Clinal adaptation and adaptive plasticity in Artemisia californica: Implications for the response of a foundation species and its arthropod community to predicted climate change

Background/Question/Methods

Local adaptation and plasticity pose significant obstacles to predicting plant responses to environmental change. Steep climatic gradients may select for clinal adaptation in plants with implications for both interactions with herbivores and response to future climate change. We examined clinal variation in plant functional traits, performance, and resistance to herbivory for the shrub Artemisia californica along a 700 km gradient characterized (from south-to-north) by a four-fold increase in precipitation and a 61% decrease in interannual precipitation variation. Plants cloned from five populations along this gradient were grown for three years in treatments approximating the precipitation regimes of the north and south range margins. We collected whole arthropod communities from experimental plants to assess the likely consequences of plant responses to changes in precipitation for plant-herbivore interactions. Additionally, we analyzed long-term climate data from 20 coastal climate stations in California to test for increased interannual variation in precipitation predicted by most climate models.

Results/Conclusions

Most traits varying among populations did so clinally; northern populations (vs. southern) had higher water-use efficiencies and lower growth rates, C:N ratios and terpene concentrations. Notably, there was variation in plasticity for plant performance that was strongly correlated with source site interannual precipitation variability. High precipitation (vs. low) increased growth and flower production more for plants from southern populations (181% and 279% respectively) than northern populations (47% and 20% respectively). Overall, precipitation variability at population source sites predicted 86% and 99% of variation in plasticity in growth and flowering, respectively. We collected >30,000 arthropods of 237 morphospecies and found population variation in herbivorous and predatory arthropods, with the latter varying clinally. Arthropods did not respond to increased precipitation. Our analysis of climate data indicates an increase in interannual precipitation variation consistent with most global change models and, unexpectedly, this increased variation is especially pronounced at historically stable, northern sites. In conclusion, precipitation environment can be a strong selective force for functional traits, including resistance to herbivory, and plasticity in response to altered precipitation. Based upon existing population variation, we predict that response to future climate change will vary along the species range, with relatively larger effects on non-plastic northern populations.