Habitat description
Open-to-semi-open graminoid-to-shrub-dominated fens and wetlands that are greatly influenced by calcium-rich alkaline groundwater seepage derived from nearby bedrock or glacially-derived material. The key indicator of these communities is the prevalence of plants that depend on high calcium levels (“calciphytes”, ex. shrubby cinquefoil, hoary willow, rough-leaved goldenrod, etc.). Calcareous Wetland communities can either occur as stand-alone features, or as a part of a mosaic within a wetland system. The common traits of these communities are expressed along a continuum, and the individual communities within this continuum can be separated by their specific characteristics:
- Calcareous Sloping Fen: Relatively small wetlands dominated by grass-like, non-woody plants on slight-to-moderate slopes, featuring more mineral soil than peat, and with only occasional small patches of woody vegetation. Small hummocks often form between rivulets of flowing seepage.
- Calcareous Seepage Marsh: Often moderate-sized wetlands on flatter ground; usually in association with beaver flowages or the peripheries of other wetland features. Vegetation is often a mix of calciphytes and more generalized wetland species, and the structure ranges from that of emergent marsh to shrub swamp. Substrate is mucky.
- Calcareous Basin Fen: Peatlands dominated by grass-like non-woody plants occurring in well-defined basins. Basin Fens are the most nutrient poor of the Calcareous Wetlands, have well-developed moss communities, and, along with calciphytes, often support vegetation more often associated with acidic bogs and fens (ex. pitcher plant, sundew, sweet gale, etc.).
- Red Maple-Black Ash-Tamarack Calcareous Seepage Swamp: Semi-open-to-forested wetlands featuring a mix of trees (characteristically black ash and tamarack), shrubs and calciphytic non-woody plants, typically occurring in basins in association with streams. These communities feature well-defined hummocks with a rich moss community based upon deep mucky soils.
Although the chemical harshness of the Calcareous Wetland communities helps to slow generalist succession, occasional low severity fire is important to maintaining vegetation structure and composition over the long-term. That said, these communities are also extremely sensitive to other disturbances, such as changes to hydrology, water chemistry, and nutrient loading.
Associated habitat types
Because of their direct association with Calcium-rich substrates, Calcareous Wetland communities can often be found in loose association with nearby Red Maple - Black Ash - Bur Oak Swamp,Calcareous Rocky Summit/Rock Outcrop Community, and Rich, Mesic Forest communities.
Characteristic communities and species
More information on the specific natural communities (Swain 2020) within this habitat type is available from the following fact sheets:
- Calcareous Basin Fen (S1)
- Calcareous Seepage Marsh (S2)
- Calcareous Sloping Fen (S2)
- Red Maple-Black Ash-Tamarack Calcareous Seepage Swamp (S2)
Due to the combination of their extreme chemical harshness and their relative rarity on the landscape, Calcareous Wetland communities support some of the richest suites of specialized and imperiled species in Massachusetts.
Ecological processes
The primary driver of these communities is a direct influence from calcium-rich alkaline groundwater. The relative harshness of this groundwater (pH ranging from 6.0-8.1 with a high concentration of calcium and often Magnesium) creates an environment where only specialized (in this case calciphytic) vegetation can thrive. Occasional low severity fire can also be important to maintain the composition and structure of these communities (especially the graminoid/forb-dominated aspects) by further setting back seral and generalist succession. In the presence of calcium-rich alkaline groundwater and occasional fire, local site conditions will influence the expression of the specific community through such elements as topography, water-temperature, substrate condition and associated nutrients and chemical inputs.
Threats
Changes to hydrology, invasive species, succession, changes to water chemistry, and nutrient inputs are primary threats to this habitat type.
Calcareous Wetlands are also expected to be particularly sensitive to the impacts of Climate Change. Specifically, changes in precipitation and the seasonality of precipitation events, as well as changes to groundwater levels, may dramatically impact the delicate hydrology of these sites. Changes in precipitation may also impact the concentration of Calcium and other minerals, as well as the pH of ground and surface water at the community’s location. Additionally, increasing water temperatures may affect the decomposition process, which will have impacts on the peat-based community variants. Finally, an increase in atmospheric nitrogen deposition will further aid the establishment and spread of aggressive generalist and invasive species.
Restoration & management recommendations
Before undertaking a project, it is important to establish clear goals that are compatible with site conditions. Factors to consider include identifying the resources required for restoration and long-term maintenance, and securing community, stakeholder, and institutional support. For more information, please see: Habitat Management Priorities and Planning.
