2012 Theses Doctoral

Multiple ecosystem services in smallholder agriculture

Sircely, Jason

Recent research into the ecological origins and social implications of ecosystem services, the benefits ecosystems provide to society, is predicated on the downward trends observed for many services. Current work increasingly emphasizes how interactions among ecosystem services (synergies and trade-offs) affect the delivery of multiple services, from soil fertility at field scales to watershed-scale hydrological function, to global climate regulation. Meanwhile, research on the relationship between biodiversity and ecosystem functioning has identified substantive linkages between the biodiversity of ecosystems and the services they provide, provoking interest in aligning biodiversity conservation with the sustained delivery of ecosystem services. To better understand the delivery of multiple ecosystem services relevant to smallholder farmer livelihoods in the tropics, the production of wood and livestock forage and the structure and fertility of soils were examined in grazed and improved fallows in western Kenya. The objectives were to investigate fine-scale interactions among ecosystem services, and the potential of fallow plant diversity to enhance levels of multiple services simultaneously, i.e., ecosystem service multi-functionality. To examine influences of fallow vegetation and management on soil structure and fertility, the biomass and functional traits (green tissue N, lignins, polyphenols, lignins+polyphenols) of fallow plants and the condition of soils were quantified. Positive associations of tissue content of lignins+polyphenols with soil organic carbon (SOC) and steady infiltration rates indicated a role of recalcitrant carbon compounds in slowing SOC turnover and supporting soil structural development, while grazed fallows had lower infiltration than improved fallows, likely a result of livestock trampling and soil compaction. To investigate whether woody overstory plants and fallow management generate interactions between the production of wood and livestock forage, the relationships of overstory biomass, overstory diversity as a proxy for niche complementarity, grazing intensity, and soil conditions with the biomass and quality (crude protein:lignin) of forage species were studied. Overstory competition in densely planted improved fallows resulted in an apparent trade-off with forage biomass, however in grazed fallows overstory and forage biomass were positively associated, likely due to negative effects of grazing on both; in addition, reduced forage quality and biomass of quality forage species were attributable to selective grazing. Finally, to assess whether plant diversity enhances joint levels of multiple ecosystem services, two indices of ecosystem multi-functionality were designed for four ecosystem functions: wood biomass, forage biomass, soil base cations, and infiltration. Evidence for positive effects of diversity on multi-functionality was clearer in grazed fallows, while in improved fallows environmental favorability appeared more influential. The difference in the diversity-multi-functionality relationship among fallow types appeared related to contrasting management, disturbance, species composition, and the productivity of vegetation and soils. The results suggest that plant diversity and site productivity can enhance multiple ecosystem services in smallholder fallows, yet diversity effects may vary with management, species composition, and site conditions.