The effects of irrigation on revegetation of semi-arid coastal sage scrub in southern California

To test the appropriateness and applicability of irrigation for restoration of coastal sage scrub, a semi-arid vegetation type native to southern Cali fornia, a field study comparing four irrigation schedules-no irrigation, sp ring irrigation only, summer irrigation only, and irrigation year round as needed-was established. A seed mixture of six native shrub species was broa dcast, and the effect of irrigation on emergence and establishment was eval uated. Restoration of arid and semi-arid vegetation is challenging because of the severe environmental conditions inherent to these ecosystems. In par ticular, the low and unpredictable nature of precipitation can limit the ch ances for successful establishment of perennial species. Under conditions w here supplemental irrigation is available, irrigation may make the differen ce between successful or failed restoration. However, increasing the availa bility of water through irrigation may result in poor plant adaptation to s urviving arid conditions or only temporary success followed by failure once supplemental water is withdrawn. In this study. irrigation stimulated germ ination in the irrigated plots about a month earlier than the plots depende nt on natural rainfall, but this had little lasting effect. Spring and summ er irrigation did not improve survival compared to the plots receiving no s upplemental water. The largest effect of irrigation was a reduction in surv ival of several species grown under irrigated conditions as compared to non irrigated conditions. In plots where water was applied all year long a sing le species, Artemisia californica, represented nearly 100% of the species p resent at the end of two growing seasons. Irrigation may speed revegetation under some conditions, but was not very effective in establishing natural vegetation structure.