types of disturbance to marsh structure. 

 Areas of dense vegetation become 

 completely eliminated wherever traffic 

 persists (Figure 30). Since most drivers 

 steer clear of the muddier habitats, their 

 damage is usually most visible in the high 

 marsh and transitional communities. The 

 impact on vegetation composition is 

 two-fold. Species restricted to these 

 habitats are eliminated, and 

 opportunistic, weedy annuals invade during 

 the rainy season when vehicle use is 

 sparse. 



Salt marsh bird's beak ( Cordylanthus 

 maritimus ssp. maritimus ) most likely owes 

 its endangered status to disturbance of 

 this type. Marsh vegetation is notably 

 sensitive to mechanical damage (Mclntyre 

 1977) and the bird's beak is especially 

 easy to break off (Dunn 1981). The latter 

 is restricted to a narrow elevational 

 range (less than 30 cm at Tijuana Estuary) 

 at the upper limit of tidal influence near 

 areas used by off-road vehicles. 



Introduced species often invade 

 native communities following disturbance, 

 and some become a common part of that 

 community even when disturbance is 

 peripheral. Two ice plants, Gasoul 

 crystallinum and G. nodiflorum , germinate 

 readily in roadways at the upper marsh 

 boundary, while brass-buttons ( Cotula 

 coronopifolia ) has become a common member 

 of southern California coastal marshes. 

 All three species are native to South 

 Africa. Two European grasses, Parapholis 

 incurva and Polypogon monspeliensis , are 

 sometimes abundant in upper marsh 

 habitats. 



Rarely does one see exotic species in 

 lower marsh habitats, although smooth 

 cordgrass ( Spartina alterniflora ) has been 

 successfully introduced from the Atlantic 

 coast to the State of Washington (Frey and 

 Basan 1978), In southern California, 

 there is one case of low marsh invasion, 

 which still puzzles ecologists. A New 

 Zealand mangrove ( Avicennia sp.) was 

 deliberately planted in Mission Bay to 

 provide live material for physiological 

 study. It subsequently spread and became 



a dominant feature of the marsh, occupying 

 the channel banks and lower parts of 

 cordgrass's distribution. Finally, 

 concerned that the tall (2-3 m) plants 

 would attract predators of the endangered 

 light-footed clapper rail, the Clapper 

 Rail Recovery Team called for the 

 mangrove's removal. The puzzle? If 

 exotic mangroves can occupy the tidal 

 creeks at this latitude, why haven't 

 native mangroves from Baja California 

 migrated north or other vascular plant 

 species developed tolerance to these low 

 elevations? 



2.6 A CONCEPTUAL MODEL OF MARSH COMMUNITY 

 DEVELOPMENT 



Observations of species compositional 

 changes in southern California marshes can 

 be combined to develop a conceptual model 

 of community development. Since most 

 observations deal with pickleweed 

 ( Salicornla virginica ) and cordgrass 

 ( Spartina foliosa ) , these two dominants 

 and their interactions are emphasized. 

 But first a word of caution. Conceptual 

 models are useful in summarizing ideas and 

 clarifying questions which need to be 

 approached experimentally. Until such 

 models have withstood repeated attempts to 

 disprove them, they should not be accepted 

 as fact. Rather, they should be viewed as 

 a set of hypotheses and discarded when 

 better explanations come along. 



The following model (Figure 31) 

 suggests a likely progression of events 

 which is predicted from observations of 

 establishment ability and competitive 

 interactions for pickleweed and cordgrass. 



2.7 VASCULAR PLANT PRODUCTIVITY 



Across the nation coastal marshes 

 have been acclaimed as highly productive 

 natural ecosystems. The cordgrass 

 ( Spartina alterniflora ) marshes of Georgia 

 and Louisiana are particularly we^Ll- 

 studied and estimates of up to 3 kg/m /yr 

 above-ground productivity are on record 

 (see review of Turner 1976, Kirby and 



38 



