Predicting the effects of sea level rise and salinity changes on west coast tidal marsh plant and avian communities
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Tidal Marsh Vegetation Responses to Salinity and SLRSalinity and tidal inundation are the two critical factors that drive vegetation community structure in estuarine tidal wetlands (Atwater et al. 1979; Mitsch and Gosselink 2000; Cronk and Fennessy 2001, Pennings et al. 2005). Atwater et al. (1979) first reported that freshwater wetlands of the Delta are characterized by greater plant species diversity than the salt marshes of the lower estuary. There is a dramatic, non-linear increase in plant species diversity and productivity in the fresh region of the Bay-Delta (Figure 1). Sites that are most saline have relatively low species diversity (Hopkins and Parker 1984, Sanderson et al. 2000), but do contain threatened and federally listed species, such as soft bird’s beak (Cordylanthus mollis ssp. mollis). Brackish sites that are less saline are not markedly more diverse, but wetlands located further up the estuary are substantially more diverse and have greater numbers of locally uncommon and rare species than lower estuary sites (Vasey, Parker, Callaway, and Schile, unpublished data). The greater diversity at freshwater sites underscores the potential ecological importance of freshwater tidal wetlands in the upper estuary and their potential vulnerability to salt water intrusion. Within California, a high proportion of imperiled and endemic species can be found within coastal ecosystems, including tidal marshes (Seabloom et al. 2006). Given the large number of locally uncommon and rare species in the brackish and freshwater tidal wetland ecosystem, as suggested by Lyons et al. (2005), the loss of these wetlands could have severe consequences for ecosystem functions in this region. Atwater et al. (1979) also documented large-scale changes in brackish to near freshwater wetland plant communities during a drought year, indicating the potential importance of dispersal effects on future distribution changes. That result parallels more recent studies from other wetland systems that have suffered temporary shifts toward more saline conditions, for example, along the Gulf Coast (Wang 1999; Flynn et al. 1995; Howard and Mendelssohn 1999, 2000; Thomson et al. 2001; Visser et al. 2002). Freshwater and oligohaline plant species will be the most sensitive to any increases in salinity (e.g., Baldwin et al. 1996). Plant zonation within estuarine wetlands is based primarily on inundation rates, largely as determined by elevation across the wetland. Mahall and Park (1976b, 1976c) showed that both salinity and soil aeration changed with elevation and that both were critical in determining the relative abundance of S. foliosa and Sarcocornia pacifica (formerly Salicornia virginica) in San Francisco Bay. Detailed surveys at San Quintín Bay, Baja California found that salt marsh plants respond to elevation differences as small as 8 cm (Zedler et al. 1999). Sanderson et al. (2000) found similar sensitivity of salt marsh plant distributions to elevation in San Francisco Bay and also identified the importance of tidal channels in influencing plant distributions. At the low end of the marsh, plants typically are stressed by excessive inundation and anaerobiosis, affecting both productivity and overall distributions (Chapman 1974; Mendelssohn and Morris 2000). Wetland plants have many specific adaptations that allow them to tolerate anaerobic conditions. Many species have well developed aerenchyma that allows oxygen to diffuse to roots and rhizomes (Armstrong 1979), and some species (including Schoenoplectus, and Juncus spp.) can transport oxygen to roots via pressurized ventilation and convective gas flow (Grosse et al. 1991). Spartina alterniflora and other species have physiological adaptations to deal with low oxygen levels (Mendelssohn et al. 1981). Even with these adaptations, increased inundation rates associated with increases in global SLR will stress marsh plants, reduce productivity, and potentially shift plant distributions (Scavia et al. 2002; Schile, Callaway, Parker, and Vasey, unpublished data). Knowledge of the relationships among seed dispersal, seed banks, plant recruitment and physical processes is crucial to predicting potential effects of climate change on tidal wetland (Baldwin et al. 1996); both salinity and inundation regimes are significant drivers of wetland plant germination and establishment. Prolonged inundation reduces species diversity and biomass (Casanova and Brock 2000) and can have differential effects along an inundation gradient (Keddy and Ellis 1985). Research conducted in coastal marshes of Louisiana suggests that higher salinity and prolonged inundation reduces germination (Baldwin et al. 1996), and these effects are amplified with disturbance (Baldwin and Mendelssohn 1998); however, comparable research in the western coast of North America has not been conducted to adequately address concerns of SLR and increased salinity on marsh plant recruitment. In addition to