The influence of elevated CO2 and O3 on fine roots and mycorrhizas of naturally growing young Scots pine trees during three exposure years

Abstract

SummaryYoung Scots pine trees naturally established at a pine heath were exposed to two concentrations of CO2 (ambient and doubled ambient) and two O3 regimes (ambient and doubled ambient) and their combination in open‐top field 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 ingrowth 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 mycorrhizal types were calculated. Elevated O3 and CO2 did not have significant effects on the biomass production of Scots pine coarse (Ø > 2 mm) or fine roots (Ø < 2 mm) and roots of grasses and dwarf shrubs. Elevated O3 caused a transient stimulation, observable 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 CO2 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 CO2 inhibited the transient stimulating effect of ozone, and a significant increase of old mycorrhizas was observed. Our 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 realistically elevated ozone does not cause a measurable limitation to roots within a period of three exposure years.