Predicting optimal stand density in NZ plantations

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Radiata pine when it is planted too closely together grows tall and spindly while wide spacing encourages branch growth and land may be underutilised. Somewhere in between is an optimum final crop stand density that allows forest owners and growers to maximise the volume and value of the logs produced.

Scion scientists have recently looked at the factors that affect the optimum final crop stand density (Sopt) and how this can vary across New Zealand. Their starting point was the two productivity indices normally calculated from sample plot measurements: Site Index (SI) derived from tree height and age, and the 300 Index (I300), which is a standardised measurement of total stem volume. By linking the productivity indices with environmental information such as climate and soil types they were able to develop productivity maps, or surfaces, of these two productivity indices covering the whole country.

Optimal stand density is also affected by the decision to produce either clearwood timber, where the trees are pruned, or structural grade timber from unpruned trees. With the premium for pruned logs declining, the Scion work focussed on optimising conditions for growing valuable large-diameter, “S27”, structural saw logs with small knots. S27 logs were defined as having a small end diameter greater than 270 mm with the diameter of the largest branch being no larger than 70 mm.

Using the simulation system Forecaster, the researchers calculated the volume of S27 logs that could be grown under a standard framing silvicultural regime for various combinations of SI, I300 and stand density. Forecaster can predict growth, yield and form, including branching, for radiata pine across a range of sites and management regimes in New Zealand. Using these simulations, they were able to develop a relatively simple model of Sopt from I300, SI and rotation length.

Image: Figure 1. Spatial variation in predicted optimal stand density under a framing regime for stands grown to 28 years. Predictions were constrained to the potential range of P. radiata through excluding areas with mean annual temperature <7.9°C, artificial surfaces, and water bodies.

Across New Zealand, the average predicted Sopt for growing S27 logs was found to be 614 stems per hectare, with Sopt increasing from north to south. (see Figure). Given that the average stand density for unpruned sawlog regimes is around 500 stems per hectare, there is definite scope for increasing the volume of high-value log products grown in New Zealand’s planted forests, and value extracted per hectare.

Recognising that 614 stems/hectare is an average, site-specific recommendations can be made by taking account of local variations in productivity indices, and thus local environment, into account. Remote sensing technologies such as LiDAR, can provide forest managers with accurate prediction of local SI and I300 values and consequently more accurate predictions of Sopt.

The Sopt model suggests there is considerable scope for increasing plantation value across New Zealand through optimising stand density by site to values that are higher than those typically prescribed. Forest owners and managers can use the model to plan more targeted operations to optimise stand density and maximise the value of their crop.

Source: Watt, M S, Kimberley, M O, Dash, J P & Harrison, D (2017).Spatial prediction of optimal stand density for even-age plantation forests using productivity indices. Canadian Journal of Forestry Research, 47: 527–535

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