Many foresters planning pre-harvest inventory in Pinus radiata in New Zealand consider the option of placing secondary basal area tally plots (aka prism plots) in addition to the normal primary plots. This can be valid way of conducting inventory in some circumstances, although this article would like to highlight some considerations foresters should take before implementing this technique.


For and against adding secondary basal area (BA) tally plots into an inventory.

  • Angle-gauge prism (BA Prism) point samples provide an easy and cost efficient way of collecting basal area.   However where determination of stocking is also a prime aim of the inventory bounded plots are more efficient and useful.
  • Stem basal area provides a good correlation with stem volume, and hence you can arrive at a volume-basal area ratio (by using the sub sample of basal area and volume from the primary plots).    This can then provide additional information to improve basal area and volume calculations at time of cruise.
  • Stem basal area and log volume out-turn does not have a good correlation as this can be driven more by stocking more than BA.   Hence one of the reasons for most of our inventory being based on bounded plots as these will provide a far better assessment of stocking that variable radius BA plots.
  • One of the most important issues with basal area plots is they cannot be easily grown forward, as most growth models require more information over and above just BA.   So secondary BA provides an improved estimate of standing volume now but any future analysis where the primary plots are grown on the secondary BA plots cannot be easily used.
  • Operational problems with BA prism use arise when in untended stands where is a lot of persistent heavy branching at DBH height, or where there are large amounts of undergrowth.   Also in steep hill country all BA assessments must account for slope, and hence an increasing number of trees will be to be correctly checked for slope distance, as each tree will have a different slope distance based on it diameter.    It has been known in the past that cruisers often took BA plots more casually and hence degrading the accuracy of the inventory, although this is a training issue it should be a serious consideration.


Inventory Analysis and Data Collection Software Considerations

  • YTGEN doesn’t currently allow any integration of prism point samples of secondary plots into yield request summaries (as of 16/01/2007). The reasons for this not being a current feature of YTGEN are as follows : (1) No one standard way of using this to adjust primary plot yields, especially where log volume estimates are required. (2) No good way of handling BA plots once a population is grown on past the cruise date, which is more often than not a feature used in inventory yield analysis.
  • PLOTSAFE does allow user to collect prism point samples “tree tallies” for secondary plots.   This information is available to the user via the export to CSV option and is stored at the plot level.   This data does not get exported to YTF population files.   Users can then manually apply BA adjustments based on their own needs.