Chapter 10

Transformations to Multiaged Stand Structures

10.1 Introduction Transformations, or conversions, from even-aged to multiaged stand structures have

become a common objective for many forests. These transformations are driven by a variety of objectives. International accords are encouraging more diverse forests. For example, a greater emphasis on multipurpose forests was a directive of the Helsinki Process. Multiaged forests are also encouraged in some forest certification protocols (McMahon 1999, Hartsfield and Ostermeier 2003) or by forest laws (McGinley et al. 2012). Many plantations have little diversity in tree size, species, or stand structure so they may be targets for transformation. Other planted forests may be considered for transformation because they lack sufficient diversity or are desired as multipurpose forests. The explicit objective therefore may not be to develop multiaged or uneven-aged stands, but instead to develop stands with greater amounts of structural heterogeneity or complexity which, in turn, may provide other benefits for wildlife, aesthetic values, resistance to abiotic disturbances, or resistance to insects and pathogens. Another justification may be to simply avoid the unpopularity of clearcutting treatments (Kimmins 1993). Transformation is not new as might be implied by the events and trends noted above: Schütz (2001b) noted the long history of transformation in the forests of central Europe. As forestry has shifted from emphasis on even-aged or multiaged systems, there have been efforts to transform one system to the other for many decades.

Both “transformation” and “conversion” are commonly used to describe the process of directing even-aged stands towards multiaged structures. The term transformation will be used in this book because it implies a less complete total change in stand structure. This is consistent with the premise that a great many different silvicultural systems exist on a continuum rather than as a limited number of options or discrete categories. Additionally, the more subtle term “transformation” fits the concept that sometimes minor changes in management regimes can result in large changes in stand structure. The term transformation was also used in the title of the 1999 and 2004 conferences on this topic (Cameron et al. 2001, Pommerening 2006). Conversion is often used to describe management that directs changes in species composition such as the conversion of Norway spruce to mixed-species compositions in central Europe (e.g., Speicker et al. 2004).

The objectives for both transformation to more complex age structures, and conversions to more complex species compositions are driven by similar demands for greater structural heterogeneity or “naturalness” in stand structure. Hence both objectives are very compatible and may be pursued at the same time. Transformation treatments of even-aged to multiaged stands may not be greatly different than restoration treatments described in Chapter 11. Multiaged stands may be a “natural” structure in many places that have been traditionally managed as even-aged. Hence the transformation regime may be a form of restoration. Here the discussion is

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focused more narrowly on a specific type of restoration where even-aged stand structures are directed towards multiaged structures. 10.2 The Silviculture of Transformation

Forestry has traditionally held a narrow of view of the possible range of multiaged stand structures. Transformation can therefore be perceived as moving stands from a great many possible even-aged stands or stages of even-aged stand development to a relatively few multiaged options (Figure 10.1). Instead, there is a broad assortment of possible multiaged stand structures and many potential pathways for transformations. This broad assortment of target multiaged stand structures is compounded by a great diversity of initial structures or possible even-aged structures from which the transformation process may be initiated. Additionally, there may also be multiple options or pathways for transformation from one initial structure to the same target structure (O’Hara 2001). As a result, there are a great many variations on what a transformation might entail, and no simple procedure that is widely applicable to all of these situations.

The target stand structure is the structure that best meets management objectives. In the case of transformation, the target structure is a moving target because the multiaged structure changes over time as it moves through cutting cycles. The target structure is therefore a range of conditions that exist within the multiaged regime. A typical objective of any silvicultural regime is a sustainable stand structure. In the case of transformation, the objective of the target structure is a sustainable management regime over the long-term.

Management decisions should always be sensitive to the initial state of the forest. This is especially true for transformations. Kenk and Guehne (2001) described several initial conditions that might affect transformation in the Black Forest in Germany such as site conditions, susceptibility to storm damage, and root disease. Each of these is suggestive of a different transformation strategy. The constraints of existing stand structures or financial resources are likely to limit both the range of target stand structures and the speed of achieving these transformations. Land managers may therefore pursue simple or complex target stand structures for transformation. Simple multiaged structures might be two aged stands or two strata stands whereas a complex structure may have many age classes, canopy strata or species. Managers will require the freedom to pursue a variety of transformation strategies based on the initial stand structure, the target structure, and the available resources for the transformation. The availability of resources to support transformations will also determine outcomes: in situations with plentiful resources, intensive transformations may result, but if resources are limited, the transformation might be more extensive.

