The in¯uence of elevated CO 2 and O 3 on ®ne roots and mycorrhizas of naturally growing young Scots pine trees during three exposure years ANNE KASURINEN,* HELJA È - S I S K O H E L M I S A A R I ² and T O I N I H O L O P A I N E N * *Department of Ecology and Environmental Science, University of Kuopio, PO Box 1627, 70211 Kuopio, Finland, ²Finnish Forest Research Institute, Vantaa Research Centre, PO Box 18, 01301 Vantaa, Finland Abstract Young Scots pine trees naturally established at a pine heath were exposed to two con- centrations of CO 2 (ambient and doubled ambient) and two O 3 regimes (ambient and doubled ambient) and their combination in open-top ®eld chambers during growing seasons 1994, 1995 and 1996 (late May to 15 September). Filtered ozone treatment and chamberless control trees were also included in the treatment comparisons. Root in- growth cores were inserted to the undisturbed soil below the branch projection of each tree at the beginning of the fumigation period in 1994 and were harvested at the end of the fumigation periods in 1995 and 1996. Root biomasses were determined from different soil layers in the ingrowth cores, and the infection levels of different mycor- rhizal types were calculated. Elevated O 3 and CO 2 did not have signi®cant effects on the biomass production of Scots pine coarse (é > 2 mm) or ®ne roots (é < 2 mm) and roots of grasses and dwarf shrubs. Elevated O 3 caused a transient stimulation, obser- vable in 1995, in the proportion of tuber-like mycorrhizas, total mycorrhizas and total short roots but this stimulation disappeared during the last study year. Elevated CO 2 did not enhance carbon allocation to root growth or mycorrhiza formation, although a diminishing trend in the mycorrhiza formation was observed. In the combination treatment increased CO 2 inhibited the transient stimulating effect of ozone, and a sig- ni®cant increase of old mycorrhizas was observed. Our conclusion is that doubled CO 2 is not able to increase carbon allocation to growth of ®ne roots or mycorrhizas in nutrient poor forest sites and realistically elevated ozone does not cause a measurable limitation to roots within a period of three exposure years. Keywords: ectomycorrhizas, elevated carbon dioxide, elevated ozone, ®ne roots, open-top chambers, Scots pine (Pinus sylvestris L.) Received 20 May 1998; resubmitted 21 December 1998 and accepted 8 January 1999 Introduction The increase of atmospheric carbon dioxide (CO 2 ), ozone (O 3 ) and a number of other trace gases is predicted to change the Earth's climate within the next century (Bowes 1993; Chameides et al. 1994). Tropospheric con- centration of CO 2 and O 3 is predicted to rise 0.5±2.0% per year (Runeckles & Krupa 1994). The research to date concerned with the effects of atmospheric CO 2 enrich- ment on forest ecosystems has generally emphasized the above-ground processes of tree seedlings, and the results have indicated mainly the stimulation of photosynthesis and growth (O'Neill et al. 1987; Bazzaz 1990; Tissue et al. 1996). Although the direct effects of increased above- ground CO 2 concentration on soil microbial processes are unlikely, below-ground plant parts and microbial pro- cesses are likely to be affected through altered resource allocation at elevated CO 2 (Rogers et al. 1994; Paterson et al. 1997). Ectomycorrhizal species have generally shown increased biomasses, a greater rate of mycorrhiza formation as well as higher percentage of root infection under increased CO 2 (O'Neill 1994). Majority of the studies dealing with the effects of elevated CO 2 and O 3 have been carried out with small tree seedlings and have lasted only a relatively short R Correspondence: Toini Holopainen, fax + 358/17 163230, e-mail toini.holopainen@uku.® Global Change Biology (1999) 5, 771±780 # 1999 Blackwell Science Ltd. 771