Fine Root Growth and Vertical Distribution in Response to Elevated CO2, Warming and Drought in a Mixed Heathland–Grassland

Belowground plant responses have received much less attention in climate change experiments than aboveground plant responses, thus hampering a holistic understanding of climate change effects on plants and ecosystems. In addition, responses of plant roots to climate change have mostly been studied in single-factor experiments. In a Danish heathland ecosystem, we investigated both individual and combined effects of elevated CO2, warming and drought on fine root length, net production and standing biomass by the use of minirhizotrons, ingrowth cores and soil coring. Warming increased the net root production from ingrowth cores, but decreased fine root number and length in minirhizotrons, whereas there were no significant main effects of drought. Across all treatments and soil depths, CO2 stimulated both the total fine root length (+44%) and the number of roots observed (+39%), with highest relative increase in root length in the deeper soil layers. Our results suggest that under future climate, plants may allocate considerable resources into roots compared to aboveground biomass. Increased carbon (C) allocation to roots may have a great impact on the overall ecosystem C balance and must be considered in modelling of future ecosystem responses to climate change. To provide models with necessary validation data, more studies are needed to investigate if higher C allocation to roots will lead to long-term C storage in more recalcitrant soil C pools or if this potential increase in soil carbon storage may be offset by increased priming activity and turnover rates for soil organic matter.