The many different possible initial states and target stand structures result in a multitude of potential alternative treatment regimes leading to great uncertainty, particularly at later stages of the transformation process. This is exacerbated by the general shortage of experience with transformation to multiaged stand structures. Adaptive management (e.g., Yousefpour et al. 2012) is therefore highly appropriate for transformation regimes. There needs to be a general

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acceptance that these transformations, particularly the more complex regimes, will have to be modified with experience and new information. Transformation regimes and adaptive silviculture should be compatible partners in the transformation process.

General multiaged management regimes consist of descriptions of the number and sizes of trees to cut, and the spatial patterns of trees and openings. Transformations are concerned with the same variables. However, transformations are typically presented as a sequence of treatments that direct the stand towards the desired stand structure. This desired structure may be simple or complex, and the sequence of treatments will usually correspond to this continuum of complexity by being short or long in duration. Transformation treatments are described here as a dichotomy between directing stands towards simple or complex stand structures.

10.2.1 Transformations to simple stand structures

The obvious first step in any transformation from an even-aged stand is to regenerate a new age class or cohort of trees. However, there may be reasons to proceed cautiously as is sometimes the case with shelterwood systems. With these systems, the shelterwood harvest may be preceded by a preparatory treatment to increase windfirmness of residual trees, develop advance regeneration, or enhance vigor and seed production potential of trees (Smith et al. 1997, Nyland 2002). A similar treatment may be useful as part of a transformation regime whether the intention is to create a relatively simple or a complex stand structure.

The shelterwood harvest treatment is often an excellent way to regenerate a second cohort and if the overstory or shelterwood trees are retained, a two-aged stand is the result. Similarly, a seed tree cut with residual or retention trees results in a two-aged stand. In eastern white pine, Wetzel and Burgess (2001) showed that a shelterwood can successfully regenerate white pine provided seedlings received sufficient light and a minimum of 50% of above canopy light (PAR) maintained sufficient seedling growth. Site preparation and weed control further enhanced growth of eastern white pine. Broadleaved species, such as European beech or lime can be planted beneath Scots pine stands in Germany to transform stands with minimal overstory treatment (Kenk and Guehne 2001). Retaining some Scots pine results in a two-aged stand and if they are removed, the stand has been converted from pure Scots pine. Malcolm et al. (2001) recommended irregular shelterwood and group selection systems for transformation of conifer stands in the UK. A variable retention harvest would also result in a two-aged stand. Regardless of what this initial treatment is called, the treatment needs to regenerate a new age class while retaining some of the previous stand.

The creation of canopy gaps or openings is another simple approach to transform an even-aged stand to a more complex structure. These gaps can instantly transform the structure and with regeneration, will result in a two-aged stand. This approach may be particularly useful with shade intolerant species. If the objective is only to achieve structural complexity, then gaps are a quick means to achieve this. However, if the objective is to also produce a consistent level of timber production while maintaining a two-aged structure, then a regulated approach to the entire stand would require half the stand be harvested in group cuts. This is probably excessive

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for most objectives. Gap creation may be best in a more complex stand management strategy where gaps can be spread out spatially and temporally in the stand. If a regulated level of timber production is not the objective, then creation of small numbers of gaps is a potential method of adding complexity in a relatively simple way. An alternative to simple multiaged stands are stratified, mixed species stands or simple modifications to even-aged regimes. Stratified mixtures are even-aged stands, but with stratified canopies because of variable growth rates of different species. Oliver and Larson (1996) discuss these types of stands in detail (also see Chapter 4.5.1). These even-aged stands may meet many of the objectives that are driving transformations. They provide stratified canopies, have mixed species compositions, and may be easy to manage. Hence they are a simple alternative means of developing a complex stand structure. Extending even-aged rotations can also enhance stand structural diversity and, at least if coast Douglas-fir, will result in only minor reductions in production (Curtis et al. 1995). However, these even-aged stands typically begin and end with a clearcut, which may be an overriding constraint.

10.2.2 Transformations to complex stand structures

Transforming even-aged stands to complex structures requires a more detailed sequence or system of treatments. These complex structures may include several age classes, multiple canopy strata, many species, or a full range of diameter classes. Achieving this complexity generally involves a sequence of treatments rather the single treatment that may be used to achieve a two-aged stand.

The first treatment is usually to regenerate a new age class. Nyland (2003) stated this point as “silviculturists should have little trouble envisioning how to start a transition, or even plan a second cut and what it should accomplish.” Nevertheless, there may be good justification to begin the process with a treatment that enhances the stability or regeneration potential of the stand. Older even-aged stands may be at high stocking levels and vulnerable to wind and snow damage. Advance regeneration that is necessary for forming the new cohort may not be present or of sufficient vigor.

