How much of genetic diversity is desirable in mass production of forest reproductive material? How mass production of forest reproductive material reduces genetic diversity? Relation between genetic diversity and mass production of forest reproductive material is discussed in a holistic manner. In industrial forest plantations, narrow genetic diversity is desirable and reproductive material is produced at clone level. On the other hand, in conservation forestry a wide genetic diversity is imperative. Beside management goals, a desirable level of genetic diversity is related to rotation cycle and ontogeny of tree species. Risks of failure are lower in short rotations of fast growing species. In production of slow growing species, managed in long rotations, the reduction of genetic diversity increases the risk of failure due to causes unknown or unexpected at the time of planting. This risk is additionally increased in cases of seed transfer and in conditions of climate change. Every step in production of forest reproductive material, from collection to nursery production, has an effect on genetic diversity mainly by directional selection and should be considered. This review revealed no consistent decrease of genetic diversity during forest reproductive material production and planting.
Genetic diversity (GD) provides the basis for adaptation and resistance to stress and changing environment, and is therefore essential for the long-term survival of forests (
There is no risk of losing genetic resources if forest populations regenerate spontaneously (
Processes involved in the production of forest reproductive material (FRM, such as seed processing and nursery production) can change the composition and the ratio among individual families in seed lots, seedling stock and in the planted forest at the end. Large changes leads to reduction of GD and unpredictable genetic gain in breeding programs (
There is little evidence that artificial forest regeneration leads to a reduction of GD at the stand level, regardless the seedlings were originated from natural stands, seed stands or seed orchards (
Transfer of FRM during afforestation/reforestation has a great influence on GD by changing gene frequency or introducing genes where they were not present before. The traditional “free movement of germplasm” in Central Europe is a complicating factor when discussing GD (
This paper provides an overview on relation between GD and FRM in a holistic way: from source selection, transfer of FRM, seed processing and seedling production to reforestation/afforestation.
Genetic diversity can be considered at different hierarchical levels: species, provenance (population), family and individual level. GD at lower hierarchical levels depends on factors that prevent panmixia or completely random mating between all individuals (
Maintenance of interspecific diversity in afforestation reduces establishment risks, increases biodiversity and the ability of natural regeneration; the value of forest products can be increased or decreased but it always complicates forest management with different management procedures for different species (
There are potential advantages to be gained by using carefully designed species mixtures in place of monocultures (
Forest tree species are highly heterozygous and contain a high portion of total genetic variation within populations (provenances), while the interpopulation component of variation rarely exceeds 5% (
The number of families (half-sib lines) in reforestation determines the degree of GD and adaptibility of the new stand or plantation. This number depends on the seed source (seed stands or orchards), seed processing and nursery production. Since GD can be assessed by the effective population size (
Genetic diversity within individuals depends on outcrossing and gene dispersal efficiency. In genetically variable tree species it is likely that every seed produced has a different genotype and considerable variation among individuals can be generated in offspring from just only a few parents (
A distinction should be made when discussing the number of clones in seed orchards and in clonal forestry (
There is no genetic variation within a monoclonal stand, thus diversity must be managed at the estate level (
The answer to this question depends on the management objectives, rotation and the breeding level (
From the genetic point of view, we can distinguish three ways of artificial establishment of forests, and therefore the mass production of FRM for their needs. These are seedling forestry, family forestry and clonal forestry. In seedling forestry, the number of genotypes in new forests is equal to the number of planted seedlings. In family forestry, the number of genotypes is equal to the number of seedlings from controlled crossing (which is usually small) before vegetative multiplication. In clonal forestry, the number of genotypes is equal to the number of clones used for planting. Obviously, GD must be reduced in order to obtain genetic gain and because of that, it is very important to find the balance between these two objectives (GD and genetic gain) in forestry.
Each subsequent breeding level increases genetic gain but narrows genetic diversity. Increase of risk of failure follows the increase of rotation time. This risk is somewhat lower in case of production and use of good quality FRM.
