Succession of Mycorrhizal Associations on Individual Roots of Radiata Pine

Abstract

Dual and treble mycorrhizal associations on the short roots of Pinus radiata D.Don are described. A survey of mycorrhizas in the top 6 in. of soil in a 42-year-old stand of Pinus radiata showed that in 42.5 cu in. samples there were from 4,600 to 10,000 large fragments of mycorrhizas of all types. Clearly discernible dual associations comprised from 1 to 4 per cent of the fragments in the samples. The smooth-black, bristly-black, and white-rhizomorphic types of mycorrhizas frequently had replaced the reddish-grey, smooth-white, and reddish-brown types. Intimate mixtures of fungal types on and in the mycorrhiza mantle were demonstrated with light and electron microscopic techniques. Histological studies of the dual associations showed that the second fungus apparently had superseded the first when root growth was resumed after a dormant period. It is suggested that replacement of one mycorrhiza type by another occurs only after continuity of the Hartig net in the cortex is broken by a zone of dead lignified cells.