Treatment sequences attempt to gradually direct the stand toward the target stand structure. There may be multiple options or pathways for achieving the target structure. For example, Nyland (2003) diagrammed two alternative management pathways for transforming a hypothetical even-aged stand to the same multiaged stand (Figure 10.2). These pathways may include treatments such as commercial harvests, non-commercial thinnings or improvement cuts, regeneration operations, or others. Kenk and Guehne (2001) described transformation regimes in Germany that included treatments such as precommercial thinning, underplanting, direct or artificial seeding, wedge cuttings to reduce wind damage, and Femelschlag treatments to release understory trees. The silvicultural “toolbox” for transforming even-aged to multiaged stands is quite large and may even include small clearcuts when wind damage is an issue (Heinrichs and Schmidt 2009).

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Stocking control is an important part of transformations. In a mixed oak – shortleaf pine forest in Missouri, USA, Loewenstein (2005) concluded that transformation from degraded, even-aged stands could be accomplished in three steps. The first entry might regenerate a second age class at age 30 by leaving an overstory density of 40%. At age 60, a third cohort could be regenerated by reducing overstory density to 30%, and the second age class density to 20%. Both of these age classes would be treated. The third age class would eventually be allocated 10% of the growing space. In this example, the target distribution of growing space or density in the transformed stand would be a 3:2:1 ratio among the three cohorts and would take approximately 80 years.

In mixed loblolly and shortleaf pines in the southeastern USA, Baker et al. (1996) described a 30-year, four step transformation process. They used stocking parameters from the BDq method (see Chapter 7.3.3) to guide the treatments (Figure 10.3). Whereas Loewenstein (2005) concluded the stocking guide for the fully transformed stand was inappropriate to guide the transformation process, Baker et al. (1996) used it in their hypothetical example. Both cases use a BDq method, but the differences were primarily related to rigidity with which the guide is followed. The basal area or “B” parameter guides total residual stocking at each entry, but the maximum diameter and q factor are only used later in the process.

Rehabilitation is a similar treatment as transformation in that it tries to move a stand from one structure another. However, rehabilitation is used move a stand from a degraded condition to an improved condition. Baker and Shelton (1998) used the term to describe a series of experiments in mixed shortleaf and loblolly pines in the southeastern USA where degraded stands were transformed into multiaged stands. On better sites, they showed that stands could be transformed back to productive, well-stocked multiaged stands in only 15 years. Their rehabilitation was evaluated based on an overall stocking target rather than a structural target. Nevertheless, these examples demonstrate that transformation may assume pathways from a variety of initial conditions to a wide variety of final stand structures. Treatment options are highly varied, and so are the tools used to guide transformations. 10.3 Sustainability and Transformation

When a stand is transformed, there is the general expectation that the new multiaged stand regime will be sustainable over the long-term. This means the new regime will successfully maintain the stand structure and the treatments necessary to maintain the structure will be feasible and environmentally sound. However, the transformation treatment itself is simply a transition exercise that need not be a sustainable regime in isolation from the intended multiaged regime that will eventually result. For example, some intermediate steps in the transformation process may not be feasible or the intermediate structures many not be sustainable.

Among the potential pitfalls of transforming even-aged to multiaged stands is the potential for using this process as a justification for removing only the large trees or a treatment analogous to traditional diameter-limit cutting (Chapter 7.3.1). In addition to the negative effects on value and tree size diversity in the new stand, these treatments may also have dysgenic effects

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where the best parent trees are removed (Chapter 14). If natural seedling regeneration is expected, then there is a potential for loss in genetic quality in the stand. The risk of dysgenic selection is reduced if vegetative regeneration is used or if a seed bank exists. However, if natural seed fall is needed, then the treatments may remove the most valuable trees, reduce the genetic quality in the stand, and also remove the best seed producers. An exception to this is described in Box 10.1where removal of the largest trees was expected to reduce production, but did not. In addition to the potential genetic effects and removal of more valuable trees, there is also the potential to adversely affect species composition by not only removing more desirable species, but also creating environmental conditions that favor the regeneration and persistence of these undesirable species (Kelty et al. 2003).

10.4 Constraints on Transformations

Tree stability is a major constraint affecting transformations from even-aged to multiaged stands. Tree stability is often represented with ratios of height to diameter (H:D in equal units) which is also a simple measure of stem taper. Trees will approach critical stability thresholds in either even-aged or multiaged structures if growing space is limiting (Faber and Sissingh 1975, Cremer 1982, Wonn and O’Hara 2001). Schütz (2001b) developed a flowchart to aid decision-making for stands with potential stability issues (Figure 10.6). Even-aged stands may exist at high densities where individual tree stability is low and the potential for successful transformation is reduced. An even-aged stand considered for transformation must therefore already have stable trees as it is difficult to regain stability once it is lost (Wilson and Oliver 2000).

