Tidal Marsh Vegetation of China Camp, San Pablo Bay, California

China Camp (Marin County, California) preserves extensive relict stands of salt marsh vegetation developed on a prehistoric salt marsh platform with a complex sinuous tidal creek network. The low salt marsh along tidal creeks supports extensive native stands of Pacific cordgrass ( Spartina foliosa ). After hydraulic gold mining sedimentation, the outer salt marsh accreted. It consists of a wave-scarped pickleweed-dominated ( Sarcocornia pacifica ) high salt marsh terrace, with a broad fringing low marsh dominated by S. foliosa , including intermittent, variable stands of alkali-bulrush ( Bolboschoenus maritimus ). Most of the extensive prehistoric salt marsh plains within the tidal creek network also support mixed assemblages of S. pacifica , but high marsh zones along tidal creek banks support nearly continuous linear stands of gumplant ( Grindelia stricta ) and saltgrass (Distichlis spicata ) with more diverse salt marsh forb assemblages. Salt pans with submerged wigeongrass ( Ruppia maritima ) are scarce, local, and small. The landward edge of the tidal marsh forms rare examples of ecotones with adjacent terrestrial vegetation, including those of alluvial valleys (ripari-an scrub and woodland, freshwater marsh, sedge-rush meadows) and hillslope grassland and oak woodland vegetation. Narrow high tidal marsh ecotones that borders terrestrial grasslands are locally dominated by creeping wildrye ( Elymus triticoides ) and Baltic rush ( Juncus balticus ), mostly on south-facing slopes. Brackish tidal marsh ecotones above ordinary high tides are associated with freshwater discharges from groundwater and surface flows. Brackish marsh ecotones support large clonal stands of sedge, bulrush, and rush vegetation ( Carex praegracilis, C. barbarae, Bolboschoenus maritimus, Juncus phaeocephalus, Schoenoplectus acutus ), intergrading with terrestrial freshwater wetlands and salt marsh. The terrestrial ecotone assemblages at

vegetation. Narrow high tidal marsh ecotones that borders terrestrial grasslands are locally dominated by creeping wildrye (Elymus triticoides) and Baltic rush (Juncus balticus), mostly on south-facing slopes. Brackish tidal marsh ecotones above ordinary high tides are associated with freshwater discharges from groundwater and surface flows. Brackish marsh ecotones support large clonal stands of sedge, bulrush, and rush vegetation (Carex praegracilis, C. barbarae, Bolboschoenus maritimus, Juncus phaeocephalus, Schoenoplectus acutus), intergrading with terrestrial freshwater wetlands and salt marsh. The terrestrial ecotone assemblages at China Camp are comparable with those of other prehistoric tidal marshes in the San Francisco Estuary, but China Camp lacks most native clonal perennial Asteraceae and halophytic annual forbs of the region's remnant high tidal marsh ecotones. Few globally-rare salt marsh plant populations have been reported from China Camp within the National Estuarine Research Reserve (NERR) and state park boundaries, but some species occur that are regionally uncommon in San Francisco Bay tidal marshes. To date, non-native tidal marsh plant invasions have been relatively minor and localized within China Camp.

INTRODUCTION
China Camp State Park (San Rafael, Marin N County, San Pablo Bay, California (38o02'37" N 122o30'12"W) includes a tidal salt marsh approximately 40.5 ha in area (Goman 2001;Goman and others 2008). Tidal marsh vegetation at China Camp is well known for its "old-growth" component: it contains a large remnant prehistoric marsh area with a complex, sinuous tidal channel network that escaped 19th-and 20th-century diking and agricultural reclamation, and preserved a nearly continuous stratigraphic record of estuarine marsh peat and mud deposition during the late Holocene epoch (Goman and others 2008;Malamud-Roam and Ingram 2004). Less well-known is another distinctive relict tidal marsh feature of China Camp tidal marsh that escaped diking and intensive agricultural modification in the 19th century: the tidal marsh vegetation gradients (ecotones) with adjacent terrestrial vegetation (oak woodland, valley grassland, freshwater marsh) on mostly intact soil profiles. Among San Pablo Bay's few relict tidal prehistoric tidal marshes (Petaluma Marsh [Petaluma River], Fagan Slough [Napa River], Whittell Marsh [Point Pinole]), China Camp retains the greatest extent and diversity of terrestrial (upland and wetland) ecotone vegetation, with high native plant species diversity and complex vegetation patterns.
The following is a qualitative descriptive account of China Camp tidal marsh vegetation conditions based primarily on long-term field observations between 1991 and 2011, a period that included prolonged intervals of drought and high rainfall associated with long-term marsh salinity fluctuations, and one extreme storm erosion event (1997-1998. The aims of this account include the following: 1. to describe vegetation patterns in relation to geomorphic and hydrologic patterns and processes (tidal marsh landforms, dynamics, substrates, and drainage patterns), comparable with other prehistoric remnant tidal marshes of the Bay Area (Whitcraft and others 2011); 2. to identify qualitative short-term patterns of vegetation change; 3. to provide a local flora for the vascular plant species of the prehistoric China Camp tidal marsh plain and its terrestrial ecotones (see Appendix A), and 4. to summarize regionally distinctive or unique aspects of plant species composition of China Camp tidal marsh vegetation.
The physical landforms of the tidal wetlands can provide a meaningful eco-geomorphic framework to interpret and compare of San Francisco Estuary tidal marsh vegetation structures, and the many physical processes that influence it (Whitcraft and others 2011). The following description of the tidal marsh vegetation is organized primarily in relation to the diverse geomorphic structure and processes of the compound prehistoric remnant and modern tidal marsh ecosystem.

