Belowground carbon allocation, root trait plasticity, and productivity during drought and warming in a pasture grass

Drought and warming can reduce forage production by altering belowground carbon allocation. However, a trade-off between belowground production and root trait plasticity can offset negative effects on aboveground plant productivity. Sustaining grassland production in a changing climate requires an understanding of plant adaptation strategies, including trait plasticity under warmer and drier conditions. However, our knowledge to date disproportionately relies on aboveground responses, despite the importance of belowground traits in maintaining aboveground growth, especially in grazed systems. We subjected a perennial pasture grass, Festuca arundinacea, to year-round warming (+3 degrees C) and cool-season drought (60% rainfall reduction) in a factorial field experiment to test the hypotheses that: (i) drought and warming increase carbon allocation belowground and shift root traits towards greater resource acquisition and (ii) increased belowground carbon reserves support post-drought aboveground recovery. Drought and warming reduced plant production and biomass allocation belowground. Drought increased specific root length and reduced root diameter in warmed plots but increased root starch concentrations under ambient temperature. Higher diameter and soluble sugar concentrations of roots and starch storage in crowns explained aboveground production under climate extremes. However, the lack of association between post-drought aboveground biomass and belowground carbon and nitrogen reserves contrasted with our predictions. These findings demonstrate that root trait plasticity and belowground carbon reserves play a key role in aboveground production during climate stress, helping predict pasture responses and inform management decisions under future climates.

Automated summary

Drought and warming can reduce forage production, but a trade-off between belowground production and root trait plasticity can offset the negative effects. However, there is a lack of understanding of belowground traits in maintaining aboveground growth in grazed systems. In this study, we found that drought and warming reduced plant production and biomass allocation belowground, while specific root length and root diameter were affected by both drought and warming.