Response of Chromolaena odorata to timber tree densities in an agrisilvicultural system in Cameroon: aboveground biomass, residue decomposition and nutrient release

In low-input land-use systems combining timber trees with shade-tolerant un derstorey crops, it is essential to establish the effects of tree density u pon weeds and the contribution of weeds to competition and nutrient cycling . Adjusting tree density may alter weed competition for light, water and nu trients, the storage of nutrients by weeds through growth and the release o f nutrients via decomposition of slashed weed residues. The effects are rep orted of (1) two timber stand densities (TSDs) and, (2) retention versus bu rning of slashed vegetation at field preparation, upon Chromolaena odorata (kondengui), the dominant weed, in an agrisilvicultural system in southern Cameroon. The system comprised the timber species, Terminalia ivorensis, an d plantain as an understorey crop. Total weed biomass was measured at the f irst weeding, 3 months after planting (MAP), and total weed biomass and abo veground biomass, partitioning and nutrient accumulation of C. odorata were measured at the second and third weedings, 7 and 12 MAP. Mass and nutrient loss were measured from stem and leaves placed separately in decomposition bags. Here, burning was not an appropriate weed management tool, as it res ulted in greater weed biomass. A high TSD did reduce weed biomass. However, foliar nutrient concentrations were generally higher in high TSDs. Thus, r etaining trees in the high TSD did not lead to any reduction in nutrient up take by the C. odorata stand. Furthermore, in the high TSD, mass loss and n utrient release characteristics had higher coefficients, and a higher perce ntage of the total nutrients accumulated were released in the early stages of incubation, all increasing the risk of nutrients being lost from the top soil. Thus, from a weed competition perspective, there were few benefits to retaining a high timber stand density. (C) 2000 Elsevier Science B.V. All rights reserved.