Historical and Prehistoric Tidal Marsh Units
The relict prehistoric (late Holocene) tidal marsh platform, and its terrestrial ecotones, occur within interior and landward portions of China Camp Marsh, including the partially drowned alluvial valleys (Back Ranch Meadows, Miwok Meadows) that are enclosed by North San Pedro Road and connected to tidal flows by culverts ( Figure 1). The late Holocene marsh platform is associated with the original, highly sinuous, tidal marsh creek networks that extend between drowned hills, forming hillslope islands and islandlike peninsulas (Figure 1), and grade into the alluvial valleys. The outer (eastern) portion of the tidal marsh, in contrast, consists of two younger and distinct marsh sub-units developed in historical times: a broad, fringing high tidal marsh platform or terrace, bounded by an active or relict wave-cut marsh scarp; and a highly-dynamic, outer fringing, low tidal
The outer high salt marsh terrace vegetation is historical: it formed bayward of the prehistoric tidal marshes in San Pablo Bay as a result of tidal marsh progradation. This progradation was initially induced by Gold Rush sedimentation (19th-century hydraulic mining outwash) under the influence of both tidal and wave deposition of fine sediment that accumulated from hydraulic mining spoils discharged from the Sacramento River (Atwater and others 1979;Doane 1999). Net progradation of the outer salt marsh terrace at China Camp continued throughout most of the 20th century, interrupted by marsh-shoreline retreat episodes that began to prevail over progradation by the 1990s (Jaffe and others 1998;Doane 1999). The outer marsh terrace lacks the complex sinuous tidal channel network and high channel density of the prehistoric tidal marsh platform. The break between the contrasting tidal drainage patterns-widely spaced, shore-normal channels with limited dendritic branching of the historical outer salt marsh, and the highly irregular, branched, sinuous, late-Holocene tidal channel system-approximately demarcates the age and structural boundaries

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4 between these two marsh sub-units ( Figure 1). The high salt marsh terrace is approximately 200 m wide along most of China Camp Marsh between Jake's Island and Chicken Coop Hill, where it fronts the original prehistoric tidal marsh platform and creek system. Southwest of Chicken Coop Hill, the high marsh terrace narrows to less than 120 m wide, and occupies almost the entire tidal marsh profile. 5 plied) and an outer low fringing salt marsh composed of monotypic Spartina foliosa (Hopkins and Parker 1984;Li and others 2005) and annually variable, mixed stands of S. foliosa and Bolboschoenus maritimus, transitional between salt and brackish marsh (Goman 2001). The high salt marsh terrace substrate is composed of stiff bay mud (silty estuarine clay; Reyes clay soils) deposited by both tidal and wave processes, enriched with fibrous organic matter (Goman 2005;Goman and others 2008). Fresh bay mud deposits on top of matted pickleweed litter are evident on the outer terrace after winter storms that also deposit wave-transported wracks of tidal litter. The low fringing salt marsh, approximately 40 to 50 m wide most years, is formed by direct rhizome spread of S. foliosa into the soft muds of adjacent bay mudflats, and is subject to cyclic storm wave erosion followed by tidal sediment accretion during post-storm recovery phases. Wave action in winter generally strips the S. foliosa stands of senescent leaf litter, leaving the high salt marsh terrace relatively exposed to winter storm wave attack. The two zones are bounded by a relict wave-cut scarp.

