Tree species screening trials in Western Kenya

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, the study 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. Results showed that in both lower Nyando and lower Yala blocks, indigenous tree species had a higher survival rate than the exotic tree species. In lower Nyando, Acacia species (indigenous) had the highest survival rates compared to other indigenous tree species and Grevillea robusta (exotic) had the lowest survival rate. In lower Yala, Senna siamea (naturalized) had the highest survival rate, followed by Termi nalia brownii (indigenous) and Markhamia lutea (indigenous) which all did very well. Casuarina equisetifolia (exotic) and Croton macrostachyus had the lowest survival rate in this area. These findings support the hypothesis that indigenous tree species have a higher survival rate than exotic species on degraded soils. However, exotic species from similar ecological sites could survive as well in the degraded soils. For instance, Senna siamea is not indigenous but has been naturalized in the area to the level of the residents seeing it as indigenous and it had the best performance in lower Yala. In both blocks generally, the exotic tree species had a higher growth rate than indigenous species based on height, stem diameter, canopy length and canopy width parameters. Thus these results did not support the second hypothesis that indigenous species would have a higher growth rate than exotic species on degraded soils. There was a serious problem of the continued perception of exotic species as superior to indigenous species by farmers. This made farmers to value and take care of exotic species in the trial plots with practices such as thorough manure and mulch application and weeding. Indigenous quadrants were neglected and in some cases uprooted. The high growth rate of exotic species could have been as a result of external sources of nutrient supply through farmers interventions on the exotic quadrants such as manuring, mulching, weeding and proper management of exotic trees. It was however encouraging to note that upto 61% would prefer to plant indigenous species instead of exotic after observing results of the trials. Livestock browsing especially on the indigenous tree species may have influenced canopy size comparisons although it was hard to tell this through statistical analysis. Termites may also rise to pose a major challenge on the growth of the majority of the trees in both blocks. Exotic species were the most attacked while Acacia species were least attacked. Indigenous species were not affected by drought compared to exotic species. In lower Nyando, indigenous species were also affected by waterlogging. Many control plots are necessary in Type III on-farm trials if multiple causes of variation such as these, a common characteristic of natural resource management research, are envisaged in the analysis. With constant monitoring farmers can follow the protocol and more findings can be concluded through these types of trials.