Tree-root systems and herbaceous species-characteristics under tree species introduced into grazing lands in subhumid Cameroon

The effect of tree species on the characteristics of the herbaceous stratum, during the first five years of a fallow, was evaluated in the North of Cameroon (average annual temperature 28.2 °C, total annual rainfall 1050 mm). Treatments included a natural grazed herbaceous fallow, a natural ungrazed herbaceous fallow and three planted tree fallows (Acacia polyacantha Willd. ssp. campylacantha (Hochst. ex A. Rich.), Senna siamea Lam. and Eucalyptus camaldulensis Dehnh.), which were protected against grazing. Because tree species influenced light interception in different ways, as well as having different root patterns, they had different effects on the herbaceous stratum in terms of species composition and biomass. The grazed herbaceous fallow maintained the greatest species richness. Protection against grazing or the introduction of tree species associated with the absence of grazing induced both a progressive evolution to a particular species composition. The ungrazed herbaceous fallow consisted mainly of Andropogon gayanus Kunth, which provided the greatest biomass (8 t dry matter ha^–1 at the end of the fallow period). E. camaldulensis provided little shade and the lowest fine root mass in the top layer allowing the growth of A. gayanus and thus a greater herbaceous biomass (3.5 t DM ha^–1) than that found under the other tree species. Under the heavy shade of A. polyacantha, the herbaceous stratum consisted mainly of annual Pennisetum spp. (2.2 t DM ha^–1) and showed the greatest N concentration (1.3%), probably due to N₂ fixation by the tree species. After the fourth year, despite the relatively open tree canopy, S. siamea, which showed the highest fine root mass, had a strong depressive effect on the herbaceous stratum. This revised version was published online in June 2006 with corrections to the Cover Date.