Department of Botany, Faculty of Biological Sciences, University of South Bohemia, České Budějovice
Supervisor: Prof. Milan Chytrý (Masaryk University Brno)
In the middle elevations of the Czech Republic, deeply incised river valleys form distinct geomorphological feature, with steep slopes and sharp upper edges contrasting to the otherwise flat or softly undulating surrounding landscape. Most of these valleys are of late Tertiary and early Quaternary origin when the uplift of the Bohemian Massif resulted into increased erosion power of rivers (Kopecký 1996). Geomorphology of these valleys was further shaped during Pleistocene periods of glaciation when intensive frost weathering occurred as a result of periglacial climate (Kopecký 1996). Main abiotic features of these valleys are related to rugged topography and specific microclimatic conditions: steep slopes with exposed rocky outcrops, diversity of landform shapes, variability in slope aspect with sharp contrast between warm south and cold north facing slopes, and also frequent temperature inversions, resulting from the valley shape and pronouncing the contrast between cold and wet valley bottom and dry continental upper valley edges. Important biotic consequences of these features are (1) high diversity concentrated in these valleys due to concentration of various, often ecologically contrasting habitats, (2) occurrence of relict species, reflecting the role of valleys as a refuge during glacial and postglacial period, (3) function of river valley as migration corridors between mountains and lowlands, with migration of both downstream and upstream direction (the latter facilitated due to the frequent occurrence of suitable dry and warm habitats within the valleys in higher altitude), and finally also (4) conservation of vegetation less affected by human activities in hardly accessible sections of the valleys. Specific features of the vegetation pattern in these valleys were summarised under the heading “river phenomenon” in the descriptions provided by Czech vegetation scientists in the 1960's (Blažková 1964; Jeník & Slavíková 1964). A concentration of strong ecological gradients within limited space of the valley together with the fact, that these valleys are the main source of the topographical heterogeneity in the middle elevations of the Czech Republic, makes them an attractive model for studies searching for environmental correlates of vegetation and plant diversity patterns at the landscape scale.
This thesis tries to describe the vegetation of deep river valleys from two different viewpoints: from the local perspective, trying to untangle effects of environmental factors on the pattern of vegetation and species richness within the valleys, and from the landscape perspective, putting species richness of topographically heterogeneous valleys into the context of surrounding homogeneous landscape. This thesis consists of four papers – three case studies and one methodological study; one of them is already published, one is in press, one is submitted and one is a manuscript. These are the main questions covered by individual papers:
Paper 1 (published in Preslia) brings a quantitative description of the vegetation-environment relationships in deep river valleys, using data from two areas differing markedly in both climatic and floristic characteristics. The performance of two main groups of environmental variables, topographical and soil, as explanatory variables in models describing the vegetation patterns in these valleys, is assessed by a set of canonical correspondence analyses. The link between particular environmental variables and main ecological gradients is analyzed by correlation analysis with Ellenberg indicator values. New method was invented for analysis of joint effect of two environmental variables on vegetation, in this case of aspect and the height above river valley.
Paper 2 (manuscript) analyzes the pattern of local species richness within two deep river valleys and its environmental correlates. Using General Linear Models, we built two sets of models, one using only spatial variables and aiming to arrive to spatially explicit model of species richness within the valley, and the second using ecological (topographical and soil) variables. Similarities and dissimilarities between the two valleys are interpreted in terms of local ecological processes and differences in composition of regional species pools. Local species richness is compared to the size of regional species pool for individual forest habitat types, using published estimates of species pool for particular habitat types, based on data from large vegetation database and modified by expert knowledge.
Paper 3 (at the time of defence submitted manuscript, later published in Ecography) tries to answer a more ambitious, general question: does the species richness change along the gradient of landscape topographical heterogeneity between heterogeneous river valleys and homogeneous surroundings? Observed pattern is interpreted as a result of fragmentation, spatial mass effect and alternatively also shift in habitat ecological conditions and processes related to the patterns of species richness along environmental gradients. Ratio of habitat generalists and specialists is used as an indication of spatial mass effect.
Paper 4 (published in Journal of Ecology) describes correction of the method used for the assessment of species habitat specialization. The method was invented by Fridley et al. (2007) and is based on co-occurrence data from large vegetation databases. However, I found that the original algorithm does not give reliable estimates of habitat specialization, as the used additive measure of beta diversity is affected by the size of the species pool. I proposed correction of this method, supported by results of both simulated and real data analyses. Corrected version of the algorithm was used in Paper 3 for estimation of species habitat specialization.
The answers to the main questions outlined in the introduction can be summarized in the following points: