Global climate change is expected to lead to increased drought severity and frequency. While some plant species are vulnerable to drought, other species are resistant to stressful environments. Dominant species often have high genetic diversity and intraspecific variation, which may allow genotypes to fluctuate in abundance in response to drought stress, preventing loss of ecosystem function. Different genotypes may cope with drought differently by succumbing to drought at different water stress levels. Using the dominant C4 tallgrass species Andropogon gerardii, we ask the question, do common genotypes of A. gerardii differ in their threshold responses to a gradient of water stress? We selected three A. gerardii genotypes (G1, G2, and G3) from a single population that have previously exhibited different responses to drought. We subjected plants to a gradient of moisture conditions (four levels, non-limiting to typical drought) over ten weeks. Using a Bayesian interaction model, we determined the response effect size between each treatment for each genotype. We examined instantaneous physiological (photosynthesis, stomatal conductance, chlorophyll fluorescence) and cumulative phenotypic (root and shoot biomass allocation, root structure, dry matter content) responses to the different water treatments. Recovery and flowering following the drought were also examined.
A. gerardii genotypes differed in their drought-induced response thresholds. As expected, all genotypes were most stressed (lowest productivity, photosynthesis, conductance, and root growth) under the most limiting water treatment. In terms of cumulative responses, G1 succumbed to drought more rapidly for above and belowground biomass and leaf dry matter content; G1 also invested more rapidly in greater root volume. However, G3 invested more rapidly in the ratio of above:belowground allocation and invested earlier in higher root surface area. However, genotypes experienced the same threshold for succumbing to drought in terms of root dry matter content, specific leaf area, root diameter, root tips, conductance, and photosynthesis. This indicates that while there are similar coping strategies among genotypes under drought, some variation in drought coping strategy does exist. Furthermore, these genotypes could fill different niches under variable environmental conditions, despite genotype similarity and high plasticity. Because G1 is conservative with vegetative carbon allocation, but generally flowers more, we expect that the vegetative growth-flowering growth tradeoff to be important for the whole-species response to drought. Both plasticity and compensatory dynamics among different genotypes for growth and flowering could be important for predicting A. gerardii’s success under more variable climate conditions.