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Development of the Douglas-fir Productivity Spatial Surface using the Process-based model 3-PG

By Dean Meason, Barbara Höck, Yue Lin, Priscilla Lad, Marie Heaphy , August 2017.

Download SWP-T038 (pdf)

Executive summary

The objective of this report was to provide a New Zealand-wide productivity surface for Douglas-fir. This output addresses limitations of an earlier study of Douglas-fir spatial productivity that was based on very a limited number of sites.

Key Results

The major results of the study were:

  • Douglas-fir permanent sample plots (PSP) providing a comprehensive coverage of New Zealand’s Douglas-fir growing areas were sourced and refined into a set of representative measurements for spatial growth modelling (calibration) and verification of the model (validation)
  • Daily climate data matching PSP measurements were successfully modified and integrated into the model
  • Allometric equations for stem and foliar mass by stem diameter at breast height (1.4m, DBH), and the wood density by age equation were successfully updated
  • 3-PG was successfully parameterised using data from 32 sites from 23 locations throughout the country with an average mean DBH error of 10% across these sites.
  • The Scion parameterised model was tested against a 25 site validation dataset from throughout the country and had an average mean DBH error of 38% across these sites. However, the majority of this error was due to the poor performance of the model for two sites.
  • The Scion parameterised model was an improvement to the Waring et al. (2008) parameterised model.
  • A Douglas-fir productivity surface was successfully developed using 3PG2S showed higher productivity areas in the Nelson/Marlborough, Southern Canterbury, Taranaki and northern Gisborne regions.
  • The Scion parameterised model provides a good foundation to understand and quantify site sensitivities for New Zealand grown Douglas-fir and to quantify any gap between actual and potential productivity.
  • On site data collection and development of an independent site fertility index is required to improve the accuracy of the model.

Implications of Results for Client

This project has provided SWP members with a fully parameterised and integrated process-based model for Douglas-fir. The model provides precise spatial information of Douglas-fir productivity throughout New Zealand and a range of management scenarios can be tested without the costlyestablishment of a large number of PSP’s and silvicultural trials. 3-PGS2 is flexible enough to easily incorporate any improvements in data, especially accurate soil characteristics and soil fertility data.

The GIS-capable output allows SWP members to easily integrate the Douglas-fir productivity surface into existing GIS data management systems and into Google Earth. This allows productivity data to be related to locations of interest.


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