Root response to CO2 enrichment and nitrogen supply in loblolly pine

This paper examines how elevated CO₂ and nitrogen (N) supply affect plant characteristics of loblolly pine (Pinus taeda L.) with an emphasis on root morphology. Seedlings were grown in greenhouses from seeds during one growing season at two atmospheric CO₂ concentrations (375 and 710 μL L^-1) and two N levels (High and Low). Root morphological characteristics were determined using a scanner and an image analysis program on a Macintosh computer. In the high N treatment, elevated CO₂ increased total plant dry weight by 80% and did not modify root to shoot (R/S) dry weight ratio, and leaf and plant N concentration at the end of the growing season. In the low N treatment, elevated CO₂ increased total dry weight by 60%. Plant and leaf N concentration declined and R/S ratio tended to increase. Nitrogen uptake rate on both a root length and a root dry weight basis was greater at elevated CO₂ in the high N treatment and lower in the low N treatment. We argue that N stress resulting from short exposures to nutrients might help explain the lower N concentrations observed at high CO₂ in other experiments; Nitrogen and CO₂ levels modified root morphology. High N increased the number of secondary lateral roots per length of first order lateral root and high CO₂ increased the length of secondary lateral roots per length of first order lateral root. Number and length of first order lateral roots were not modified by either treatment. Specific root length of main axis, and to a lower degree, of first order laterals, declined at high CO₂, especially at high N. Basal stem diameter and first order root diameters increased at high CO₂, especially at high N. Elevated CO₂ increased the proportion of upper lateral roots within the root system.