Dual mycorrhizal symbiosis: an asset for eucalypts out of Australia?

The dual symbiotic habit of the genus Eucalyptus is well documented: while arbuscular mycorrhizae are always dominant in seedlings and young trees, they are replaced by ectomycorrhizae in older trees. Here, the mycorrhizal status of eucalypts of different ages was assessed over the year in three plantations in the humid Mediterranean climate zone of Algeria (using morphological, anatomical, and histological criteria) and quantified in the dry and wet seasons over three years (using the grid-line intersect method of Giovannetti and Mosse (1980, New Phytol. 84: 489–500, doi: 10.1111/j.1469-8137.1980.tb04556.x ) on randomized fine root segments). Unexpectedly, the results revealed the persistence of arbuscular mycorrhizae in old trees and their dominance over ectomycorrhizae during the dry season, while the opposite pattern was observed in the wet season. Composite mycorrhizae displaying intimately mixed arbuscular mycorrhizae and ectomycorrhizae anatomical structures also occurred and were more frequent during the dry season. Eucalypts were clearly able to form a wide diversity of symbiotic associations with arbuscular mycorrhizal and ectomycorrhizal fungi locally associated with native trees, shrubs, and herbaceous plants. Therefore, the mycorrhizal status should not be a limiting factor to the silviculture of eucalypts in this area of Algeria because the dual symbiotic habit provides complementary benefits in a poor soil and in an alien environment.