Relationships of overstory trees and shrubs with forage species portray ecosystem service interactions in smallholder fallows

Interactions among ecosystem services are increasingly perceived as important to ecosystem service delivery. Synergies and trade-offs among ecosystem services arise through direct ecological interactions or indirectly through correlated responses to other factors. To investigate whether and how interactions of overstory trees and shrubs with livestock forage species growing beneath generate ecosystem service interactions, overstory and forage species were examined in smallholder fallows in western Kenya. In 18 grazed and 21 improved fallows, we estimated biomass and quantified soil properties. We assessed whether the overstory reduces forage biomass and quality through competition, and whether overstory niche complementarity ameliorates competition or enhances facilitation. In improved fallows, forage biomass declined with overstory biomass, indicating competition and a wood-forage trade-off. In grazed fallows, biomass of higher quality forage species increased with overstory biomass, indicating a synergy, likely indirect. Niche complementarity, quantified as taxonomic and functional diversity, did not appear influential. Forage quality was not associated with overstory characteristics, but declined with grazing intensity. The contrasting relationships between overstory and forage species among fallow types appear ultimately attributable to the presence and intensity of grazing and the dense overstory in improved fallows.