Influence of biophysical aspects on the performance of sustainable land management measures in the Usambara highlands of Tanzania

Samenvatting

Chapter 1 is an introductory chapter. This chapter first reveals that soil erosion problems in the West Usambara Mountains Highlands are long and historical and that efforts of curbing them began in the early 1900s. The chapter eventually shows that despite these efforts, implementation of SLM measures meant to reduce soil degradation have remained dismal. The chapter then draws four specific objectives on which this thesis is based. The chapter argues that although social and economic factors affecting land degradation in the West Usambara Mountains are known, perceptions of farmers, development workers and policy makers towards it are not fully understood. Further, that the magnitude of soil erosion in smallholder agro-ecosystems of the area is also unclear. The chapter questions the extent on which remote sensing and modeling tools could locate the adopted SLM measures on a landscape scale to determine their effectiveness in reducing erosion and promoting yields. The questions raised in Chapter 1 eventually pave the way to the subsequent chapters that address the research questions presented in this chapter.

Chapter 2 employs pixel-based (Maximum Likelihood Classification) and object-based image analysis (OBIA) remote sensing techniques to reveal land use patterns and adoption of soil conservation technologies in two sample blocks of 100 km2 each. The Chapter eventually models soil erosion in the two blocks and estimates effectiveness of the adopted SLM measures on a landscape scale. The Chapter finds the Maximum Likelihood Classifier to be reliable in generating land use thematic layer maps from which soil conservation measures can be studied with ease in mountainous areas. The OBIA-technique was found to be effective in identifying, classifying and mapping the adopted SWC technologies on a landscape scale. Though results indicated large differences in the adoption of soil conservation technologies between the two blocks, it was observed that despite the 80year long efforts of making farmers adopt SWC technologies, the area covered by bench terraces, grass strips and fanya juu terraces was < 20%. The chapter however found effectiveness of the SWC technologies across the two sample blocks to remain comparable. It concludes that adoption of the SWC technologies in the West Usambara highlands is largely influenced by biophysical conditions and is not related to the quality of the technologies being implemented.

Chapter 3 combined ground soil surveys, GIS and erosion modeling to locate and map smallholder agro-ecosystems in the West Usambara Mountains. The Chapter located six dominant agro-ecosystems in the area and found that Agro-forestry and other tree-based agro-ecosystems dominate the area due to historical land use changes and later institutional interventions across the West Usambara Mountains. This chapter finds combined use of soil surveys, GIS and modeling to be reliable in locating, mapping and assessing soil losses in smallholder agro-ecosystems which cannot be located by satellitebased remote sensing resources. The chapter reveals that smallholder agro-ecosystems in the West Usambara Mountains differ significantly in slope, vegetation cover, soil conditions and soil losses. Soil loss in agro-ecosystems dominated by annual crops was found to be 18 times higher compared to Natural Forests in the area due to lower soil cover and inefficient conservation and cultivation practices. The chapter concludes that improved adoption of soil conservation measures and vegetation cover technologies across the agro-ecosystems can reduce soil losses across the West Usambara Mountains by 37% and increases soil organic carbon levels by 16%.

Chapter 4 studied perceptions towards land degradation by administrative personnel and two smallholder communities in the West Usambara Mountains. The chapter finds that though farmers and administrative personnel have comparable perceptions on land degradation outcomes and interventions required against it, they differ in the way they perceive its causes. The chapter also reveals that though the two communities differed significantly in their perceptions on the causes, outcomes, and the required interventions against land degradation, they had a comparable perception on the effectiveness of selected SLM measures. Furthermore, farmers in the two locations can be categorized in effort-based SLM categories which use individual efforts of maintaining productivity of household fields. This chapter eventually associates differences in adoption of SWC technologies between the two communities to biophysical differences that separate their communities based on agro-ecological conditions, rainfall availability and soil properties. The chapter finds promised availability of irrigation water to have been the most influential factor which motivated farmers to install bench terraces. The chapter finally recommends the Extension department to consider variability of resources and biophysical conditions when advocating adoption of SLMs in the West Usambara Mountains. 

Chapter 5 reveals that because soil erosion is a serious problem in the highlands of East Africa, sustainable land management (SLM) measures have been widely promoted to reduce erosion and increase crop yield. The adoption of SLM measures however has remained low. The chapter aimed to understand the contribution of biophysical factors on adoption of the SLM measures and therefore measured biophysical conditions of fields with SLM measures, impact of different SLM measures on soil degradation and effects of SLM measures on crop productivity. Results of this chapter show that superior SLM measures are implemented on less steep slopes where they stabilize field slopes and reduce soil losses by water erosion. Though the chapter associates high amounts of farmyard manure used on fields with superior SLM measures to higher crop yields it also reveals that the use of manure had no significant improvements of soil fertility status of the respective fields because the amounts applied are too small to allow build-up of soil nutrients.

Chapter 6 is a synthesis chapter. This chapter brings together findings made in Chapters 2 through 5 and discusses their implications to scientific knowledge and development needs in the West Usambara Highlands and the Eastern Africa region. The chapter argues that for Lushoto district adequate nutrient application and recommended plant spacing is crucial if soil fertility and higher yields are to be sustained especially in fields with bench terraces. Further, this chapter cautions that recommendations made by administrative personnel in Lushoto regarding soil erosion control and SWC technologies must be taken with caution. This implies that extension services in the West Usambara Highlands should not adopt a blanket approach in advocating SLM measures. The chapter concludes that despite a long and historical campaign in making farmers of the West Usambara highlands adopt SWC technologies only a small proportion of the area has been conserved (<5%) and that, due to the link between biophysical conditions and adoption of SWC technologies, communities located in areas with abundance of geo-ecological resources like favorable soil conditions, high availability of water resources, parcels of valley bottom land for vegetable growing and high annual rainfall tend to take soil conservation technologies less seriously than fellow communities in less favorable conditions. 

Lastly this chapter advocates for the Extension department in the West Usambara Mountains to opt for alternative soil conservation approaches other than bench terraces, fanya juu terraces, cut-off drains and grass strips. This implies discontinuation of these current measures on steep lands (slopes > 40%). The chapter advocates instead for alternative measures like traditional agro-forestry systems, correct spacing of cultivated crops and inclusion of fodder trees/crops for livestock in the local farming systems. It is for this reason that chapter 6 requires policy makers and the extension departments in the region legislates and enforce a ban towards continued cultivation of annual crops (like maize and bean) without the company of agro-forestry and other tree based systems. Given the disappointing results obtained till date in reversing land degradation in the West Usambara Mountains, the chapter ends with a plea that rural development policies and extension strategies should therefore focus much more on enhancing farmers’ motivation and capacities to invest in their land, rather than promoting certain technologies.