Understanding the dynamics and distribution of root system components and how they are affected by pasture–tree interactions in silvopastoral systems are important for better management of agroforestry systems. The biomass and morphology of coarse root components were studied for 3- and 4-year-old Pinus radiata clonal and seedling trees growing with or without lucerne (Medicago sativa). Root:shoot ratio and lateral and vertical root biomass were greater by 1.5, 2.3 and 6.1 times, respectively, in clonal than in seedling trees, particularly in the no understory treatment compared to the lucerne treatment. Fractional allocation of root biomass to lateral and vertical roots was higher in clonal than in seedling trees by 1.1 and 2.6 times, respectively, while allocation to the root core was 1.4 times higher in the seedlings than the clone.Competition from lucerne reduced fractional allocation of root biomass to lateral roots by 40% in the seedling tree in 1993 and increased allocation to the root core. Competition was more intense at age 3 than at 4 years. No competition effect on lateral roots was observed with the clonal trees.Radiata pine root systems showed strong morphological plasticity to respond to changing soil conditions. Ripping coupled with thinning increased lateral root growth in the ripped zone so that by age 4 years 60% of lateral roots was in this zone. This was probably due to alterations in soil structure and reduced competition, particularly for moisture. Similarly higher soil moisture led to more lateral root biomass growth in the no understory compared to the lucerne treatment and on the south side of the trees compared to the north side. These results illustrated that selection of genotypes and planting material as well as management techniques such as soil cultivation, selection and placement of pasture understory, and thinning can all be used to manipulate rooting patterns and tree productivity in agroforestry systems. Furthermore, selection of planting material can also be used to reduce tree toppling in radiata pine.