Summary
We review here our own research and related work on hybridization between two widespread and largely sympatric European oak species (Quercus petraea and Q. robur). There is a near total lack of local differentiation in chloroplast DNA markers between them. A model is proposed to account for this lack of differentiation: invasion by one species of the range occupied by the other through pollen swamping. In support of this model, ecological, palaeoecological and population genetic studies indicate that one species (Q. robur) disperses its seeds better than the other one, that pollen flow is much more efficient than seed flow in oaks and that hybridization and introgression are asymmetric, hence reinforcing the ecological dynamics by facilitating the dispersal of Q. petraea in regions already colonized by Q. robur. ‘Resurrection’ of Q. petraea following this wave of hybridization appears to be rapid. More generally, available evidence indicates that hybridization could constitute an important mechanism of dispersal in both natural and human-induced plant invasions.

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Hybridization as a mechanism of invasion in oaks

How temperate forests will respond to climate change is uncertain; projections range from severe decline to increased growth. We conducted field tests of sessile oak (Quercus petraea), a widespread keystone European forest tree species, including more than 150 000 trees sourced from 116 geographically diverse populations. The tests were planted on 23 field sites in six European countries, in order to expose them to a wide range of climates, including sites reflecting future warmer and drier climates. By assessing tree height and survival, our objectives were twofold: (i) to identify the source of differential population responses to climate (genetic differentiation due to past divergent climatic selection vs. plastic responses to ongoing climate change) and (ii) to explore which climatic variables (temperature or precipitation) trigger the population responses. Tree growth and survival were modeled for contemporary climate and then projected using data from four regional climate models for years 2071–2100, using two greenhouse gas concentration trajectory scenarios each. Overall, results indicated a moderate response of tree height and survival to climate variation, with changes in dryness (either annual or during the growing season) explaining the major part of the response. While, on average, populations exhibited local adaptation, there was significant clinal population differentiation for height growth with winter temperature at the site of origin. The most moderate climate model (HIRHAM5-EC; rcp4.5) predicted minor decreases in height and survival, while the most extreme model (CCLM4-GEM2-ES; rcp8.5) predicted large decreases in survival and growth for southern and southeastern edge populations (Hungary and Turkey). Other nonmarginal populations with continental climates were predicted to be severely and negatively affected (Berce, France), while populations at the contemporary northern limit (colder and humid maritime regions; Denmark and Norway) will probably not show large changes in growth and survival in response to climate change.

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Adaptive and plastic responses of Quercus petraea populations to climate across Europe.

The purpose of the study was to estimate among- and within-population variation for juvenile growth and for growth rhythm traits, bud flushing and leaf colouring, in open-pollinated families of Quercus robur L from six Lithuanian populations under different environmental conditions Assessments were done in the nursery up to 4 yrs, and after replanting in three field trials at the age of 6 yrs A highly significant population effect was found for bud flushing Large family variance components were estimated for bud flushing and height The family×site interaction was significant and more pronounced for growth cessation and height than for bud flushing Depending on the trait, from 13 to 33% of families contributed significantly to the family×site interaction Strongly significant site×population interaction was estimated only for bud flushing Families with early bud flushing were taller, although the correlation was weak Correlations between family means in bud flushing and bud flushing of parent trees

Genetic Variation and Phenotypic Plasticity of Quercus robur Populations and Open-pollinated Families in Lithuania

Background and Purpose: The objective of this research was to determine variability of some remaining populations and groups of trees of pedunculate oak (Quercus robur L.) at the molecular-genetic level in Bosnia and Herzegovina. The analysis was made by means of highly polymorphic nuclear microsatellite markers, which were used in some earlier research involving pedunculate oak embryos. The obtained results will provide guidelines for proper management with and regeneration of pedunculate oak forests, and for their re-introduction. In particular, this research will contribute to further improvement and conservation of pedunculate oak using in situ and ex situ methods. MaterialandMethods: This study analyzes molecular genetic variability of 12 natural populations of pedunculate oak (Quercus robur L.) in Bosnia and Herzegovina. Molecular genetic variability was analyzed on the basis of DNA by means of the following four nuclear microsatellite markers: ssrQpZAG1/5, ssrQpZAG9, ssrQpZAG36 and ssrQpZAG108. The analysis included a total of 108 alleles. Results and Conclusion: Considerable differences were observed in the frequencies between populations. The same differences were confirmed with analysis of other parameters, such as the effective number of alleles, the fixation index and genetic differentiation. It should be pointed out that this research confirmed higher allele heterozygosis in the populations in relation to pedunculate oak populations in Western Europe. This result suggests that the investigated populations have not lost much of their genetic potential for adaptation. This could be attributed to the vicinity of the studied populations to their glacial refugium. This is the reason for their higher resistance and sturdiness in comparison with the populations of western and central Europe. They posses more genetic variability despite the fact that they had for centuries suffered a strong anthropogenic influence, which brought them to their survival limit and the verge of extinction. The obtained results will allow us to preserve the genetic structure of this valuable species in Bosnia and Herzegovina.

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Genetic variability of pedunculate oak (Quercus robur L.) in Bosnia and Herzegovina.

The study was conducted to evaluate the genetic parameters in adaptive and growth traits within five Swedish populations of Quercus robur L, and three populations of Fagus sylvatica L. The Quercus robur populations had 27-30 families in each, sampled at latitudes 55" 34' to 60" 12' and the Fagus sylvatica populations had 28-30 families in each, sampled at latitudes 55" 28' to 57' 33'. The 1 -year old seedlings of Quercus robur were planted in nursery experiments at two distant sites at latitudes 59" 40' and 56" 38'. The 1-year old seedlings of Fagus sylvatica were planted only at the southern site. The plants were assessed yearly at ages 3-6 and some years twice for growth rhythm traits. Large and significant family effects were obtained for all traits, except for growth cessation and height in some Fagus sylvatica populations. Large coefficients of additive variation were obtained. The family x site interaction in Quercus robur populations was more pronounced for growth cessation and height, than for bud flushing with a larger family x site component for the southern populations. Stable and strong genetic correlations were obtained for bud flushing in different years and weaker, but stable correlations for growth cessation. Bud flushing and height genetic correlations showed a tendency to change with age from positive or slightly negative to negative. This trend was most pronounced in Quercus robur populations. These correlations were almost always negative in Fagus sylvatica populations. Late growth cessation was correlated with height. Thanks to the large and significant family effect, all populations in both species demonstrated high potential for adaptation and long-term breeding, which is most beneficial for gene conservation.

Within-population variation in juvenile growth rhythm and growth in Quercus robur L. and Fagus sylvatica L.

Provenance trials on Quercus robur L. and Quercus petraea (Matt.) Liebl. in Rhineland-Palatinate (Germany): preliminary results of phenotypic and genetic surveys.

B. R. Stephan

Papers

Provenance trials on Quercus robur L. and Quercus petraea (Matt.) Liebl. in Rhineland-Palatinate (Germany): preliminary results of phenotypic and genetic surveys.

Development of provenances of Quercus petraea and Quercus robur from acorns to six-year old plants in relation to species specific traits.