Stand dynamics of old crop radiata pine

Authors: Graham West
Publication: New Zealand Journal of Forestry, Volume N.Z.J.For. 2025, Issue N.Z.J.For. 70(1) 2025, pp 3-10, May 2025
Publisher: New Zealand Institute of Forestry

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Abstract: A concern by many that radiata pine was not suited to be grown for long-term carbon forests has led to several attempts to change the Permanent Carbon Forests settings in the New Zealand Emissions Trading Scheme (ETS). To help inform this contentious issue, this paper provides an update on the growth and survival of the oldest radiata pine in New Zealand measured in Permanent Sample Plots (PSPs). Recent growth measurements of four PSPs averaging over 100 years old were added to a previous analysis, undertaken 12 years ago, to examine stand dynamics and mortality trends. This analysis confirmed earlier reports that tree stockings had reduced significantly with natural competition-induced mortality, so that by age 80 years stands are reduced to approximately 200 live stems per hectare (sph) or less. Tree stockings have since remained reasonably constant with little windthrow or mortality. At the current age Diameter-at-Breast Height (DBH) was still growing, but height growth had slowed. Annual volume increment is currently around 21 m³/ha/yr. To better understand why these stands show remarkable resilience two new metrics were applied. Reineke’s size-density relationship, or rule, is used widely in North America to explain stand dynamics. New Zealand radiata pine conforms well to this rule and creates the maxim that growing large trees at high stockings is not biologically possible, as they will always self-thin. This dynamic appears to be likely to occur with most exotic forest species, but the timing of the effect may be species-dependent. To explain the apparent wind firmness of these centurions a proxy measure using a slenderness ratio (Ht/DBH) was calculated. The trend in this metric shows the typical pattern of increasing up to age 40, but thereafter has surprisingly decreased to levels where wind damage is expected to be relatively minor. Relative to their height, green crowns are now small even though the stands are widely spaced. It appears from this limited data source that stands of radiata pine adapt to the predicament their potential huge size creates, and to ensure their survival they self-thin to encourage low slenderness with small crowns. The trend to low stockings allows light into the stand at older ages and aids the potential transition to indigenous species if the appropriate seed source is available.

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