Wood properties and ring width responses to long-term atmospheric CO2 enrichment in field-grown loblolly pine (Pinus taeda L.)

Loblolly pine (Pinus taeda L.) were grown in the field, under non-limiting nutrient conditions, in open-top chambers for 4 years at ambient CO₂ partial pressures (pCO₂) and with a CO₂-enriched atmosphere (+ 30 Pa pCO₂ compared to ambient concentration). A third replicate of trees were grown without chambers at ambient pCO₂. Wood anatomy, wood density and tree ring width were analysed using stem wood samples. No significant differences were observed in the cell wall to cell lumen ratio within the latewood of the third growth ring formed in 1994. No significant differences were observed in the density of resin canals or in the ratio of resin canal cross-sectional area to xylem area within the same growth ring. Ring widths were significantly wider in the CO₂-enrichment treatment for 3 of 4 years compared to the ambient chamber control treatment. Latewood in the 1995 growth ring was significantly wider than that in the ambient control and represented a larger percentage of the total growth-ring width. Carbon dioxide enrichment also significantly increased the total wood specific gravity (determined by displacement). However, when determined as total sample wood density by X-ray densitometry, the density of enriched samples was not significantly higher than that of the ambient chamber controls. Only the 1993 growth ring of enriched trees had a significantly higher maximum latewood density than that of trees grown on non-chambered plots or ambient chambered controls. No significant differences were observed in the minimum earlywood density of individual growth rings between chambered treatments. These results show that the most significant effect of CO₂ enrichment on wood production in loblolly pine is its influence on radial growth, measured as annual tree ring widths. This influence is most pronounced in the first year of growth and decreases with age.