Opportunities for conserving Calcareous Wetlands is incredibly limited due to their overall rareness and conditional specificity. Therefore, protecting and/or restoring each extant occurrence in the state is now very important. Most of the Commonwealth’s larger sites are receiving at least some management, though many of the smaller sites are in need of attention.
Historically, Calcareous Wetland communities persisted on the landscape due to their chemical harshness, their exposure to periodic low-severity fire, and in some cases, episodic natural grazing. However, these communities are also very sensitive to many anthropogenic disturbances, and as a result, nearly all post-settlement occurrences need some level of restoration to address modern stressors. When considering management, the first step taken should be a thorough assessment of current site conditions and the identification of the specific stressors that need to be overcome. These communities are somewhat complex in their function, and in many instances, there will be multiple stressors impacting an occurrence. Therefore, it’s especially important to focus not just on the symptoms of compounding stressors, but to instead understand the root causes and work to address the underlying factors.
Water Quality and Quantity Restoration
- Hydrology: Calcareous Wetlands occur within a very narrow range of optimal groundwater-influenced hydrology. Therefore, even small changes to hydrology will have profound impacts on the vegetation composition and structure of the community. Having too much surface water at a site is a very typical modern stressor of these communities, and will often be expressed in obvious ways, such as pooling water and the aggressive presence of generalist species like cattails. Beaver dams at the outflow of sites has become common at many sites, and thoughtful remediation of beaver impacted hydrology can be an important management action. Increased runoff from development, and artificially constricted outflows (causeways, undersized culverts, etc.) are two other common stressors that regularly can oversaturate Calcareous Wetlands. Conversely, reducing the natural amount of water on a site can also result in harm to the community by drying the substrate and reducing the harsh chemical and pH influences on the site. Both of these outcomes will lead to a reduction in the calciphytic wetland obligate plants that define the community, and will often lead to a shift toward more generalist species dominance and changes to the overall community structure. Ditching (including historic ditching), up-flow restrictions and excessive groundwater extraction are all common ways that Calcareous Wetlands are stressed by reductions in the presence of groundwater. Partial or complete filling of ditches may be an appropriate mitigation strategy in some cases. It should be noted once hydrologic stressors are identified, any hydrologic remediation should only be undertaken after a thorough assessment of the site’s hydrology – including the expected outcome of the remediation – has been completed.
- Water chemistry: The chemical profile of any Calcareous Wetland is a delicate balance driven by natural groundwater influences that can be easily upset by unnatural inputs. There are myriad anthropogenic activities that can alter the chemical profile of these wetlands, and the outcome of these activities are typically expressed by an increase of aggressive native generalist and invasive plants, along with the decline of obligate calciphytes. Nutrient loading from agricultural and residential runoff, as well as the chemical inputs from nearby roads (especially road salt), can result in major compositional changes to a site. The modern increases in atmospheric deposition of elements such as nitrogen can also have important negative consequences to the integrity of Calcareous Wetlands. Reducing unnatural chemical inputs to a site by reducing these inputs in the greater watershed is a key remediation practice. Additionally, understanding the hydrology of a site can also greatly assist in the reduction of these inputs by identifying direct source points, and when possible, correcting altered hydrological inputs that are delivering these chemical inputs (such as culverts and drainage swales). In cases where addressing the direct chemical input is not possible (such as with atmospheric deposition), addressing the symptom caused by these inputs is the key action (such as treating invasive and aggressive native generalist plants).
- Invasive Plant Control: The need for vegetation control in Calcareous Wetlands is typically the result of one or more stressors to that community (especially altered hydrology and/or chemistry). Addressing the negative vegetative expressions (increases in invasive and aggressive native generalists) is critical in maintaining the long-term viability of these occurrences, but this approach should also be coupled with trying to understand and ultimately remediate the cause of the stress. In other words, focusing only on the symptom of the stress will never result in actually overcoming the degradation caused by the source of the stress. Below are considerations for dealing with common invasive and aggressive native generalist plant situations in Calcareous Wetlands:
- Cattail: Though cattails are mostly native to Massachusetts (an aggressive non-native hybrid is becoming a serious and growing issue), the presence of cattail in a Calcareous Wetland is strong indicator of hydrologic and/or chemical stressors in that system (especially nutrient loading and elevated water levels). Unchecked, cattail will quickly invade these systems and convert them to near monocultures. Addressing hydrologic and chemical stressors is critical for the long-term integrity of the site, but this alone will not eliminate extant cattail occurrences. Herbicide - specifically glyphosate in the late summer – is an important tool for controlling invading cattail occurrences (expect it to take several years of treatment for established stands). Once cattail monocultures are reduced by herbicide application, their footprints will be quickly recolonized by the desirable seedbank of native obligates. Careful foliar applications are appropriate in monocultures, while more discrete applications (like glove wipe) are appropriate in lower density situations.