Vegetation of the Outer High Salt Marsh Terrace and Fringing Low Marsh
High Salt Marsh Terrace Vegetation. S. pacifica growth forms on the high marsh terrace are tall (0.3 to 0.4 m), dense, highly branched, and shrubby, rather than prostrate or decumbent low forms common on peaty middle marsh plains of the prehistoric tidal marsh platform. Most S. pacifica stands on the terrace are either nearly monotypic or low in diversity (Hopkins and Parker 1984). Regionally, S. pacifica is associated with the middle and high salt marsh zones, where it is usually dominant (Grewell and others 2007;MacDonald 1977;Peinado and others 1994).
Vegetation gaps with exposed peaty bay mud, but minimal persistent wrack deposits, appear to be either recolonized by S. pacifica, or occasionally by less common perennial halophytic marsh forbs, including Limonium californicum, Distichlis spicata, Frankenia salina, and Jaumea carnosa. Other species comprising the high salt marsh terrace vegetation include Polypogon monspeliensis, Cotula coronopifolia, Spergularia salina, and Spergularia macrotheca. Grindelia stricta is uncommon in the high salt marsh terrace most years. Native salt marsh forbs are generally uncommon on the strongly Sarcocornia-dominated terrace, and are associated with gaps in pickleweed canopies, and well-drained banks of tidal channels and ditches that occur infrequently within the high salt marsh terrace southwest of Jake's Island. Intermittent heavy litter wrack deposits persisting through the growing season appear to initiate dieback patches (vegetation gaps) in the thick pickleweed canopy. Dieback patches are often initially colonized by annual Atriplex prostrata while thick wrack debris persists. Vegetation gaps with bare bay mud substrate, exposed after litter mats are dispersed or decomposed, appear to become local, infrequent sites of native salt marsh forb colonization. Large areas of Sarcocornia dieback (conspicuous "gray marsh" patches), were evident on segments of the high marsh terrace that were influenced by prolonged waterlogging from high rainfall and frequent over-marsh tidal flooding, and by heavy wrack deposits in the El Niño winter of 1997-1998. Sarcocornia dieback in waterlogged and wrack-deposited marsh areas was followed by high abundance of A. prostrata the following summer (personal observation). Old wrack deposits in the outer marsh terrace include very old, persistent (embedded in marsh sediment), large woody debris, which consist of trunks and limbs of trees, wrecked docks, and pilings. Large woody debris provide topographic highs in the outer high marsh terrace, and support erratic occurrences of upland weeds perched above the marsh, rooted in the decaying wood, such as Bromus diandrus.
Cuscuta pacifica var. pacifica (syn. C. salina var. major), a leafless annual shoot-parasite of Sarcocornia and other halophytes (Grewell 2008), is episodically abundant in the high salt marsh terrace, but with high variability among years. It forms conspicuous, brilliant orange mats that temporarily shroud or smother Sarcocornia, usually reaching peak abundance in mid-to late summer, particularly in dry, hot summers ( Figure 3). Cuscuta mats are usually most frequent and extensive on the landward portions of the high salt marsh terrace, often where marsh soil is visibly drying and cracking (likely indicators of hypersalinity). Vegetation gaps in Sarcocornia (local dieback or reduced cover and density that influence competition and species diversity) may follow severe Cuscuta infestations in California coastal salt marshes (Callaway and Pennings 1998;Grewell 2008), and Cuscuta appears to contribute to the vegetation gaps in the high salt marsh terrace of China Camp, along with wrack-deposition processes. Because of the strong seasonality and annual variability in Cuscuta cover, it has sometimes been under-estimated as an important or seasonally codominant component of China Camp's tidal marsh vegetation (e.g., Li and others 2005).
The geomorphic setting (marsh shoreline orientation to dominant winds, wide mudflats, and open water wave fetch) of China Camp Marsh appears to influence the density and complexity of tidal channel development in the outer high marsh terrace, and thus indirectly influences channel-patterned, local high marsh vegetation structure and plant diversity. The relative homogeneity of the outer high marsh terrace's nearly monotypic S. pacifica vegetation is related to its scarcity of natural tidal channels and associated drainage gradients that support the local higher frequency of Grindelia stricta, Distichlis spicata, Frankenia salina, and other high salt marsh forbs. Low-density, simplified tidal drainage patterns are typical of prograded salt marshes subject to strong wave action along the north shore of San Pablo Bay (Atwater and others 1979), including China Camp (Doane 1999).The density of narrow, closely spaced, short ("herringbone" pattern) shore-normal tidal channels at the outer marsh edge increases westward towards Gallinas Creek, as the marsh edge becomes increasingly sheltered from exposure to the wave erosion and deposition processes (maximum open bay fetch) of San Pablo Bay. The channel-deficient, waveexposed marsh edge of China Camp Marsh is similar to the structure of prograded, flat, relatively featureless fringing salt marsh terraces between Sonoma Creek and Petaluma River on the northeastern shore of San Pablo Bay. These marsh terraces increase in channel density, and channel-patterned high salt marsh diversity, as they grade toward the wavesheltered mouths of Sonoma Creek and the Petaluma River, as China Camp Marsh does along Gallinas Creek.

