Article Title

Distribution and Stability of Grasslands in the Los Angeles Basin





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Present grassland distribution (as of 1 980) was mapped from modem aerial photographs for 21 7.5 minute quadrangles in the Los Angeles Basin. These patterns were compared with the distribution of grasslands mapped from aerial photographs from 1 928 to 1936. Grasslands increased in all but three quadrangles, the greatest increases being in the northwestern portion of the basin. Vegetational changes, however, were not unindirectional as shrubland, notably coastal sage scrub, replaced grassland in sections of nine quadrangles. In general, areas subject to frequent fires and grazing moved from shrubland to grassland while grassland areas with infrequent disturbance were invaded by shrubs. An intensive study was undertaken in a quadrangle in which vegetation had changed in both direc- tions. Nine islands of coastal sage scrub surrounded by grassland were selected for study. Vegetation pattern was not dictated by topographic position or soil charateristics. Density of seedlings was high within patches of mature shrubs but seedlings were largely absent outside their boundaries. Shrub sizes in transects across the ecotone suggested that in eight of the nine coastal sage scrub patches shrubland was not invading grassland. Recent burning (within 10 years), coupled with intensive grazing, appears to inhibit the invasion of shrubs into adjacent grasslands. We hypothesize that the vegetation of the Los Angeles Basin is a mosaic of community types differing in their tolerances to disturbance.