Coastal Dune Restoration Vegetation Monitoring Report

Parsons, Lorraine; Spaeth, Michael; Ender, Cody

By the late 1990s, more than 60% (1,400 acres) of the approximately 2,200 acres of coastal dune, bluff, and scrub habitat in Point Reyes National Seashore (Seashore) had been invaded by European beachgrass (Ammophila arenaria) and iceplant (Carpobrotus spp.). These species were most likely planted between the early to mid-1900’s to stabilize highly mobile coastal dunes and bluffs for the protection of adjacent ranches and roads and have since spread substantially along the Seashore’s coastline. As coastal dunes support at least four federally listed species, as well as a number of other species designated rare or of special concern, the Seashore embarked on an ambitious program of coastal dune restoration starting in 2001. Since then, the Seashore has removed approximately 271 acres of non-native invasive plants from approximately 525 acres of coastal dunes. Three of the projects recently conducted include Abbotts Lagoon Coastal Dune Restoration Project (initiated in 2011), B Ranch North Restoration Project (initiated in 2013), and AT&T Coastal Dune Restoration Project (initiated in 2015). Invasives removal has been conducted manually through hand removal, mechanically through use of heavy equipment, and chemical through foliar application of herbicide. To evaluate success of restoration efforts, park staff implemented pre- and post-restoration vegetation monitoring. All of the projects regardless of approaches showed high rates of initial success in greatly reducing cover of the primary invasives (European beachgrass and iceplant), however, re-establishment of native dune plant species was poor. In areas restored mechanically, “flipping” or inversion of highly rhizome-contaminated soil horizons with less contaminated soils from below left vast expanses of bare sand for many years, with native plant colonization possibly inhibited by drought conditions, wind-driven remobilization of now unstabilized sands, and/or possibly a parasitic microbe in soils. The only colonizer of these barren expanses early-on was the federally endangered Tidestrom’s lupine (Lupinus tidestromii), a member of the pea family which is capable of fixing its own nitrogen -- and potentially serving as a nursery species for other dune plants. In herbicide treatment areas, particularly in the backdunes, extremely slow decomposition of standing dead European beachgrass biomass restricted establishment of other plant species due to lack of bareground or available “open space.” Vegetation recruitment was higher in herbicide-treated foredune areas due to sand overwash, which buried dead beachgrass clumps and created new substrate for plant establishment. Since the drought ended in 2016-2017, native plant cover has started to climb in restored dune areas, but so has cover of so-called “secondary invaders,” which are taking advantage of the substantial drop in primary invader cover and disturbance associated with restoration to gain a foothold. Ultimately, restoration efforts may need to focus on more than just invasives removal to restore an intact, functioning, and resilient native coastal dune ecosystem. Restoration planning should take into account plant-soil feedback dynamics and lingering physical and soil chemistry legacy effects left by invaders that may impede or even preclude recolonization by native plants. Until effective measures are developed to restore soil chemistry and biota as well as plants, land managers might best be served to focus restoration efforts on low-risk, high-yield restoration efforts such as removal of invasives in lightly or very recently invaded areas or herbicide treatment of European beachgrass or iceplant in foredune areas where sand overwash creates a new substrate for native plant establishment unimpacted by lingering legacy effects.

Coastal Dune Restoration Vegetation Monitoring Report

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