Is it plastic or just fantastic? Understanding the link between local adaptation and drought response in a dominant grass species

Background/Question/Methods

The ability of dominant grasses to tolerate or acclimate to drought will become a major driver of changes in community composition, carbon and water cycles and ecosystem services as we encounter increasing severity and frequency of droughts in the North American grasslands. Although some variability in drought tolerance within a species is expected, it is unknown whether the local adaptation of populations can explain the expected variability. The objective of this study is the quantification of plasticity of drought tolerance in different populations of Bouteloua gracilis, a C4 perennial grass that dominates grasslands across a range of climates and is a major contributor of ecosystem function and services within these systems. Sixteen populations of B. gracilis were selected from sites along a gradient in growing season aridity index (GSAI) and grown at two different soil moisture levels for fourteen weeks in June-October 2016. Changes in total biomass, leaf osmotic potential (Y_OSM), midday water potential (Y_MID), photosynthesis (A_MAX) and stomatal conductance (g_s) were measured to assess adjustment to soil moisture for each population.

Results/Conclusions

Plants from more arid sites (lower GSAI) had higher biomass under both soil moisture conditions (R-squared = .53 and .52) and sensitivity to changes in soil moisture conditions showed a weak relationship with GSAI (R-squared = .14, p=.08). Increases in biomass were not explained by a corresponding increase in A_MAX, g_s or instantaneous water use efficiency (A_MAX/ g_s). Plants from more arid populations had more negative Y_MID values, while plants from less arid sites showed more negative Y_OSM (R-squared = .43 and .67 respectively). These relationships indicates that different strategies may exist within a species in response to lowered soil moisture and that these strategies may be related to climate of origin . Whether these contrasting strategies confer disadvantages under novel environmental conditions is of critical importance in understanding the performance of this dominant species and the associated ecosystems under forecasted climatic conditions.