Root development and whole-tree allometry of juvenile trees of five seed lots of Pinus radiata D.Don: implications for forest establishment on erosion-prone terrain, East Coast region, North Island, New Zealand

Abstract

Background: 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 r 2 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.

Effect of changes in forest water balance and inferred root reinforcement on landslide occurrence and sediment generation following Pinus radiata harvest on Tertiary terrain, eastern North Island, New Zealand

“…Similarly, the proportion of ground area covered by the canopy for stands of P. radiata established at the recommended planting density of 1250 stems ha -1 is based on annual measurements of the tree crowns of fifty 1-year-old trees, fifty 2-year-old trees, twenty-five 3-year-old trees, and fifty 4-year-old trees from an earlier study site located at Okiwa Forest (Fig. 1) (Marden et al 2016). From this study, we also drew on annual measurements of the mean maximum lateral spread of the root systems of fifty 1-year-old trees, fifty 2-yearold trees, twenty-five 3-year-old trees, and ten 4-yearold trees to calculate the proportion of ground occupied until the roots of adjacent trees come into contact (i.e., a root occupancy index of 1) (Kelliher et al 1992a).…”

“…However, the roots of juvenile P. radiata are generally asymmetrically distributed around the stem (Marden et al 2016), and because there are often several 'extralong' roots present, the contribution of the lateral roots of the replacement plantings to net soil-root reinforcement at the time of overlapping with adjacent trees (i.e., site occupancy index = 1) is likely to have been overestimated. Furthermore, while the root systems of 4-year-old trees exhibit strong vertical tap-and sinkerroot development, only ~4% of root biomass, 16% of the total root length, and 4.5% of the total root crosssection area occur below the depth of landslide failure planes (Marden et al 2016), where they would probably have a greater influence on slope stability (O'Loughlin & Ziemer 1982;Watson & O'Loughlin 1990;Watson et al , 1999Ekanayake et al 1997). Thus, for juvenile replacement plantings <5 years old, insufficient root development below the landslide failure plane probably accounts for the continuing occurrence of landslides and elevated sediment generation rates on slopes harvested 3 and 4 years before storms (Table 2).…”

“…Thus, any forest management strategy that aims to maintain a near-closed canopy and an effective live soil-root reinforcement system by delaying thinning until after the canopy has closed, and at which time evapotranspiration rates reach a maximum of 35% of annual rainfall, has the potential to reduce the risk of soils reaching field capacity sufficiently to minimise the occurrence of storm-initiated failures on slopes identified as high risk. That said, estimations of the period until net root reinforcement is restored are complicated by the naturally high spatial variability in root characteristics and architecture (Marden et al 2016). These are influenced by factors including heterogeneities in soil type and texture (Watson & O'Loughlin 1990), impenetrable bedrock at shallow depths, high water tables that limit vertical root development (Ray & Nicoll 1998), slope aspect and steepness (Chiatante et al 2003), and the prevailing wind (Nicoll & Duncan 1996;Marden et al 2016).…”

“…In New Zealand, previous studies of cause-effect relationships of storms following the harvesting of P. radiata forests have generally focused on establishing the relationships between landslide occurrence, site factors, and root decay rates (O'Loughlin & Watson 1979, O'Loughlin & Ziemer 1982Watson et al 1999), whereas data on sediment generation by landslides during the post-harvest period is less common (e.g., O'Loughlin 1972;O'Loughlin et al , 1978Hicks & Harmsworth 1989;Basher et al 2011;Marden & Rowan 2015). Similarly, analyses of soil-root reinforcement interactions have tended to focus on the measurement of growth parameters of the root systems of wellestablished trees at specific ages (O'Loughlin 1985;Watson & O'Loughlin 1990;Watson et al 1995), and until relatively recently, there has been very little information available on the below-ground growth performance and/or root reinforcement of juvenile trees (Watson & Tombleson 2004;Marden et al 2016).…”

“…To establish the approximate duration of the "window of vulnerability' (Sidle & Ochiai 2006) of harvested areas to storm-initiated landslides, we correlate the extent of damage and sediment generation rates derived from harvested areas after storms at Mangarara and Wakaroa forests (Fig. 1) with: (i) known root decay rates of harvested P. radiata (O'Loughlin & Watson 1979), and (ii) time series measurements of the growth rates of the lateral roots of immature P. radiata (Marden et al 2016) established at Okiwa Forest (Fig. 1) and on the same terrain as at Mangarara and Wakaroa forests.…”