Adaptive root morphology as a drought response in Bromus inermis

Grassland ecosystems across the globe are increasingly threatened by climate change, which is predicted to exert different pressures on native and invasive plants. Plant responses to changing environmental conditions are often measured or predicted using their morphological and anatomical traits, however, few studies account for the intraspecific trait plasticity that plants exhibit in response to environmental stressors, including drought. In this study, we examine whether a six-year experimentally induced drought altered plant species composition and diversity in a grassland in western Manitoba, Canada, and whether smooth brome (Bromus inermis Leyss.), an invasive perennial grass, exhibited differential root morphology and architecture as a result of drought.

We conducted a plant inventory, harvested aboveground plant biomass, and collected, washed, and scanned roots of smooth brome individuals sampled from a long-term, extreme drought experiment. Scanned images of the roots of twenty smooth brome individuals were analyzed to compare the morphological and architectural traits, including the proportion of rhizomes produced from root crowns, in plants growing in drought and ambient conditions.

Long-term drought increased the alpha diversity but not the beta diversity of experimental grasslands. For smooth brome, drought increased the number of crown buds that produced rhizomes, and the total length and surface area of roots. Smooth brome also increased its allocation of root length and surface area to very fine roots under drought.

Smooth brome exhibited significant differences in morphological root traits following drought, indicating a phenotypically plastic strategy of water acquisition. Our work reinforces the importance of incorporating intraspecific variation in root traits into measurements of plant responses to drought stress. Understanding how plants respond to drought is critical to predicting how climate change will continue to impact the composition and diversity of grasslands across North America, including the spread of exotic invasive species.