Christian Wehenkel

Is there a trade-off between species diversity and genetic diversity in forest tree communities?

The two most important components of biodiversity, species diversity and genetic diversity, have generally been treated as separate topics, although a coordination between both components is believed to be critical for ecosystem stability and resilience. Based on a new trait concept that allows for the assessment of genetic diversity across species, the relationship between species diversity and genetic diversity was examined in eight forest tree communities composed of different tree genera including both climax and pioneer species. It was intended to check whether a trade-off exists between the two diversity components as was found in a few studies on animal species.Using several isozyme-gene systems as genetic markers, the genetic diversity across species within each of the tree communities was determined by two measures, the commonly used intraspecific genetic diversity averaged over species and the recently developed transspecific genetic diversity per species. Both data sets were compared with the corresponding community-specific species diversity resulting in a positive relationship between the two diversity components. A statistically significant positive correlation was established between the transspecific genetic diversity per species and the species diversity for three isozyme-gene systems. Beyond that, consistent results were obtained using different parameters of the diversity measure which characterize the total, the effective and the number of prevalent variants. The number of prevalent variants reflected most significantly the non-randomness of the observed diversity patterns.These findings can be explained by the observation that the pioneer tree species reveal a by far higher genetic diversity than the climax tree species, which means that an increase in species diversity, due to the addition of several pioneer species at the expense of one or two climax species, goes along with an increase in the level of genetic diversity. Forest tree communities with the highest degree of species diversity exhibit therefore the highest transspecific genetic diversity per species. This result was discussed with regard to the particular composition and stability of forest tree communities.

Analysis of biodiversity across levels of biological organization: a problem of defining traits

Biodiversity is a term that comprises the appearance, structure and function of all levels of biological organization, including genes, species and ecosystems. The vast majority of measures of biodiversity (usually termed ‘diversity indices’) considers only number, proportion and distribution of species which belong to a specified group and exist in a defined area or ecosystem. Genetic diversity as a part of biodiversity within species (or populations) was either not regarded in this respect or was treated (by geneticists) as a separate entity of diversity quantified with separate measures. Little attention has been given to the integration of both types of diversity, within and among species, in a single measurement (termed ‘transspecific’ diversity). In order to attain this integration on a general basis, an operational trait concept is developed which allows the determination of variation in traits observable in members not only of the same species but also of different species. The concept rests on methods of investigation that can be adapted to a broader range of organisms without modification of their characteristics. Once a trait is specified on this basis, any meaningful measure of diversity can be applied to assess biodiversity across levels of biological organization. The utility of the concept is demonstrated by application to the results of an earlier study on associations between species and genetic diversity in a forest tree community. Attributes of isozymes which are visible in electrophoresis are used as a transspecific genetic trait.

Spatial genetic structure in the very rare and species-rich Picea chihuahuana tree community (Mexico)

Abstract In natural plant populations, the spatial genetic structure (SGS) is occasionally associated with evolutionary and ecological features such as the mating system, individual fitness, inbreeding depression and natural selection of the species of interest. The very rare Mexican P. chihuahuana tree community covers an area no more than 300 ha and has been the subject of several studies concerning its ecology and population genetics. The overall aim of most of these studies has been to obtain data to help design preservation and conservation strategies. However, analysis of the fine-scale SGS in this special forest tree community has not yet been conducted, which might help enrich the above mentioned conservation programs. In this study, we examined the SGS of this community, mostly formed by P. chihuahuana Martínez, Pinus strobiformis Ehrenberg ex Schlechtendah, Pseudotsuga menziesii (Mirb.) Franco, and Populus tremuloides Michx, in 14 localities at both the fine and large scales, with the aim of obtaining a better understanding of evolutionary processes. We observed a non-significant autocorrelation in fine-scale SGS, suggesting that the genetic variants of all four tree species are randomly distributed in space within each sampled plot of 50 x 50 m. At the larger scale, the autocorrelation was highly significant for P. chihuahuana and P. menziesii, probably as a result of insufficient gene flow due to the extreme population isolation and small sizes. For these two species our results provided strong support for the theory of isolation by distance.

Different diversity measures and genetic traits reveal different species-genetic diversity relationships: A case study in forest tree communities

