Results and Discussion

The major environmental variables controlling the distribution of vascular plants in tidal marshes of the northern San Francisco Bay estuary are elevation and water salinity (Atwater and Hedel 1976). Elevation of marsh surfaces relative to tide levels determines the soil moisture content and frequency, duration, and depth of submergence, whereas the salinity of the water flooding a marsh determines the soil salinity (ibid .). Water salinity is an important influence in the regional distribution of tidal marsh plants; high soil salinity causes many plants to disappear toward San Francisco Bay, resulting in tidal marsh communities composed of only 13 or 14 native plant species (Atwater etal . 1979). Where water salinities are rather low, as in the Sacramento/San Joaquin Delta, tidal marsh communities are more diverse, containing some 40 plant species, most of which are relatively salt-intolerant—largely the same species that occur in freshwater marshes in California (ibid .).

Water salinities in the vicinity of the study area vary both seasonally and annually in response to the amount of freshwater flow from the river systems. Figure 1 shows the variation in mean monthly water salinities at Collinsville. Judging from the salinity data for Collinsville, water salinity is probably not a major limiting factor for plants in the intertidal zone there. Except in unusual circumstances (such as the drought of 1976–77) the water in this area varies from essentially fresh to only slightly brackish. Because of the regular flushing action of the tides and the rapid runoff from riverbanks at low tides, soil-salt concentrations resulting from evaporation would not be expected to be significantly higher than the water salinity of the river. That soil salinities are not high in the intertidal zone of this area can be inferred from the absence of salt-tolerant plants such as Distichlisspicata and Frankenia grandifolia from the upper reaches of the intertidal zone.

Figure 1.
Mean monthly water salinities (in parts per thousand) of the
Sacramento River at Collinsville, California. The bottom line
averages monthly means over the period 1967–80. The top
line represents monthly means for 1977, the year with the
highest salinities on record (from USDI Bureau of
Reclamation, Tracy Field Division).

The major ecological factor influencing the distribution of plants in the intertidal zone of the study area is considered to be elevation with respect to tide levels. Tidal heights (in decimeters) at the confluence of the Sacramento and San Joaquin Rivers (near the study area) are as follows (based on data in Simpson etal . 1968; definitions from Atwater etal . 1979):

10.1—mean higher high water (average height of the higher of the daily high tides);
–3.4—mean lower low water (average height of the lower of the daily low tides);
18.3—estimated maximum high water;
–7.6—estimated minimum low water.

Plants tolerant of relatively long periods of submergence (e.g., Scirpusacutus , S .

californicus , and Typha spp.) occupy lower sites along the river (below mean higher high water), whereas species less tolerant of long submergence (e.g., Carex barbarae , Hydrocotyleverticillata var. triradiata , and Lythrumcalifornicum ) occur at higher elevations in the intertidal zone (at or above mean higher high water).

A complete list of the vascular plants of the three plant communities considered in this study (tidal streambank, riparian woodland, and annual grassland) is found in Appendix A.

A tabulation of the flora of the three communities is given in table 1. The riparian woodland community contains the largest number of species (78) followed by the tidal streambank (49) and annual grassland (38) communities. The annual grassland community is included here primarily to illustrate the highly disturbed nature of the site. The low total number of species present (38) and the very high percentage of introduced species (82%) attests to its disturbed condition.

The riparian woodland community also supports a large non-native component (29 species). Over 30% of species are introduced plants, the majority of which also occur in the adjacent annual grassland community.

The tidal streambank community supports the smallest number of introduced species (8), only 16% of the total species. This figure compares favorably with the proportion of introduced species in the California flora as a whole, but is low compared to most California cismontane areas (table 2). Only the floras of Mount Diablo and Mount Hamilton Range have similar non-native components. The published floras of these two areas, however, are almost 40 years old; the percentages of introduced taxa present in both areas are almost certainly higher today.

