COS 21-4 - Phenotypic and genotypic variability of a dominant perennial grass in response to changes in precipitation seasonality

Abstract: Understanding the genotypic and phenotypic plasticity of plant source populations under varied precipitation conditions can improve restoration success in changing climate scenarios. Available native plant materials may or may not be adapted to emerging climate conditions for a restoration site, thus identifying source populations that are best adapted to present and future moisture regimes can improve restoration success. The North American Monsoon creates a gradient of precipitation seasonality across the southwestern US, ranging from summer-dominated precipitation in southern New Mexico to winter-dominated precipitation in northern Utah. Pleuraphis jamesii, a dominant rhizomatous perennial grass, has naturally occurring populations across this gradient and is a useful species for semi-arid grassland restoration because it provides forage for wildlife and livestock, stabilizes soils, and tolerates drought.

We investigated trait and genetic variability among P. jamesii individuals from four source populations spanning the precipitation seasonality gradient under three experimental watering conditions in a common garden: (1) spring-dominated watering, (2) summer-dominated watering, and (3) intermediate watering where equal water was applied during spring and summer. We measured a suite of traits including above and below-ground biomass, phenology, and number of inflorescences, and performed DNA sequencing to investigate genetic differentiation of source populations. We hypothesized that traits would vary by source population and by watering treatment, and that individuals would have larger biomass and more inflorescences when watering treatment matched the precipitation seasonality of the source location. We also hypothesized that populations that differed by traits would also differ in genetic composition.

We found that biomass, phenology, and inflorescence counts varied by source population, but the variation did not correspond to the precipitation seasonality of the source population. However, across all source populations, below-ground biomass responded significantly to watering treatments, with lowest values in spring-dominated and highest values in summer-dominated watering treatments. The four source populations did not vary in their responses to watering treatments. The two source populations from the summer-dominated side of the precipitation seasonality gradient were genetically similar, while the two northern populations were both genetically distinct. Of the traits we measured, only above-ground biomass differed in ways consistent with population genetic variability.

Our results indicate that P. jamesii does not exhibit local adaptation to precipitation seasonality, suggesting that if resistance to changes in precipitation timing is a restoration goal, source population need not be a primary consideration. However, if specific traits are desired for restoration goals, targeting specific source populations may improve outcomes.