Seasonal below-ground carbon balance for Pinus radiata trees growing at ambient and elevated CO₂ concentration

Abstract

Atmospheric CO₂ concentration is increasing at c. 0.5% y⁻¹, predominantly as the consequence of fossil fuel combustion and deforestation. By the end of the 21st century, based on current emission rates, the atmospheric CO₂ concentration will reach c. 500 µmol mol⁻¹. Many studies have shown that total plant productivity is enhanced at elevated CO₂ concentration and there is some evidence to suggest that allocation below-ground is also increased. However, the effect this may have on the below-ground carbon fluxes is poorly understood. This thesis investigated the effects of Pinus radiata growth at elevated CO₂ concentration on the seasonal below-ground carbon balance for young Pinus radiata trees in the first two years after planting.

The study was conducted at the Forest Ecosystems Elevated CO₂ Project facility at Bromley, Christchurch. Genetically identical Pinus radiata D. Don trees were grown at ambient (350 µmol mol⁻¹) and elevated (650 µmol mol⁻¹) CO₂ concentration in large open top chambers. Bi-weekly fine root measurements were made to investigate firstly whether tree growth at elevated CO₂ concentration increased carbon allocation below-ground, secondly to determine whether the seasonality of the rates of fine root production and loss changed, and thirdly to determine whether the fine root distribution was modified. Root measurements were made from minirhizotrons placed horizontally at four depths in the soil. A linear relationship was determined between root numbers observed from minirhizotrons and root length density and root carbon density in the soil.

Estimates of the seasonal change in carbon flux from the soil surface for tree plots were made to determine if the rate of carbon loss from the tree root systems increased at elevated CO₂ concentration, and whether this could be attributed to increases in the rate of fine root growth. A model describing the relationship between carbon flux density (ƒ) at the soil surface with distance from the tree stems was used to estimate the annual carbon flux from the trees on a unit ground area basis. Carbon flux density was measured monthly using a chamber placed on the soil surface at 0.35 m from the stem which was attached to a gas analyser and, was estimated at the stem from soil CO₂ concentrations at four depths using a one-dimensional gas diffusion model.

More carbon was allocated to root production for trees growing at elevated CO₂ concentration. After two years, 36% more roots had been produced at elevated CO₂ concentration than at ambient CO₂ concentration, although the difference was not significant. In the first year, fine root (<1 mm diameter) production at a depth of 0.3 m was observed to occur six weeks earlier than for trees at elevated CO₂ concentration. However, the same difference did not recur at the beginning of the second growth season. Seasonal changes of root production were largely explained by changes in soil temperature. Root loss only occurred after one year from when the trees were planted and total root loss after two years tended to be greater at elevated (14%) than at ambient (9%) CO₂ concentration. The life span of fine roots was significantly reduced at elevated CO₂ concentration and half-lives for roots were estimated to be 951 d at ambient and 333 d at elevated CO₂ concentration. Root longevity was also a function of the time in the season when the roots first appeared. Horizontal and vertical fine root distribution tended to differ between the two treatments. Relatively more fine roots were concentrated close to the stem for trees growing at elevated than those at ambient CO₂ concentration. In addition, relatively more fine roots occurred deeper in the soil profile for trees growing at elevated than for those at ambient CO₂ concentration.

Carbon loss from the root systems of trees growing at elevated CO₂ concentration, measured as CO₂ flux from the soil surface, tended to be higher than that for trees at ambient CO₂ concentration. For the second year of growth, the estimated annual total flux from the tree plots was 13% higher in the elevated treatment (1895 g y⁻¹) than in the ambient (1671 g y⁻¹) CO₂ treatment. The higher carbon fluxes at elevated CO₂ concentration were largely explained by increased fine root production. There were strong positive relationships between measurements of ƒ and root production, and ƒ and stem growth. Daily changes in ƒ were strongly and positively related to temperature. The seasonal change of ƒ was a function of soil temperature and changes in root biomass carbon. Both ƒ and root production declined exponentially with distance from the stem. These results are relevant when developing sampling procedures to estimate tree or plot carbon fluxes.

This study has shown that there was an increase in below-ground carbon allocation for young Pinus radiata trees growing at elevated CO₂ concentration and, as a consequence of this, carbon efflux from the soil increased. However, long-term studies of trees growing at elevated CO₂ concentration are needed to determine whether these changes will affect long-term carbon balance and lead to increases in soil carbon storage.