The potential for nutrient uptake by forest trees is a function of site fertility, fine roots and mycorrhizal symbiants. This study focuses on the effects of moderate doses of liming on the mycorrhizal status of oak (Quercus petraea and Q. robur) roots. In ten in situ trials, where liming has been applied 1-27 years prior to sampling, the effects on fine roots and their mycorrhizal status were assessed from 1994 to 1996. All sites, except one, were situated on acidic soils with low base saturation. Fine roots were evaluated in terms of fine root biomass, length and specific root length. Mycorrhizal status was evaluated in terms of total number of mycorrhizal tips, number of tips per root length and main ectomycorrhizal morphotypes, both in absolute numbers and in relative proportion. At two of these sites, in close vicinity to each other, consecutive sampling was carried out during four periods between 1995 and 1996. The fact that some variation existed between the sampling seasons was verified, but lime-induced treatment effects were similar for all seasons. Data from all ten sites showed liming to slightly but significantly increase total number of mycorrhizal tips (ranging from 5.81 to 109.3 x 10 hoch 9 ha-Ü and from 9.63 to 125.6 x 10 hoch 9 ha-Ü for the control and liming treatment, respectively), but this was mainly an effect of a significantly increased total fine root length (ranging from 26.8 to 104.9 x 10 hoch 6 m ha-Ü for the control and liming treatment, respectively) than of a higher number of tips per metre of root length. The latter variable was not significantly affected by liming on an overall basis (ranging in general between 100 and 600 tips m-Ü, with a highest value up to 1700 tips m-Ü). Liming in general decreased the relative proportion of smooth mycorrhizae in favour of hairy types (increasing significantly on average from 0.5 to 1.0% in the control to 2 to 6% in the lime treatment). Furthermore, within the smooth types, some changes occurred. Hence, the absorption surface of the uptake system was enhanced by an increase in fine root length and a differential stimulation of hairy types of ectonymorrhizae, but not by a greater number of mycorrhizal tips per unit of root length. Soil pH explained part of the shift within the smooth types, but not the increase in the hariy types. In the present study, liming consistently modified the mycorrhizal status of the oak roots and stimulated fine root development.
