Our Science and Stewardship Department has been busy re-envisioning the future of Windswept Bog, one of the Foundation’s most popular properties, since cranberry cultivation was retired at this site in early 2019. We have been working with restoration ecologists at the Massachusetts Division of Ecological Restoration’s Cranberry Bog Program (Mass. DER) and soil scientists at the USDA Natural Resource Conservation Service (USDA) on preliminary concept plans. Once completed, these plans will then enable us to proceed with engineering design, permitting and funding proposals – all of which need to happen prior to initiating construction. Although we are still in the early phases of this process, we would like to share some of the overall restoration needs and goals necessary for designing a watershed-level restoration project such as this.
There are currently 39 acres of retired cranberry bog and 111 acres of non-cranberry bog wetlands on the 231-acre Windswept property, which is contiguous with several thousand acres of protected conservation land in the Middle and Eastern Moors. The overarching goal of this wetland restoration project is to re-establish wetland connections within the retired bogs themselves and between the retired bogs and the adjacent natural wetland habitats. By reestablishing wetland connectivity, functionality of the watershed in this portion of the island will be restored to a more natural, resilient system.
Although farmed cranberry bogs are essentially wetlands, there are site conditions established during bog construction as well as ongoing cultivation practices that limit the ability of these wetlands to function naturally, without the need for extensive water level manipulation. Individual bogs are separated from each other by constructed dikes and berms with flumes and culverts that control water flow and limit natural connections. The process of broadcasting a thin layer of sand across the bog surface during the winter months to stimulate the cranberry vines to produce new growth elevates the bog over time, resulting in isolation from the groundwater. Within each bog, there is an extensive system of linear internal and perimeter ditches that channelize water flow, preventing the entire bog surface from receiving water unless it is purposefully flooded. In order to restore natural wetland functionality to these manipulated agricultural wetlands, these limiting factors – flow restrictions, increased elevation and channelization – need to be addressed through restoration techniques.
To address flow restrictions, all cranberry bog restoration projects include some component of berm, dike and water control structure removal. At Windswept, this will likely include either complete or partial removal of most of the constructed dikes that currently separate the individual bogs from each other. Since one of the Foundation’s goals for this project is to maintain public access and enjoyment of the property, some of the existing dikes currently utilized as walking trails may be partially left in place, with boardwalks installed over the sections removed to provide water flow connectivity. Additionally, there are several locations where bogs around the perimeter of the property are separated from adjacent natural wetlands by dikes and berms. These will also likely be breached in some fashion to allow water to flow naturally between wetlands. These connections both restore functionality to the wetlands themselves and provide corridors for wildlife such as turtles, fish and eels to move through and utilize these improved habitats.
The addition of sand across bog surfaces over many years creates unfavorable conditions for wetland plant species. In the absence of restoration work, pitch pine trees often colonize within abandoned cranberry bogs in southern New England, resulting in these sites slowly evolving into upland habitats. Because we are seeking to restore naturally functional wetland habits at Windswept, a component of the restoration work for this project will likely include removing at least a portion of the accumulated sand layer (estimated to be an average of 17 inches deep). Once removed, the surface of the bogs would be “roughened” to mix any remaining sand into the organic soil layers below and create micro-topography, resulting in more variable wetland habitat across the former bog surfaces. These disturbed habitats would likely be rapidly colonized by native wetland plants via recruitment from the seed back or colonization from adjacent undisturbed habitats.
The sand excavated from bog surfaces can be re-purposed to fill internal and perimeter ditches within the individual bogs. Filling these ditches allows water to flow across the entire bog surface instead of being channelized, enhancing wetland habitat. At some locations, a shallow meandering stream through the center of the bog may be created during the restoration process to slow water flow and to allow for more natural conditions. During the original construction process at Windswept Bog, elevations across the site were manipulated so that water flowed via gravity through the constructed bogs from the south and east sides of the property towards the north and west. The design process for this restoration project will likely take advantage of these existing flow patterns.
The primary source of water for the Windswept Bog system is Stump Pond, a freshwater pond created during the cranberry bog construction process in the early 1900’s. Although a human-made wetland, the pond is a beautiful component of the property and supports ecologically valuable habitat that is unique on Nantucket. Retaining this pond is a priority goal for the Foundation. This can be achieved by redesigning the water control structures currently in place to maintain a stable pond water level while providing for overflow into adjacent wetlands during periods of higher water. In addition to flow from Stump Pond, water enters the Windswept Bog property from at least 11 inflow pipes or culverts under Polpis Road and eventually exits the site through an extensive freshwater marsh system located just east of the intersection of Polpis and Almanac Pond Roads. From here, a culvert under Polpis Road connects to several meandering streams that eventually empty into Polpis Harbor.
The restoration of natural wetland habitat functionality on this property will likely improve water quality within the entire watershed, including the outflow into Polpis Harbor. In addition to inputs from the retired bogs themselves, water inflows onto the property from adjacent homes, lawns, septic systems and roads in the surrounding watershed are a source of excess nutrients. Slowing water flow through the site by restoring connected, ecologically functional wetlands and soils vegetated with native plants will enhance the wetland’s natural ability to take up and store extra nutrients prior to discharge into the harbor watershed.
Mass. DER and USDA are funding the development of site-specific plans that provide recommendations for improving wetland connectivity, reducing bog surface elevation and filling human-constructed ditches, as discussed above. However, there are additional information gaps that need to be filled to fine-tune these over-arching management recommendations. Although NCF’s former bog manager has provided a comprehensive understanding of how surface water flows across the site, the amount and extent of groundwater influence still needs to be determined by conducting a property-wide hydrology study. Additionally, NCF Science and Stewardship Department ecologists are in the process of collecting extensive data on the many rare and unique species currently present on the property – including spotted turtles, upland and wetland plants and breeding and migratory birds. Knowledge of the locations and population extents of these rare resources will enable plans to be refined to reduce negative impacts to these species and their habitats on the property.
The Foundation is excited to be working with our partners on this comprehensive restoration project, and are extremely grateful for the expertise and support we have received from Mass. DER and USDA. We have together made important progress on the development of detailed plans for this work. However, there are many subsequent steps that still need to be taken prior to the start of construction work, including detailed surveying, extensive permitting and applications for grants to cover funding for the extensive costs.
We look forward to sharing additional information on the progress of this project in future blog posts and newsletters!
The Nantucket Conservation Foundation is a private, non-profit land trust that depends on contributions from our members to support our science projects, conservation property acquisitions and land management efforts. If you are not already a member, please join us now! www.nantucketconservation.org