Adverse Genetic Impacts Of Silviculture On Wood Quality

Nowadays, plantation forestry is an imperative to meet wood requirements efficiently on the finite land available for wood production. And besides, the adverse effects of silviculture on wood properties tend to be accompanied by heightened expressions of genetic variation in wood properties. By Rowland D Burdon and John R. Moore, New Zealand Forest Research Institute

The need for plantation forestry, to meet global needs for wood supplies and various environmental services is well documented. 

Profitability must nonetheless be addressed. Three main determinants of it are productivity in terms of the amount of wood grown per unit time, price per unit volume of that wood, and harvest age through earlier harvests reducing the effective growing costs. 

While profitability of forest plantations can be assessed through measures such as ‘stumpage’ (harvest revenues less harvesting and transport costs), two other measures are widely seen as more rigorous. 

These are Net Present Value (NPV) (for a given discount rate) and Internal Rate of Return (IRR). These latter measures account for the time cost of money and the time taken between incurring costs and realising revenues. 

Therefore, harvest age can have large influences on NPV, particularly at high discount rates, and on IRR. Intensive forest management has been used as a means of raising forest productivity. 

Productivity is widely equated with the volume of stemwood produced per ha per annum. This criterion, may suffice for producing appearance-grade solid-wood products. However, it will often give an inflated measure of relative difference in stem biomass production, which will be more relevant, say, for pulp production and reducing rotation length. 

The main interventions that can raise productivity fall into two broad categories, namely genetic improvement and silvicultural measures. 


Improvement On Genetic

Genetic improvement involves selective breeding, delivery systems for genetic gain, and customised deployment of improved breeds or clones to sites and silvicultural regimes. 

Also, genetic improvement is directed at all determinants of profitability, and the appropriate emphasis on individual determinants depending on the particular context. Regarding silvicultural interventions, good plantation establishment is viewed as a basic precondition. 

The scope for genetic improvement in any one trait is governed by the heritability multiplied by the coefficient of variation (CV). 

In radiata pine plenty of scope exists for genetic improvement in productivity and wood properties, although there are constraints on the genetic gain achievable on both fronts. For growth traits the scope is dominated by high variability, with correspondingly high CVs, which compensate for typically low to moderate heritability. 

The expectations have been borne out by demonstrated gains in productivity from genetic selection. Also, dramatic genetic improvement was achieved, even from initial plus-tree selection, in tree form and general log quality; this had the flow-on benefit of averting the requirement for high initial planting densities, with reduced planting and tending costs. 

Such gains, along with genetic gains in growth rate, have led to widespread uptake on the part of industry; nurserymen and forest growers have been paying price premiums for genetically improved seed and nursery stock, the premiums depending on the level of genetic improvement. Continuing genetic gains can be expected. 

However, their magnitude in per-hectare yields is currently uncertain, because faster individual-tree growth rates will accelerate between-tree competition and the approach to crop ‘carrying capacity.’ Inherent productivity will need to be defended on at least some sites, often by breeding for disease resistance, and yet, genetic gains in resistance or tolerance will tend to be achieved at the expense of some potential genetic gains in productivity for disease-free situations.

Wood properties generally show moderate to high heritability. This can translate into much larger CVs for wood-performance traits like modulus of elasticity (stiffness) and modulus of rupture (strength). 

However, despite the scope for genetic improvement of wood properties, emphasis on their genetic improvement came relatively late compared with improvements in growth traits. 

This resulted from a combination of factors, notably unwillingness of industry parties to accept sacrifice of potential gains in wood volume production when there are few if any current price signals in the market to warrant this, and the difficulties and costs of assay for some important wood properties. 

But with a reduction in rotation age and aggressive thinning regimes, a general decline in wood properties became a major concern, while assays for wood properties have become much cheaper and more powerful, so in recent years tree breeders have sharply increased the emphasis on the genetic improvement of wood properties.


Genetic Parameters & Their Implications

The genetic parameters include the genetic and non-genetic variances that determine the heritability of individual traits, and corresponding between-trait covariances that contribute to genetic and non-genetic correlations. 

Given good information on the genetic parameters and on the comparative economic worth of genetic changes in breeding-goal traits, it is in principle possible to optimise the comparative emphasis on selection traits. 

Also, one can in principle predict, for a given intensity of selection, genetic gains in various traits and the aggregate value of the genetic gains. In practice, the tree breeder faces much uncertainty about genetic parameter values and economic-worth functions. 

Genetic correlations are often estimated very imprecisely, and can be appreciably site-specific, while comparative economic weights among breeding-goal traits can be very uncertain. 

Especially troublesome is a combination of adverse between-trait genetic correlations and very uncertain economic-worth functions for individual traits; this can even mean genetic ‘gains’ that are of negative financial worth. 

In practice, real gains will tend to be less than predicted ones because selection is always based on imperfect information on inheritance of traits. 


Impacts Of Silvicultural Interventions

The impacts of silvicultural interventions on wood quality—indeed, more generally on quality-related traits—in radiata pine are quite well understood in qualitative terms. 