Mass production of FRM is based on vegetative or generative reproduction. In this paper, we consider sexual reproduction with distinction between FRM from open and controlled pollination (crossing). Controlled crossing is limited in terms of quantity, and its use is often restricted to breeding programs. A combination of controlled crossing with mixed propagation can be used for the mass production of reproductive material,
There are several methods for the production of improved reproductive material from open pollination. The mostly used are seed production areas (SPA) or seed stands (SS), parts of forest selected from natural stands or old plantations. The next level in tree breeding is seed orchards (SO) of trees without pedigree and, finally, clonal plantations of next generations from tested genotypes. Stands and orchards for the production of seeds from open pollination are suitable for use of the additive genetic variance (or general combining ability -
GD of seed lots is influenced by the size of the parent population, the balance in the parental reproductive success, the kinship of its members, and by the level of inbreeding (
There are concerns about the effective number of parents in the SS and SO because only a portion of individuals contributes to the gametes pool and transmits its genes to the next generation (
Properly managed SS can be used as a source of high quality seeds for reforestation until genetically improved seed from SO becomes available. In most cases, the establishment of new forests with FRM originating from SS provides a level of GD similar to wild population from which they comes (
SS must consist of one or more groups of trees properly spaced and in sufficient numbers (
SO represent a link between tree breeding and afforestation (
There is concern about the GD of seed lots from SO, because if a lower heterozygosity results from an increased level of inbreeding, we can expect a reduction of fitness (
The total number of alleles (
The number of alleles per locus (
The percent of polymorphic loci (
The expected heterozigosity (
The above results indicate that phenotypic selection at the early stage of breeding of highly polymorphic species does not significantly reduce genetic variability, likely due to the sampling of trees for plantation establishment from widely distributed natural populations (
Seed collection is the critical stage in mass production of FRM for maintaining of GD. A large portion of the original GD in seed sources can be lost when seeds are collected from a small number and/or from inappropriate trees.
Two methods of collecting seeds are commonly applied (
A uniform harvesting across the whole collecting area is recommended (
Mother (seed) trees should be carefully selected during collection and this is usually done by phenotypic selection. Phenotypic selection successfully conserves genetic variation in natural populations and presents an easy and inexpensive way to provide material for further breeding (
Despite a sufficient number of properly selected seed trees is chosen for collection, the family ratio in the seed lot may be uneven. Number of viable seeds from different families may differ because of the unequal number of collected seeds, fruits or cones, different number of seeds in the fruits or cones and different viability due to the state of maturity, insect attacks, infections and maturation (
Collecting seeds in non-mast years leads to a reduction of genetic diversity, as seeds from a small number of parents are included which generally are not representative of the population. Due to the large variation in the genetic structure of seed crops between years (
Seed processing can lead to a decrease in GD by reducing the participation (or even the elimination) of some families from the initial seed lot. Most methods of seed processing are custom to average sized seed. Indeed, seeds closer to the average for any trait subject to processing, are less likely to be accidentally eliminated (
Unlike the systematic, random elimination has less impact on the genetic constitution of seed lot, because the seed probability to be eliminated does not depend on its physical or physiological traits. An example of random elimination is seed storage, when storability of large and small seeds or seeds from different families is the same (
Seed grading is usually based on seed size and weight, and leads to rejecting a certain amount of small, but still viable seeds. This procedure can change the genetic constitution of the whole seed lot, discarding entire families with smaller seed size. However, even in the absence of seed grading a similar effect may occur, as the plants developed from small seeds are smaller and show a higher mortality rate (
The size and weight of seeds are under strong parental genetic control (
The size and weight of seeds are positively associated with viability and germination parameters (
Seed storability is under strong genotype influence and the preservation of viability during storage largely differs among families (
Seed germination may strongly decrease GD of seed lots due to different germination capacity, speed and vigor among families and individual seeds, and may lead to either under- or overestimation of the effective population size of seed lots (
Seed germination parameters are under strong genetic control both in gymnosperm (
Seed germination is an important parameter for seedlings production. This is well explained by
A high GD included in seed lots after collection may be contrasting with the uniformity desired for mass production of seedlings in the nursery. Indeed, seeds with uniform germination and seedlings with consistent growth are easier to be cultivated in the nursery. In addition, nursery operations are more effective when carried out in a homogenous environment.
