REGULAR ARTICLE Depth-dependency of trembling aspen and paper birch small-root responses to eCO 2 and eO 3 L. K. Rhea & J. S. King Received: 14 June 2011 / Accepted: 4 December 2011 / Published online: 21 January 2012 # Springer Science+Business Media B.V. 2012 Abstract Background and Aims Projected changes in the atmo- spheric concentrations of CO 2 and tropospheric O 3 over the next 50 years are of significant concern due to the linkages in the cycling of carbon and water in forested ecosystems. Responses of tree roots to elevated CO 2 (eCO 2 ) and O 3 (eO 3 ) have been characterized primarily by studies of relatively shallow roots, yet deeper roots often play a disproportionately large role in water acquisition relative to their biomass. We under- took the present study to determine if there were signif- icant root responses to eCO 2 and eO 3 below the maximum soil depths typically studied. Methods In the current study, we characterized small root biomass and morphometric responses to eCO 2 and eO 3 at the Aspen-FACE Experiment in Rhine- lander, Wisconsin down to a depth of one meter. Results Elevated CO 2 caused relatively undifferentiated growth stimulation. Elevated O 3 stimulated root growth in the AA community at depth, while in the AB community there was a reduction in root growth in the shallow soil layer that was reversed in the deeper layers. Conclusions Root responses below depths typically studied were qualitatively similar than those within shallower soils for eCO 2 , but were sometimes com- pensatory for eO 3 . Keywords Carbon dioxide . Ozone . Small roots . Fine roots . Biomass . Morphometry . Trembling aspen . Paper birch Introduction Understanding the effects of elevated atmospheric CO 2 (eCO 2 ) and tropospheric O 3 (eO 3 ) on both above- ground and belowground forest net primary produc- tion is important for predicting climate feedbacks via carbon sequestration and is critical for estimating fu- ture water availability, as the cycling of carbon and water are intimately linked. Elevated CO 2 has been shown to increase aboveground and belowground growth and water use efficiency (WUE), while eO 3 generally has the opposite effect (Curtis and Wang 1998; Anderson 2003). The effects of combined eCO 2 and eO 3 have been less consistent, but also less well studied (Rogers et al. 1994; King et al. 2001, 2005; Kasurinen et al. 2004). Adequate characteriza- tion of belowground responses to climate change is, however, still lacking (Brunner and Godbold 2007; Iversen 2010). Plant Soil (2012) 355:215–229 DOI 10.1007/s11104-011-1094-2 Responsible Editor: Alain Pierret. L. K. Rhea (*) : J. S. King Department of Forestry and Environmental Resources, North Carolina State University, Campus Box 8002, Raleigh, NC 27695, USA e-mail: rhea.lee@epamail.epa.gov J. S. King e-mail: john_king@nscu.edu J. S. King Department of Biology, University of Antwerp, B-2610 Wilrijk, Belgium