Salt Marsh-Upland Ecotones in Central California: Vulnerability to Invasions and Anthropogenic Stressors

Abstract

Ecotones, zones of abrupt biological transition typically reflecting strong physical gradients, may be particularly sensitive to changes in environmental conditions. Our characterization of the ecotone between salt marshes and uplands in the Elkhorn Slough watershed in central California revealed that extent of appropriate habitat for native high marsh species endemic to this zone is extremely limited. The ecotone is highly invaded, with nonnative upland weeds accounting for a significant proportion of cover. We investigated responses to two anthropogenic landscape management strategies, restriction of tidal exchange through water control structures and cattle grazing. Moderate tidal restriction resulting in muted tidal exchange dramatically decreased ecotone width, native marsh plant richness, and cover by native ecotone specialists. Even stronger tidal restriction resulting in very low tidal exchange led to a seaward shift of the ecotone into the area formerly occupied by mid-marsh vegetation; upland plants now occupy the former ecotone zone so net loss of wetland habitat has occurred. Cattle grazing led to a very substantial increase in bare ground, a significant decrease in native marsh plant richness and a significant increase in non-native plant cover. Thus, both of these management regimes can have significant negative impacts on rare salt marsh ecotone extent and biodiversity.

Evaluating Tidal Marsh Sustainability in the Face of Sea-Level Rise: A Hybrid Modeling Approach Applied to San Francisco Bay

“…The high marsh zone is high in plant diversity, relative to mid and low marsh, and hosts several endangered plant species, including soft birds-beak ( Chloropyron molle , formerly Cordylanthus mollis ), and many endemic species [84] . Much of this habitat has already been lost or degraded due to urban and agricultural development, restriction of tidal exchange, and the erection of levees, contributing to the endangered status of the plant and animal species that depend upon it [50] , [85] .…”

Impacts of Cattle on the Vegetation Structure of Mangroves

“…Similar to patterns of abundance for trees, there were few seedlings in areas with cattle along the landward side of the mangrove forest (Hoppe-Speer et al 2015). Trampling by livestock in saltmarshes has been shown to reduce establishment of seedlings due to a loss in soil structure increasing the likelihood of small marsh plants being uprooted (Jensen 1985;Turner 1987;Tessier et al 2003;Wasson and Woolfolk 2011). In addition to trampling, cattle were also likely grazing on mangrove seedlings because, similar to the stunting of saplings, they were shorter and had more leaves in these areas, suggesting a compensatory response to breakage of the apical shoot (Strauss and Agrawal 1999).…”

“…Skilleter and Warren 2000;Chapman et al 2005). There appears extremely limited capacity for mangrove regeneration along the landward side of forest because while cattle remain in these relatively small paddocks there will be sustained impacts (Wasson and Woolfolk 2011). Indeed, all life history stages of the mangrove A. marina appeared influenced by cattle access and grazing.…”

Long-term management is needed for conserving plant diversity in a Wadden Sea salt marsh

“…The effects of grazing on plant diversity may be less apparent or negative in America (e.g. Wasson and Woolfolk 2011;Di Bella et al 2014), because salt marshes in America are usually dominated by one or a few plant species (Conde et al 2006;Pennings et al 2001). By contrast, salt marshes in Europe harbor more plant diversity (Bakker et al 2015).…”