Does elevated CO2 alter root architecture and biomass after 5 years in a mature temperate woodland?

Anthropogenic CO2 emissions have resulted in elevated CO2 (eCO2) in the atmosphere, and this rise is predicted to continue1. Increases in CO2 have fertilised forest ecosystems and led to an uptake of CO2 into plant and soil biomass. Early findings at BIFoR FACE (Free-Air Carbon Dioxide Enrichment) showed increased photosynthetic uptake2, fine root net primary productivity3 and soil respiration4, indicating increased carbon (C) allocation belowground and mirroring previous forest FACE experiments. Roots play a key role in whole-plant functions, biogeochemical cycling and interactions with biotic factors, thus based on the early findings we expect that the increased C allocation belowground will have an impact on root biomass and architecture. Root biomass combined with root architecture (such as root diameter and length) are of high importance to elucidate the impacts of eCO2 on primary productivity, interactions in the rhizosphere, carbon sequestration and nutrient cycling5,6. This study assesses the impact of elevated CO2 on root biomass and architecture at the BIFoR FACE the first 5 years of operation (2017-2022).&#160;&#160;Changes in root biomass and architecture were monitored via soil coring three times a year (spring, summer and autumn) to 30 cm (per horizon). The root biomass in assessed as per dry weight in four different root diameter classes (<1, 1-2, 2-5 and >5 mm) and the root architecture was assessed via fresh root scanning.&#160;Root biomass exhibited a prompt and sustained increase under eCO2 during the first 5 years of CO2 fumigation, with the increase being more pronounced for the three smaller diameter classes (<1, 1-2 and 2-5 mm). Moreover, the increase was relatively higher in the O and B soil horizons. Due to limited abundance of larger roots in the top soil layers, no clear patterns have been observed for the largest root class (>5 mm). Increases in root biomass could suggest increases in total root length, root diameter and tissue density, enhancing trees&#8217; capacity to acquire more soil resources such as water and nutrients, or resource storage.&#160;References1Intergovernmental Panel on Climate Change; Core Writing Team; Pachauri, R.K.; Meyer, L.A. (Eds.) Climate Change 2014: Synthesis Report, Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change; Intergovernmental Panel on Climate Change: Geneva, Switzerland, 2014; 151p.2Gardner, A., Ellsworth, D., Crous, K., Pritchard, J., Mackenzie, A.R. (2021). Is photosynthetic enhancement sustained through three years of elevated CO2 exposure in 175-year-old Quercus robur? Tree Physiology, 42 (1), 130-1443Ziegler, C., Kulawska, A., Kourmouli, A., Hamilton, L., Shi, Z., MacKenzie, A.R., Dyson, R.J., Johnston, I.G. (2022). Quantification and uncertainty of root growth stimulation by elevated CO2 in mature temperate deciduous forest. Science of the Total Environment, 854,4Kourmouli, A., Hamilton, L., Pihlblad, J., Barba, J., Bartlett, R., MacKenzie, AR., Hartley, I., Shi, Z. (2023). Initial carbon and nutrient responses to free air CO2 enrichment in a mature deciduous woodland. (submitted) 5Norby, R. J., &&#160;Jackson, R. B.&#160;(2000).&#160;Root dynamics and global change: Seeking an ecosystem perspective.&#160;New Phytologist,&#160;147,&#160;3&#8211;12.6Wilson, S. D.&#160;(2014).&#160;Below-ground opportunities in vegetation science.&#160;Journal of Vegetation Science,&#160;25,&#160;1117&#8211;1125.