. The ecology of Tijuana Estuary, California : a national estuarine research reserve . 12 3 4 5 Height Class Figure September height distribu- tions for cordgrass over a three-year period. Stem heights were pooled for ten quadrats x 4 stations. Height classes are 30- cm intervals (approximately 1 -ft), , 1 = 0-30 cm, 2 = 31-60 cm, 3 = 61-90, 4 = 91-120, 5 = 120-150 cm. The number of stems was 406 in 1989, 446 in 1990, and 579 in 1991. Plants of the constructed marshes at both the Chula Vista Wildlife Reserve and the Sweetwater Marsh National Wildlife Reserve have reduced statu
. The ecology of Tijuana Estuary, California : a national estuarine research reserve . 12 3 4 5 Height Class Figure September height distribu- tions for cordgrass over a three-year period. Stem heights were pooled for ten quadrats x 4 stations. Height classes are 30- cm intervals (approximately 1 -ft), , 1 = 0-30 cm, 2 = 31-60 cm, 3 = 61-90, 4 = 91-120, 5 = 120-150 cm. The number of stems was 406 in 1989, 446 in 1990, and 579 in 1991. Plants of the constructed marshes at both the Chula Vista Wildlife Reserve and the Sweetwater Marsh National Wildlife Reserve have reduced stature because soil nitrogen supplies are limiting (Langis et al. 1991, Zedler 1991b). The importance of having many tall stems (60-120 cm) has led to the recommendation that cordgrass canopy architecture be a primary criterion in judging habitat suitability. Two San Diego Bay marshes that were designed and constructed for clapper rail nesting have not yet attracted the species. Their canopies differ from those at Tijuana Estuary in having fewer tall stems; densities, on the other hand, are often similar or higher. Thus, the data from Tijuana Estuary suggest that densities alone are not the critical factor, but rather the density of tall stems. These reference data suggest minimum standards for "suitable clapper rail habitat" in southern California (Zedler, in review). Individual data sets have far less use in marsh-to-marsh comparisons than long- term records. The magnitude of year-to-year variations within a marsh must be known in order to determine if a man-made marsh is truly different from natural ones. The 1989 and 1990 data at Tijuana Estuary were critical to the comparison of constructed marshes sampled in 1989 and 1990 (Zedler, in review). Of further value is the fact that the Tijuana Estuary monitoring program provides information on a variety of factors that elucidate cause-effect relationships. Understanding the reasons for differences between natural and cons
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