The influence of elevated CO2 and O-3 on fine roots and mycorrhizas of naturally growing young Scots pine trees during three exposure years

Young Scots pine trees naturally established at a pine heath were exposed t o two concentrations of CO2 (ambient and doubled ambient) and two O-3 regim es (ambient and doubled ambient) and their combination in open-top field ch ambers during growing seasons 1994, 1995 and 1996 (late May to 15 September ). Filtered ozone treatment and chamberless control trees were also include d in the treatment comparisons. Root in-growth cores were inserted to the u ndisturbed soil below the branch projection of each tree at the beginning o f the fumigation period in 1994 and were harvested at the end of the fumiga tion periods in 1995 and 1996. Root biomasses were determined from differen t soil layers in the ingrowth cores, and the infection levels of different mycorrhizal types were calculated. Elevated O-3 and CO2 did not have signif icant effects on the biomass production of Scots pine coarse (diameter >2 m m) or fine roots (diameter <2 mm) and roots of grasses and dwarf shrubs. El evated O-3 caused a transient stimulation, observable in 1995, in the propo rtion of tuber-like mycorrhizas, total mycorrhizas and total short roots bu t this stimulation disappeared during the last study year. Elevated CO2 did not enhance carbon allocation to root,growth or mycorrhiza formation, alth ough a diminishing trend in the mycorrhiza formation was observed. In the c ombination treatment increased CO2 inhibited the transient stimulating effe ct of ozone, and a significant increase of old mycorrhizas was observed. Ou r conclusion is that doubled CO2 is not able to increase carbon allocation to growth of fine roots or mycorrhizas in nutrient poor forest sites and re alistically elevated ozone does not cause a measurable limitation to roots within a period of three exposure years.