RESISTANCE BY NATURAL VEGETATION IN THE SAN GABRIEL MOUNTAINS OF CALIFORNIA TO INVASION BY INTRODUCED CONIFERS

For a century, foresters have been attempting to expand the conifer forests of a rugged southern California mountain range by planting both native conifers, of which there are thirteen species, and non-natives, of which forty-five species have been tried. Planting began as an early project in ecological restoration under the mistaken belief that the chaparral, which blankets the lower slopes, had replaced forest destroyed by logging, burning, and sheep grazing. By 1910 it had become evident that general afforestation of the chaparral belt was out of the question, but widespread conifer planting has been carried on in burns, along roadsides, and around campgrounds. Species introduced from nearby regions, and from climatically similar distant regions, have often grown well where protected from competition and wildfire. None, however, has invaded native vegetation adjacent to planting sites. The situation contrasts sharply with the overwhelming exotic invasions in other Californian vegetation formations. Implicit in the conifer introductions was the hypothesis that conifer species diversity in the San Gabriels had been limited by geographic isolation. The failure of any non-native species to naturalize suggests that barriers to dispersal were irrelevant and that diversity of the local conifer flora was ecologically controlled.