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Optimising new PSP locations for durable eucalypts

By Justin Morgenroth, September 2018.

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Executive summary

As part of an effort to develop a forest industry based on durable hardwood eucalypts in New Zealand, a network of sample plots for numerous species has been established throughout the country. Two species of special interest are Eucalyptus bosistoana and Eucalyptus globoidea. There are 158 permanent sample plots (PSPs) established in seven trial sites for these two species. These PSPs have provided valuable data for growth and yield modelling and for examining the relationship between environmental conditions as trial sites and the species (i.e. site-species matching). However, the existing PSPs do not cover the entire range of environmental conditions in New Zealand, nor do they cover the range of conditions the species are capable of growing in. Hence, the PSP network could be strategically expanded into new areas. This research combines habitat modelling and stratified random sampling approaches to build a GIS-based habitat model for a strategic expansion of the current PSP network.

The method applied in this study consisted of three main stages. In the first stage, the study selected 17 model variables in three categories (i.e. topography, climate, and soil), defined their importance, and tested them for multicollinearity. For each model variable, data were collected for three geographic zones: (1) the native habitat of the species, (2) all of New Zealand, and (3) the existing PSPs. The second stage was to process data to build the model. The third stage applied variable restriction and stratification analyses to calculate a priority index for the complete study area. This index represented the priority for establishing new PSPs in the study area, based on under-represented environmental characteristics.

By assessing the priority index the results highlighted: (1) over-represented areas (i.e. high environmental similarity, where the environmental conditions have been described by many PSPs in the existing network), and (2) under-represented areas (i.e. high environmental dissimilarity, where the environmental conditions have been described by no or few PSPs in the existing network). The results suggest that new PSPs for the two species should be established in Rangitikei District and Taupo District. Other high-priority areas include Northland and Auckland regions, the East coast of the Gisborne region, and southeast-facing hillsides of the mountain chains in the central South Island.


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