High plant diversity in Oregon tidal wetlands and multiple threats to its persistence

Background/Question/Methods

Tidal wetlands in the Pacific Northwest occur in coastal estuaries differing widely in size, relative freshwater inputs, and degree of watershed development. To better understand patterns of plant diversity in tidal wetlands across the region and potential climate change effects on vegetation in these ecosystems, we (1) quantified species richness at the plot and estuary scale, (2) evaluated the number and types of plant assemblages in tidal marshes and swamps, (3) compared differences in composition among habitats and estuaries, and (4) evaluated diversity relationships with soil salinity and tidal elevation. Using plots randomly stratified across three major habitat classes based on National Wetlands Inventory (NWI) maps, we quantified plant composition, cover, and richness and environmental characteristics in four outer coast estuaries. Sites ranged from a highly marine dominated lagoon to a highly river dominated estuary. We determined tidal elevation with survey grade GPS and soil characteristics with standard methods.

Results/Conclusions

At least 100 vascular plant species and 12 common assemblage types were present cumulatively in the estuaries. Plot richness was high, generally exceeding comparable values in studies from other temperate and subtropical tidal wetlands. Overall species richness and composition among the estuaries was similar despite major differences in the degree of freshwater inputs to each watershed. The presence, frequency, or cover of some taxa differed by estuary, with some of these patterns likely reflecting differential salinity tolerance among species. Composition varied moderately among NWI habitat classes within the estuaries. Species richness showed a positive monotonic relationship with tidal elevation, but appeared to be less strongly associated with soil salinity. Spatial patterns of diversity suggest that estuaries have similar species pools from which levels of richness at smaller spatial scales are determined by within-estuary variation such as elevation gradients. Sea-level rise, increasing salinity, and non-indigenous species all represent potential threats to tidal wetland plant assemblages. These factors may threaten vegetation in high intertidal and oligohaline tidal wetlands in particular, areas in which we located some of the highest plant diversity.