Assessment of crop and tree species growing potential using climate, soil and topographic information

Climate and soil information is not measured directly at every point in New Zealand, so these variables need to be estimated in between the sites at which data are observed.

In general, a statistical technique is used for filling in the gaps between locations where data are available, using geographic variables such as location (latitude and longitude) and topographic information, such as elevation above sea level, to aid the interpolation.

Specifically for climate data, a thin-plate smoothing spline interpolation is used, where a smoothed surface is fitted to the data with some error allowed at each data point. In areas where there are large distances between measurement sites (e.g. in high elevation areas) the uncertainty in the estimation is likely to be highest.

Climate, soil and topographic factors, which are known to be of importance for specific crop or tree species growth, are identified from detailed growth studies performed in New Zealand and abroad by experts in plant and tree sciences. Each growth factor is broken down into three categories: optimal, marginal, and not suitable. An example is shown in Table 1, for Blackwood.

The overall suitability of an area to a particular crop or tree species is assessed using a simple additive model. Firstly, the three suitability categories (optimal, marginal, and not suitable) are assigned an integer value of 2, 1, and 0 respectively. Maps of these categories for each growth factor can then be produced (e.g., see the small maps in the Blackwood example below). The overall suitability index is then the sum of the individual growth factor suitability scores. In the case of Blackwood, there are seven important growth factors (see Table 1) so the maximum value of the overall suitability index is 14 (each of the seven factors is optimal, i.e. 7 x 2 = 14). The lowest possible index value is zero. Areas with suitability index values from 11 to 14 can be considered reasonably well suited to growing Blackwood (e.g., see the suitability map below, produced for the Gisborne District on behalf of Ngati Porou Whanui Forests Ltd).

Some growth potential maps have a "global limiter". This means that for a particular factor, areas that are deemed "not suitable" are directly translated as "unclassified" in the overall growth suitability map. An example of this is on the Blackwood growth potential map, where land above 350 m is deemed unsuitable for growth, regardless of the other growing conditions.

Projections of climate change can also be used to produce future growth potential maps. The methodology is exactly the same as that described above, whereby the individual parameter suitability scores are added together to produce a combined suitability index map. While many of the growth requirement maps remain unchanged (e.g. those to do with topographic exposure), changes to rainfall and temperature based growth requirement maps will influence the overall suitability.

It is advisable to look at the growth potential maps for broad areas which show "good potential" (in the top third of the suitability rating scores). Consider which topographic, soil and climatic factors limit the crop in these areas (from the individual growth factor suitability maps), and how you are placed to deal with these limitations.

Information about relevant markets, costs, labour requirements, available infrastructure, processing requirements, transportation options and general information about "growing the crop" is also needed, in addition to an assessment of growth potential based on the climate, soil and topography. Growers’ associations and industry groups often have extensive information about practical methods and scientific research, including networks of other growers or consultants you can approach for more advice. Following that, once you have researched all that you can, if you still have particular problems or questions regarding a certain tree species, or a specific growing arrangement on your property, you might consider approaching a professional consultant.

Due to the large number of maps that are typically produced in a project like this, it is highly desirable to have a tool which helps the user easily manage the information. In collaboration with GeoInfo Ltd., we have developed such a tool which enables the user to zoom in to a particular location, step through all the available maps, print out maps of interest, easily move to another location or zoom back out to the full map extent, and easily link to the project report (which includes all the supporting information on growing the particular crops).