Differential distribution of fine roots between Robusta coffee and shade trees influences water use

Abstract

Coffee is Uganda’s most important cash crop, commonly grown in agroforestry systems that offer shade, food, timber, and microclimate regulation. However, shade trees may compete with coffee for water and nutrients. This study characterized root distribution and water use of Robusta coffee and shade trees. A randomized complete block design was used with three treatments: coffee under Albizia coriaria (ACS), coffee under Ficus natalensis (FNCS), and open sun coffee (COSS), each replicated three times. Soil moisture was monitored using a Diviner 2000, and fine root distribution assessed through soil sampling up to 100 cm depth at varying distances from coffee plants (≤ 50, 51–199, and ≥ 200 cm). ImageJ software was used to measure root length from which root length density (RLD) and specific root length (SRL) were computed per depth layer. Significant differences in RLD were observed (p < 0.05), highest in COSS (0.11 cm/cm³) and lowest in FNCS (0.06 cm/cm³). Most fine roots were concentrated in the top 20 cm. Horizontally, FNCS showed differentiation, with coffee roots mainly at 51–199 cm, and F. natalensis roots at ≤ 50 cm. FNCS had the highest SRL (1063.9 cm/g), and ACS the lowest (597.8 cm/g), though differences were not statistically significant. Vertically, F. natalensis roots dominated the 80–100 cm layer, while coffee roots were concentrated at 20–40 cm. Actual evapotranspiration (ETa) varied with season and shade tree species but differences among systems were not significant. The findings indicate competition for water between coffee and shade trees, emphasizing the importance of shade tree management to reduce below-ground competition.