One of the representative species of this flora, the cypress Cupressus atlantica, is experiencing year by year a decrease in its biomass production and the surface areas it occupies. This species contributes to efforts to control erosion and degradation of forest soils. But it is highly sought after by local communities particularly for the quality of its wood. In addition, the young shoots are subjected to overgrazing (by sheep) which hinders the species' natural regeneration. Replanting operations have been undertaken, but have proved largely unsuccessful, with nearly 70 % of the young saplings planted dying after the first year.

To counter the threat hanging over this species, a research programme was set up in 2003, jointly between the Cadi Ayyad University of Marrakech, the High Atlas Regional Forestry Directorate and scientists from IRD research unit UR 040 (1). The IRD researchers proposed an original ecological approach, founded on the study of interactions between the cypress, the shrubby plants-lavender and thyme-associated with them and the soil microflora, in order to define new practices for cypress replanting schemes in the Moroccan Atlas.

Cypress develops symbiotically with soil micro-fungi, arbuscular mycorrhizal fungi (2). This association yields mutual benefits, the plant providing sugars for the fungi which in return helps provide the tree's supply of water and mineral salts, mainly of nitrogen and phosphorus. In controlled mycorrhization glasshouse experiments, the researchers inoculated strains of these fungi, sampled from the study zones in the Moroccan High Atlas, into young nursery plants. There resulted a distinct improvement in the development of these young cypress with inoculated mycorrhizal fungi, which had a higher level of minerals in their leaves (21 % more phosphorus, in particular). Other experiments, conducted in the field, confirmed this result: the fungi favour better nutrition for the young cypress which enables them to build up greater resistance to transplantation-linked hydric stress for when they are adult. Although it is effective, this plant-by-plant inoculation technique remains cumbersome and costly, which limits its large-scale use.

In the cypress stands of the Moroccan High Atlas, several species of lavender and thyme are associated with the trees. These pioneer species, which form sparse clumps of vegetation, are the first to colonize the stony eroded soils of these arid and semi-arid ecosystems. Soil analyses revealed that these species generate islands of fertility, resource islands, rich in nitrogen and phosphorus, as well as in mycorrhizal fungi with which these plants also live in symbiosis. However, what role do they play in the development of young planted cypress saplings?

Lavender and cypress grown together and inoculated with mycorrhizal fungi under glass showed significantly higher growth than when they were cultivated and developed separately. Moreover, the concentration of propagules (3) appeared greater around the lavender roots than around those of cypress and, even more so, than around bare soil (244.5, 179.7 and 7.82 for 100 g of dry soil respectively). Lavender therefore favours the proliferation of mycorrhizal fungi and of their mycelium in the soil. As it reproduces, this "nurse" plant multiplies the number of resource islands generated, which eventually increases the fertility of the whole of the terrain involved. It therefore creates conditions that encourage growth of the planted cypress, stabilizing the soil as it does so by its roots.

Increased understanding of the process of natural regeneration of cypress trees has allowed the application of a new method of replanting of this species in the Moroccan High Atlas. In the sites that must be re-wooded, bands of lavender were planted in strips of earth built up perpendicularly to the slope so as to retain water. The cypress saplings were planted the following year. The first results confirm the beneficial role lavender has on these young plantations: their mortality after one year proves to be very low and soil erosion remains limited. Research investigations were also conducted on other associations of species, such as cork oak and cysts, or again thujas and lavender. The latter are the subject of a trial conducted by the Rabat Regional Forestry Directorate in areas undergoing reforestation in the North of Morocco. - Institut de Recherche Pour le Developpement