Relict Wave-Cut Scarp: Vegetation Patterns and Physical Processes.
There is a relatively sharp vegetation and topographic boundary between the high salt marsh terrace and the fringing, low, salt marsh vegetation, rather than a continuous elevation gradient and ecotone. The outer boundary of the high marsh terrace is marked by relict wave-cut scarp (marsh cliff), which is formed in cohesive bay mud, with dense Sarcocornia root mesh supplying the relatively high shear strength (Pestrong 1965(Pestrong , 1972 sufficient to maintain near-vertical eroded slopes. The scarp maintains a dynamic and relatively straight outer edge of the high marsh terrace vegetation, and a sharp discontinuity with the wide, low marsh zone below it (Doane 1999; Figure 2). The scarp is approximately 0.3 to 0.5 m high, varying with phases of accretion or erosion. Stepped marsh terrace topography is typical of relict wave-cut salt marsh terrace scarps that have cyclic erosion and progradation (Allen 1989(Allen , 2000Pringle 1995). Wave-induced marsh scarp retreat rates are a function of wave power and sediment supply (Schwimmer 2001), modified by wave attenuation from the width and seasonally variable structure of fringing marsh vegetation below the scarp (Yang and others 2012; Möller and Spencer 2002). The low marsh fringe is, therefore, ecologically and geomorphically important in maintaining the relict marsh scarp profile and dynamics. Salt marsh terrace scarps may be masked by vegeta- The development of Bolboschoenus maritima stands below the high marsh terrace scarp, in the inner zone of the fringing low marsh, appears to correspond with brackish phases in years of high rainfall and low salinity. B. maritima colonies or zones in the fringing low marsh are visually distinguishable from cordgrass in brackish years by their slightly darker-green foliage, but have not been detected by remote sensing, at least in some years (Li and others 1995). The error rate of Bolboschoenus stand classification by remote sensing methods is significantly higher than that of Spartina (Zhang and others 1997). In drought years of high salinity (particularly high salinity early in the growing season), B. maritimus remains mostly vegetative, and its culms scarcely exceed the height of cordgrass before they become senescent in late spring or early summer. Variable development of bulrush canopy height and density, especially in years of high marsh salinity, may limit discrimination of bulrush stands within the Spartina-dominated low marsh at China Camp. B. maritimus can remain dormant as corn-like buds on rhizomes through several years of excessive salinity (Pearcy and Ustin 1984;Ustin 1984), so its lack of emergence each year does not imply absence or extirpation in the fringing low marsh. At China Camp, mean winter and summer aqueous salinities in the 1970s were reported to range from approximately 20‰ to 30‰ (Atwater and others 1979), a range which are inhibitory to tion during the recovery or progradation phase or slow erosional retreat (Moreira 1992). The high marsh terrace scarp at China Camp is visible mostly within the inaccessible outer marsh, in aerial images, and a few roadside viewing points ( Figure 2).
Microtopographic indicators of intermittent erosion (rills, spurs, and furrows; Allen 2000) occur along the scarp, despite the closed canopy of vegetation above and below the scarp. Erosional microtopography within the scarp is often evident in the scarp in spring, after winter storms, indicating partial reactivation of the scarp in some years. Wave scour and thinning or leveling of Sarcocornia canopies is often evident a few meters landward of the scarp crest in spring, after winter storm wave activity, and before vegetative regeneration. No other species are common in the wave-scoured zone within and behind the scarp, other than occasional Atriplex prostrata.
The now-relict scarp was particularly active during the El Niño storm erosion event of 1997-1998, which caused scour of the fringing cordgrass marsh, scarp retreat, and large wrack deposits on the high marsh terrace. Some years, dense wracks of Spartina leaf litter are episodically concentrated along the topographic break at the toe of the scarp (Figure 2), where they may either delay re-emergence of the Bolboschoenus or Spartina canopy, or cause temporary dieback, depending on the thickness and duration of the wrack deposit.
Low Fringing Tidal Marsh. The low fringing tidal marsh below the relict wave-cut scarp is mostly dominated by California cordgrass (Spartina foliosa; Li and others 2005), often with an intermediate and usually inconspicuous zone of abundant Bolboschoenus maritimus, which is mixed with Spartina or Sarcocornia in years of high rainfall and relatively low salinity ( Figure 2). Since the 1997-1998 El Niño event, the outer edge of the fringing Spartina marsh below the relict scarp has been dynamically stable or prograding (clonal spread into mudflats on a continuous gradient), with relatively minor cyclic erosion and recovery of the bayward edge. Most years, after winter wave erosion events, the outer rhizome front of the Spartina belt appears to be spreading into the mudflat. Smaller wave-cut scarps also form within the low fringing Spartina-growth of Bolboschoenus maritimus (Kantrud 1996;Ustin 1984). Average annual salinity at China Camp from 1988 to 1994 was also relatively high, ca. 27‰ (Malamud-Roam and Ingram 2004, citing USGS data), which suggests that the relative abundance and visibility of Bolboschoenus in the fringing low marsh may have been low (or practically undetectable) until after the series of wet years that followed the major El Niño event of 1997-1998.