Norway spruce stands in central Europe provide an interesting example. They are often maintained at high densities in even-aged stands resulting in relatively high H:D ratios. High H:D ratios may limit options for thinning treatments or any other intervention. Kenk and Guehne (2001) described forests in Germany where H:D ratios were higher in even-aged stands than in those managed with the Plenter system. A thinning or harvest treatment to regenerate a new cohort as part of a transformation may result in large amounts of tree loss to wind or snow if the even-aged stand density was high. This represents a major constraint on transformations in dense Norway spruce stands or other species where density is high (Mason and Kerr 2004). Tree stability can be improved, or the gradual increase in H:D ratios can be slowed with early and heavy thinning. In Norway spruce in the Czech Republic, heavy thinning was able to keep H:D ratios below critical thresholds thereby creating opportunities for transformation to multiaged structures or conversion to mixed-species structures (Slodicak and Novak 2006). A related concern is with inadvertent species conversions during the transformation process. In some ecosystems, the partial harvests that are used to regenerate new age classes of trees will create environments more suited to shade tolerant species that may be less desirable. In these situations more severe treatments or group openings may be necessary. Planting to encourage desired species may also be a useful strategy.

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Operational constraints may make it impossible or infeasible to achieve certain structural qualities. Tree spatial patterns of transformed stands may not fully match those of the natural stands they attempt to target. For example, a restoration objective may have a target stand structure that includes a particular spatial pattern of trees in addition to a target vertical stand structure. The difficulty of harvest operations on steep slopes may limit harvests to infrequent but relatively severe treatments. Specific spatial patterns may also be difficult to achieve. In the Black Forest of Germany, distinct differences in spatial patterns were evident between even-aged and uneven-aged stands, and stands undergoing transformation (Hanewinkel 2004).

There may also be limitations on transformation because of tree age. Once a transformation begins, there is generally an expectation that some residual trees from the even-aged stand will be retained for several cycles of the transformation process. Depending on the regime, this may require trees that can be retained for 100 years or more. Schütz (2001b) included tree longevity in his flowchart outlining the decision process of whether to begin the transformation process with the current stand or wait until the next (Figure 10.6). In mixed-species forests where species have variable life expectancies, such as the central hardwoods in North America, having trees of greater longevity provides more flexibility in designing the transformation regime (Loewenstein 2005). Tree longevity is therefore an important factor in making tree selections, or possibly a factor for selecting tree species during the transition process.

As with much of silviculture, economic constraints may also limit the options in transformation regimes. Some transformation regimes may occur at considerable financial costs to the landowner. Economic analyses of transformation regimes are complicated by the many assumptions that must be made. A transformation is assumed to move an even-aged stand to a multiaged stand. The age of the even-aged stand can be quite variable and this affects the financial returns from the transformation. For example, Knoke and Pluscyzk’s (2001) comparison assumed a Norway spruce stand at age 47 would be transformed. This provided some immediate returns for the transformation regime because of the harvest of merchantable timber that would be delayed in the even-aged regime. A similar analysis of a fully mature even-aged stand at the time of harvest would result in a different conclusion because the immediate returns of the clearcut harvest in the even-aged regime. Economic analyses of transformation regimes are therefore very difficult and the implications of their results are limited. Other discussions of economics of transformation regimes have been completed by Buongiorno (2001), Knoke et al. (2001), Tarp et al. (2005), and Knoke (2012).

A significant economic concern may be the relative costs associated with the new and old regimes when considered in their final form. For example, if the multiaged management system results in a less expensive natural regeneration strategy as compared to tree planting in the even-aged system, there may be substantial economic advantages for the multiaged system. Or, increased harvesting costs with operating among many standing trees in the multiaged stands may create advantages for the even-aged stand. A more conventional economic comparison between the two alternative systems (e.g., Tahvonen 2009) may therefore be more useful than

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the analysis of the transformation itself. The expenses of a transformation are important, but should probably be looked at as a simple cost (if any) of transformation, not the sole justification for selecting a new regime (Knoke et al. 2001). 10.5 Synthesis

