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FRAXIGEN Informe resumido

Project ID: EVK2-CT-2001-00108
Financiado con arreglo a: FP5-EESD
País: United Kingdom

Seed collection guidelines

A seed collection with a broad genetic base will avoid inbreeding depression; ensure that planted stands remain viable and produce seed; and maintain adaptability in future environments. Decisions are needed on which trees within a stand to collect from; whether to collect from isolated trees; and whether or not to collect in poor seed years.

We have studied reproductive biology in Fraxinus to determine the factors determining genetic diversity in seed. Each European ash species has a different sexual system. F. excelsior has male, female and hermaphrodite trees, with no clear divisions between genders. F. ornus has only males and hermaphrodites, whilst F. angustifolia is purely hermaphrodite. Controlled pollinations and molecular markers showed that selfing is possible on hermaphrodite trees, but naturally Fraxinus is highly outcrossing with negligible selfing in both mast and non-mast years. Mating between closely related trees rarely occurs, because most pollen is moved over tens of meters by insects (F. ornus) or hundreds of meters by wind (F. angustifolia, F. excelsior). Therefore there is little risk of inbreeding in natural populations. Pollen production in F. angustifolia overlaps with the opening of female flowers in F. excelsior. This means that there is potential for hybridisation in mixed stands, and the hybrid seed is most likely to occur on F. excelsior trees.

Adequate sampling depends on the number of seed collected per tree and the species' mating/pollination system. Wind pollination (F. excelsior, F. angustifolia) is likely to result in trees producing seed from many fathers, whereas insect pollination (F. ornus) is likely to produce seed of fewer fathers per tree. To make a seed collection with similar levels of genetic diversity requires collection from roughly 50% more trees in F. ornus than in F. excelsior or F. angustifolia. In F. excelsior or F. angustifolia, a collection from only ten mother trees, of at least 500 seeds per tree, will capture 95% of all the available genetic variation in the collection area, apart from rare alleles (gene variants) which comprise less than 5% of the total. In F. ornus, fifteen trees must be included in the collection to ensure the same genetic variation in the seed. Spacing between seed trees is also important for diverse collections. In F. excelsior seed trees should be at least 150m apart to ensure collection from both unrelated females and different pollen pools. With F. ornus most seed trees mate with different fathers, so it is sufficient to collect from trees separated by the seed dispersal distance (30m).

1. Seed can be collected from females and hermaphrodites, isolated trees (as part of larger collections), and in non-mast years, without risk of inbreeding. Contrary to popular belief, F. excelsior trees do not change sex.
2. In F. excelsior and F. angustifolia, seed collections should include at least ten trees separated by at least 150 m; in F. ornus, the trees need be only 30m apart but at least 15 trees should be included in the collection.
3. It is better to spread a collection over a wider area, keeping the minimum distance between seed trees, rather than restrict the number of trees or reduce the distance between trees.
4. In mixed F. excelsior/F. angustifolia stands the seed from F. excelsior trees may be hybrid.

Sustainable management of native woods or their restoration through natural regeneration requires genetically diverse seed. In F. angustifolia and F. excelsior high levels of pollen flow mean natural regeneration maintains high levels of genetic diversity, even in small fragmented woodland remnants. Even where isolated trees form the basis for regeneration, the diverse pollen pool fathering seed means regeneration will be genetically diverse. Conversely, the high level of gene flow means native woods are not 'isolated, uncontaminated, local gene pools'. Natural regeneration in a reserve also contains 'foreign' alleles from external pollen sources.

1. In F. ornus small isolated remnants (<15 seed bearing trees) should be avoided as the only source of natural regeneration, since insect pollination patterns means regeneration is likely to be lower in genetic diversity than in larger remnants.

Native populations of all three European Fraxinus species maintain high levels of genetic variation. Neither the species, nor their genetic resources are seriously threatened and in need of specific conservation programmes. Nevertheless activities such as seed collection, planting and restoration through natural regeneration need to take note of FRAXIGEN's recommendations if genetic diversity is to be maintained locally, with future productivity and adaptability unaffected.

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David BOSHIER, (Senior Research Associate)
Tel.: +44-1865-275128
Fax: +44-1865-275074
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