variation. The result is a mosaic of 

 populations that are constantly shifting in 

 space and time. 



These long-term patterns of habitat 

 change have been revealed only recently; they 

 are emphasized here because they are not 

 obvious upon short-term observation. Other 

 dynamic aspects of the estuary are more 

 easily documented. These are the daily and 

 seasonal fluctuations in tidal height, the 

 alternation of wet and dry seasons that creates 

 pools in winter and salt pannes in summer, 

 the seasonal patterns of temperature that 

 stimulate development of macroalgae in the 

 inland lagoons and tidal creeks, and the 

 migration of birds that use the intertidal flats 

 in winter but not in midsummer. Each 

 community must be viewed as a function of its 

 changing physical environment, as well as a 

 complex mixture of interacting species. 



3.2 TRANSITION FROM UPLAND TO 

 WETLAND 



By definition, transitions are areas where 

 one community type (e.g., upland) shifts to 

 another (e.g., wetland). The most extensive 

 wetland habitat at Tijuana Estuary is the 

 intertidal salt marsh. Its exact upper 

 boundary is difficult to discern, because the 

 vegetation blends into that of the upland. The 

 two communities overlap in a transitional 

 area that has elements of both (Figure 1.3). 



Most of the peripheral upland has been 

 disturbed at Tijuana Estuary. The best 

 information on what these areas might have 

 been like naturally comes from Baja 

 California (Neuenschwander 1972). At Bahia 

 de San Quintin (30°25' N., II6°00' W.), we 

 analyzed the transitional vegetation in detail 

 to determine at what point one leaves the 

 upland and enters the wetland (Zedler and Cox 

 1984). There, the wetland plants that 

 occurred highest along the slope were alkali 

 heath (Frankenia grandifolia), sea lavender 

 (Limonium californicum), and sea-blite 

 (Suaeda esteroa). The upland plants that were 

 found farthest downslope were two species of 

 box-thorn (Lycium spp.) and Frankenia 



palmeri. In California, the latter occurs only 

 in San Diego Bay; whether or not it ever 

 occurred at Tijuana Estuary is unknown. The 

 box-thorn (Lycium californicum), however, 

 is common in several of the remnants of 

 transitional vegetation at Tijuana Estuary. It 

 is easily recognized by its thorny, stiff- 

 twigged appearance and small red berries. 

 Like many shrubs of the coastal scrub 

 community, box-thorn is drought deciduous. 

 It is leafless during summer when the salt 

 marsh vegetation is at peak biomass. 



3.2.1 Plants of the Wetland-Upland 

 Transition 



A small remnant of the transition 

 community persists in a sloped corner of the 

 northern end of Tijuana Estuary. Because the 

 slope is one of the most frequently used entry 

 points for visitors to the estuary, a detailed 

 description of upland-wetland compositional 

 shifts has been developed. The area was 

 sampled in 1984 (Table 3.2) with 396 

 quadrats spanning elevations from the street 

 into the marsh. In this transition area, the 

 upland community is coastal sage scrub with 

 several shrub species, some of which are 

 evergreen (e.g., laurel sumac, Rhus laurina; 

 lemonadeberry, R. integrifolia; jojoba, 

 Simmondsia chinensis), and some of which are 

 drought-deciduous (e.g., golden bush, 

 Haplopappus venetus; California sagebrush, 

 Artemisia californica). 



The data in Table 3.2 indicate a relatively 

 abrupt boundary between upland and wetland 

 plants. The band of overlap is narrow because 

 the topography is fairly steep. While many 

 exotic plants have invaded the coastal scrub, 

 few have sufficient tolerance of inundation and 

 salt stress to invade the coastal wetlands. One 

 weedy species, the Australian salt bush 

 (Atriplex semibaccata), has an extremely 

 wide range of tolerance. It occurs throughout 

 the transition zone and well into the upper 

 salt marsh. It withstands the dry saline 

 upland as well as occasional inundation by 

 seawater, and thus has been promoted for 

 horticultural uses. Although its light foliage 

 and bright red berries are attractive, further 

 spread of this exotic species is of concern. 



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