Intraspecific variation of growth and adaptive traits in European oak species

According to various reports, 200-450 oak species including hybrid populations exist worldwide, with 24 of these - including 3 hybrid forms - having their natural range within Europe. They all belong to the subgenus Lepidobalanus. The most important section is robur with 21 species. The European species belong to both deciduous (15 species) and evergreen (9 species) oaks. Some difficulties in clear morphological and physiological definition of the species involved is caused by hybridization. Most studies into intraspecific variation exist for the species Quercus robur L and Quercus petraea (Matt) Liebl. Some information is available for Quercus cerris L, Quercus virginiana Ten (= Q dalechampii Wenz), Quercus ilex L and Quercus suber L, based on comparative plantations. In addition, a considerable number of morphological, physiological and biochemical studies based on natural populations exist which are not completely covered in this review paper. For most characters observed, oaks exhibit a wide variation. This is not only the case for morphological traits of pollen, seed, wood and plants but also for physiological traits and phenology which have great adaptive importance. Provenance experiments and progeny tests started as early as 1877. Most of these were only of local importance. This is partly due to the fact that acorns can only be stored for a limited period and flowering is irregular. However, the results available show that the choice of provenance can be important for the successful economic management of plantations. Variability of leaves, phenology, form, growth, wood and bark, roots, seed and flowering has been discussed separately. Improvement via selection and testing seems to be promising due to the considerable between-population and within-provenance variation. Vegetative propagation has been developed for some species by grafting, cutting propagation and in vitro propagation. Tree breeding approaches have also been discussed.