A significant reduction in GD may take place in the nursery, depending on nursery conditions and production methods. Production of seedlings under environmental conditions much different from those at the seed collection site may cause directional selection at this stage or after planting (
Distinction should be made between production of bareroot and containerized seedlings. Risk of reduction in GD by directional selection is lower in production of bareroot seedlings with sowing seeds in the beds. However, selection pressure is stronger in bareroot nurseries compared to container nurseries, due to harsher field conditions during germination. Uniformity of growing conditions is a keystone for commercial nursery production. There are evidences that temporarily heterogeneous environmental conditions might promote a higher survival of heterozygote genotypes, while homozygosity could be favored in relatively homogeneous conditions (
The production of containerized seedlings may also lead to a possible reduction of GD in the seedling stock. Indeed, more than one seeds are usually sown per cell and smaller seedlings are removed after germination. This results in an undesired directional selection favoring parents with less dormant and fast-germinating seeds, though their seedlings do not necessarily show better performances in the field. To this purpose,
Based on the literature examined, there are no evidences that nursery operations might reduce GD. In five reviewed studies, seedling stocks did not show significant differences in GD parameters as compared to their relative seed lots (
Finally, the reviewed studies also suggested that seedling production in the nursery maintains the GD observed in the initial seed lot, regardless to species and production method (bareroot -
Grading and culling of seedlings is a routine procedure in nursery as integral part of lifting and packaging. Culling is usually based on height, diameter and physical damage. Culling based on physical damage does not represent a directional selection (the damage often occur by chance), while culling based on size can be directional, depending on the genetic control of height and diameter (
Culling based on height and diameter partially discards inbred plants and other poor or abnormal genotypes, thus increasing the adaptability and growth capacity of the seedling stock. Such procedure does not constitute a directional selection, as inbred seedlings are usually inferior, susceptible to disease and often can not survive under field conditions. However, removing smaller plants from seedling stock can discard genotypes with possible faster growth at later stages. There are numerous examples that smaller seedlings in the nursery achieve a faster growth in the field (
In this context, standards for seedlings culling should be determined by seed origin (seed stands, seed orchards and full-sib crossing) and nursery conditions (
Reforestation activities largely depend on the goals, with distinction between forest regeneration and tree plantations. These goals must be a compromise between productive and non-productive (benefit) functions (
Artificial regeneration is the most obvious silvicultural practice resulting in possibly drastic changes of genetic structures not only in the planted stands, but also in the neighboring forests
Distinction should be made between the establishment of new forests by direct sowing and planting of seedlings. From the genetic point of view, reforestation by direct sowing has a lower impact on GD as compared with planting seedlings (
Based on the literature analyzed in this review, differences between natural (including those from natural regeneration) and planted populations are not conclusive (
The above results suggest no significant negative impact of artificial regeneration
Although the transfer of FRM is not directly related to its production, knowledge of the future use can provide important guidance for mass production. The success of reforestation in the case of transfer of reproductive material is directly dependent on the ability of a population to adapt to the new environmental conditions. Previous work has mainly focused on pairing provenances or genotypes with sites where they adapted well and where they can be well recovered. Transfer of reproductive material should be based on knowledge on planting site, species genetic diversity and biology (
Provenance tests show that transferred populations often perform as well as the local provenances or better (
There is concern that local natural adaptation and migration of plants can not keep up pace with climate change. However, evolution of trees can take place in just a few generations or less than 200 years and, in some cases, even only one generation is needed for local adaptation (
Beside the choice of the best suited FRM (in terms of origin, type, quality, planting site and goal-specific management), tracking the identity of the transferred material is necessary. Unfortunately, at the operational level, application of this theoretical model is often limited by the FRM available on the market. FRM producers (including seed collectors, seed processing stations, nurseries) tend to minimize the number of species, due to economic and management reasons. This emphasizes the need of a project specific planning period of at least five years to allow the production of appropriate FRM for the majority of species, from seed source selection based on transfer guidelines, to seedling production and planting. Additional attention should be payed in situations when a large amount of FRM is needed in short time, after the occurrence of catastrophic events (forest fire, wind or frost damage on large areas).
High genetic diversity is essential for the long-term survival of forests, providing the basis for future adaptation and resistance to stress and changing environment. A high degree of GD is also needed in the case of FRM transfer to long distances or different climates to ensure local adaptation of the transferred material.
