Simplification of shade tree diversity reduces nutrient cycling resilience in coffee agroforestry

Abstract

Abstract Agroforestry systems are refuges for biodiversity and provide multiple ecosystem functions and services. Diverse multispecies shade tree canopies are increasingly replaced by monospecific shade, often dominated by non‐native tree species. The loss of tree diversity and the nature of the dominating tree can have strong implications for ecosystem functions, for example, nutrient cycling ultimately reducing crop production. To understand direct and indirect impacts of shade trees on nutrient cycling and crop production, we studied coffee agroforestry systems in India along a gradient from native multispecies canopies to Grevillea robusta (Proteaceae) ‐dominated canopy cover. We identified 25 agroforests, across a broad rainfall and management gradient and assessed litter quantity and quality, decomposition, nutrient release, soil fertility and coffee nutrient limitations. Increasing G. robusta dominance affected nutrient cycling predominantly by; (a) changing of litter phenology, (b) reducing phosphorus (P), potassium (K), magnesium (Mg), boron (B) and zinc (Zn) inputs via litterfall, decelerated litter decomposition and immobilization of P and Zn due to low‐quality litter, (c) reducing soil carbon (C) and micronutrients (especially sulphur (S), Mg and B). Coffee plants were deficient in several nutrients (nitrogen (N), calcium (Ca), manganese (Mn), Mg and S in organic and B in conventional management). (d) Overall G. robusta‐dominated agroforests were characterized by a reduction of P cycling due to low inputs, strong immobilization while decomposition and antagonistic effects on its release in litter mixtures with coffee. Synthesis and applications. The conversion of shade cover in coffee agroforestry systems from diverse tree canopies to canopies dominated by Grevillea robusta (Proteaceae) reduces the inputs and cycling of several micro‐ and macronutrients. Soil fertility is therefore expected to decline in Grevillea robusta‐dominated systems, with likely impacts on coffee production. These negative effects might increase under the longer dry periods projected by regional climate change scenarios due to the pronounced litter phenology of Grevillea robusta. Maintaining diverse shade canopies can more effectively sustain micro‐ and macronutrients in a more seasonal climate.