Mycorrhizal populations and fine root development on Norway spruce exposed to controlled doses of gaseous pollutants and simulated acidic rain treatments

Effects after long-term fumigation and simulated acidic rain (pH 4.0) treatment in open-top chambers (OTC) during 5 vegetation periods on ectomycorrhizal (EM) frequency, fine root structure and distribution of short roots were recorded for Norway spruce. Simultaneous application of ozone and SO(2) resulted in a significant decrease (P< or =0.01) in the proportion of EM root tips and an increase in the numbers of necrotic short roots. The observed differences were partly due to more than additive effects of ozone and SO(2). On the basis of EM and non-mycorrhizal (NM) short roots in the irrigated controls (OTC-V, pH 5.6, Ah-horizon: EM + NM/cm=0.05) the acid treatments resulted in drastic effects (P< or =0.05) on the branching density and the proportion of EM root tips and NM short roots. Relative numbers of short roots infected by indigenous EM fungi with trees in the fumigation treatments at pH 4.0 were similar: 1.5-1.9 EM tips/cm and differed noticeably from the control: 5.2 EM tips/cm. Again, increasing numbers of necrotic and NM short roots were noted. Parallel measurements by the ingrowth-core technique confirm the argument, that EM formation seems to be a reliable indicator of pollution-induced stress.