Habitat description
Shrub swamps are wetlands dominated by woody plants <1 to 5 meters tall on seasonally or temporarily saturated soils. Shrub height may be uniform or mixed, and shrub density can vary from high (>75% cover) to somewhat open, with herbaceous vegetation or open water areas between shrubs. This common, widespread habitat can be found in lowlands, along the margins of rivers, streams and other waterbodies, and along or within forested or herbaceous-dominated wetlands. These wetland shrub thickets are generally flooded in spring and early summer, with water levels dropping below the soil surface by late summer or early fall. Since shrubs often form dense thickets, the herbaceous layer of shrub swamps is generally sparse and species poor. Natural disturbances including seasonally high water and beaver activity, limit the presence of trees which can be absent or scattered.
Characteristic natural communities
Shrub swamps can be divided into distinct natural communities which are highly variable due to the influences of microclimate, topography, hydrologic regimes, amount and types of mineral enrichment in surface- and groundwater, and past land use. They may be referred to as scrub-shrub wetlands, alder thickets, and buttonbush swamps. In MassWildlife’s habitat classification, certain wetlands that are shrub dominated and have characteristics such as peat accumulation or high pH from calcareous bedrock are included in acidic peatlands and calcareous wetlands habitats.
Learn more about the natural communities found in shrub swamps:
- Highbush Blueberry Thicket (S4)
- Shrub Swamp (S5)
Natural communities are given state rarity/imperilment ranks ranging from S1-S5 (S1: rarest/most imperiled).
Characteristic plants and animals
Depending on site characteristics, shrub swamps can be dominated by one of, or a combination of, the following species: alders, sweet pepperbush, buttonbush, winterberry, highbush blueberry, swamp azalea, maleberry, dogwoods, arrow-woods, meadowsweet, spicebush, willows, greenbrier, and the nonnative European alder-buckthorns. Scattered red maple or gray birch saplings can also occur. Since shrubs often form dense thickets, the herbaceous layer of shrub swamps is often sparse and species poor, and might include skunk cabbage, various ferns, sedges, and sphagnum moss, with common arrowhead in wetter areas. Water-willow grows in the more open areas of shrub swamps.
Several species of greatest conservation need are associated with shrub swamps. Shrub swamps provide some of the most productive breeding habitat for amphibians in Massachusetts, and many function as vernal pools. They also serve as important breeding or migratory stopover habitat for birds. Some shrub swamp plants serve as hosts for rare moths and butterflies such as the pale green pinion and precious underwing.
Associated habitats
Shrub swamps often occur in complexes in association with other wetland types but may also occur in isolation surrounded by uplands. They may occur with marshes or wet meadows, riparian and floodplain, vernal pools, acidic peatlands, and forested swamps. In MassWildlife’s habitat classification, certain wetlands that are shrub dominated and have characteristics such as peat accumulation or high pH from calcareous bedrock are included in acidic peatlands and calcareous wetlands habitats.
Ecological processes
Changes in the frequency, severity, and duration of flooding or saturated soil, beaver influence, and ice scour are natural disturbance processes that maintain and create shrub swamp habitat. Without these limiting factors, shrub swamps will become forested over time through succession.
Beaver play a significant role in creating and maintaining shrub swamps. When beaver populations locally deplete food resources, dams may be abandoned, resulting in succession to wet meadow, then to shrubland, and finally into forest. The ecological process begins again when beaver recolonize low-lying sites. This cycle has been much reduced in areas that have been developed, and where beaver flooding is controlled, reducing the natural processes associated with flooding regimes.
Threats
Development can lead to direct alteration of shrub swamps as well as indirect effects on water quality (pollution), and water quantity (natural system modification). Water withdrawals and diversions, undersized culverts, increased cover of impervious surface associated with development, and beaver control can alter natural patterns of water level fluctuation that maintain and create shrub swamp habitat. Nutrient inputs and other pollutants from development and agriculture can impact water quality. Invasive species like buckthorn, grey willow, and purple loosestrife can also pose a significant threat.
Climate change may impact this habitat in a number of ways. While total precipitation is expected to increase, other common predictions include warmer temperatures, longer and more severe summer droughts, shorter but more intense winter/spring floods, and reduced extent and duration of winter snow cover. Such changes could alter the hydrological regimes of many shrub swamps in the region. Expected outcomes include seasonal drying of wetland soils, which could facilitate changes in dominant vegetation. Smaller shrub swamps could be lost entirely, while those that are larger could contract in area or become fragmented. There is also a possibility of a net gain in shrub-swamp habitat. Recent research indicates that the last two decades have been the wettest years in the Northeast in 500 years. This has led to higher groundwater elevations and may lead to an increase in the extent of this habitat, but areas of lower elevation in the current extent of the habitat may flood, potentially reducing shrubby vegetation in those areas.
Conservation actions
- Proactive habitat protection: Protect land around shrub swamps, prioritizing sites supporting state-listed animals and other SGCN, as well as other wetland protection priorities identified in BioMap and other conservation planning tools (e.g., municipal open space plans).
- Habitat restoration and management: (see below)
- Law and policy: Regulate and limit the impacts of development, pollutants, and water withdrawals. Innovative approaches to incentivizing compatible development and managing water withdrawals and water quality should be considered where applicable.
- Conservation planning: Include key shrub dominated wetlands and wetland complexes in conservation planning efforts at multiple spatial scales. (See BioMap as an example.)
- Monitoring and research. Monitor the health and trends of SGCN populations, plant communities, and other wildlife. Monitoring the effectiveness of habitat restoration to inform adaptive and cost-effective management. Conduct targeted research to improve habitat and population management.
- Public outreach: Include information about the role shrub swamps and associated wetland complexes play in biodiversity conservation, flood mitigation, water filtering, and climate resiliency as part of broader communication strategies.
Restoration & management recommendations
The following is an overview of restoration options; get details about specific management practices by clicking on the provided links.
Water quantity restoration: Increased runoff from development, and artificially constricted outflows (causeways, undersized culverts, etc.) are stressors that can alter the hydrology of shrub swamps. Culvert replacement and/or dam removal and restoration may be appropriate restoration activities in some cases. Conversely, reducing the natural amount of water on a site (e.g., historic ditching, excessive groundwater extraction) can also result in harm to the community by drying out the underlying soils. Partial or complete filling of ditches may be appropriate mitigation strategies in some cases. It should be noted that 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 quality restoration: There are many anthropogenic activities that can alter the chemistry of shrub swamps, often resulting in an increase of invasive plants. Nutrient loading from agricultural and residential runoff, as well as the chemical inputs from nearby roads (especially road salt), can result in compositional changes to the plant community. Reducing unnatural chemical inputs from point and non-point sources is a key remediation practice. Addressing altered hydrology, such as culverts and drainage swales that contribute to chemical runoff and managing symptoms like invasive and aggressive plant growth are essential actions for restoration.
Invasive plant control: Addressing invasive species in shrub swamp habitats at important areas for SGCN is a priority conservation action. Programs to proactively treat established invasive species are key to restoring important habitats and should be pursued whenever possible. Protocols to prevent the establishment of invasive species, either through controlling potential vectors (contaminated soil, landscaping, etc.) or addressing pioneering invasive populations through early-detection—rapid-response programs, are important ways of dealing with invasive species before they are impacting a habitat.
Selective tree removal: In limited circumstances, selective tree removal or other techniques to manage saplings may be beneficial. For example, at a site that supports a rare shrub swamp specialist species or a highly specialized shrub swamp community. Another example where this action could be considered is where a small forested swamp is fragmenting two otherwise adjacent large patches of open habitat.