Leaf morphological differentiation between Quercus robur and Quercus petraea is stable across western European mixed oak stands
Ann. For. Sci.
Leaf morphological differentiation between Quercus robur and Quercus petraea is stable across western European mixed oak stands
Antoine Kremer 5
Jean Luc Dupouey 3
J. Douglas Deans 4
Joan Cottrell 1
Ulrike Csaikl 2
Reiner Finkeldey 0
Santiago Espinelg
Jan Jensenh
Jochen Kleinschmiti
Barbara Van Damj
Alexis Ducousso 5
Ian Forrest 1
U. Lopez de Herediak
Andrew J. Lowe 4
Marcela Tutkova 2
Robert C. Munro 4
Sabine Steinhoffi
Vincent Badeau 3
0 Swiss Federal Research Institute WSL , Zürcherstrasse 111, Birmensdorf , Switzerland
1 Forestry Commission (FC), Forest Research, Northern Research Station , Roslin, Midlothian, EH25 9SY, Scotland , U.K
2 Austrian Research Centre (ARCS) , Seibersdorf, 2444 , Austria
3 Institut National de la Recherche Agronomique (INRA), Unité d'Écophysiologie Forestière , 54280 Champenoux , France
4 Centre for Ecology and Hydrology , Edinburgh, (CEH), Bush Estate, Penicuik, Midlothian, EH26 0QB, Scotland , U.K
5 Institut National de la Recherche Agronomique (INRA), Unité de Recherches Forestières , BP 45, 33611 Gazinet Cedex , France
- Leaf morphology was assessed in nine mixed oak stands (Quercus petraea and Q. robur) located in eight European countries. Exhaustive sampling was used in an area of each stand where the two species coexisted in approximately equal proportions (about 170 trees/species/stand). Fourteen leaf characters were assessed on each of 5 to10 leaves collected from the upper part of each tree. Three multivariate statistical techniques (CDA, canonical discriminant analysis; PCA, principal component analysis; MCA, multiple correspondence analysis) were used in two different ways: first on the total set of leaves over all stands (global analysis) and second, separately within each stand (local analysis). There was a general agreement of the results among the statistical methods used and between the analyses conducted (global and local). The first synthetic variable derived by each multivariate analysis exhibited a clear and sharp bimodal distribution, with overlapping in the central part. The two modes were interpreted as the two species, and the overlapping region was interpreted as an area where the within-species variations were superimposed. There was no discontinuity in the distribution or no visible evidence of a third mode which would have indicated the existence of a third population composed of trees with intermediate morphologies. Based on petiole length and number of intercalary veins, an “easy to use” discriminant function applicable to a major part of the natural distribution of the species was constructed. Validation on an independent set of trees provided a 98% rate of correct identification. The results were interpreted in the light of earlier reports about extensive hybridization occurring in mixed oak stands. Maternal effects on morphological characters, as well as a lower frequency or fitness of hybrids in comparison with parent species could explain the maintenance of two modes, which might be composed of either pure species or pure species and introgressed forms.
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caractérise par une distribution bimodale, chaque mode correspondant à une espèce. La distribution de la première variable ne manisfeste pas de
discontiunuité pouvant indiquer l’existence d’un troisième groupe, correspondant à des arbres avec des morphologies de type intermédiaire. Une
méthode d’identification rapide basée sur les deux caractères les plus discriminants (nombre de veines intercalaires et longueur du pétiole) a été
proposée et validée sur un autre jeu de données (98 % d’identifications correctes). La structure de la variation morphologique entre les deux
espèces résulte sans doute de l’hybridation entre elles, et de l’hérédité des caractères morphologiques.
1. INTRODUCTION
Morphological variation in mixed oak stands composed of
Quercus petraea and Quercus robur has been of general
interest in Europe for decades. The two species commonly
coexist in mixed stands and foresters need rapid “easy to use”
morphological screening methods which would help to
discriminate between the species [
9, 10, 14
]. Dendrologists
require classification criteria for taxonomic purposes [
3, 17, 18,
22, 25
]. Forest biologists and ecologists seek unambiguous
traits which could be used for studying introgression between
these two interfertile species [
15, 28, 29, 34
]. The vast
amount of literature devoted to morphological variation in
mixed oak stands demonstrates the debate that has been
raised. In addition to efforts to define the most appropriate
morphological traits to be assessed [
2, 26, 30, 31
],
investigations have been conducted on relevant statistical methods
[
11, 12, 20
]. However these reports have not led to general
agreement on the methods and traits to be used for
distinguishing Quercus petraea from Q. robur, mainly because no
comparative studies were conducted across different
countries. Key issues in the controversy were also the sampling
and (...truncated)