BREEDING TRIALS: ACHIEVING GENETIC GAIN

Base breeding populations of our selected species have been established on 13 contrasting trial sites across NZ’s eastern drylands regions. These trials are the foundation of our tree improvement programme.

Large breeding populations ensure that a broad range of genetic diversity is captured from which to select multiple traits for improvement.

Breeding trials: species selection

The focus of NZDFI's genetic improvement work has been on two species:

• E. bosistoana              Coast grey box
• E. globoidea                White stringybark

Other species included in our breeding work:

• E. quadrangulata       White topped box
• E. argophloia*             Western white gum
• E. tricarpa                    Red iron bark

* E. argophloia was eliminated from the breeding programme in 2023 following a review of all  species' performance across our trial network.

Breeding trial sites – location and establishment

Between 2009 and 2012 NZDFI established base breeding populations of these species across ten sites. In 2016 and again in 2021, a further three breeding trials were established. Over 90,000 trees have been planted to ensure that broad-based selection of elite trees is possible. The key species are:

• E. bosistoana            254 families, 44,052 seedlings
• E. globoidea              161 families, 26,640 seedlings
• E. quadrangulata     104 families, 15,576 seedlings

We also planned to research and develop inter-specific hybrids - hence the more limited plantings of E. argophloia and E. tricarpa.

The 13 different sites cover the range of abiotic (cold, drought, soils etc) and biotic (pests, diseases) stress factors likely to be found in New Zealand’s north-eastern drylands. This range of sites allows analysis of the stability of superior genotypes under contrasting environmental conditions by identifying the relative contributions of genetic and environmental effects.

In 2023, an evaluation of species performance in our trials led to the decision to drop E. argophloia from the breeding species list. We have yet to make progress with E. quadrangulata as so far it has not been seen flowering in any of the trials.  In future E. tricarpa (red ironbark) and E. macrorhynca (red stringybark) may be candidates for our genetic improvement programme.

Breeding population design

Each breeding trial is planted in separate blocks of each species. The individually labelled trees for each family are laid out in a randomised design. Trees are planted at 2315 stems per hectare (1.8m x 2.4m). Each species is replicated at three different sites.

This design ensures control of environmental variation at site level, and enables between- and within-family selection. Permanent sample plots (PSP’s) have been established for measuring productivity and developing growth models.