shifts in plant distributions, there are likely shifts in productivity due to gradual changes in salinity, with lower productivity in saline marshes (Pearcy and Ustin 1984, Rasse et al. 2005). Productivity studies from the Bay-Delta are limited (Mahall and Park 1976a); however, data from across the Bay-Delta demonstrate a trend of decreased productivity with increasing salinity (Figure 1). Atwater et al. (1979) measured high annual biomass of fresh and brackish marsh dominant Schoenoplectus californicus (formerly Scirpus californicus; approximately 2500 g/m2) in comparison to salt marsh biomass for Spartina foliosa (300 to1700 g/m2, with only one site near the high end of this range) or Sarcocornia pacifica (500-1200 g/m2). Similarly, in other estuarine ecosystems, production rates are consistently lower in salt marshes (Odum 1988), likely due to the added stress of high salinities in salt marsh soils. Tidal Marsh Avian Community Responses to Salinity and SLRDue to the harsh environment created by high salinity and tidal inundation regimes, tidal marshes are generally characterized by low vertebrate species diversity, as well as the low structural diversity of these systems (Greenberg et al. 2006). However, they are also characterized by a high proportion of endemic vertebrate subspecies, specially adapted to tolerate those harsh environments (Basham and Mewaldt 1987; Greenberg and Droege 1990). Brackish and fresh marshes support more vertebrate species than salt marshes (PRBO unpublished data), but the additional species are generally more common and generalist in their habitat preferences. In the Bay-Delta, salt marshes support six avian subspecies of conservation concern—California Clapper Rail (Rallus longirostris obsoletus), California Black Rail (Laterallus jamaicensis coturniculus), Tidal Marsh Song Sparrow (Melospiza melodia samuelis, M.m. pusillula, M.m. maxillaris), and Salt Marsh Yellowthroat (Geothlypis trichas sinuosa). These same species are also found in brackish, but usually not in fresh marshes. Thus, while an increase in salinity may lead to declines in tidal marsh plant diversity, and perhaps the loss of several rare and endemic plant species, we do not expect the same pattern in avian communities, in which many species may benefit from an increase in high salinity tidal marshes. Rather, SLR may pose a larger threat to tidal marsh vertebrates. Several taxa, including California Black Rail, are known to depend on the presence of high-tide refugia from predators, which may be reduced or eliminated with SLR (Evens 1986). Others, including Tidal Marsh Song Sparrow and Salt Marsh Yellowthroat have been observed to have lower densities in smaller, more fragmented marshes (Spautz et al. 2006). Furthermore, not all tidal marsh-associated vertebrate species are likely to respond in the same manner to the effects of climate change, given the variation in salinity tolerance, vegetation associations, vulnerability to edge-associated predation, impacts of tidal inundation and flooding, and response to tidal channels. The disparate shifts in ranges of plant, as well as avian species, may therefore result in a “tearing apart” of ecological communities (Parmesan 1996), which could cascade up and down the food chain, creating other disruptions in ecosystem functions. In conjunction with habitat fragmentation, the disruption could provide new opportunities for introduced exotic species to invade. Furthermore, the spread of exotic invasive plant species such as S. alterniflora has great potential to change tidal marsh plant community structure, and exclude some species, such as Song Sparrow, that have low densities and low reproductive success in this vegetation type (J.C. Nordby unpublished data). For avian species, high diversity has been associated with high structural vegetation diversity, more than plant species diversity (Rotenberry and Wiens 1980; James and Warner 1982). However, San Francisco Bay studies have demonstrated that individual marsh plant species are important predictors of individual avian species’ abundance (Spautz et al. 2006), as has been found in other systems (Wiens and Rotenberry 1981). Statistically controlling for landscape context, geomorphic characteristics, and vegetation structure, Song Sparrow density has been shown to increase with percent cover of Grindelia stricta (saline-brackish) and Baccharis pilularis (upland), while Common Yellowthroat density has been shown to increase with percent cover of Schoenoplectus acutus (brackish-fresh), Bolboschoenus maritimus (saline-brackish), and Lepidium latifolium (invasive), as well as overall vegetation diversity (Spautz et al. 2006). Thus, to a certain extent, we might expect avian species’ distributions to shift in response to shifts in dominant or subdominant tidal marsh plant species. However, direct effects of physical factors such as salinity, inundation, and channel density, as well as landscape context, may also influence the distributions of these species and these need to be incorporated into predictive models.
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