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GxN hybrid clones A wise investment for

high-potential, mid-altitude sitesIain Thompson, Tree Breeder

Hybrid clonal forestry is an vital component of NCT’s breeding strategy. The importance of a hybrid clonal breeding programme to the Co-operative was identified a decade ago and in 2002 an agreement was signed between NCT and the CSIR to develop new hybrids. This was undertaken to increase the quality of product supplied to NCT’s customers and as such, the programme was named Project Pulp.

Existing CSIR E.grandis X E.nitens (GxN) and E.grandis X E.urophylla (GxU) hybrids were also made available to NCT’s members through a license agreement between the CSIR and NCT.

Although early progress was slow, with much of the material not sufficiently tested over the NCT growing area, Project Pulp is running smoothly and has grown into a large programme with over 50 trials covering a range of site types.

Many of these trials are young, but NCT’s result database improves on a monthly basis. This information is crucial to understanding where to plant particular clones. Information on disease, frost and wood properties is slowly being built up for each of NCT’s commercial clones.

GxN hybrid clones are selected to combine complementary characteristics of E. grandis and E. nitens species and are best suited to high potential, mid-altitude sites, exposed to light snow and moderate frost risk.

E. grandis is historically a very important pulpwood and pole species due to its rapid growth and well-studied wood properties. Unfortunately E. grandis is very sensitive to snow damage and is thus commercially limited to areas that have no or

very little snow risk. In contrast, E. nitens, which exhibits good snow

tolerance, has been widely planted in the cold, high-altitude forestry areas of South Africa, but is limited to cold areas because of its susceptibility to Mycosphaerella leaf spot disease which is more prevalent in lower, warmer areas. Therefore GxN hybrid clones have been identified, by NCT, as a possible replacement to E. grandis plantings in an altitude range of 1 000m to 1 450m.

These hybrid types offer better growth rates, improved pulping properties and elevated cold, frost and snow tolerance when compared to E. grandis. They also offer advantages over E. smithii and E. dunnii as they are currently preferred by pole producers.

In order to be economically viable, a GxN hybrid clone must perform significantly better than E. dunnii and E. smithii as these species are propagated from seed and are cheaper to propagate than hybrid clones propagated through rooted cuttings.

E. dunnii and E. smithii seedlings are widely-planted on sites suitable for GxNs, but have been characterised as slightly and moderately snow tolerant respectively, therefore some GxNs show better growth and better snow tolerance than their pure species alternatives.

With the risk of heavy snow events predicted to increase, GxN hybrids could tolerate the effects of climate change better than other pure eucalyptus species currently grown in the mid-altitude, summer rainfall regions of South Africa.

The increased growth rate of GxN clones is

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Snow damage in Eucalypt compartments

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often overlooked, but GxNs out-perform their pure species counterparts in 95% of NCT trials planted on high potential, mid-altitude sites. One trial out of 20 has not been topped by a GxN commercial clone and this trial was planted above 1 500m and the best treatment was E. nitens.

NCT currently has the data to support the argument for increased growth rates from GxN, however meaningful snow tolerance data is much rarer. However, one trial has yielded some snow data in the last two winters. The trial is planted on Carl Seele’s (former NCT Chairman) farm and is situated at an altitude of 1 460m.

Snow in the winters of 2011 and 2012 (when the trial was just under two and three years old respectively) caused enough damage to show clear and significant differences between clones and E. dunnii and E. smithii treatments (figure 1). Four of NCTs currently recommended GxN commercial clones were present in the trial and all four significantly outperformed the seedling controls with regard to snow damage.

The snow damage observed in this trial indicates that most commercially available GxNs are more tolerant to snow damage than both

E.dunnii and E.smithii. This is an important result as both these species are grown above the KZN snow line described by the ICFR.

However there are also significant differences in snow tolerance between GxNs, with some of the old company controls not performing as well as the newer NCT/CSIR clones.

The unpredictable and complex nature of snow damage makes the results from this trial site very valuable, but continued monitoring of this trial and others like it is important to increase the knowledge base of snow tolerance within the GxN hybrid type.

Based on results from current NCT trials, GxN hybrid clones show better growth rates, a better market diversity and better snow tolerance on high potential, mid-altitude sites, than currently recommended pure species.

However, significant differences exist between GxN clones and care should be taken when choosing a clone for your particular site. NCT now has a growing database on each recommended clone and this information can be accessed through NCT or through one of their affiliated clonal nurseries.

Figure 1: Percentage snow damage from two consecutive winters of snow events (GN A, B and C are company GxN controls).

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Figure 2: Young GxN clones growing in a commercial block.

Figure 3: GxN cuttings in a rooting facility at one of NCTs affiliated clonal nurseries.