Vegetation of the Prehistoric Tidal Marsh Platform
The inner belt of prehistoric tidal marsh vegetation is developed on the more complex environmental gradients and substrates that are associated with sinuous networks of stable prehistoric tidal creeks, and contacts terrestrial soils of alluvial valleys and hillslopes along its landward edges. The interior prehistoric marsh is late Holocene: the basal strata of the marsh developed since sea level stabilized near modern elevations during the last 4,500 yr BP, but the modern upper marsh soil horizons are only a few centuries old (Goman 2005;Goman and others 2008;Malamud-Roam and Ingram 2004). Pollen and sediment stratigraphy (long cores to depths up to 6 m below the modern marsh surface) reveals that most (67%) of the marsh's depositional history was dominated by relatively brackish-rather than saline-tidal marsh conditions (Goman 2005; Goman and others 2008) as well as by alternating unvegetated mud and marsh deposition episodes, which have been interpreted as disequilibrium phases of rapid sealevel fluctuations (Goman 2005). The stratigraphy of China Camp reveals that even in its brief (ca. 4,000 yr) late-Holocene history, its vegetation composition has fluctuated markedly in response to climate and sea level (Malamud-Roam and others 2007), and has not maintained a steady-state or dynamic equilibrium vegetation that resembles familiar historical conditions.
Modern salt marsh vegetation is established on geomorphically mature tidal marsh landforms and soils, including low-relief natural levees along larger tidal sloughs, nearly flat peaty marsh plains (higher organic matter content than the young bay mud of the historic prograded high marsh terrace) and dynamic steep tidal creek banks with high turnover of slump blocks and scarps (Gabet 1998;Fagherazzi and others 2004). Depressional marsh-plain salt pans (pools, ponds), such as those of Petaluma Marsh, are a typical feature of the geomorphically mature or prehistoric tidal marsh plains of the San Francisco Estuary (Grossinger 1995;Baye and others 2000), but they do not occur in the prehistoric salt marsh platform of China Camp, and there is no indication that salt marsh pan features ever developed here. The distribution of tidal marsh pans correlates with their position along the stream-influenced gradients of tidal sloughs in other San Pablo Bay tidal marshes (Grossinger 1995). Only poorly drained marsh plains with prostrate Sarcocornia and Distichlis vegetation occupy the large drainage divides where pans potentially form.

Salt Marsh Plains of Drainage Divides.
The middle marsh zone, occupying the broad, flat drainage divides between tidal channels, is a mosaic of relatively decumbent, low (0.2 to 0.3 m high) Sarcocornia pacifica assemblages, which include native salt marsh zone forbs and Distichlis spicata (Figure 4). The patchiness and species diversity of the prehistoric marsh plain contrasts with the extensive tall, dense, shrubby near-monotypic stands of S. pacifica on the outer high marsh terrace. Relatively common associates of S. pacifica in the interior marsh plain include Distichlis spicata, Cuscuta pacifica var. pacifica, Jaumea carnosa, and Frankenia salina. Infrequent species of the middle salt marsh plain include Triglochin concinna, T. maritima, Spergularia salina, S. macrotheca, Polypogon monspeliensis, Limonium californicum, Cotula coronopifolia, and Atriplex prostrata. The relative abundance of Sarcocornia and associated salt marsh forbs and Distichlis varies among years, typically with the increasing relative abundance of Sarcocornia during years of low rainfall/high salinity, which is consistent with local experimental investigations (Ryan 2009;Ryan and Boyer 2011).