Abstract Relationships between species diversity and genetic diversity, the two most important elements of biodiversity, have recently attracted considerable interest in the field of community genetics. The present study contributes to this issue by addressing three questions that seem to have been ignored so far, namely whether the use of (a) different diversity measures, of (b) different components of diversity, and of (c) different genetic traits may lead to different assessements of speciesgenetic diversity relationships. For this purpose, data on species composition and genetic traits were collected from the natural regeneration of nine forest communities, which consist of three pure and six mixed tree stands located in the Thuringian forest area. The genetic traits comprised one DNA (AFLP) and five isozyme traits all of which were determined in all species. In contrast to other studies, the species diversity was determined for two components, SD (species diversity) and NeS (effective number of genetically distinct species), and the genetic diversity was determined for three components, TSGD (the transspecific genetic diversity taken over all species of a community), ISGD and NGS (each describing a special average of intraspecific genetic diversity). Each component was quantified by measures of diversity representing four orders of the Renyi/Hillfamily. The orders correspond to the degree to which prevalence of types is considered in the diversity measure (at the lowest order, known as richness, prevalence is disregarded, with increasing order, the diversity measure reports prevalent types only). In our data, the diversity measured for each genetic trait separately showed a great range of variation across traits and components of diversity even in the same stand. The choice of the diversity component thus turned out to have a substantial effect on the assessment of the level of genetic diversity within stands. This prompted more detailed studies of the relationships between species and genetic diversity. Relationships were quantified with the help of the coefficient of co-variation, and the statistical significance of the co-variations was verified through permutation tests. The co-variations between SD and TSGD were found to be generally positive and in most cases significant, but the co-variation declined with increasing orders of diversity for most of the genetic traits. In contrast, the co-variation between SD and ISGD was not consistent for the four orders of diversity. In particular, the co-variations for the highest order were found to be negative for all traits. The results of our explorative study thus demonstrate that the assessment of levels of genetic diversity within stands as well as species-genetic interrelations critically depend on the choice of the diversity component, of the order of diversity, and of the genetic trait. These observations lend support to different and even opposing hypotheses on the processes potentially generating species-genetic relationships. Therefore, strategies in the conservation of biodiversity, for example, are suggested to be related more specifically to the components and orders of diversity to be safegarded and to consider the functions of genetic traits in relation to adaptationally relevant environmental factors.

The Role of Climatic Variables for Estimating Probability of Abundance of Tree Species

ABSTRACT n We analyzed the influence of climatic variables on the abundance of native tree species in 1,490 sampling plots systematically distributed in the Sierra Madre Occidental (state of Durango, Northwestern Mexico, 26°50′ and 22°17′N and 107°09′ and 102°30′W). We used the Weibull distribution and the finite Gaussian mixture model to study the climatic limits of 15 tree species in relation to seven variables thought to affect species abundance. We found that although they may occur in the same geographical region, some species display a wider range of ecological tolerance than others. Of the 15 species under study, only two (Quercus magnoliifolia and Q. arizonica) can be considered generalists in relation to some climatic variables, while the other 13 species behaved as specialists, implying a narrower range of distribution. The analytical techniques used enabled us to demarcate the zones in which the probability of abundance of each species is highest in relation to the climate variables considered. The findings could be used to help define climate for the 15 studied tree species of economic and ecological interest.

Genetic structure of remnant black poplar (Populus nigra L.) populations along biggest rivers in Serbia assessed by SSR markers

Abstract Black poplar (Populus nigra L.) is a keystone species of riparian softwood forests along riversides in vast areas of Europe, Western Asia and Northern Africa. Since the end of the 20th century, black poplar has been recognized as an endangered species throughout Europe due to the loss of its natural habitat and possible crossbreeding with hybrid poplars. Using twelve nuclear SSR loci, we analysed the genetic structure of four native populations from three river valleys in the northern part of Serbia. All tested loci were highly polymorphic, displaying 8 to 25 alleles per locus, overall 179 detected alleles and an average effective number of alleles 5.87. Observed heterozygosity (overall Ho = 0.703) has been lower than the expected (overall He = 0.808) in each population, which indicates positive mean of fixation index values (overall Fis > 0 (0.132)). An AMOVA analysis revealed that the highest degree of genetic variation occurred within populations (95.33 %) while the genetic variation between populations was really low (4.67 %). High gene flow and no significant loss of allelic diversity have been recorded in the studied populations in Serbia.

Differentiation of six Eucalyptus trees grown in Mexico by ITS and six chloroplast barcoding markers

Abstract Different species of the genus Eucalyptus, originally native to Australia, are being cultivated in different parts of the world due to their fast growth and beneficial wood properties. In Mexico, probably up to 25 different Eucalyptus species (many of them with unknown species declaration) were introduced early in the 20th century. Many Eucalyptus species are cross compatible and information about provenances of the single eucalypt species is rare. In this study, an experimental plantation established in 1984 and located in Northeast of Mexico was chosen as example to re-assign the species name of six randomly selected Eucalyptus trees growing in this plantation. First, a phylogenetic tree was constructed from complete chloroplast sequences of 31 Eucalyptus species available in the NCBI database. The phylogenetic tree includes three of the nine Eucalyptus species known to be introduced to Mexico, namely E. camaldulensis, E. saligna and E. grandis, which belong to a clade named “Symphyomyrts”. By employing combined BLASTN and UPGMA analyses of six chloroplast (cp) regions, three of the six unknown eucalypt samples (Euc4, 5, 6) cluster together with E. microtheca and E. cladocalyx, whereas the other three (Euc1, 2, 3) were more similar to a group containing E. camaldulensis, E. grandis and E. saligna. UPGMA analysis of the ITS region overall shows the same rough clustering, but provide more detailed information for two samples being most likely assigned to E. camaldulensis.