Patterns of woody-plant mortality have been linked to environmental changes, such as increased moisture deficits, salinity, and episodic outbreaks of insect herbivores. While many studies have focused on specific stresses, site variation in susceptibility raises a less studied question of whether local adaptation to stress creates variation in carbon allocation that can influence plant susceptibility to herbivory. Tamarix spp. are a group of dominant, non-native tree species in western North America that are experiencing dieback driven by episodic herbivory of the introduced tamarisk leaf beetle (Diorhabda carinulata). As beetles have spread across this landscape there appears to be variation in mortality driven by carbon starvation, including across areas with different salinities. Here we ask whether plants from sites with increasing salinities have different patterns of gas exchange, productivity and carbon allocation when grown in a common garden. We evaluated these traits using reciprocal salinity treatments with asexually propagated plants from adjacent high and low salinity sites along the Colorado River. Plants were grown in a greenhouse and traits were evaluated under increasing salinity. We anticipated that Tamarix is adapted to its local environment; and local adaptation to specific conditions will yield predictable responses to episodic herbivory.
Both in situ collections and greenhouse trials of plants from high and low salinity sites suggest local adaptation exists and is manifested in different allocation patterns. For example plants from high salinity sites allocated relatively greater amounts of resources to belowground biomass in the greenhouse regardless of the salinity they were grown in (F1,12 = 4.4, p = 0.055). Plant material collected in situ from the low and high salinity sites during the dormant season showed increased allocation to labile carbon storage (F1,18 = 5.7, p = 0.027). In the greenhouse plants from high salinity sites had reduced photosynthetic rates (F1,9 = 19.65, p = 0.002), while at the same time exhibiting greater water uptake indicating a lower water use efficiency. Future studies will evaluate how resource allocation patterns to storage or growth in response to increased salinity interact with episodic herbivory. The results from these studies suggest that Tamarix is experiencing environmental selection across small spatial scale gradients, and that the heterogeneous rates of mortality from herbivory across the landscape may be linked to heritable traits related to carbon allocation in response to salinity and other site factors.