Active Restoration Promotes Wetland Establishment and Plant Species Richness on Former Riparian Cranberry Farms More Effectively than Passive Retirement

Abstract:

The area of cranberry farms (bogs) in Massachusetts (USA) is declining due to low cranberry prices and competition with other regions. Because bogs were typically created from former wetlands, retired cranberry bogs, particularly those along historic stream channels and on former riparian swamps, are prime locations for wetland restoration. Two trajectories are possible for retired cranberry bogs: (1) passive revegetation, or (2) active restoration of bogs and adjoining stream channels. Recent active restoration projects have removed water control structures, filled drainage ditches, built sinuous stream channels, and created microtopography on the former bogs. Re-establishment of wetland vegetation is a primary goal of restoration. Active restoration is expensive, but it is unclear if passive revegetation alone will restore wetland vegetation in sites with a strong agricultural legacy. Other potential benefits of active restoration include increased coastal resilience and nitrogen removal. Determining outcomes of passive retirement will help managers prioritize spending on active restoration projects.

We compared vegetation changes over time in passively and actively restored retired cranberry bogs. We surveyed vegetation in 34 cranberry bogs that had been passively retired for 0 to >70 years and followed the vegetation before and after active restoration at one site in detail. Native plant species richness increased after retirement in passively retired sites, peaked in bogs retired for 10-20 years, and then declined over time. Cover of woody and upland plant species also increased with time since retirement. Overall, many of the sites trended toward moist, non-wetland woodlands. In contrast, native plant species richness and wetland plant species richness increased dramatically in the first and second years following active bog restoration. The total cover of wetland species also increased. The actively restored bog had fewer woody species, more graminoids, and more wetland-associate forbs after restoration. These results suggest that in many retired bogs and other agricultural settings, active restoration techniques that result in locally raised water tables and varied microtopography will promote revegetation of native wetland plants compared with passive restoration.