Intraspecific root plasticity in agroforestry systems across edaphic conditions

Land-use practises converting forests to tree-crop systems commonly result in large expanses of intensively managed landscapes. However, some farming practices retain trees and other forest structural components during conversion as a means to confer favorable conditions through agroecological intensification. Understanding root plasticity in situ in response to such a multi-species rooting environment is important to avoid interspecific resource competition. This, however, is an often-understudied parameter due to methodological constraints. Here, we investigate two dominant parameters of root ecology, coarse root distribution and soil water acquisition, of the economically important tree-crop, Theobroma cacao, in monoculture and in mixture with shade trees (agroforestry systems) at two edaphically contrasting sites [sandstone (sandy loam) and phyllite-granite (loam) derived soils] in Ghana, West Africa. In monoculture and in mixture, we employed ground-penetrating radar to detect cocoa coarse root distribution and plant-soil δ18O isotopic signatures to estimate soil water acquisition zones. In monoculture, detected cocoa coarse root vertical distribution differed between sandy loams and loams, with a less dispersed distribution and a higher mean coarse root depth in sandy loams. Detected vertical coarse root distribution was also strongly differentiated between cocoa in monoculture and in mixture; cocoa exhibited restricted root allocation to a smaller zone in the presence of a shade tree, in sandy loam soils. In monoculture, cocoa plant δ18O isotopic signature matched a narrow soil δ18O isotopic zone, while this matched plant-soil zone expanded for cocoa in mixture, illustrating larger soil water acquisition zones in the presence of a shade tree but exclusively in sandy loam soils. We show that under certain conditions, root modification in the presence of a secondary species may limit competition as tree-crop root plasticity differentiates belowground allocation and resource acquisition zones in an agroforestry system. © 2013 Elsevier B.V.