The percentage of introduced species present in the tidal streambank community is especially low given the disturbed nature of the site. As previously indicated, this community has developed since the construction of levees between 1900 and 1940. The riparian woodland and annual grassland communities have also developed during this same time period. Their floras, however, exhibit a significantly higher percentage of introduced plant species. Annuals comprise a large percentage of the introduced species in the annual grassland (59%) and riparian woodland (42%) communities. An additional 32% of the introduced species of the annual grassland and 35% of those in the riparian woodland are perennial, non-rhizomatous herbs. Both of these life forms, especially the annuals, appear to be at a competitive disadvantage in the tidal streambank environment. Fifty-eight percent of the tidal streambank flora consists of rhizomatous herbs (fig. 2). Of the 29 species of rhizomatous herbs present in the tidal streambank community, only three (10%) are introduced.

It thus appears that the primary reason for the low number of introduced species in the tidal streambank community (relative to the other two communities) is the restricted capability of introduced plants (most of which are annuals or non-rhizomatous perennials) to establish under conditions of periodic or prolonged inundation. This fact is further emphasized by the relative paucity of introduced species in areas within the intertidal zone which have been more recently disturbed by riprapping. Although cover and density in riprapped areas are far lower than on undisturbed levees, the species which are found in these areas are predominantly native.

The life form spectra of the tidal streambank and riparian woodland plant communities (fig. 2) highlight several differences between these two communities. Almost 25% of the species present in riparian woodland are woody species, as opposed to 4% (2 species) of the total species found in the tidal streambank community. Twentyfive percent of the riparian woodland species are annuals—62% are introduced—while 14% (6 species) of the intertidal flora are annuals—43%

Figure 2.
Life form spectra of the tidal streambank and riparian woodland plant communities. Height of the bars corresonds to the percentage
of the total species in the community represented by each of the five life forms. A—annual; PH—perennial herb (including biennials);
RH—rhizomatous perennial herb (including herbs spreading by stolons and creeping root systems); S— shrub; and T—tree.

are introduced. The percentage contribution of perennial, non-rhizomatous herbs to the floras of both communities is almost identical, 24% for the tidal streambank community and 25% for the riparian woodland (the absolute species numbers are 10 and 21, respectively). Perhaps the most significant difference between these two communities is the much greater proportion of rhizomatous herbs in the flora of the tidal streambank plant community (58% of the total species compared with 28% for riparian woodland community).

Certain life forms enjoy an apparent competitive advantage in the tidal streambank community. Perennials account for 86% of the total flora, suggesting that one major limiting factor is the difficulty of seedling establishment under the ebb and flow of tidal waters. This would put annuals at a distinct disadvantage. The annual strategy may also be a handicap in another way: in an azonal community where water is not limiting, dry season dormancy is not only unnecessary but is probably detrimental. Rhizomatous species are more successful than non-rhizomatous species, a fact which may be at least partially explained by the greater ability of the former to apomictically spread once established. Even the two woody species present in the intertidal zone, Salix lasiolepis and the introduced Rubusdiscolor , are capable of extensive vegetative reproduction. Thus 63% of the tidal streambank flora is capable of vegetative reproduction.

In terms of floristic composition and life form spectra, the tidal streambank community in this rather disturbed area is remarkably similar to that of other, less disturbed areas in the Sacramento/San Joaquin estuary (compare the species list for Browns Island in Knight 1980). However, the plant cover and density of the tidal streambank community of the study area are certainly lower relative to less disturbed examples of this community elsewhere, although this fact is yet to be quantitatively documented.

Many of the species of the tidal streambank community (e.g., Typhalatifolia , Scirpusacutus ) have very wide distributions and occur in several types of moist to wet habitats. However, a few of the tidal streambank species in the study area exhibit restricted distributions and occupy only the intertidal habitat. Asterchilensis var. lentus and Lilaeopsis masonii are both recognized as rare and endangered by the California Native Plant Society (Smith etal . 1980). Both of these taxa and a third, Grindeliapaludosa , formerly considered rare and endangered, are entirely restricted to intertidal areas in the Sacramento/San Joaquin estuary. Although these plants are not particularly rare in the habitats in which they occur, their continued existence may be threatened by human alterations of their narrow habitats. The practice of riprapping streambanks results in a significant loss of habitat; potential increases in water salinity of the estuary as a result of

proposed future freshwater diversions may have deleterious effects on these plants.