Spatial variation and understorey competition effect of Pinus radiata fine roots in a silvopastoral system in New Zealand

In designing agroforestry systems, the combination of tree genotype (or species) and pasture species and the spatial arrangement of trees are important considerations. The spatial variation of fine root length density (FRLD) of three radiata pine (Pinus radiata D. Don) genotypes, referred to here as clone 3, clone 4 and seedlings, was studied in a three-year-old temperate silvopastoral experiment. The genotypes were planted with three understorey types: ryegrass (Lolium perenne) mixed with clovers (Trifolium spp), lucerne (Medicago sativa), and control (bare ground). Also fine root distribution of both tree and pasture species with soil depth and in relation to tree row (0.9 m north or south of and within the ripped tree row) was studied. Greater FRLD was found in clonal than in seedling trees in the bare ground treatment but not in the two pasture treatments, and in the 0-0.1 m but not in the 0.1-0.2 or 0.2-0.3 m soil layers. Clonal trees had a greater ability to develop a more extensive root system, especially in the 0-0.1 m soil layer, but that advantage disappeared when they were planted with pasture species since competition from the pasture species was most severe in the 0-10 cm layer. The FRLD of lucerne was greater than that of ryegrass/clovers, consistent with the greater aboveground biomass production of lucerne. Pasture species FRLD was greater on the south (wetter) than on the north side of the ripline or in the ripline. The interception of prevailing southerly rain-bearing wind by tree crowns resulted in the south side being wetter than the north side. Results indicated that production and distribution of fine roots of both tree and pasture species responded to changes in the microclimate. We suggest that to optimize pasture/tree biomass production planting trees in the north-south direction is better than in the east-west direction at the studied site.