There is a strong, international interest for greater use of multiaged forest management (O’Hara 2001). It is part of the surge in interest in more complex stand management strategies. This surge is part of a cyclic swing in forest management (Chapter 2). There may be a temptation to let the term “transformation”, or even “conversion”, define the intentions of land managers attempting to meet objectives for complex stand structures. A single transformation regime that was widely applied would probably achieve stand-level complexity but may not be well-suited over many stands because it might homogenize landscapes. Instead, it should be recognized that there are many forms of complex stand structures that may vary greatly within a forest type and will certainly vary between forest types. The complex stand structure of a Norway spruce forest may have little in common with the complex structure of ponderosa pine. Likewise, the tools used for transformation should be highly variable and encompass a full range of potential treatments. The analogy of the “silvicultural toolbox” is appropriate here: there are many tools for transformation from patch cuts to thinning to shelterwood treatments, and many more. There are also many target stand structures that can be the objective of transformation. Some alternative structures may be easier to attain and more economical than others. Transformation silviculture should also be highly adaptive rather than assuming pre-established plans will suffice without adjustments later in the transformation process.

A reoccurring theme in this book is the need for flexibility and that there are many options for multiaged stand structures. However, most transformations have tended to set diameter distributions as their targets. But any stand structure is possible and some will be more likely to meet management objectives than others. Transformation targets should be flexible as should the treatment regimes that are used to achieve these targets.

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Figure 10.1. Conceptual diagrams displaying alternate ways of viewing options for transformation from even-aged to multiaged stands. The left is a conventional view where there are many options for even-aged stands but not for multiaged stands. The right displays the idea that there are very many possible multiaged stand structures.

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Figure 10.2. Diagram of alternate pathways for transformation from an even-aged to a multiaged stand. The pathway on the left uses a uniform cutting and the one on the right a patch cutting system. Other alternative pathways are often possible as well (from Nyland 2003).

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Figure 10.3. Steps in transformation of a hypothetical loblolly/shortleaf pine stand from even-aged to multiaged. This transformation takes a normal diameter distribution and transforms it a distribution resembling a reverse-J with three cutting treatments over 30 years (adapted from Baker et al. 1996).

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Figure 10.6. Flowchart displaying a decision process for transformation when windthrow is a concern (from Schütz 2001b).

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Box 10.1 Transformation of Even-Aged Norway Spruce on the Schlägl Forest

The Sonnenwald Forest Management District in Austria provides a unique case of transformation to multiaged. The Schlägl Monastery managed the Sonnenwald Forest using clearcutting and even-aged management until the late 1970s when a transformation to a multiaged forest began. The forest is nearly pure Norway spruce with some European beech (Figure 10.4). Stand treatments have been described as “target diameter harvests” that removed larger trees from the even-aged stands as an ongoing means of transformation. The objective was a diameter distribution resembling the equilibrium curve (Schütz 1975, Sterba 2004) which resembles a rotated-sigmoid curve. A 3.5 ha plot was established (the Hirschlacke plot) and mapped in an even-aged, 120-year-old stand undergoing the target diameter harvest regime.

The initial reaction to these treatments was a concern they would reduce stand growth by removing the larger, faster growing trees (Sterba et al. 1981). Also, as a diameter-limit harvest, the treatments may have been dysgenic and would certainly remove the more valuable trees in the stand thereby leaving a high-graded stand.

After two harvest treatments and two thinnings over less than 25 years, stands were approaching the target equilibrium diameter distribution (Figure 10.5). The change in the diameter distribution is primarily in the increase in numbers of small trees, but the removal of larger trees has not affected the stocking in the larger diameter classes. The estimated stocking for the equilibrium is 475 m2/ha. The tree data from the Hirschlacke plot were used to examine individual tree growing space efficiency. Growing space efficiency provides the ability to assess different components in a stand such as age classes, canopy strata, or species (Chapter 7.2.1). Sterba and Zingg (2001) used a diameter-based measure of available growing space and found small trees with long crowns were most efficient.

The Schlägl example demonstrates the flexibility and resilience in central European Norway spruce forests where reliable regeneration provides several options for managers. Despite the potential dual genetic and economic risks, a transformation that removed the larger, more valuable trees was successful at moving stands towards the target multiaged structure. This example also demonstrates the utility of using individual tree efficiency to assess management activities. Analyses of growing space efficiency revealed the target diameter harvests did not have an adverse effect on volume production because the small trees were unexpectedly efficient. Sterba’s (2004) simulations indicated a number of alternative equilibrium curves could be met in a 100 years. The Sonnenwald Forest Management District was previously managed as a regulated even-aged forest, at the time of initial transformation it consisted of even-aged stands of a variety of ages. The complete transformation process for the entire forest will therefore take many decades.

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Figure 10.4. An even-aged stand at Sonnenwald Forest prior to transformation.

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Figure 10.5. Changes in diameter structure from 1977 to 1997 at the Hirschlacke plot (from Sterba 2004).