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Guidelines for ash seed source selection

European and national guidelines currently encourage the planting of local tree seed, over seed from non-native sources. One objective of FRAXIGEN was to provide scientific evidence to inform this policy. We carried out a range of field and laboratory studies to investigate patterns of genetic variation and local adaptation in three species (Fraxinus excelsior, F. angustifolia, F. ornus) and six countries (U.K., Sweden, Spain, Greece, Romania and Slovakia).

Our studies of flowering, fruiting and leaf flushing showed that different stands flush at different times, and this is related to latitude, longitude and elevation. For example, Romanian material planted in provenance trials of F. excelsior in southern England flushes earlier than local material and can be damaged by late frosts, causing forking. Late frosts can also kill flowers and reduce fruit set, even in local material. F. angustifolia is particularly at risk because it flowers early in the year (January-March).

Avoid moving material over large latitudinal or longitudinal ranges since it is likely to be poorly adapted to local climate, causing forking and poor seed set.

(i) Reciprocal transplant experiments:
We established field trials in UK (F. excelsior) and Greece (F. angustifolia, F. ornus) to assess the geographic scale of adaptive variation. Local material was tested against material collected at sites increasingly further away, and the trial was replicated at all the sites. We measured germination percentage of direct sown seed, and survival and growth of one-year-old seedlings. Preliminary results showed no correlation of height growth with patterns of ecological or geographic variation. No local adaptation was observed at the scale of the experiments, covering distances up to about 250km.

1. Provenance description for ash can operate at the scale of at least 200-300km without compromising local adaptation.
2. Good provenances can be moved within this range without compromising performance.

(ii) Molecular markers:
We assessed levels of neutral genetic variation in stands of ash across Europe using both chloroplast and nuclear markers.

(a) Chloroplast markers
Chloroplast markers, being maternally inherited, can be used to investigate the movement of species via seed dispersal over long time periods. They provide a good picture of broad scale population structure over large distances.

Fraxinus angustifolia and F. ornus both have southerly distributions in Europe, with postglacial migration predominantly from east to west, originating from glacial refugia in southeastern Europe and western Asia. Fraxinus excelsior is today widespread across central and northern Europe. Glacial refugia were probably located in Iberia, Italy, the eastern Alps and the Balkan Peninsula. Migration was predominantly northerly and from more than one direction from these refugia, after the retreat of the ice.

1. Where conservation of genetic diversity is the highest priority it should aim to be guided at the largest scale by the distribution of haplotypes
2. In countries where only a single ash haplotype is present, chloroplast markers can be used to detect some non-native material.
3. Most haplotypes have widespread distributions, giving little support to a strategy of dividing large regions of provenance into small seed zones.

(b) Nuclear markers
We used nuclear microsatellite markers to assess the overall level of genetic variation in ash species, the differences within stands and between stands, and how this variation is distributed across Europe.

We found high levels of genetic diversity within stands in all three Fraxinus species. Fraxinus excelsior and F. angustifolia showed few genetic differences between stands, suggesting high levels of gene flow via pollen movement (as expected for species with wind-dispersed pollen). At the northern edge of the range of F. excelsior, stands are more isolated and have lower levels of diversity. They are more differentiated because there is less pollen flow between them. Fraxinus ornus pollen is dispersed by insects. The genetic differences between stands in any one locality are few, but they increase with geographic distance. The foraging behaviour of the insect pollinators tends to move pollen between neighbouring trees. Closer individuals are therefore more likely to be related than more distant ones.

1. High levels of gene flow between stands argue against the strategy of dividing large regions of provenance into small seed zones.
2. Geographically distant stands show more genetic differences in insect-pollinated Fraxinus ornus than wind-pollinated F. excelsior and F. angustifolia.

Informations connexes

Reported by

Forestry Commission Research Agency
Forestry Commission Research Agency, Northern Research Station
EH25 9SY Roslin
United Kingdom
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