Viability of assisted vs. unassisted development of tidal fresh swamp

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The combination of low woody debris recruitment, low seed source, and introduced species will prevent the development of tidal swamp similar to reference conditions, without extensive intervention.

Over 90% of freshwater and oligohaline tidal swamps have been extirpated in Puget Sound, limiting both seed source on restoration sites, and the availability of reference conditions. There is evidence that swamp development is dependent on the capture of large woody debris as platforms for woody plant recruitment. Some initial study has suggested that wood recruitment may be limited on sites with remnant levees, and that wood levels in rivers are far lower than historical levels (Hood 2007). Revegetation in freshwater tidal settings is further challenged by a number of very competitive introduced species (e.g. reed canarygrass and purple loosestrife) that have been observed to persist following restoration of tidal inundation (Tanner et al 2002), and may limit development of woody vegetation in wetland restoration, and the intensive planting increases the dominance of native species (WDOE CITE). The traits, tolerances and natural community structures of freshwater tidal species have been poorly described by Hood (pers. coms.), and so even horticultural introduction of more diverse communities is largely experimental.

Delta Strategy Analysis


The uncertainties in how this topic affects delta restoration has resulted in its inclusion in the ESRP River Delta Adaptive Management Strategy. This three criteria analysis should build off the analysis above, and supports development of learning projects.

Importance Viability Policy Relevance

We don't have clear evidence describing the importance of freshwater tidal vegetation structure or composition in providing delta services (see Delta salmon utilization). We know that reed canarygrass and purple loosestrife provide strong competition that modifies tidal freshwater composition. We have reference sites to describe target states of vegetation, and examples of sites that are not on a clear path to recovering those conditions despite restoration of hydrologic conditions. There is some evidence that wind-dispersed conifers require relatively close seed source for recruitment.

Fisher Slough (2011) and Spencer Island provide examples of unassisted revegetation in tidal fresh settings that can verify that native spruce swamp does not recover under current conditions. There are multiple opportunities to test the efficacy of assisted revegetation.

Manual revegetation of a large tidal freshwater site would constitute an increase in project cost, that might not be supported under current funding policy. However, this may depend more on verification of the relative importance of tidal fresh systems, than extensive proof of poor natural regeneration.