High Salt Marsh Vegetation Bordering Tidal Creeks.
One of the most conspicuous salt marsh vegetation patterns corresponds with the drainage, elevation, and sedimentation gradient patterns associated with tidal creeks (Allen 2000). Narrow and often dense and continuous bands of robust, semi-evergreen Grindelia Francisco State University, pers. comm., 2012) and early 1990s. The continuous, tall, flood-emergent canopy structure of Grindelia is important flood escape cover habitat for endangered California clapper rails that travel and forage along tidal channels (Albertson and Evens 2000), as well as for small mammals that inhabit the marsh plain, which include the endangered salt marsh harvest mouse (Shellhammer 2000). Sarcocornia also develops shrubby, tall growth forms (up to approximately 0.6 m high) in the high marsh zone that borders tidal creeks, similar to its growth forms on the high salt marsh terrace. It occurs in sporadic, discontinuous patches, often with low vigor, along the high marsh ecotones that border hillslopes. Frankenia and Distichlis stands are also relatively abundant, robust, and frequent near the crests of channel scarps and natural creek levees.

Low Salt Marsh Vegetation Dynamics of Tidal Creek
Banks. The structure of tidal creek bank vegetation is influenced by cyclic erosion and accretion processes that are driven by the tidal hydrology of channels. Erosional slump blocks are frequent along China Camp's mature tidal creeks (Figures 5 and 6), particularly after stormy winters. Cohesive, peaty

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slump blocks displace high salt marsh vegetation from bank tops and natural levees to intertidal elevations too low to sustain them, causing dieback and death. Slump blocks subsequently become colonized by Spartina foliosa (Gabet 1998) and trap fine sediment (Figure 7), causing localized cyclic salt marsh succession back to Sarcocornia and associated species (Gabet 1998). This process results in the dynamic stability of creek banks, with a high turnover of lowand high-marsh vegetation types, and unstable intermediate stages of conversion between them, at any given creek bank location. Channel bank edges vary from near-vertical erosional scarps with overhanging dense Sarcocornia-Distichlis-Frankenia canopies, to dense Spartina low marsh (Figures 5 and 6).
During years of high rainfall and low channel salinity during the growing season, Bolboschoenus maritimus locally establishes or regenerates in sporadic but sometimes large (up to 5 m long) colonies within tidal creek banks, among dominant Spartina foliosa stands. B. maritimus is otherwise restricted to brackish marsh ecotones associated with terrestrial ecotones of the prehistoric marsh platform.

Brackish Tidal Marsh
Extensive areas of transitional salt-to-brackish tidal marsh occur in Miwok Meadows and Back Ranch Meadows (Figures 1 and 7), landward of N. San Pedro Road, where daily tidal flows are partially choked but maintained by culverts connected to tidal creeks bayward of the road, resulting in a damped tidal range of about 60 cm in channels. Choked tidal flows, combined with substantial terrestrial freshwater discharges (shallow groundwater, runoff, and ephemeral streams), reduce salinity in the brackish marsh. The brackish tidal marsh plains of Miwok Meadows Marsh and Back Ranch Meadows Marsh are also remnants of the prehistoric tidal marsh platform that intergrades with a drowned alluvial valley that support freshwater marsh and willow riparian scrub.
The Miwok Meadows Marsh plain supports a mosaic of patches dominated by Distichlis, Sarcocornia, Bolboschoenus, and Jaumea, with Grindelia again dominating narrow zones that borders tidal creeks. Beneath the Sarcocornia canopy, a sparse ground layer of Isolepis cernua and Juncus bufonius occurs sporadically, particularly near the terrestrial ecotone. During drought years, saline or hypersaline condi-    North San Pedro Road (Figure 8), but pans are otherwise very rare in the fully tidal marsh bayward of the road. Pan beds in the brackish marsh are persistently flooded by spring tides and winter rainfall for most of the year. They support dense stands of Ruppia maritima (submerged aquatic vegetation beds) in spring and early summer. Ruppia usually develops abundant epiphytic filamentous algal growth by summer, and goes dormant. The relatively stable marsh edges of brackish pans are associated with Distichlis, Sarcocornia, and Jaumea. Bolboschoenus marsh edges invade pan beds in years of low salinity. In contrast, the few small and shallow salt marsh channel pans at China Camp lack Ruppia.
A fully tidal brackish marsh with local dominance by tules and cattails occurs at the south end of China Camp Marsh (Figure 1), where local canyon drainage and a seep has established a narrow, dense stand of sharply zoned Schoenoplectus acutus, Typha latifolia, and Bolboschoenus maritimus, which grades directly into a Sarcocornia marsh that supports locally frequent Triglochin maritima-apparently the largest and most robust populations of T. maritima at China Camp ( Figure 9). This disjunct occurrence of tulebulrush tidal marsh occurs immediately below a wil-A B low grove at the limit of tidal flooding below N. San Pedro Road. Tule-bulrush marsh zones are otherwise now found only in northern San Pablo Bay, primarily in upstream reaches of Napa Marsh, and very locally in upper Petaluma Marsh. B. maritimus also occurs along the landward edge of the tidal marsh plain where freshwater seepage from hillslopes or roadsides appears to cause local brackish gradients (Figure 9) that borders the terrestrial ecotone.

