Effect of exotic tree plantations on free living and plant parasitic soil nematodes and population changes with eucalypt hybrids and plantation age.

Introduction T he effect of changing environment on microfauna in plantations compared to savanna was studied for free-living and plant parasitic nematode poputations. The first are of importance in the micro-food web, and although little is known about it, change in organic matter quality is assumed to affect their populations. The populations of plant parasitic nematodes, which are bound to one or several host plants, are expected to be influenced by the new planted and undergrowth species and by the disappearance of savanna species. Thus if planted eucalypts were host-plants for one of the native plant parasitic nematode species, population densities of this parasite could increase and their possible pathogenic effect on the crop should be considered. Knowledge of nematodes associated with the eucalypts' rhizosphere in natural environments and in plantations is very limited, according to Majer et al. (1997), who mentioned nematodes as the most numerous animal class in the soil but did not quote any references on this topic. In eucalypt plantations in the Congo, a qual itative approach showed two species of plant parasitic nematodes belonging to the genus Xiphinema (Family: Longidoridae) as parasites of eucalypt roots: X. parasetariae and X. souchaudi (Baujard et al. 1998). The words "nematode" and "eucalypt" can be found linked very often in bibliographic data bases. This is related to the permanent quest for nematicidal properties of natural substances among plants, including eucalypts, and the need to reduce the use ofsynthetic chemical nematicides for environmental considerations. However, such studies were not concerned with nematode parasites of eucalypt roots. The study had two aims: (i) to assess the changes in species distribution and population densities of nematodes associated with changes in plant cover; (ii) to study a possible pathogenic effect of these changes on the eucalypt crop.