Intraspecific root plasticity in agroforestry systems across edaphic conditions

Abstract

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.