Rooting characteristics of Pinus radiata D. Don as influenced by understorey competition in an agroforestry system

Content Partner: Lincoln University. Fine root length density and coarse root biomass and architecture of tissue cultured clonal and seedling planting materials of Pinus radiata D.Don was estimated in an agroforestry system with understorey treatments (no understorey, Medicago sativa, and Lolium perenne + white clover) at the Lincoln University agroforestry experiment site, New Zealand. Seasonal growth patterns of fine and coarse roots, branches and needles of radiata pine trees were related to soil moisture and temperature as influenced by planting material, understorey and micro-site conditions. The climate was summer-dry temperate with 660 mm annual rainfall evenly distributed throughout the year. The monthly mean evapo-transpiration was lower than the rainfall in winter, whereas in spring and summer it exceeded rainfall by 250%. Most of the rain-bearing winds come from the south, resulting in the soil on the south side of the tree rows being cooler and wetter compared to the north sunny side. At age 3, clone 3 trees were taller than clone 4 and seedling trees although the basal area was very similar among these planting materials. The fine root length density of clone 3 and clone 4 trees was higher than seedling trees particularly at 0-10 cm depth. The fine root length density was higher in the ripline compared to the north and south side, and was higher on the south side compared to the north side. Fine root distribution was thus a function of planting materials, site preparation and soil moisture distribution. Lucerne had higher rooting density than the ryegrass/clover plants, and so provided greater competition to the trees. Coarse root architecture studies showed that root:shoot ratio, and biomass of root core, vertical and the lateral roots, particularly in the ripline, were higher in clone 3 than in the seedlings. As a result of understorey competition, seedlings had a lower vertical and lateral root biomass in the lucerne treatment compared to the no understorey treatment, whereas in clone 3 root biomass was less affected by lucerne competition. The lateral root biomass was higher in the ripline compared to the sides, and it was higher in the south compared to north. Root biomass growth between 1993 and 1994 was much higher in clone 3 than seedling trees, particularly with the vertical roots; also much higher in the no understorey than with the lucerne treatments. The fractional allocation to the core did not change for clone 3, whereas it was l