Environmental conditions are generally the best predictors of terrestrial communities, and have long been used to define patterns of plant species distributions. Both extreme temperatures, whether hot or cold, and water availability have been shown to select for certain trait correlates that result in the increased fitness of a plant species. In this dissertation I conducted several experiments across large and small landscapes to determine tradeoffs and trait variation exhibited in Tamarix when collected across a range of temperature and water availability.
Tamarix provided a unique opportunity to study how rapidly these trait variations become established in woody plants, due to it being an introduced plant. Although it was already established at many sites in North America by the late 1800’s, it is a perennial woody plant that resprouts from underground tissues after fire and flood disturbances and stand replacement does not occur often. Because of these traits most stands are likely only separated from the founding originals by only a few generations.
Using populations sourced from across a broad thermal range in Arizona I was able to find evidence that there was variation in phenology, allocation to storage, and reproductive output across the different populations of Tamarix. The plants were grown in a common garden in Yuma, AZ and I witnessed that plants from colder sites had delayed bud break and flowering in the spring and allocated greater amounts of carbon to storage, at the expense of growth and reproduction. Delayed phenology protected the plants from frost events that occur in the spring at the colder sites, and the increased allocation to carbon could serve as a reserve to regrow plant tissues in the spring after late frost events.
To investigate trait variation at a smaller scale I used plants sourced from two sites with different salinities. The sites were located at the Cibola National Wildlife Refuge on the Lower Colorado River in southeast California. The sites were only separated by 1.5 km, but the salinity increased six fold across that distance. Increasing salinity is analogous to decreasing water availability as high solute concentrations reduces the plant available water. Using both in situ and greenhouse experiments I found that plants from high salinity sites exhibited more conservative water use traits than those from low salinity sites. I also found that plants grew best at salinities closest to that of the source site. Increased salinity reduced net photosynthesis and growth in the low salinity populations, while high salinity populations grew best when solute concentrations were high.
To determine how these tradeoffs would interact with an additional stressor we used the tamarisk leaf beetle (Diorabda carinulata) in a salinity x herbivory experiment. We found that the plants from the high salinity sites were most impacted by the beetle herbivory. The more conservative traits exhibited by the high salinity populations resulted in lower biomass accumulation between herbivory events and the plants were unable to recover from repeated defoliation events. Taken together these three studies show that trait variation exists across Tamarix populations and that there are apparent tradeoffs between the traits that reduce the fitness of the individuals when exposed to additional stress.
