Shade tree root traits in cocoa agroforestry systems are associated with their functional leaf phenology groups

Analysing root traits to identify below ground acquisition mechanisms and relating them to above ground traits, such as leaf phenology, can improve the understanding and design of resource use efficiency in drought resilient agroforestry systems. Shade trees play a key role in regulating above and below ground resource use dynamics in agroforestry systems. Specific shade tree functional traits such as leaf phenological development, crown architecture and leaf traits such as specific leaf area and nitrogen content have been related to shade tree impact on productivity, ecosystems service provision and drought resilience of agroforestry systems. Understanding the influence of many different shade tree species on resource use and the productivity outcome resulting from their interaction with cocoa plants have, so far, mainly focused on aboveground traits. Yet, there is an urgent need to put adequate emphasis to the equally important belowground processes, root systems, and root-rhizosphere interactions. Root trait research is significantly limited in tropical communities, constraining understanding of belowground processes and interactions within complex systems such as agroforestry. There is lack of understanding of strategies in belowground resource acquisition among functional groups of shade tree species. In this study, two key roots traits, i.e. fine root length density and fine root diameter of 13 common shade trees species belonging to 6 functional (leaf phenology) groups and cocoa were evaluated under farmer field conditions. Fine root samples were acquired for 4 replicates of each shade tree species through extensive root coring up to 60 cm depth and at three horizontal shade tree impact zones (inner, mid and outer). Scanned sorted shade tree and cocoa plant root images were analysed with WINRHIZO.  All cocoa plants irrespective of their associated shade tree functional group exhibited resource acquisitive (non-conservative) fine root traits, i.e. with higher root length density and smaller diameter. Similarly, shade trees in the ‘brevi deciduous during dry season’ functional group exhibited the notable paradox of leaf flushing during dry season characterized by higher, leaf area-related, water uptake in the dry season exhibited non-conservative root traits. Evergreen and complete deciduous functional groups showed a conservative root trait showing lower fine root length density and larger diameter. Shade trees with conservative root traits are therefore considered complementary to the cocoa plant acquisitive traits, thereby enhancing belowground resource use efficiency and drought resilience in cocoa agroforestry systems.