Terrestrial Ecotones of Tidal Marsh
The brackish tidal marshes of China Camp are formed by terrestrial freshwater discharge gradients between freshwater wetlands with tidal salt marsh, and represent the broadest terrestrial wetland ecotones of China Camp Marsh. Where other local terrestrial vegetation types (valley grassland, oak woodland, mixed evergreen forest, riparian woodland, and freshwater marsh; Howell and others 2007) contact or intergrade with tidal salt marsh, they also form narrower zones of terrestrial-tidal marsh ecotones on original soils, some of which support distinctive and regionally uncommon plant assemblages. Dikes, ditches, and agricultural conversion have intensively altered or completely destroyed former terrestrial ecotones that were prevalent in most of the San Francisco Estuary's tidal marshes, leaving a small number of prehistoric tidal marsh ecotones in the San Pablo Bay and Suisun Marsh, and extremely few in San Francisco Bay (Baye and others 2000;Boyer and Thornton 2012). China Camp retains some of the most intact and diverse, distinctive examples of terrestrial ecotones of tidal marsh in the estuary, although they cannot be assumed to represent the historic range of terrestrial ecotone variability in the North Bay.

Elymus triticoides and Juncus balticus Ecotones.
A distinct vegetation zone occurs in the terrestrial ecotone along high tidal marsh, composed of codominant perennial, rhizomatous, dense sod-forming native rush and grass species Elymus triticoides (syn. Leymus triticoides) and Juncus balticus. The abrupt, conspicuous Elymus-Juncus zone between the upland hillslope bunchgrass/annual forb grassland and the tidal marsh is usually less than 2 to 3 m wide at China Camp (depending on slope; Figure 10). Since neither dominant species occurs in the adjacent ter-restrial or tidal vegetation beyond this zone at China Camp, the status of this assemblage as an "ecotone" is ambiguous; it may properly be regarded as a distinct tidal marsh edge vegetation type rather than a vegetation transition zone. Distichlis spicata from the tidal salt marsh does extend locally into the Elymus-Juncus zone, but it is not generally dominant there. The E. triticoides zone, either single-species dominant or in association with Juncus balticus, recurs as a characteristic local remnant feature at many other prehistoric tidal marsh terrestrial ecotones formed on alluvial fans or clay loam hillslopes in the San Francisco Estuary (e.g., upper Newark Slough, San Francisco Bay; Whittell and Giant marshes, Point E. triticoides ecotones appear to have expanded at China Camp in the last two decades, as at Rush Ranch. The relative abundance of Juncus balticus in hillslope edges of tidal marsh increases in the vicinity of hillside seeps and road culverts, and on northfacing slopes ( Figure 11); E. triticoides tends to dominate the assemblage otherwise. Both clonal perennial graminoid species exhibit relatively low salt tolerance compared with adjacent salt marsh halophytes, but they extend into the upper zone of Sarcocornia-Distichlis salt marsh, possibly aided by clonal integration (Evans and Whitney 1992). Neither extends deep into the tidal marsh plain, and neither colonizes creek bank high marsh zones at China Camp tidal salt marshes. In contrast, J. balticus is common or locally dominant in brackish marsh plains of Suisun Marsh. The Juncus-Elymus zone is notably poor in typical invasive non-native species widespread in tidal marsh levees of other Marin baylands. Grindelia is generally infrequent or absent in the dense sod formed by rhizomatous Elymus-Juncus ecotone, but it locally occurs at the lower edge, particularly near indicators of freshwater seeps.  (Salzman and Parker 1985), which occurs locally at the south-facing toe of Jake's Island slopes, and at China Camp beach. This species also has an element of terrestrial ecotones at the Rush Ranch tidal marshes that borders Suisun Slough (Whitcraft and others 2011

