Restoration tradeoff: expanding native grasslands vs. increasing invasive cover
Von Holle. B. and G. Motzkin. 2007. Historical land use and environmental determinants of nonnative plant distribution in coastal southern New England. Biological Conservation 136:33-43. PDF.
Summary
This paper takes an early look at the factors that influence invasion of natural habitats by nonnative plants in coastal upland habitats including regional biodiversity hotspots: sandplain grasslands and coastal heathlands. The authors collected vegetation and soils data from 776, 20 X 20 m plots in coastal upland habitats of southern New England and adjacent New York. Nonnative species were present in ~30% of the plots. In plots with nonnative species, the mean number of nonnative species per plot was 2.8 with an average total cover of 6.3%.The results indicate that the distribution of nonnative plants is influenced by a suite of interdependent current and historical factors. The key finding relevant to sandplain grasslands and heathlands is that extant open-canopied areas (which include sandplain grasslands) harbored significantly greater nonnative species richness and cover than closed-canopy habitats. In addition, historically cultivated sites and sites with current or past soil disturbance had greater nonnative species richness, as did sites with richer soils and higher native plant species richness. Higher nonnative cover was associated with abiotic conditions such as surficial geology and soil calcium and phosphorus. Because many rare coastal sandplain plants reach their greatest abundance on extant open-canopied habitats, efforts to restore native plants in these areas will involve tradeoffs between the benefits of expanded habitat for these species and increased risk of invasion by nonnative species.
With climate change threatening sandplain grassland habitats in coastal New England, restoration and expansion efforts are ongoing throughout the region. Understanding the drivers of invasion can help managers assess site-specific invasion risk and consider trade-offs in restoration techniques.
Take home points
Restoration efforts in open-canopied habitats should consider the impacts of soil disturbance which increases the risk of nonnative species colonization.
Knowing the land use history of a management area helps to assess the level of risk associated with nonnative species.
Management implications
The authors argue that the most effective nonnative plant management in this landscape may simply be to encourage natural revegetation or establishment of native plant cover without severe soil disturbance. However, given the biodiversity importance of open sandplain habitats, the tradeoff of managing increased invasion risk can be warranted.
If soil scarification or other soil disturbance is performed for restoration purposes, which is shown to be necessary for sandplain grasslands in the absence of fire regimes (Bois et al. 2023), these sites should be closely monitored for nonnative species encroachment.
Using native plant seed additions in the xeric, nutrient-poor soils of this region can assist with the reestablishment of native diversity and the resistance of native vegetation to further spread of nonnative species.
Related papers: Bois et al. 2023; McKone & Hernandez 2021
Keywords
Impact study; sandplain grasslands; invasive plants; land use history, soil disturbance