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 ass imilates and water use efficiency of Douglas fir [Pseudotsuga menziesi i (Mirb.) France] was investigated. Juvenile Douglas firs were exposed to a long-term treatment at 350 and 700 pi l(-1) CO2 for 14 months an d subsequently crosswise transferred to phytotrons for a short-term tr eatment with 350 and 700 mu l l(-1) CO2 for 4 wk in an atmosphere cont inuously labelled with (CO2)-C-14. No interactive effects on total net uptake of (CO2)-C-14 between long-term treatment and short-term treat ment were observed. The short-term treatment with 700 mu l l(-1) CO2 i ncreased the total net uptake of (CO2)-C-14 by 22%, compared with the 350 mu l l(-1) CO2 treatment. The long-term pretreatment did not affec t the total net uptake, suggesting that photosynthetic acclimation had not occurred. However, expressed per unit of needle mass a 14% reduct ion was observed in the trees pretreated at 700 mu l l(-1) CO2. This w as not because of a reduced sink strength of the root system. This red uced uptake per unit of needle mass after long-term treatment may have implications for carbon storage in forest ecosystems. The results sho wed that an initial growth stimulation can eventually be annulled by d eveloping physiological or morphological adaptions. (CO2)-C-14 the roo t/soil respiration increased in the short-term treatment with 700 mu l l(-1) CO2, indicating a stimulated use of current carbon compounds ei ther by roots or microorganisms. The water use efficiency during the s hort-term treatment with 700 mu l 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-t erm treatment and by the long-term pretreatment by about 15%. Some of the observed effects appeared to be persistent, such as decreased wate r use per unit needle mass, whereas others, stimulation of total net ( CO2)-C-14 uptake and water use efficiency, were transient.