrients like nitrogen and phosphorous. The circumstances responsible for higher fertility in this community relates to pH and hydrology. Having intermediate pH levels (6-7) results in greater nutrient availability, while seasonal fluctuation in water table (oxygenation) can allow a greater turnover in macronutrients accumulated within the substrate. Sites with stable water tables tend to be dominated by red maple (Acer rubrum) and black ash (Fraxinus nigra), and is similar to other peatland communities

in species composition, being relatively nutrient poor. Areas with stable water tables along the northern and western borders may have conifers such as white pine (Pinus strobus) and eastern hemlock (Tsuga canadensis) for hardwood-conifer swamp (Pinus strobus - (Acer rubrum) / Osmunda spp. Swamp Forest). Tree and shrub establishment is most favored in less frequently inundated areas such as on microtopographic highs, which develop later in succession from upturned root masses of wind-toppled trees. Therefore, the wettest portions of a site require longer times for forest to become established.

The prairie fen (Dasiphora fruticosa ssp. floribunda / Carex sterilis - Andropogon gerardi - Arnoglossum plantagineum Fen, Cladium mariscoides - (Carex lasiocarpa, Hypericum kalmianum, Oligoneuron riddellii, Eleocharis elliptica) Fen) occupies areas in which groundwater maintains an almost constant soil saturation without ponding. Topographically elevated sites within the basin may have exposed marl that is more seasonally saturated. Fens support a unique short-statured flora of calcium specialists (calciphiles). The low nutrient environment is driven in part by the high pH levels; the extremely high levels of calcium limits the solubility of phosphorous and iron. The constant hydroperiod from a flow of groundwater maintains anoxic conditions that prevent the release organically stored nutrients.

Without disturbance, fens may succeed through shrub swamp phases to rich tamarack swamp (Larix laricina - Acer rubrum / (Rhamnus alnifolia, Vaccinium corymbosum) Swamp Forest). Continued succession will result in shade-intolerant tamarack (Larix laricina) to be replaced with shade-tolerant red maple (Acer rubrum), and potentially to a version of hardwood swamp. Under prior cooler climatic periods, northern whitecedar (Thuja occidentalis) would succeed tamarack as rich conifer swamp (Thuja occidentalis - Acer rubrum / Cornus sericea Swamp Forest). Rich Conifer Swamp was formerly infrequent in MLRA 98 prior to being cutover, but is more prevalent in adjacent MLRAs to the north (MLRAs 94A and 96). Relict stands of northern whitecedar do still exist in this MLRA, but after widespread logging excessive browse due to increased populations of white-tailed deer (Odocoileus virginianus) has severely limited its ability to regenerate.

More acidic conditions are transitional to peat bog (Chamaedaphne calyculata / Carex oligosperma - Eriophorum virginicum Acidic Peatland), and poor conifer swamp (Picea mariana / Ledum groenlandicum / Carex trisperma / Sphagnum spp. Open Bog Woodland), or another version of hardwood swamp (Acer rubrum, saccharinum) - Fraxinus spp. - Ulmus americana Swamp Forest) with acid indicators like Quercus palustris, Nyssa sylvatica, and Vaccinium corymbosum.

Rare areas with brackish groundwater influenced by Paleozoic salt deposits may be recognized as the Inland Salt Marsh ecological site represented by the Schoenoplectus maritimus - Atriplex patula - Eleocharis parvula Saline Marsh association. However, at present these saline sites have not been delineated as their own map units.

Unconverted stands of wet meadows, marshes, and swamps can be degraded by the addition of excess nutrient runoff, altering species composition; or through the introduction of Eurasian exotics such as purple loosestrife (Lythrum salicaria), reed canarygrass (Phalaris arundinacea), common reed (Phragmites australis ssp. australis), and glossy buckthorn (Frangula alnus).

Conversion to cropland involves the drainage of the land, followed by removal of vegetation, plowing the upper 20 to 30 cm of soil, and introduction of fertilizer. Results of an unpublished dynamic soil properties study conducted in 2015 by the Grand Rapids Soil Survey Office (in cooperation with the National Soil Science Lab in Lincoln, NE), suggests that farming mucky soils can result in soil becoming heavily compacted, high in nitrates and low in carbon-to-nitrogen ratios. Sites restored with native plants and with the hydrology partially restored, retained the higher nutrient levels and compaction characteristic of actively cultivated sites. Despite restoration with native plants, formerly cultivated sites may have significant coverage of invasive reed canarygrass (Phalaris arundinacea). Although opportunities of seed dispersal and disturbances are factors in invasive species presence, other studies suggest a connection with the high nutrient loads (Green and Galowitch, 2002).

The emerald ash borer (Agrilus planipennis) is causing a major transition in plant composition through the mortality of green ash (Fraxinus pennsylvanica) and black ash (Fraxinus nigra) from silver maple-green ash hardwood swamp to red maple-tamarack swamp phases respectively. The introduced pest causes 100 percent mortality of stems greater than 2.5 cm in diameter within five to ten years of infestation. This mortality will reduce the frequency of reproductively mature trees and thereby severely limit new seed production. As of 2020, many former ash stands have no living ash stems greater than 10 cm, but have vigorous seedling and sapling layers, and some basal sprouts from adult trees. There are some signs of success from biocontrol studies using introduced parasitoid wasps, but models of worst-case scenarios show the potential extirpation of ash populations within the next 30 years (Kappler et al., 2019).