Tree species screening trials in Western Kenya

The Western Kenya Integrated Ecosystem Management Project (WKIEMP) was initiated with support from the World Bank through a grant from the Global Environment Facility (GEF). The project, which became effective in July 2005, sought to improve the productivity and sustainability of land use systems in selected watersheds in the Yala and Nyando river basins through adoption of an integrated ecosystem management approach (Boye,2008; Verchot 2008). Land in lower Yala and lower Nyando areas had over the years become severely degraded because it was not appropriate for low-input subsistence agriculture and also due to lack of conservation and mitigation measures. The WKIEMP’s tree species screening trials activity therefore sought to support on- and off-farm conservation strategies and to build the capacity of local communities and other institutions in identifying and managing ecosystem issues as well as in the implementation of conservation and/or mitigation measures. Generally, the target populations often expressed preferences for exotic species like Eucalyptus spp., Casuarina equisetifolia and Grevillea robusta. While these species are highly productive, it was not clear whether these species are the best choices for degraded sites. Secondly, there was a lack of knowledge by farmers about the potential productivity of indigenous species. Farmers often remarked that indigenous tree species are not as productive as fast-growing exotics. Yet they also readily admitted that they did not have experience with these species in a plantation setting and that their observations were based on volunteer trees in the landscape that were not properly protected and cultivated. Many technical manuals suggest that indigenous species are better adapted to the local environment and should have better survival and growth rates in a region, particularly on marginal lands (Mulizane et al. 2005, Carpenter et al. 2004, Olukoye et al. 2003). Additionally, we recognized the risks associated with introduction of exotic species into new landscapes, although the preferred exotic species have shown no tendencies to be invasive. Finally, while farmers tended to believe that exotic species grow better, we have observed that there is a lot of variation in indigenous species germplasm. We believed that with proper selection the potential exists that these species could perform as well or better than exotic species on degraded sites. All of these ‘beliefs’ needed to be substantiated through demonstration. Thus, ICRAF organized a series of species screening trials within the framework of the WKIEM Project. The trials were also to serve as demonstration plots to sensitize farmers to the potential of indigenous trees. The main objective of the tree species screening trials was to assess the performance of appropriate indigenous tree species relative to farmer-preferred exotic species on degraded lands. The hypotheses were: H1: Indigenous trees will have higher survival rates during the first year of planting than exotic species on degraded soils. H2: Indigenous trees will grow faster on degraded soils than exotic species. Additionally, we sought to explore the potential to develop a predictive model for tree performance on degraded lands through a widespread network of trials that cover a large geographic area and that span a wide variation in climate, soils and other biophysical conditions.