Facilitative and Interfering Effects of Arthrocnemum Subterminale on Winter Annuals

The relative importance of interspecific interference and facilitation in natural plant communities is controversial, in part due to the difficulty of manipulating these interactions in the field. I investigated the relative intensity of facilitative and interfering effects of Arthrocnemum subterminale, a subshrub common in the upper zones of California salt marshes, on three species of winter annuals by measuring spatial associations, manipulating Arthrocnemum canopy cover and root distributions, and comparing aspects of the physical microhabitat under Arthrocnemum to that in the open. The annuals Hutchinsia procumbens and Parapholis incurva were positively associated with Arthrocnemum, and survival and aboveground biomass of these species declined when shrub canopies were removed. Spergularia marina was negatively associated with Arthrocnemum, and survival increased when shrub canopies were removed. Combined effects of facilitation and interference were observed for Parapholis survival and biomass, which were reduced by root interference even though the overall effect of Arthrocemum on Parapholis was positive. Arthrocnemum canopies elicited direct effects on the winter annuals, but also altered substrate characteristics. Soil was mounded under the shrubs, averaging 3.14 cm higher in elevation than the surrounding open soil. Soil salinity was 27% lower under Arthrocnemum canopies, soil moisture was 13% higher under shrubs, and shrub understories received less light than open marsh soil. Although Arthrocnemum had species—specific interfering and facilitative effects, and interference and facilitation operated simultaneously, my results suggest that facilitative effects of Arthrocnemum were more significant than interference in the transition zone of the upper marsh. Physical conditions in the open reduced overall annual biomass and survival, but were ameliorated by Arthrocnemum, which altered the species composition and productivity of the upper marsh plant community.