Ecophysiology of clonal and seedling trees of Pinus Radiata D. don in an argoforestry system

Abstract

Above ground biomass production, crown architecture and canopy development were studied in seedling origin (seedling trees) and a tissue culture origin clone (clonal trees) of Pinus radiate trees, growing with and without pasture understorey (Medicago sativa). The objective was to improve our understanding of above ground processes influencing tree growth in an agroforestry situation. This study was carried out at the Lincoln University Agroforestry Experiment located in Lincoln (43° 38' S, 172° 30’ E; 11 m a.s.l.), New Zealand, where the climate is temperate with uniformly distributed annual rainfall of about 660 mm and the soil is a free draining silt loam.

Although, the clonal trees were 0.5 m taller and had greater stem basal area than the seedling trees at age 3, there were no differences in accumulated total above ground biomass and biomass increment. However, due to differences in allocation of biomass to crown components, the clonal trees had a higher harvest index and a higher foliage efficiency (FE: biomass production per unit foliage area).

The crown architectural study found that the clonal trees had a higher branch basal area and more medium size branches than the seedling trees. The branch basal area to foliage area relationship indicated that the clonal trees had less foliage area per unit branch basal area than the seedling trees. They also had longer internodes and a higher crown shape ratio (crown height/crown width) than the seedling trees. These differences in crown architecture and foliage distribution affected the FE of the different planting materials.

When trees were grown with the lucerne understorey, the accumulated above ground biomass was decreased by 67% compared to the no-understorey treatment. But the reduction in basal area and stem wood mass were only 42 and 50%, respectively. Allocation of above ground biomass into crown components was also affected by planting materials. Wood fraction was higher and branch fraction was lower in clonal trees than seedling trees resulting in a higher harvest index. Branch basal area and foliage area relationship indicated less foliage area per unit branch basal area, leading to improved FE. These changes were greater in the seedling than the clonal trees.

These changes in biomass and crown architecture were extended to look at the interception of solar radiation, the efficiency of carbon assimilation and productivity after the live crown pruning of trees. Growth losses due to pruning were higher in the clonal than in the seedling trees when they were grown in the no-understorey treatment. But due to the hindrance of tree growth by lucerne competition, growth losses due to pruning were smaller than where there was no-understorey.

New foliage growth was related to water stress of the trees, but in pruned trees the effects of water stress were decreased. However, there was no effects of pruning on the temporal growth pattern of foliage and branches, specific leaf area and needle density, nor, on above-ground biomass allocation to foliage, branches and stem wood.

Pruning also resulted in an increased FE. This improved FE was due to increased photosynthesis rate (thus light conversion efficiency) and increased light interception per unit foliage area. Photosynthesis and light conversion efficiency was increased more than 30% and 35%, respectively, in pruned trees compared to the unpruned trees. Light intercepted per unit foliage area was about 35% higher in pruned trees, but n decreased with increasing foliage area after pruning. More detailed light interception studies are needed in agroforestry systems to determine the optimum pruning strategy, taking into account of impact on tree and understorey growth and wood quality.

Forest managers usually manipulate stem and branch growth by altering planting density. However, this study found that understorey competition can be used in agroforestry situations to achieve desirable tree characteristics such as reduced branch size and improved harvest index.