Characterisation of within-tree and within-ring resin-pocket density in Pinus radiata across an environmental range in New Zealand

Resin pockets are found in the xylem of conifers belonging to four main genera and can generally be classified into two types. Type 1 are radially narrow discontinuities in the wood while type 2 are radially flattened, contain less callus tissue, and are open to the external environment at early stages in their development. Although resin pockets are a major cause of degrade for appearance grade timber little is known about how the frequency of type-1 and type-2 resin pockets varies within trees and within rings. In this study, we collected data from 15- to 18-year-old Pinus radiata D.Don stands at four sites covering a wide environmental gradient. Resin pocket frequency was determined by cutting the lower 5 m of six trees at each site into 50-mm sections. Each of these sections was then imaged. Resin pockets were identified as type 1 or type 2 and the location of the resin pockets in three dimensions was recorded. Using these detailed measurements, the objectives of this study were to characterise: (i) three-dimensional variation in type-1 and type-2 resin pocket frequency within trees; and (ii) the position of type-1 and type-2 resin pockets within rings. The frequency of type-2 resin pockets was double that of type-1 resin pockets, and this ratio did not vary significantly between sites. Within trees, resin pocket density varied markedly in the radial but not the longitudinal or circumferential dimensions. At all four sites, variation in the radial dimension was characterised by an absence of resin pockets in the inner rings and fluctuating resin pocket densities in the outer rings. The age at which substantial resin pocket formation began ranged from 4 years on the fastest growing site to 8 years on the slowest growing site. On the driest sites, resin pocket incidence consistently peaked in the latter part of the growth ring in all trees, but on the windiest and wettest site the distribution was more irregular and varied between trees. The distribution of type-1 and type-2 resin pockets was highly segregated within the growth ring with mean positions occurring respectively at ca. the 30th and 80th percentile of the growth ring width.