Modelling the effects of genetic improvement on radiata pine wood density
- Mark O. Kimberley,
- John R. Moore and
- Heidi S. Dungey
New Zealand Journal of Forestry Science
Density is a key wood quality trait, which is moderately to highly heritable, and has been the focus of selective breeding efforts in radiata pine (Pinus radiata D.Don) in New Zealand. Forest managers require information on realised gain in wood density in order to help them make decisions about which tree stocks to plant, how to manage stands and when to harvest in order to achieve certain wood quality outcomes. The aim of this study was to quantify realised genetic gain in radiata pine wood density and to incorporate it into existing modelling systems for predicting growth and wood quality.
A national model of radiata pine wood density, which predicts wood density at breast height as a function of ring number from the pith and a "local parameter", was modified to account for the effects of genetic improvement. The value of this local parameter was estimated for 679 radiata pine families with differing levels of genetic improvement (as quantified by their GF Plus rating for wood density) that were growing in 18 trials established by the Radiata Pine Breeding Company. The value of Wood Density Index (defined as the breast height outerwood density at age 20 years) was calculated from the estimate of the local parameter. Simulations were performed to show the impact of genetic improvement on whole-log average density and the variation in density within a log.
There was a strong positive relationship between GF Plus rating for wood density and Wood Density Index (R 2 = 0.73), with a one-unit increase in GF Plus corresponding to a 2.16 kg m-3 increase in wood density. An increase in GF Plus density rating from 18 to 27 would result in an increase in breast height outerwood density at age 20 years of 18.5 kg m-3. Over the same range of genetic improvement, average whole-log density is predicted to increase by 14-16 kg m-3. Validation of the model using independent data from older trees showed that it was able to correctly predict the effect of genetic improvement. It also indicated that the assumption that the expression of genetic gain is constant across sites with different wood density potentials is valid, although data from additional trials located across a wider range of sites is required to confirm this.
The effect of genetic improvement on wood density has been quantified and included in growth and yield modelling systems. This enables forest managers to estimate wood density in radiata pine plantations for any site and management regime established using tree stocks with a specific wood density rating.
In New Zealand, management of radiata pine (Pinus radiata D.Don) forests has shifted dramatically over the last 90 years. Un-thinned stands that were established in the 1930s were typically grown at high stand densities and on rotations of 45 years or more. However, economic analyses undertaken during the 1960s showed that alternative silvicultural regimes comprising early pruning coupled with pre-commercial thinning could provide very good economic returns (Fenton and Sutton 1968). This resulted in stands being grown at wider spacing, with heavy thinning to concentrate growth on a smaller number of trees. Rotation lengths were shortened considerably from 45 down to 30 years and, in some cases, 25 years (James 1990). It was recognised at least as early as the 1970s that these changes in silvicultural practices would give rise to trees with a higher proportion of corewood, larger branches in the upper un-pruned logs and a reduction in wood density (Burdon et al. 2004; Cown 1992; Harris et al.1976). Selective breeding to improve wood density was recommended as one means to compensate for the reduction in wood quality associated with changing silvicultural practices (Harris et al. 1976).
New Zealand's radiata pine tree breeding programme started in the 1950s when the first plus tree selections were made (Burdon 2008; Carson 1996; Jayawickrama and Carson 2000). The long-standing goal of this programme has been the production of large, fast-grown and well-formed logs, which gave rise to the growth and form (GF) breed (Jayawickrama and Carson 2000). Early radiata pine tree breeding efforts did not focus on wood density or other wood quality traits even though wood density was highlighted as being an important property due to its relationship with a number of other solid wood properties such as stiffness, strength and hardness, as well as with pulp yield and several paper properties (Panshin and de Zeeuw 1980), and it was known that density is a moderately heritable trait (Jayawickrama 2001; Wu et al. 2008; Zobel and Jett 1995). The focus was primarily on growth and form traits until the 1970s (Jayawickrama and Carson 2000), and the adverse genetic correlation that exists between wood density and growth (Jayawickrama 2001) would have contributed to a reduction in wood density. Only since the 1970s has the importance of wood density been recognised and selective breeding been undertaken for this trait (Burdon2010; Dungey et al. 2009; Jayawickrama and Carson 2000; Wu et al. 2008). Density is now a key trait in New Zealand's radiata pine breeding programme and is included in the GF Plus rating system, which was developed in 1998 by the Radiata Pine Breeding Company (RPBC), Rotorua, New Zealand.
The GF Plus rating for wood density is derived directly from breeding values (an explanation of the system can be found at http://rpbc.co.nz). Ratings are given to seedlots based on the contribution and breeding values of parents represented (Scion, unpublished data). Breeding values are derived from phenotyping individual trees in progeny trials in New Zealand and southeast Australia. An example of methods and types of analyses undertaken for breeding value estimation can be found in Cullis et al. (2014). Under the GF Plus rating system, higher GF Plus ratings indicate greater genetic merit for wood density. However, the relationship between GF Plus rating for wood density and the realised values of this trait for a tree of a given age, growing on a particular site and under specific silvicultural regime has not been quantified. Such information is of use for forest managers in determining the level of genetic improvement needed to produce logs that meet certain wood density thresholds. This may enable structural quality logs to be produced on sites that are currently considered marginal for producing such material. In this paper, we modify an existing model of intra-stem variation in radiata pine wood density (Kimberley et al. 2015a) to include the effect of genetic improvement and show how it can be incorporated into a decision support tool that can be used by forest managers to evaluate different options around planting stock.
A dataset containing wood density values from families with known GF Plus wood density ratings was supplied by the Radiata Pine Breeding Company. This dataset included information from 18 separate genetics trials that were mainly single-tree plot designs (Table 1). All but one of the trials were located in the North Island of New Zealand, with the remaining trial (FR203_2) located in New South Wales (NSW), Australia. These trials included 679 unique families with many of the families established at more than one trial site. Mean values of wood density for each family were calculated from measurements made on at least 30 trees per family (25,091 individual trees in total). Wood basic density was generally determined for each tree based on a 5-ring core sample taken at breast height (7- or 8-ring core samples were taken at three sites). These cores were mostly collected from rings 1-5 or 6-10 from the pith (Table 1), and this information was used to estimate breeding values and GF Plus density rating for each family (Radiata Pine Breeding Company, unpublished data).
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Source: New Zealand Journal of Forestry Science
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