Elevated CO 2 and hybrid poplar: a detailed investigation of root and shoot growth and physiology of Populus euramericana, ‘Primo’

Abstract

Exposure of the hybrid poplar clone ‘Primo’ ( Populus deltoidex × Populus nigra) to 580 μl 1 −1 carbon dioxide for just 68 days significantly ( P ≤ 0.05) increased stem height by 13% compared with trees grown in ambient CO 2 concentrations. The stem diameter was significantly ( P ≤ 0.05) increased and both total biomass and woody stem biomass also showed higher values (38% and 31% increases respectively) in elevated CO 2. Trees in elevated CO 2 had more leaves and a greater total leaf area, whilst the specific leaf area was decreased in elevated CO 2 on four out of five occasions and was significantly ( P ≤ 0.05) lower after 68 days, an effect indicating that leaves were thicker and/or heavier. Rates of photosynthesis ( A) measured after 49 and 67 days of exposure revealed that trees in the elevated CO 2 treatment had lower values of A when measured at either 350 or 580 μl 1 −1 CO 2. Sequential harvests at intervals during the study in which the root and shoot components were analysed separately allowed the construction of root:shoot ratios and allometric coefficients; there was no significant effect on the allometric coefficient and the root:shoot ratio was significantly increased on only one occasion. However, measurements of the ‘apparent’ root length suggested that root lengths were greater in the CO 2 treatment. There was a significant increase in the number of fine root tips visible down the surface of specially designed rooting tubes ( P ≤ 0.05), indicating more fine roots or an increase in fine root branching. The growth rates of individual fine or large roots over 24 h were unaffected, again suggesting that increases in biomass may be due to more root segments rather than longer individual roots. Root water relations were also examined and showed a tendency towards solute accumulation and increases in turgor pressure ( P) and effective turgor ( P e) at times when root growth was stimulated, although these were not consistent. Cell wall plasticity of the tips of large roots was significantly ( P ≤ 0.01) reduced in elevated CO 2, possibly indicating a greater tendency to divert resources to the formation of root branches. The results of the study are discussed in the light of the possible consequences of changes in poplar growth and physiology for forestry practice in an increased CO 2 environment.