Designing topographic heterogeneity for tidal wetland restoration

Abstract Topographic heterogeneity affects abiotic and biotic components of vegetated ecosystems and has the potential to provide functions and services such as promoting floral and faunal biodiversity. Yet, few studies have documented the design, implementation, and outcomes of mound-type features in estuarine and tidal freshwater wetland restoration projects. Here we report data from a synoptic survey of soil temperature and moisture on mounds at tidal wetland restoration sites in the Pacific Northwest, together with the results of a literature review, and the insights of regional restoration practitioners regarding ecological and practical considerations for mound construction. Few papers reviewed addressed conditions for plant establishment on mounds, such as soil moisture, which is important in climates with annual dry seasons, like the Pacific Northwest. We report 2015 data on soil moisture and temperature for the tops, sides, and toes of 15 mounds constructed between 2001 and 2013 of fill dirt, or dirt and logs, at 5 tidal wetland restoration sites. Two of the restoration projects studied are located in the lower Columbia River and estuary, one is in the Puget Sound, and two are in outer Pacific coastal estuaries in Oregon. The heights of mounds ranged from 0.5 m to 1.5 m above the marsh plain, and the area of individual mounds ranged from 21 m 2 to 5185 m 2 , while most published information treated somewhat smaller topographic features (termed mounds, hummocks, tussocks, ridges, levees, fans, or terraces). Differences in the minimum and maximum values of moisture between the toe and the top of mound in summer ranged from 2.9% to 40%. Statistical analysis strongly suggested stratification of soil moisture by elevation. Analysis of soil temperature was less conclusive, but temperature appeared to be positively correlated with elevation. Soil moisture relative to mound aspect (cardinal direction) was significant in some cases, but inconsistent between mounds. We also report our observations from a third recent restoration project on the tidal Columbia River, and observations made by other practitioners who have designed and installed mounds from San Francisco Bay to northern Puget Sound. Our findings will support design of wetland restoration sites where topographic heterogeneity is an objective.