Effects of Groundcover Management Systems on Root Distribution and Demography in an Avocado Orchard

We conducted minirhizotron observations of root demography and distribution for three years in a newly planted ‘Haas’ avocado (Persea americana Mill) orchard on ‘Mexicola’ seedling rootstocks, established on a steep hillside in central Chile. Our objective was to investigate whether different groundcover management systems (GMS) affect avocado tree performance and environmental externalities. The general hypothesis was that avocado root phenology, production, morphology, vertical distribution, and lifespan would be affected by GMS treatments. A more specific hypothesis was that avocado tree roots would be negatively affected by belowground interspecific competition from groundcover species that were planted to minimize soil erosion and runoff. The trees were grown in four GMS: 1) Bare Soil (BS), a combination of pre- and post-emergence herbicides; 2) Vegetation Strip (VS), post-emergence herbicide applied in a 1-m wide strip centered on the tree row plus a groundcover mixture seeded between tree rows; 3) A complete groundcover (GC), the same groundcovers as in the VS treatment covering the entire surface of the plots. More roots were observed in the non-bearing year (2009–10) than in the bearing year (2010–11). Trees in BS plots produced the most aboveground shoot and fruit biomass, and they had more shallow and larger diameter roots. Trees in VS and GC plots had more roots in the 31–50 cm depth, and roots of smaller diameter. Lifespans of roots emerging in fall were 54% and 57% greater than those emerging during summer and spring, respectively; and soil depth and root diameter were positively correlated with root survival. Compared with the GC plots, the lifespans of thinner roots (< 0.2 mm) in the BS and VS plots were 61% and 47% greater, respectively. These results demonstrated morphological plasticity of avocado root systems in response to different growing conditions (compared to their native humid subtropical habitat) and to resource and non-resource competition when grown in mixed herbaceous groundcover stands.