Recent and significant environmental changes have greatly affected native recruitment and re-establishment in chaparral and sage scrub plant communities. Established shrubs of these water-limited environments may be important in facilitation, where neighboring plants may benefit from shared resources and protection from herbivory. Living plants strongly influence community structure and interactions; however, there is little information suggesting that dead shrubs and trees in drought-affected landscapes may provide similar services as live shrubs. We conducted an experiment in Cal Poly Pomona’s Voorhis Ecological Reserve (VER) in Pomona, CA to determine whether seedling survival and growth depend on abiotic factors (microclimate conditions) or on biotic factors (herbivory). Two native woody shrubs (Artemisia californica and Salvia mellifera) and four annual native species (Amsinckia intermedia, Deinandra fasciculata, Phacelia distans, and Pseudognaphalium californicum) were outplanted and sown, respectively, in five blocks with three nurse treatments (live shrub, dead shrub, and exposed areas), and a nested caged and uncaged treatment in each nurse treatment. Environmental sensors and trail cameras were installed to measure abiotic factors and estimate herbivore occupancy. Leaf water potential, plant survival and height, and abiotic data were analyzed to determine abiotic and biotic effects on growth and survivorship under nurse shrub and caged treatments.
There were differences in abiotic conditions of each nurse treatment throughout the year; overall, exposed treatments had highest solar radiation and soil temperatures, and lowest soil moisture. Cage treatments significantly increased growth of A. californica (F1,688= 192.020, P < 0.001) and S. mellifera (F1,688= 8.400, P < 0.001). Nurse treatments had a significant effect on A. californica water stress, (F1,138 = 0.007, P < 0.001), while a nurse and cage treatment interaction significantly affected S. mellifera water stress (F1,121= 1.488, P < 0.001). Caged A. californica under dead shrubs grew the most, but were most water stressed at certain times, due to variation of abiotic conditions in each nurse treatment throughout the year. Plants under shrubs, though water-limited, were shaded during summer. Caged S. mellifera in exposed areas grew the most, and were least water stressed under dead shrubs. From our results, we conclude that nurse shrubs may reduce environmental stress during seedling establishment and herbivores are a significant barrier to recruitment in all habitats. Collection and analysis of data is ongoing.