- Clonal Forbs and Graminoids: Like with cattails, the dominance of other clonal generalist native plants is signaling stress in a Calcareous Wetland. Clonal plants - like sensitive fern, lake sedge, and some species of asters and goldenrods - can quickly overwhelm a stressed fen and greatly reduce the viability of associated calciphytic obligates. Understanding and addressing the underlying stressor is important (altered hydrology, adverse chemical inputs, lack of appropriate disturbance such as fire), just as it’s important to directly treat the invading vegetation. Thoughtful use of herbicide is typically the best way to directly address these situations, and a best-case management scenario will also include remediating any underlying stressors.
- Pioneering Native Woody Vegetation: Historically, the chemical harshness of these sites, along with periodic fire on the landscape, worked in tandem to suppress woody vegetation. In modern times, however, altered hydrology and the absence of fire have often led to an increase in woody vegetation in these systems. This changes wetland structure and greatly reduces the viability of calciphytic obligates. In Calcareous Wetlands with flowing surface water, an increase in woody vegetation can also attract beavers to the site, who will quickly restrict the water flow and alter the site’s hydrology. Cutting and treating the stumps of generalist woody vegetation, or carefully applying a foliar herbicide, are two effectives ways of dealing with established generalist woody vegetation. However, it should be noted that all Calcareous Wetland communities support associated obligate calciphytic shrubs (ex. hoary and autumn willow, poison sumac, etc.) that should not be eliminated. Staying ahead of woody succession in currently functioning Calcareous Wetlands is an important pro-active step in maintaining community integrity, and can be accomplished through the application of prescribed fire, and in some cases, very carefully planned grazing.
- Invasive Plants: Phragmites, red canary grass, glossy buckthorn and purple loosestrife are the primary invasive species that impact Calcareous wetlands in Massachusetts. However, many of the invasive species known to occur in Massachusetts can be present and adversely affect these communities; even species that are generally considered to be indicators of terrestrial environments. The careful application of herbicide is be the most effective way to treat nearly all invasive species occurrences in Calcareous Wetlands, though it should be expected that it will take several years of treatment to completely eradicate any established invasive occurrences due to persistent seedbanks and the clonal nature of certain species. It should also be noted that the continued presence of invasive species in Calcareous Wetlands may be an indicator of an underlying stressor such as altered hydrology or unnatural chemical inputs, and therefore addressing both the invasive species and the underlying stressors is critical to restoring the long-term viability of a site.
- Prescribed Fire: Periodic low severity fire was an historically important element in maintaining the structure and specialized composition of Calcareous Wetland communities. Fire suppresses the establishment of generalist woody vegetation and reduces thatch buildup that can otherwise inhibit the germination of specialist plants, as well as result in the alteration of soil composition and chemistry. Whenever possible, prescribed fire should be considered an important part of any long-term management plan related to Calcareous Wetlands. However, burning in these communities poses a unique set of challenges, and attention to burn severity is especially important so that fire doesn’t ignite underlying peat.
- Grazing in Calcareous Sloping Fens: The thoughtful application of grazing can be a useful tool to aid the initial restoration of certain Calcareous Sloping Fen occurrences that have shifted toward a more generalist composition. The other Calcareous Wetland communities are typically too wet, have too sensitive a substrate, and/or have an incompatible structure/composition to reasonably support grazing. Grazing in certain sloping fens can be especially effective at suppressing/reducing woody vegetation and stressing clonal vegetation that is outcompeting obligate calciphytes. However, grazing alone will not be an effective long-term strategy for maintaining the integrity of these communities, as the continued pressure will eventually negatively impact calciphytic obligates while introducing a continually increasing nutrient load to the system. Grazing may also inadvertently introduce invasive species to a system, either through seeds transported within an animal’s fur, hooves or digestive tract, or through supplementary feed brought to a grazed site. Introducing grazing to a sloping fen for management purposes is a complex proposition, and finding a prescription that balances grazing pressure, nutrient inputs, physical disturbance to the substrate, and vegetative response is not only difficult, but also requires close monitoring and the ability to remain constantly adaptive in response to regularly observed results. In other words, grazing might be a useful tool in certain circumstances, but only a very well-thought-out approach with clear and adaptable objectives should even be considered. Even in situations where grazing is deemed a reasonable approach, it should be viewed as a complementary activity used in tandem with other techniques, such as herbicide treatment, prescribed fire and hydrologic remediation.
Additional resources
- Coming soon