Hybrid Zones Between Two European Oaks: a Plant Community Approach

Abstract

Phenomena of hybridization can affect the ecology and evolution of the species involved in the process, as well as their communities. Although numerous papers focus upon the problem of taxonomy, few of these have attempted to study hybrid zones in relation to the analysis of their communities. On the Iberian Peninsula, hybridization phenomena among different oak species are frequent. It is, however, between Quercus faginea Lam. and Quercus pubescens Willd. where the most noteworthy hybridization phenomenon occurs. In this respect, we are familiar with the existence of different introgression levels but we are unaware of whether these hybrids are the transitory result of the interspecific genetic flow or whether these are maintained by means of extrinsic selection processes. Study of plant communities' flora and environment might shed light upon this issue. Comparison between plant communities dominated by one of the parental species and those dominated by individuals of hybrid origin might enable us to establish the presence or absence of an environment that is potentially selective in favour of the hybrids. Thus the possible existence of extrinsic selection. Furthermore, this information will help us to understand plant community distribution in an area that is difficult to interpret. To this purpose, we used multivariate ordination techniques (DCA and CCA) based upon a total of 395 floristic releves covering the whole range of the parental species on the Iberian Peninsula and upon climatic and edaphic variables for each of these releves. We also compared the groups obtained in relation to floristic similarity (Jaccard index), richness and diversity (ShannonWeaver index). Forests associated with Quercus pubescens are related to heavy summer precipitation, whereas Quercus faginea forests correspond to lower values of this variable and higher ones for continentality. Between both formations, there is a broad hybrid zone, with diffused borders that are related to an environmental gradient of Mediterranean influence. In this region, two types of forest communities were distinguished, which enabled us to divide the hybrid zones into two territories. Our results allowed us to locate the hybrid zone in an ecotone. The differentiation between habitats appears to indicate models of ecological selection. These models require, by definition, the presence of an environmental gradient between the parental zones. We are, however, aware of the need for future experiments in order to establish whether the hybrids are better adapted than the parental species. Only with availability of all this information can intrinsic selection be rejected.

Effects of abiotic and anthropogenic factors on the spatial distribution of Quercus faginea in the Spanish Central Pyrenees

“…The species does not occur above 1500 m because freezing temperatures hinder the establishment and growth of seedlings. As in our study, Sánchez de Dios et al (2006) found that Q. faginea forests on the Iberian Peninsula were associated with continental areas that have low precipitation. The phenology of Q. faginea is similar to that of species that produce roots that can access deep water reserves (Castro and Montserrat 1998).…”

Present and future extension of the Iberian submediterranean territories as determined from the distribution of marcescent oaks

“…This method has several advantages over other models used for similar interpolations (Sánchez Palomares et al 1999) and has been used in similar studies (Benito Garzón et al , 2007Garzón et al , 2008Sánchez de Dios et al 2006). …”

“…The Iberian Peninsula is one of the European regions where transition territories are of greatest importance, both because of their extent and the importance of the plant communities to which they are home (Costa Tenorio et al 1997;Sánchez de Dios et al 2006). Owing to their transitional nature, submediterranean territories are very sensitive to change (Gosz 1992).…”

Genetic evidence of reproductive isolation in a remote enclave of Quercus pubescens in the presence of cross-fertile species

“…In this way we might omit a true hybrid zone (i.e. an area where hybrids would outperform both parental species), which can be located in an ecotone between preferable habitats of parental species (Sánchez de Dios et al 2006). Future studies need to address this aspect as well.…”

“…Related oak species can hybridise relatively easily, and hybrids are usually fertile (Whittemore and Schaal 1991;Rushton 1993;Tovar-Sánchez and Oyama 2004;Curtu et al 2007;Peñaloza-Ramírez et al 2010). Furthermore, related oak species can differ in ecological optima (Whittemore and Schaal 1991;Timbal and Aussenac 1996;Petit et al 2004;Sánchez de Dios et al 2006;Gugerli et al 2007), so that they often occupy different niches and can reveal variable adaptation to local environments, making room for exogenous selection (Barton and Hewitt 1985).…”