Contradictory behaviour of site index when utilised in basal area projection functions for <i>Pinus radiata</i> in the Hawkes Bay region of New Zealand

Authors: Richard Woollons
Publication: New Zealand Journal of Forestry, Volume N.Z.J.For. 2006, Issue N.Z.J.For. 51(4) 2007, pp 28-32, Jan 2007
Publisher: New Zealand Institute of Forestry

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Abstract: In the Hawke's Bay, Pinus radiata growth models have the property that basal area/ha is predicted to decrease a little with a higher site index.

This illogical outcome is explored by examination of PANPAC permanent sample plot data, obtained from five forests. There is abundant evidence of site index decreasing at higher altitudes. The data are reduced to allow more balanced comparisons of the forests at various elevations. A basal area/ha projection equation shows a highly significant negative site index coefficient that increases the precision of the model by 17 %. To examine the data further, the equation is simplified so that site index appears as a straight line model with the slope parameter determining the sign of the site index coefficient. The slope of the model is shown to be negatively correlated with site index.

Examination of the growth data in the five forests suggests that the effect is likely to be a result of higher stockings resident in the high altitude plots together with low lying plots probably being under-occupied. The negative site index coefficient is thus the result of climatic and historical silvicultural effects. 

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