Freshwater-Brackish Wetland Terrestrial Ecotones.
Willow scrub, wetland sedge meadow, and oligohaline emergent (freshwater) marsh form terrestrial ecotones with brackish tidal marsh in the low-gradient alluvial valley of Back Ranch Meadows, landward of N. San Pedro Road (Figures 9 and 12). These form a patchy, irregularly zoned ecotone that fluctuates in position and composition during drought and high rainfall cycles (Figure 13). Willow thickets (Salix lasiolepis) with ground layers dominated by shadetolerant and slightly salt-tolerant Carex praegracilis intergrade with fresh-brackish high marsh assemblages, rush-sedge meadows, and emergent freshwater marsh, including Typha latifolia, Carex praegracilis, Eleocharis macrostachya, Isolepis cernuus, Juncus balticus and J. mexicanus, J. effusus, J. phaeocephalus, and Potentilla anserina. These sedge-rush assemblages intergrade or overlap in patches with a brackish marsh plain dominated by Sarcocornia, Distichlis, and Jaumea (Figure 7). This brackish-fresh wetland gradient is flooded with estuarine waters during extreme high winter tides. During drought years, the Typha stands in the freshwater-oligohaline-brackish high tidal-marsh ecotone below the willow thicket at Back Ranch Meadows die back, and are invaded rapidly by Distichlis and Sarcocornia ( Figure 13). Carex praegracilis also expands in the willow ground layer when the canopy thins during droughts, and light increasingly penetrates to the ground layer. Sedgerush meadow dominated by C. praegracilis in the brackish ecotone, in contrast, appears to resist rapid displacement during droughts. Brackish marsh ecotypes of Achillea millefolium and Scrophularia californica, widespread in Petaluma, Napa-Sonoma, and Suisun brackish tidal marshes, are apparently absent at China Camp.
Groundwater seeps from upland and alluvial slopes are evident in local patches or zones of brackish marsh ecotone vegetation along some hillslope and roadside locations in years of high rainfall. These are indicated by atypical high marsh zones with locally abundant Bolboschoenus maritimus (Figure 9).

Uncommon Native Tidal Marsh Plants
China Camp Marsh supports only two plants currently considered to be rare or special-status taxa.   Marsh tidal marsh edges. Zeltnera trichantha (syn. Centaurium trichantha), a slender annual forb, occurs locally at several locations in the high tidal or brackish marsh zone of China Camp near Turtle Back Hill and Back Ranch Meadows, bordering hillslopes and road fills. This is the only known current salt marsh location remaining in San Pablo and San Francisco Bays for this species, which was historically present in their tidal marsh edges (Baye and others 2000).

Exotic and Invasive Tidal Marsh Plants
Compared with most San Pablo Bay tidal marshes, China Camp has escaped-to date-the brunt of regional invasions by exotic salt marsh species, mostly from Europe and Asia. In recent years, for example, perennial pepperweed (Lepidium latifolium), Mediterranean tarplant (Dittrichia graveolens) and Australian or Pacific bentgrass (Agrostis avenacea) have rapidly invaded disturbed levees and high tidal marsh edges in San Pablo Bay. L. latifolium is limited to local disturbed brackish marsh edges at the north and south end of China Camp, above and below N. San Pedro Road. The scarcity of L. latifolium and other ubiquitous upland ruderal forbs of North Bay levees (e.g., Raphanus sativus, Brassica nigra, and Conium maculatum) in the terrestrial ecotones of China Camp Marsh likely results from the marsh's lack of bay mud levees (deposition of artificially drained estuarine clay-silt sediments on steep slopes above tidal elevations) and the periodic levee maintenance disturbance by capping with anoxic, sulfidic bay mud (Baye 2000

CONCLUSIONS
China Camp's tidal marshes conserve regionally rare and exceptionally well-preserved examples of salt marsh vegetation within a relatively intact late-Holocene tidal marsh platform, including multiple types of tidal marsh-terrestrial ecotones with distinctive native plant species assemblages. China Camp's remnant tidal marsh plant assemblages have some significant similarities with other prehistoric tidal marshes in the San Francisco Estuary. Most other prehistoric tidal marsh remnants in the Estuary, however, have strongly altered or agriculturally reclaimed (diked) terrestrial edges that are dominated by mostly nonnative vegetation. China Camp's tidal marsh vegetation, in contrast, has retained some complex terrestrial ecotones with predominantly native terrestrial vegetation, and lacks a history of intensive agricultural modification. The complex vegetation structure of China Camp's prehistoric tidal marsh contrasts with the relatively homogenous, broadly zoned salt marsh vegetation of the historic, prograded, outer high salt marsh terrace, which was formed with relatively simple topography and drainage patterns. China Camp Marsh has suffered relatively little from regional invasions by non-native tidal marsh plants, even in recently formed (post-reclamation era) portions of its tidal marshes. China Camp Marsh provides outstanding opportunities for research on mature, diverse tidal marsh vegetation, and is important as a regional reference site for tidal marsh management and restoration planning.