Stand density index – a useful tool for New Zealand redwood plantings
New Zealand Tree Grower February 2007.
Measuring growing space occupancy in redwoods is not new. In 1933, a researcher named Reineke reported data for even-aged stands in California, presenting a stand density index for various species.
He found that stands reach an upper limit of stocking relating to average tree size. This concept can be used to define maximum final crop stockings for a target average tree size. For example, when all growing space is occupied in even-aged redwood stands, there is only space for approximately 800 stems per hectare with an average diameter at breast height (DBH) of 50 cm. Alternatively, there is space for 600 stems per hectare with an average DBH of 60 cm, or 475 stems at 70 cm DBH.
Lower stocking makes less complete use of available growing space but may be needed to satisfy other objectives, such as keeping lower branches alive longer or producing trees of uniform size. Intense competition during the rotation creates winners and losers, giving a wider range of diameters at harvest.
The stand density index concept of stocking control should be transferable to New Zealand plantations. Data from one particular redwood stand in New Zealand show that this stand is only at 75% of the upper limit of the US stand density, with 200 stems per hectare and an average DBH of 100 cm, yet a very wide spread of diameters exists. This implies that 75% of maximum stand density index may be a sensible upper limit for wood production in New Zealand. This would be 360 stems per hectare at 70 cmDBH, 460 stems per hectare at 60 cm and a maximum of 620 stems per hectare to achieve a 50 cm average DBH.
The maximum stand density index for redwoods is higher than the stand density index for most species due to the shade tolerance of redwood, the thick bark, and lack of stocking related pests. The stand density index for a whole stand can be divided into stand density index for each group of multi-aged stands, proving to be a useful concept for stocking control in even-aged and multi-aged plantations and natural stands.