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.

肯尼亚西部综合生态系统管理项目（Western Kenya Integrated Ecosystem Management Project, 简称WKIEMP）由世界银行（World Bank）通过全球环境基金（Global Environment Facility, 简称GEF）提供赠款支持启动。该项目于2005年7月正式生效，旨在通过采用综合生态系统管理方法，提升亚拉河与尼扬多河流域选定集水区的土地利用系统生产力与可持续性（Boye,2008; Verchot 2008）。
亚拉河下游与尼扬多河下游地区的土地因不适宜开展低投入自给农业，且缺乏保护与缓解措施，多年来已出现严重退化。因此，WKIEMP的树种筛选试验活动旨在支持农场内外的保护策略，提升当地社区与其他机构识别、管理生态系统问题，以及实施保护或缓解措施的能力。
目标群体通常偏好桉树属（Eucalyptus spp.）、木麻黄（Casuarina equisetifolia）、银桦（Grevillea robusta）等外来树种。尽管这些树种生产力颇高，但尚不清楚它们是否为退化立地的最佳选择。其次，农户对乡土树种的潜在生产力缺乏认知。农户常认为乡土树种的生产力不及速生外来树种，但他们也坦言，自身并未在人工林场景中使用过这些乡土树种，其观察仅基于景观中未受妥善保护与培育的自然萌生树木。
诸多技术手册指出，乡土树种更适应本地环境，在区域内尤其是边际土地上应具备更高的存活率与生长速率（Mulizane et al. 2005, Carpenter et al. 2004, Olukoye et al. 2003）。此外，尽管本次研究关注的外来树种未表现出入侵倾向，但我们仍意识到将外来树种引入新景观存在潜在风险。最后，尽管农户普遍认为外来树种生长更佳，但我们观察到乡土树种的种质资源存在大量变异。我们认为，经过合理筛选，乡土树种在退化立地上的表现有望达到甚至超越外来树种。所有这些“认知”均需通过示范加以验证。因此，国际农业林业研究中心（International Centre for Research in Agroforestry, 简称ICRAF）在WKIEMP框架下组织了一系列树种筛选试验。这些试验同时作为示范田，向农户宣传乡土树种的应用潜力。
本次树种筛选试验的核心目标是评估适宜乡土树种相较于农户偏好的外来树种在退化土地上的表现。
研究假设如下：
H1：乡土树种在退化土壤上的第一年定植存活率将高于外来树种。
H2：乡土树种在退化土壤上的生长速率将快于外来树种。
此外，我们还旨在通过覆盖广阔地理区域、涵盖多样气候、土壤及其他生物物理条件的广泛试验网络，探索开发退化土地树木表现预测模型的可行性。