Carbon allocation and water use in juvenile Douglas fir under elevated CO₂

In this study the impact of an elevated CO₂ level on allocation of assimilates and water use efficiency of Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] was investigated. Juvenile Douglas firs were exposed to a long-term treatment at 350 and 700 μ 1^-1 CO₂ for 14 months and subsequently crosswise transferred to phytotrons for a short-term treatment with 350 and 700 μ l ^-1 CO₂ for 4 wk in an atmosphere continuously labelled with ¹⁴ CO₂ . No interactive effects on total net uptake of ¹⁴ CO₂ between long-term treatment and short-term treatment were observed. The short-term treatment with 700 μ 1^-1 CO₂ increased the total net uptake of ¹⁴ CO₂ by 22%, compared with the 350 μ l ^-1 CO₂ treatment. The long-term pretreatment did not affect the total net uptake, suggesting that photosynthetic acclimation had not occurred. However, expressed per unit of needle mass a 14% reduction was observed in the trees pretreated at 700 μ l^-1 CO₂ . This was not because of a reduced sink strength of the root system. This reduced uptake per unit of needle mass after long-term treatment may have implications for carbon storage in forest ecosystems. The results showed that an initial growth stimulation can eventually be annulled by developing physiological or morphological adaptions. ¹⁴ CO₂ in the root/soil respiration increased in the short-term treatment with 700 μ l^-1 CO₂ , indicating a stimulated use of current carbon compounds either by roots or microorganisms. The water use efficiency during the short-term treatment with 700 μ l^-1 CO₂ increased by 32 %, but was not affected by the long-term pretreatment. Water use per unit needle mass during the short-term treatment was decreased both by the short-term treatment and by the long-term pretreatment by about 15%. Some of the observed effects appeared to be persistent, such as decreased water use per unit needle mass, whereas others, stimulation of total net ¹⁴ CO₂ uptake and water use efficiency, were transient.