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Patterns of vegetation diversity in deep river valleys of the Bohemian Massif

PhD thesis

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 summarized under the heading “river phenomenon” in the descriptions provided by Czech vegetation scientists in the 1960's (Blažková 1964; Jeník & Slavíková 1964). 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 interesting model for studies searching for environmental correlates of vegetation and plant diversity patterns at the landscape scale.

Outline of the thesis

This thesis tries to describe the vegetation of deep river valleys from two different viewpoints: from local perspective, trying to untangle effects of environmental factors on pattern of vegetation and species richness within the valleys, and from landscape perspective, putting species richness of topographically heterogeneous valleys into the context of surrounding homogeneous landscape. 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:

  1. What is the relationship between species composition of vegetation and the main ecological gradients in deep river valleys?
  2. Which environmental factors are the best predictors of the local species richness in these valleys and how can be diversity-environment relationship influenced by differences in regional species pool?
  3. What is the effect of landscape topographical heterogeneity on the local species richness and which ecological processes may cause this effect?

Paper 1 (published in Preslia) brings quantitative description of the vegetation environment relationships in deep river valleys, using data from two areas differing markedly in both climatic and floristic characteristics. 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 set of canonical correspondence analyses. 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.

Summary of results

The answers to the main questions outlined in the introduction can be summarized in the following points:

  1. Relationship between species composition of vegetation and main environmental factors within the valleys:
    1. vegetation in deep river valleys is structured along two main complex ecological gradients: the moisture-nutrients-soil pH and the light-temperature-continentality; the first one is related to the elevation above valley bottom, the second one is related to aspect;
    2. the effect of aspect is pronounced the most in the middle parts of the valley slopes, while being lowest at the shaded valley bottoms;
    3. among the other important topographical variables are (in addition to the elevation above valley bottom and aspect) slope and landform shape of the plot in downslope direction; among important soil variables are occurrence of Fluvisols, Cambisols and skeletic soils, soil depth and measured soil pH;
  2. Relationship between local species richness and environmental variables within the valleys:
    1. the highest local species richness within the valley is located at the valley bottom and at the south and west facing upper edges of the valley slopes;
    2. soil pH is a strong predictor of species richness, but only in case of Vltava river valley with predominating acid soils with values of pH<4.5; in case of Dyje valley, where the soils are generally more basic (perhaps as a result of drier and warmer climate due to lower elevation), the effect of soil pH on species richness is negligible;
    3. important factor related to the high local species richness in case of the Dyje valley is continentality, resulting probably from the higher proportion of continental species in regional species pool of Dyje valley due to its geographical location at the boundaries between Hercynian and Pannonian floristic district;
    4. local species richness is positively correlated with the size of regional species pool estimated for particular habitat types (with exception of oak-hornbeam forests); this indicates that estimates of species pool size itself may be a good predictor of real local species richness;
  3. Relationship between landscape topographical heterogeneity and local species richness of particular vegetation types:
    1. generally, nutrient-poor vegetation types are more species rich in topographically heterogeneous landscape, while the opposite is true for nutrient-rich vegetation types;
    2. nutrient-poor vegetation types (e.g. oak forests) have high proportion of habitat generalists, indicating that their higher species richness in heterogeneous landscape may be result of pronounced spatial mass effect;
    3. the pattern of local species richness along the gradient of landscape topographical heterogeneity may be also attributed to the shifts in stand ecological conditions: at heterogeneous landscape, the stands have higher soil reaction (valid for almost all vegetation types), and also higher productivity (valid only for nutrient-rich vegetation types).
  4. Additionally to the three case studies also one methodological study was included (Paper 4). It points up the problem of the method for estimation of species habitat specialization, as originally published by Fridley et al. (2007), showing that the result is affected by the size of species pool. Corrected version, using multiplicative beta diversity measure alternatively to the originally used additive measure, is proposed.