However, quantifying the detailed relationships is challenging, and doubtless hampered by the fact that the quantitative relationships can be specific to individual situations, so it has generally not been achieved. 

But one can safely say that the impacts are often large, involving some highly adverse effects of interventions that boost site productivity and/or reduce harvest age. 

The negative effects tend to be accompanied by exaggerated expression of genetic variation in traits, in what is termed level-of-expression (LoE) genotype-environment interaction.

Incidentally, for traits that must be assayed by visual scores rather than in absolute values, genetic gain predictions in the presence of LoE interaction may best be made by varying economic weights according to LoE. 

Overall, despite large gaps in quantitative data, the general implications of silvicultural impacts on wood quality for tree breeding are clear.

Accordingly, our focus is on boosting soil fertility during the main period of stand growth, manipulation of stocking, and the impacts of reducing harvest age.


Boosting Productivity Through Fertiliser Treatment

The scope for such a major boost in productivity by manipulating soil fertility is suggested by the ‘farm site effect’ observed in New Zealand, with yields of radiata pine being 20–40 percent higher on ex-pasture sites with elevated soil fertility compared with sites that had carried forest or shrubby vegetation. 

Such levels of productivity generally much exceed those achieved in fertiliser trials on forest sites. The reasons for the full farm site affect are unclear, although they presumably involve the soil microbiota and soil carbon forms. 

But whatever the causes of this effect, its magnitude creates a strong incentive to find whether it is possible to mimic it and how to do so sustainably, provided its ill-effects on tree form and wood properties can be minimised or offset. 

The evidence suggests that the farm site effect on productivity declines percentagewise with stand age. Maclaren and Knowles calculated a decline in the boost in predicted stem volume production from 33 percent at age 21 to only 13 percent at age 31, on a farm site that ‘was not extremely fertile’. 

Aside from possible estimation errors, it is not ascertained how much this decline resulted from a ‘wearing-off’ of the elevated soil fertility or was an effect of the architecture of crop development on the impact of elevated soil fertility at different stand ages on stemwood volume production.

While increases in productivity resulting from boosted soil fertility can be major, they tend to be at the expense of wood properties and other quality-related traits such as stem straightness, knot size and freedom from malformation. 

Indeed, there is good reason to believe that such fertility would be associated with more reaction wood, much reduced stiffness, and poorer dimensional stability, and more internal checking. 

Such soil fertility also adversely affects tree-form traits, further accentuating the importance of quality-related genetic selection criteria. Moreover, all these quality-related traits are notoriously degraded by exposure on fertile sites, creating even more call for genetic selection to defend wood quality.


Different Level Of Stocking Control

Low stocking strongly favours individual-tree diameter growth and thus earlier attainment of target log-size specifications. 

However, it can militate strongly against wood quality in both increasing percentage of corewood and having more direct effects on wood-quality components, an exception being that low stocking allows the benefits of pruning to be fully realised. 

Also, by reducing productivity, low stocking levels will tend to reduce carbon sequestration, although insofar as they allow earlier harvests they bring forward the dates of carbon sequestration through wood in service. 

Low stocking, along with elevated site fertility, not only tends to depress wood quality, but it can also inflate the expression of among-tree genetic variation in quality-related traits. Also, the adverse effects on productivity of low stocking can increase the size and severity of timber defects through ill-effects on tree form. 

As mentioned earlier, a notorious but largely undocumented effect is the disastrous form of trees with the long-internode branching habit on sites that are both exposed and highly fertile. 

In such situations the productivity gains resulting from the fertility can be eroded by increased logging waste caused by stem malformation.

The impacts of high stocking on various aspects of wood quality, however, are more complicated, and may vary according to local conditions. 

High stocking has been repeatedly associated with higher wood stiffness (modulus of elasticity), largely independent of effects on corewood percent. 

Despite enhancing wood stiffness, high stocking can lead to higher percentages of corewood, although such effects on corewood percent seem generally minor. 

In one word, the results of Cown and Dowling strongly indicate that the benefits of higher stocking on wood stiffness outweigh any downside in outerwood percentage. That said, higher stocking will tend to reduce piece size and the thickness of clearwood zones achievable by pruning. 

Both the overall piece size and the relative volume of clearwood affect the recovery of clear lumber in a mill.


Length Of Rotation

Economic attractions can exist for harvesting even before the culmination of mean annual increment, although they may be less for the forest estate than for individual stands. 

For most wood properties and end-products longer rotations are advantageous in radiata pine, except when they incur excessive heartwood. 

Heartwood in radiata pine is neither reliably durable nor reliably treatable with preservative, and often adversely affects appearance.

Some ulterior justification for longer rotations can arise in increased carbon sequestration. However, shorter rotations are typically very prejudicial for wood properties, leading to greatly increased corewood proportions.


Effects Of Maturation 

There is some possible scope for defending tree form significantly by deploying propagules with some maturation (‘physiological ageing’). 

Some maturation may also help defend some wood properties, through reductions in microfibril angle and compression wood incidence, which should mitigate longitudinal shrinkage and help maintain stiffness, but would tend to accentuate spiral grain angle. 

