Coastal Dune Restoration: When is Invasive Plant Removal Just Not Enough?

Mechanically restored coastal dunes with European sea-rocket, a secondary invasive species, in the foreground. Ecologists at Point Reyes have been working with researchers to find potential interactions involving soil microorganisms that might be hindering restoration.

NPS / Lorraine Parsons

January 2020; Updated December 2020 - By the late 1990s, two non-native, invasive plant species had invaded 60% of Point Reyes National Seashore’s 2,200 acres of coastal bluff, dune, and scrub. European beachgrass (Ammophila arenaria) and iceplant (Carpobrotus edulis) were ovewhelming critical habitat for many federally listed species. So in 2001, the park embarked on an ambitious coastal dune restoration effort. Ecologists used both mechanical and chemical approaches to remove 271 acres of beachgrass and iceplant from 525 acres of dunes. However, restoration in herbicide-treated beachgrass areas further from the ocean has proceeded slowly. Standing dead biomass of beachgrass has been persisting for more than four years. In addition, it is often being replaced by new weeds, or secondary invaders. Point Reyes ecologists have been working with researchers at the University of Arizona and University of California, Davis, to find out why. In particular, they've been sarching for potential interactions involving soil microorganisms that might be hindering restoration.

Soil microorganisms play a significant role in coastal dune habitats. For example, they can interact with native plants to help resist low numbers of invasive plants. But factors like disturbance, or a dramatic rise in invasive plant abundance (e.g., from deliberate planting), can overwhelm that resistance. Once established, invasive plants can change soil chemistry and biota. Those changes, or "legacy effects," may then help them maintain a competitive advantage by impeding the growth of native dune plants.

Comparing soil microbial communities in areas with different invasion intensities and restoration approaches, the researchers found evidence of several strong links. Microbial communities in heavily invaded sites were significantly altered. They had a lower abundance of presumed nitrifiers, fermentative bacteria, fungal parasites, and fungal dung saprotrophs. They also featured more cellulolytic bacteria and a class of arbuscular mycorrhizal fungi. In general, these changes did not fully dissipate following herbicide-based invasive plant removal. Mechanical restoration, involving inverting or “flipping” rhizome-contaminated surface soils with soils from below, more effectively removed soil microbial legacy effects. However, it may have induced other chemical changes that slowed or precluded re-establishment of native dune plants.

Iceplant has many well-documented chemical "legacy effects" on areas where it has become established. For example, it increases organic matter, increases soil carbon, reduces pH, reduces soil nitrogen, reduces water availability, changes soil biota, and suppresses the growth of native plants.

NPS / Lorraine Parsons

The scientists' findings, published in a special issue of Restoration Ecology, suggest that restoration efforts focused only on plant removal may not be enough. Soil microbial legacy effects can derail efforts to re-establish functioning, intact, and resilient native dune ecosystems. In particular, they may impede efforts to re-establish native dune plants. They may also promote re-invasion by primary target species, or invasion by new weeds better adapted to the changed soil conditions.

Faced with these challenges, what are dune restoration managers to do? Extra steps like iceplant duff removal or soil amelioration might help. Alternatively, managers could focus on low-risk, high-yield restoration activities. For example, removing invasives from sparsely to moderately invaded areas, or in foredune areas where sand overwash can counteract beachgrass legacy effects, can increase chances for success.

Point Reyes is continuing to partner with the University of Arizona. Now, they're working on a follow-up study on the microbial decomposer community. In addition, park ecologists have been assessing dune soils to better understand the role of soil chemistry. Like soil microbial legacy effects, chemical legacy effects of beachgrass invasion may also slow or derail restoration efforts.

For more information

• Parsons, L.S., Sayre, J., Ender, C., Rodrigues, J.L.M. and Barberán, A. 2020. Soil microbial communities in restored and unrestored coastal dune ecosystems in California. Restoration Ecology, 28: S311-S321. https://doi.org/10.1111/rec.13101
• Contact Lorraine Parsons

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