In clonal plants, modular growth allows for the production of new individuals (ramets) that are genetically identical to each other and to their clonal ancestor [1]. This behaviour is observed in the tree species Prunus avium L. (wild cherry). It endows the species with a mixed sexual and asexual reproduction system in which each individual does not only produce seed but is also able to spread vegetatively by root suckers.
Typical of pioneer tree species such as wild cherry is that they colonize early forest successsional stages as a result of forest disturbances (larger gaps in the forest canopy). Although this species reaches ages of up to 80 years and more, it is generally replaced by climax tree species during progressing succession of the local disturbance (e.g. beech, Fagus sylvatica L.). This requires an effective strategy allowing stably repeatable rounds of local establishment, dispersal and local extinction, particularly in natural central European continuous cover forests. Consequently, the establishment of local populations including several generations is unlikely due to a lower competitive ability compared to climax tree species (e.g. limited shade tolerance, early declining growth rates). While this expectation has been confirmed by a number of observations [2], it does not explain the advantages of a mixed reproductive strategy involving clonal reproduction in particular. Moreover, central European forests are since long strongly affected by different forest management strategies, several of which do not reflect the supposed natural reproduction patterns. In fact, P. avium can be observed primarily in two systems of forest management, in "group selection shelterwood systems" (= high forest system, HFS in the following) and in relics of former "coppice with standards" systems (= CWS in the following).
In the group selection shelterwood systems, natural regeneration is achieved by irregular, groupwise lowering of stand density in old stands. Such gaps are gradually increased over a time span of 30 to 50 years until total clearing. This technique allows the regulation of the abundance of tree species differing in shade-tolerance and thus the mixture of species typical for various stages of forest succession. Besides single-tree selection forests, this kind of forest treatment is supposed to represent similar forest tree reproduction patterns expected for natural central European forest ecosystems without human interference.
The former CWS combined the supply of firewood as well as construction timber (two-storied system). Firewood was produced by exploiting ("= coppicing") the even aged lower storey within short intervals (5 to 40 years) promoting tree species capable of vegetative reproducetion (e.g. stump shooting, root suckering). Only some high quality trees, the so called "standards", were left (about 25 to 50 trees per hectare) leading to an uneven aged overstorey of irregularly distributed stems and irregular canopy cover [3].
Hence, in order to understand the ecological basis of the mixed sexual and asexual reproduction system of wild cherry, it is desirable to analyse local occurrences of cherry for patterns of sexual and asexual reproduction among adult individuals. The focus on the adult stage is justified by the fact that successful establishment can only be stated after individuals reached adulthood. In this way it would be possible to test the range of validity of the common hypothesis that wild cherry does not spend more than one generation at the same location.
An assessment of this hypothesis therefore critically depends on the possibility to distinguish between adult individuals derived from asexual and from sexual reproduction. Basically, suitable methods of clonal analyses are provided by genetic markers of sufficient variability, such that an efficient rejection of the hypothesis of sexual reproduction is possible. More precisely, the clonal origin of two individuals with identical multilocus-genotypes is assumed if the hypothesis is rejected that two copies of such a genotype can be produced independently by sexual reproduction. The common methods of clonal analyses are based on the absence of homologous (allelic) and non-homologous (non-allelic) gene associations [see [4-7]]. These assumptions are a priori not realized in wild cherry because of its gametophytic self-incompatibility system (GSI) in combination with its capacity for clonal propagation.
In fact, the assumption of independent association of genes is already questioned by the early work of [8] about the effects of partial self-fertilization (see also [9] for a more detailed recent treatment). Beyond this, stochastic associations among genes can be generated by all major evolutionary and adaptational processes, such as selection, mating and random drift. The present paper will therefore be concerned with the application and critical evaluation of recently developed methods that take genic associations into consideration [10].
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