Growth,loss, and vertical distribution of Pinus radiata fine roots growing at ambient and elevated CO2 concentration

Increased below-ground carbon allocation in forest ecosystems is a likely consequence of rising atmospheric CO₂ concentration. If this results in changes to fine root growth, turnover and distribution long-term soil carbon cycling and storage could be altered. Bi-weekly measurements were made to determine the dynamics and distribution of fine roots (< 1 mm diameter) for Pinus radiata trees growing at ambient (350 μmol mol^–1) and elevated (650 μmol mol^–1) CO₂ concentration in large open-top chambers. Measurements were made using minirhizotrons installed horizontally at depths of 0.1, 0.3, 0.5 and 0.9 m. During the first year, at a depth of 0.3 m, the increase in relative growth rate of roots occurred 6 weeks earlier in the elevated CO₂ treatment and the maximum rate was reached 10 weeks earlier than for trees in the ambient treatment. After 2 years, cumulative fine root growth (P_t) was 36% greater for trees growing at elevated CO₂ than at ambient CO₂ concentration, although this difference was not significant. A model of root growth driven by daily soil temperature accounted for between 43 and 99% of root growth variability. Total root loss (L_t) was 9% in the ambient and 14% in the elevated CO₂ treatment, although this difference was not significant. Root loss was greatest at 0.3 m. In the first year, 62% of fine roots grown between mid-summer and late-autumn disappeared within a year in the elevated CO₂ treatment, but only 18% in the ambient CO₂ treatment (P < 0.01). An exponential model relating L_t to time accounted for between 74 and 99% of the variability. Root cohort half-lives were 951 d for the ambient and 333 d for the elevated treatment. Root length density decreased exponentially with depth in both treatments, but relatively more fine roots grown in the elevated CO₂ treatment tended to occur deeper in the soil profile.