Distributional boundaries of salt marsh ecotone plant community are influenced by competitive interactions at both harsh and benign ends of an environmental gradient

Background/Question/Methods

In California salt marshes, the high marsh-upland ecotone is limited to a narrow elevational zone that extends across a strong environmental gradient caused by tides. Theory predicts that the relative importance of abiotic and biotic factors in setting species’ distributions changes as conditions shift from harsh to benign. We tested this theory at both the species and community levels, taking advantage of the limited distribution and abrupt boundaries of the high marsh-upland ecotonal community. The underlying environmental gradient was quantified with seasonal analyses of soil pH, salinity, moisture, total and available N, and organic content. To identify whether plant competition set the upper and/or lower distributional limits of the community, transects extending from the marsh, across the ecotone, and into the upland were subjected to removal of all non-ecotonal species from half of the transect, while the other side of the transect constituted the control. Treatments were maintained, and community composition data was collected seasonally for two years. In a companion experiment, individuals of ecotone species Frankenia salina, Distichlis spicata, and Jaumea carnosa were planted with and without competitors at five species-specific elevations: 1) above the landward distribution boundary, 2) at the landward boundary, 3) at the distributional middle, 4) at the seaward boundary, and 5) below the seaward boundary. Biweekly survival data and final above-ground biomass were collected from February through October.

Results/Conclusions

The gradient in abiotic conditions correlated strongly with elevation and exhibited strong thresholds that corresponded with the boundaries of the ecotonal community. In contrast to theoretical expectations, the removal transects indicated that competitive interactions from neighboring species set distributional boundaries of the ecotone at both the physiologically stressful (seaward) end of its distribution, as well as at the benign (landward) end. In the outplant experiment, two species (D. spicata and J. carnosa) were limited by competition under benign conditions, while all three species were limited by competition under stressful conditions. While examination of these interactions at the most sensitive, earliest life stages could provide additional insights into the factors that limit species distributions in this system, our results contradict the generalization that tolerance to stress alone sets species boundaries at the harsh end of environmental gradients.