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

Abstract

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 lower for seedlings in 1994 compared to 1993.

Topsoil moisture content rather than temperature appeared to be the major factor in control of root growth and development. Higher soil moisture content in the 10 to 30 cm depth was associated with earlier and higher maximum rates of fine root tip initiation than in the 0-10 cm depth, and this resulted in a higher maximum rate of total fine root elongation in the 10-30 cm depth. Similarly, a higher soil moisture content in the no understorey treatment was associated with a higher maximum rate of fine root elongation and coarse root cross sectional area growth, particularly with clone 3 than in the lucerne treatment. The maximum rate of total fine root elongation for clone 3 was 35% higher in the no understorey compared to lucerne; seedlings did not differ with understorey treatments. This illustrated the vigorous root regeneration character of clone 3 compared to seedlings. Also, the maximum rate of fine root elongation occurred earlier in the no understorey than in the lucerne treatments. The effect of lower soil moisture content on the north as opposed to the south side of lucerne and both north and south sides in the case of the no understorey treatments further resulted in a lower maximum rate of total fine root elongation. This indicated that lucerne was very competitive for soil moisture particularly on the north side of the ripline.

The growth pattern of the above ground (i.e., fascicle elongation, branch cross sectional area growth) and below ground (white root tips initiation, total fine root elongation and coarse root cross sectional area growth) components followed the sequence: (a) firstly, early spring growth of branches provided room for fascicle emergence and elongation, (b) secondly, fine root initiation occurred in late summer, and cross sectional area of branches and then coarse roots reached their maximum growth, (c) thirdly, fine root elongation occurred in autumn and reached its maximum growth in winter.

The result of this study suggest that if the primary aim of tree growers is to produce the maximum amount of timber under an agroforestry system it is necessary to select planting materials with vigorous rooting characteristics in order to support the above ground growth of trees. Planting materials similar to clone 3 seem to be ideal because of their higher fine root length density and coarse root biomass compared to the other planting materials with or without understorey competition. The study of fine root regeneration, and coarse root cross sectional area growth of clone 3 showed a higher maximum growth rate compared to seedlings. These rooting characteristics were linked to the higher above ground growth of clone 3. Furthermore, agroforesters have to select an understorey based on its competitive nature with the tree's root system. In this study, ryegrass/clover as an understorey crop produced less root competition to trees than lucerne.