However, too much maturation makes mass-propagation too difficult and expensive, and reduces vigour in successfully rooted propagules. Exploiting the advantages of maturation formed part of a proposal to institute a ‘Farm Site’ breed of radiata pine which, however, did not get implemented.

A relatively subtle effect of harvest age arises in addition to effects of maturation state of the planted propagules. It does so because wood properties in relation to ring number from the pith can vary with height from the ground [9], according to how maturation state increases with height. 

With very short rotations, the proportion of juvenile wood, which occurs close to ground level, might be appreciably elevated. This could contribute further loss of in stiffness, and possibly poorer dimensional stability, but a lower spiral grain angle and lower fibre coarseness.


Combined Effects & Synergisms

The effects of site and silviculture can be strongly reinforcing, even synergistic, although such effects may be very case-specific. The pathways for the effects on wood properties are various. 

Both elevated fertility and lower stocking levels can undoubtedly have very adverse effects on tree form (affecting knot sizes and grain distortion), wood stiffness (through more reaction wood and more directly), and dimensional stability in drying and in service (also through more reaction wood and more directly). 

These effects reinforce the adverse effects of the reductions in harvest age that they can bring. Importantly, the adverse effects of silviculture on wood quality appear to be generally accompanied by heightened expression of genetic variation in quality-related traits. 

Indeed, the effects of genetics and silviculture may be synergistic, representing second-order level-of-expression interaction among genotype, site and silviculture as specified. 

Anyway, the heightened need to defend wood quality in the presence of joint influences is fortunately accompanied by the enhanced scope for genetic selection to defend it.


Silvicultural Practices and Adverse Genetic Impact On wood Properties

In conclusion, the emphasis has been on needs for structural, solid-wood products, which include plywood and laminated products as well as conventional lumber. 

These products are of key importance in the utilisation of radiata pine wood, and are subject to consistent influences from adverse genetic correlations with growth-rate variables and various effects of intensive silvicultural practices. The effects on wood properties and their putative pathways of action apply primarily to solid-wood products. 

With appearance-grade products, which can command quite high prices, the situation is more complicated. For them, some adverse genetic correlations are involved with branching habit and growth rate, but mechanical stiffness is not crucial, although dimensional stability is important and subject to some of the same influences as stiffness. 

The implications for productivity and product quality in relation to stocking and harvest age are also more complicated. 

The effects on wood properties, and their putative pathways of influence, apply primarily to solid-wood products. While a significant portion of the crop is pulpwood, going into pulps and reconstituted wood for which corewood can have some advantages, pulpwood in New Zealand is quite a minor proportion of the radiata pine crop and fetches essentially by-product prices.

Adverse genetic correlations between wood properties and stem volume production appear to be almost pervasive in radiata pine. 

They, therefore, limit the genetic gain that is simultaneously achievable on both fronts, requiring proactive selection even to defend against drops in wood quality. 

At the same time, some key influences of silvicultural measures to boost productivity and reduce harvest age are highly prejudicial to wood quality, through both increasing the corewood percentage and other, more direct effects on some wood properties. 

The combined effects of adverse genetic correlations and net silvicultural influences place the tree breeder under heavy pressure to pursue genetic improvement in wood properties, even at the expense of considerable potential genetic gain in productivity. The breeder’s work will need to be complemented by judicious deployment of improved breeds or clones to sites and silvicultural regimes.


New Future Technology

There will be more new technologies applied in the future. On one side, genetic selection has been progressively refined, by more sophisticated algorithms for using phenotypic information on not only the selection candidates but also their relatives. 

The phenotypic information originally represented field measurement data combined with visual ratings, but has become increasingly supplemented by laboratory-based determinations. 

Nowadays there is increasing pursuit of genomic selection, based upon whole-genome scans, which can allow provisional evaluation of candidate genotypes in advance of phenotypic information. 

Other genomic applications, which include pedigree analysis, will doubtless be useful for radiata pine breeding. In addition, remote-sensing technologies offer the prospect of obtaining very large volumes of field phenotypic data at modest cost. 

However, such developments do not really alter the challenges posed by the genetic trade-offs between productivity and quality-related traits. 

There is some prospect, though, of genomic selection offering earlier and better identification of so-called correlation breakers.

On the other side, the topic is addressed essentially in the context of the current level of domestication of radiata pine. 

For the longer term, there are ways in which domestication might be much intensified. For instance, it is likely that net productivity can be lifted by conferring reproductive sterility. 

This would also remove an important objection to the use of either genetic engineering through effectively containing transgenes or any other DNA sequences or combinations involved in creating Genetically Modified Organisms. 

But new technology for genetic manipulation and propagation systems, while it may provide more powerful tools, is not seen as fundamentally changing the issues surrounding the corewood phenomenon in conjunction with the pressures to grow short-rotation forest plantation crops.

As mentioned earlier, the context of genomic selection may facilitate selection of ‘correlation breakers’ (strongly favourable deviants from adverse genetic correlations) by identifying genomic regions where pleiotropic effects are minimal. 

However, the consistency of between-trait genetic correlations across all populations of radiata pine indicates that the genetic correlations predominantly reflect pleiotropy rather than just the chromosomal linkage that is more readily overcome.

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