Drought and warming alter belowground carbon allocation and root morphology in the pasture grass Festuca arundinacea

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Manjunatha H. Chandregowda, Mark G Tjoelker, Elise Pendall, Amber C. Churchill, Haiyang Zhang and Sally A. Power, Hawkesbury Institute for the Environment, Western Sydney University, Richmond, NSW, Australia, Amber C. Churchill, Department of Ecology, Evolution and Behavior, University of Minnesota, St. Paul, MN

Background/Question/Methods
Rising temperatures and severe drought are expected to reduce primary production under future warmer, drier climates in many regions across the globe. To sustain grassland production under more extreme climatic conditions, it is crucial to understand how plants adjust morphologically and physiologically to higher temperatures and reduced soil water availability. Most studies have focussed on aboveground traits, so a significant knowledge gap relating to belowground morphological and chemical traits remains. We hypothesised that plants would adjust to a warmer, drier climate by allocating more carbon to storage in belowground organs and by shifting their root morphologies towards more acquisitive traits to increase their capacity to take up water and nutrients. We tested this hypothesis by exposing a pasture grass, Festuca arundinacea, to a factorial combination of elevated temperature (ambient + 3 ºC) and a 60% reduction in winter and spring rainfall, at the Pastures and Climate Extremes (PACE) facility near Sydney, Australia.
Results/Conclusions
We found that drought and warming reduced belowground biomass by 18% and 40%, respectively, compared to controls, and that effects were independent and thus additive (55% reduction in warmed and droughted plots). Belowground net primary production was also reduced by drought (52%), warming (35%) and their combination (77%). There were significant interactions between drought and warming such that drought increased specific root length and decreased mean root diameter under warmer conditions. The significant interactions between treatments increased starch in root tissue during drought under ambient temperature. The independent effects of drought and warming reduced starch in root crowns by 21% and 18%, respectively, and their combination by 23%. Principal components analyses indicated that climate sensitivity in F. arundinacea was associated with acquisitive root traits (such as high SRL, root N and crown N) and drought recovery was associated with high carbon storage in root crowns. These results highlight the importance of including belowground traits in vegetation models to improve the mechanistic basis for predictions of grassland responses to changes in temperature and rainfall regimes, and thereby inform decisions over species and cultivar choice for future climates.