Many steps in the mass production of FRM may potentially lead to a reduction of GD in seedling stocks. Phenotype selection, transfer of FRM and breeding, are selective practices that favor specific genotypes. Seed processing and storage, as well as nursery conditions and operations, can also favor certain families and discard others. Furthermore, grading of seed and seedlings can result in unwanted directional selection of the FRM. However, based on the litarure analyzed in this review, no consistent decrease of genetic diversity has been observed during forest reproductive material production and planting.
The adoption of appropriate collection strategies can maximize the genetic diversity in seed lots, aimed to avoid population genetic bottlenecks and maintain the largest effective population size. Collecting seeds from trees of different ages, as well as the mixing of seeds collected in different years, may contribute to maintain GD. Mixing of various classes of seeds before sowing is recommended in cases of afforestation for conservation purposes. Seed collection, processing and seedling production at family level, followed by mixing of seed/seedling families before their use is the safest way to preserve genetic diversity, though this complicates production practices and unduly increases their costs.
Nursery procedures aimed at providing the greatest number of plants per unit of seed, the highest percentage of acceptable trees and the maximum survival of outplanted seedlings, may reduce the risk of narrowing GD. Selection pressure on seedlings tends to reduce when growing conditions are favorable, so that weaker genotypes, non-competitive in natural conditions, can develop into high-quality seedlings. Regarding seedlings production, culling undersized seedlings has the greatest impact in terms of reduction in GD of the FRM. Standards for culling, especially based on height, should be adjusted by taking into account lower hierarchical levels of GD (provenance, population) and the final use of seedlings. Nursery production practices should provide a uniform planting material, with minimal need for culling.
Reforestation success relies on large local diversity, with a choice of appropriate species and proper transfer of FRM. The genetic diversity of the planting material is the result of previous operations carried out during its production. In this context, sowing seeds instead of planting seedlings reduces the risks of loss of GD as the result of directional selection during seed processing and seedling production. However, planting of seedlings is recommended, because it ensures a higher survival rate and a greater chance of success. High survival and high-density planting in reforestation programs promote natural selection in the new population.
Influence of breeding on genetic diversity.
Differences in the parameters of genetic diversity among natural populations (NP) and seed orchards (SO). (
Species | Type | Heterozygosity Parameters | Markers | Source | ||||
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NP | 34.7+ | - | - | - | 0.283+ | Isozymes |
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SO | 39.5+ | - | - | - | 0.266+ | |||
NP | - | - | 1.1 | 11.1 | 0.054 | Isozymes |
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SO | - | 1 | 1.2 | 11.1 | 0.058 | |||
NP | - | 4 | 2.0 | 69.2 | 0.216 | Isozymes |
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SO | - | 6 | 2.7 | 84.6 | 0.238 | |||
NP | - | 3 | 1.82 | 66.9 | - | Isozymes |
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SO | - | 6 | 2.77 | 100 | - | |||
NP | - | 4 | 2.14 | 52.6 | 0.171 | Isozymes |
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SO* | - | 2 | 2.28 | 62.5 | 0.172 | |||
SO** | - | 1 | 2.25 | 56.3 | 0.163 | |||
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NP | 46 | 7 | 2.7 | 64.7 | 0.210 | Isozymes |
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SO | 40 | 1 | 2.4 | 64.7 | 0.207 | |||
NP | 39 | 9 | 2.17 | 55.6 | 0.164 | Isozymes |
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SO | 31 | 1 | 1.72 | 50.0 | 0.157 | |||
NP | 55 | 12 | 2.04 | 59.3 | 0.114 | Isozymes | ||
SO | 45 | 2 | 1.67 | 44.4 | 0.114 |
