Ecotones are areas of abrupt physical and biological transition, usually between two ecosystems defined by characteristic vegetation. They often host particularly rich and sometimes unique biodiversity. But they are also particularly vulnerable to human disturbance, because they tend to be narrow zones and because they face threats originating in both of the overlapping ecosystem types. Disturbances can lead to shrinkage or loss of these transition zones, or to alterations in community composition, for instance if conditions favor one of the overlapping communities at the expense of the other.
The ecotone between salt marshes and adjacent uplands is a narrow zone that hosts the highest marsh plant diversity of the estuarine ecosystem, as well as many salt-tolerant upland plants. At Elkhorn Slough, this ecotone is only a few meters wide, sandwiched between the marsh plain with 100% pickleweed cover and the uplands with 100% terrestrial vegetation.
The ecotone appears to serve as an important corridor for wildlife at Elkhorn Slough. Many different animal species have been documented in the ecotone. See wildlife at the marsh-upland ecotone >>
The marsh-upland ecotone occupies a very narrow elevational band just barely within the reach of the highest tides (above Mean Higher High Water). This zone is shown in red in the accompanying figure, just a tiny wedge above the much more extensive salt marsh. We have observed tidal flooding of the ecotone only on the highest “King Tides” of the year.
K. Wasson and A Woolfolk of ESNERR investigated the effects of two human alterations on the Elkhorn ecotone, namely cattle grazing and tidal restriction with water control structures. The results of this investigation can be obtained from our paper published in Wetlands (Salt Marsh-Upland Ecotones in Central California:
Vulnerability to Invasions and Anthropogenic Stressors, pdf, 478KB). We found that the width of the salt marsh-upland ecotone was dramatically reduced in both the muted tidal exchange and cattle treatments (under 1 m average) relative to the control (over 2 m average). Bare ground was much more abundant (>50% of ecotone) in the cattle treatment than in any other treatment (<5% on average). This decrease in habitat extent is of concern because the ecotone is so rare to begin with, and yet serves important functions in terms of nutrient cycling and animal use. Species richness and diversity were also reduced in the muted tidal exchange and cattle treatments. In addition, invasion of the marsh community by non-native species was significantly increased by cattle. In a related study led by R Martone, we examined the effects of trampling, tidal restriction, and fertilization on the marsh upland ecotone (Impacts and interactions of multiple human perturbations in a California salt marsh, pdf, 540KB). We found that trampled sites are invaded by non-native weeds, and those that are tidally restricted show longer recovery times following trampling.
These changes to plant communities are of concern because the high marsh ecotone is where salt marsh diversity peaks; despite its limited extent, we found 65 total species in the ecotone. Native salt marsh richness has been shown to contribute to recruitment of native species, canopy complexity, biomass, and nitrogen accumulation in California marshes. There are six species of “ecotone specialists” found primarily in this transition zone between marsh and uplands at Elkhorn Slough. See “Ecotone specialists” below.
UCSC Graduate Student Carla Fresquez has been conducting experiments to examine what limits the distribution of these species at Elkhorn Slough, examining the role of both physical and biological factors.
In addition to examining spatial differences among sites with different management practices, we have conducted long-term monitoring to track changes in the location of the ecotone over time. Published 4/13 in Estuaries and Coasts: Ecotones as Indicators of Changing Environmental Conditions: Rapid Migration of Salt Marsh–Upland Boundaries. pdf, 472KB. We found that when tidal exchange is limited from diked sites, such as Estrada or Porter Marsh, upland weeds migrate downwards, occupying former wetland habitat.
At sites with natural tidal exchange, we observed landward migration of the ecotone over the past decade. This appears to be due to increased tidal inundation over the past decade – there have been more “king tides” that inundate this highest portion of the marsh in recent years. Our study demonstrates that the ecotone is resilient to sea level rise – able to rapidly migrate upward – and thus is a sensitive indicator for climate change.