Interactions between soil acidification, plant growth and nutrient uptake in ectomycorrhizal associations of forest trees

Interactions between soil acidification and ectomycorrhizal associations of forest trees are reviewed, placing special emphasis, where appropriate, on recent Swedish research. Mycorrhizal fungi represent an important component of the biodiversity of forest ecosystems and are themselves subject to the influence of acidification. In addition, these symbiotic fungi also possess the capacity to mediate the extent to which their host plants are influenced by soil acidification. The ways in which different mycorrhizal fungi respond to acidification are still very poorly understood. Ectomycorrhizal fungi growing in symbiotic association with different hosts may respond in very different ways from that of a pure fungal culture and the overriding problems of identifying and quantifying them in soil have so far restricted progress in understanding changes occurring in the field. Controlled laboratory experiments have improved our knowledge of interactions involving identified species. Acidification can influence ectomycorrhizas either directly through altered soil pH, or indirectly through changes in soil nutrient availability, metal solubility, or carbon flow. Each of these factors can operate directly on the individual symbionts or exert its effect indirectly on one symbiont by directly influencing the other symbiont. The ability of ectomycorrhizal fungi to mediate the effects of acidification on forest trees is of special significance when discussing changes in mycorrhizal community structure and the possible effects of a given level of soil acidification. This mediation is achieved in a number of ways including modification of the soil chemical environment, altering patterns of plant nutrient uptake and increasing tolerance to, or detoxifying, the increased levels of heavy metals or aluminium often associated with soil acidification. The possible mechanisms behind these interactions are discussed with special reference to increased aluminium concentrations. Methodological problems associated with demonstration of different mechanisms are also discussed. Improved knowledge of the functional basis of observed mycorrhizal responses and effects is a pre-requisite for better under standing and prediction of the dynamics of forest ecosystems and their responses to anthropogenically generated stresses.