Abstract:

Establishment of exotic forest on erosion-prone pastoral hill country in the East Coast region of New Zealand escalated following a major cyclonic storm in 1988. Pinus radiata D.Don is the predominant species used for erosion control. It has been suggested that planting densities could be reduced if faster growing P. radiata seedlings and cuttings from genetically improved seed lots were used. Methods: A field-based trial was established to measure and compare annual growth rates between P. radiata seed lots (GF 16 and GF 19 seedlings and cuttings grown from GF 23 seed) used during the period of early forest establishment (1960s onwards) in this region and genetically improved seed lots (GF 27 seedlings and GF 28 cuttings) developed at a later date. Over a 4-year period, the above-ground parameters (diameter at breast height, root collar diameter, tree height and canopy diameter) were measured before whole trees (canopy and root systems) were destructively sampled. Results: Root collar diameter (over bark) was most highly correlated with tree height, with r2 ranging between 0.95 and 0.98 for all the seed lots, and all regressions were highly significant (P < 0.001). In any year of the trial, there were no differences among seed lots with respect to root collar diameter, total above-ground biomass or total below-ground biomass. By year 4, mean below-ground biomass comprised 17% of total biomass. There was no consistently significant difference among seed lots in the distribution of total below-ground biomass or root length with distance from the root bole, relative to depth, or in either their maximum lateral root spread or maximum root depth until year 4 when GF16S and GF27S had a greater (P = 0.05) maximum root spread than GF28C while the latter had developed a significantly greater (P = 0.05) proportion of its root biomass and root length within the top 0.5 m of the soil profile. The root cross-sectional area of vertical roots increased significantly with age but decreased with depth. Root orientation formed a ‘bilateral fan-shaped’ architecture. Conclusions: The absence of consistently significant differences among seed lots suggests that during the early 4 year post-establishment period, no one seed lot would provide earlier soil reinforcement or result in a superior level of slope stability than would any of the other seed lots trialled. Thus, irrespective of seed lot preferences, any reduction in the current recommended planting density for erosion-prone hill country would further increase the period of vulnerability and risk of young plantings to damage by storm-initiated landslides.

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