Salinity-Herbivore-Plant Interactions: Effects of Plant Health, Beetle Defoliation, and Local Adaptation on Tamarix Growth
Randall Long1*, Tom Dudley2, Adam Lambert3, Kevin Hultine4
1Ecology, Evolution, and Marine Biology, University of California Santa Barbara, Santa Barbara, CA
2Marine Science Institute, University of California, Santa Barbara, Santa Barbara, CA
3Marine Science Institute, University of California, Santa Barbara, Santa Barbara, CA
4Research, Conservation and Collections, Desert Botanical Garden, Phoenix
Tamarix, a non-native tree, is abundant in riparian areas throughout the western US and is highly successful in adverse environments that combine high salinity and arid conditions, with genotypes being locally adapted to the site conditions. In addition, Tamarix is repeatedly defoliated over the growing season by the tamarisk leaf beetle (Diorhabda spp.). This defoliation has resulted in variation of dieback, with soil salinity being shown to be correlated with dieback. To investigate whether there are synergistic interactions between salinity and herbivory we conducted a greenhouse experiment using two genotypes of Tamarix from different salinities at the Cibola National Wildlife Refuge. The plants were grown under reciprocal salinities and then defoliated using Diorhabda carinulata. Beetle preference for plant-salinity interactions were measured using choice trials to test if beetles preferred healthier plants as local adaptation to site salinity exists in Tamarix. Biomass accumulation was measured to test our hypothesis that there would be a synergistic effect of salinity and herbivory, with a prediction that low salinity genotypes would be most affected in high salinity.
A Pearson’s Chi-squared test was used to evaluate if beetle choice was influenced by salinity, and was found to be significant (2(2) =20.67, p 0.001,  =0.05). With beetles preferring plants that were grown in the salinity in which they were collected from, supporting our hypothesis that beetles preferred healthier plants. With respect to the synergistic effect of salinity and herbivory we found that there was a significant interaction between beetle herbivory and salinity on biomass accumulation of both leaves (ANOVA: F1,56 =19.84, p0.001) and stems (ANOVA: F1,56 =2.88, p =0.095). Combined these results indicate that beetles preferentially feed on healthy plants, but that increased salinity leads to synergistic effects in reducing total biomass.