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