Differences in the parameters of genetic diversity between the seed lot (SL) and the seedling stock (SS). (
Species | Type | Heterozygosity Parameters | Markers | Source | |||
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U1 | - | - | - | 0.44 | RAPD |
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NR2 | - | - | - | 0.39 | |||
SS | - | - | - | 0.43 | |||
SL | 38 | 2.2 | 70.6 | 0.219 | Isozymes |
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SS | 39 | 2.3 | 64.7 | 0.215 | |||
NR | - | - | 87 | 0.190 | Isozymes |
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SS | - | - | 95.5 | 0.203 | |||
SOS3 | 10 | 8.79 | - | 0.653 | Microsatellites |
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SS | 7 | 6.98 | - | 0.641 | - | ||
SO4 | - | - | - | 0.29 | Isozymes |
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SS | - | - | - | 0.28 | |||
SO | - | - | - | 0.75 | Microsatellites | ||
SS | - | - | - | 0.72 |
Differences in the parameters of genetic diversity between initial population or natural population and new plantations. (
Species | Type | Heterozygosity Parameters | Markers | Source | ||||
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Natural | - | - | - | 0.419 + | 0.359 + | Isozymes |
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Plantation | - | - | - | 0.423 + | 0.408 + | |||
Initial Krotoszyn | 77 | 3.08 | 92 | 0.254 | 0.254 | Isozymes |
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Plantation Krotoszyn | 78 | 3.12 | 92 | 0.242 | 0.231 | |||
Initial Gubin | 69 | 2.76 | 80 | 0.257 | 0.256 | |||
Plantation Gubin 1975 | 74 | 2.96 | 96 | 0.234 | 0.233 | |||
Plantation Gubin 1982 | 65 | 2.56 | 80 | 0.244 | 0.237 | |||
Unmanaged | - | - | - | 0.44+ | - | RAPD |
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Natural regeneration | - | - | - | 0.39+ | - | |||
Plantation | - | - | - | 0.43+ | - | |||
Unmanaged | 12.2 | - | - | 0.73 + | 0.46 + | SSR | ||
Natural regeneration | 11.5 | - | - | 0.72 + | 0.47 + | |||
Plantation | 11.5 | - | - | 0.74 + | 0.46 + | |||
Natural regeneration | - | 1.83 | 32.6 | 0.160 | 0.137 | Isozymes |
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Plantation | - | 1.83 | 35 | 0.149 | 0.138 | |||
Natural | - | - | 83.7 | 0.248 | - | RAPD |
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Plantation | - | - | 78.7 | 0.234 | - | |||
Natural | - | 5 | - | 0.52 | 0.5 | Microsatellites |
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Plantation | - | 4.93 | - | 0.52 | 0.5 | |||
Old natural | - | 1.89 | 88.7 | - | 0.381 | RAPD |
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Natural regeneration | - | 1.84 | 83.8 | - | 0.349 | |||
Plantation | - | 1.72 | 72.2 | - | 0.297 | |||
Progeny from phenotipic selection | - | 1.67 | 66.5 | - | 0.259 | |||
Natural old | 109 | 10.9 | - | 0.637 | 0.492 | Microsatellites |
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Natural regeneration | 108 | 10.8 | - | 0.643 | 0.500 | |||
Plantation | 102 | 10.1 | - | 0.632 | 0.479 | |||
Progeny from phenotipic selection | 100 | 10 | - | 0.634 | 0.788 | |||
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Initial | - | 3 | - | 0.169 | 0.183 | Isozymes |
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Progeny from 10 | - | 2.95 | - | 0.190 | 0.189 | |||
Progeny from 20 | - | 2.95 | - | 0.182 | 0.182 | |||
Progeny from 30 | - | 3 | - | 0.181 | 0.184 | |||
Progeny from 40 | - | 2.91 | - | 0.183 | 0.182 | |||
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Natural regeneration | - | 2.56 | 77.3 | 0.320 | 0.237 | Isozymes |
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Artificial regeneration | - | 2.51 | 72.7 | 0.315 | 0.230 | |||
Natural | - | - | 77.8 | 0.244 | - | RAPD |
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Plantation | - | - | 79.3 | 0.241 | - | |||
- | - | - | - | - | - | Microsatellites |
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Plantation | 6 | 5.97 | - | 0.518 | - | |||
Nonmanaged | - | - | 82.0 | - | 0.26 | RAPD |
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Managed | - | - | 72.5 | - | 0.26 | |||
Progeny | - | - | 59.7 | - | 0.22 | |||
Natural | 3 + | - | - | 0.255 | - | Isozymes |
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Plantation | 1 | - | - | 0 | - |