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The ecology of downy oak (Quercus pubescens Willd.) and its hybrids on limestone at the northern limits of its distribution - a study of the interrelations between soil, climate, and vegetation

The Downy Oak (Quercus pubescens Willd.) in Southern Germany is a rare tree species. It hybridizes with Sessile Oak (Quercus petraea) and occurs in association with several introgressive hybrids. While it forms zonal forests in the sub-mediterranean region, it reaches the northern limits of its distribution north of the Alps. There it is restricted to dry and base-rich sites. When studying the site preferences of Downy Oak, the ecotone from Downy Oak-forests to neighboring forest types is of special interest. Apparently, changes of certain ecological threshold values lead to the replacement of the dominant canopy trees and forest communities associated.
Aim of this study was to identify parameters which explain the ecotone between Beech (Fagus sylvatica) and Downy Oak-forests, with the main focus on soil water storage capacity and climate. Special attention was given to the differentiation of morphologically pure Downy Oak (Quercus pubescens) and its morphological hybrids (Quercus petraea x pubescens). The differences of the site amplitudes for these two “taxa” were determined. Several phytosociological methods were applied, including   “classic” approaches, cluster and factor analyses. The five study sites on forested slopes with stands of Downy Oak are located in the submontane zone of the Klettgau, and in the montane zone of the central Swabian Alb (Southern Germany). Within each of the oak forests, the adjacent ecotone and beech forest, relevés (10 x 10 m) were placed in a systematic grid system. Within each relevé, parameters of soil, species, and stand structure were recorded. The initial analysis was conducted on an individual stand basis, due to the considerable local differences in species composition. In a first step, for each of the five stands a cluster analysis was used to delineate  relevé groups based floristic similarities. As a result, for each forested slope a Beech forest type could be distinguished from a Downy Oak forest, and a third group of relevés representing a transitional species assemblage. Using factor analysis, this transition could be explained as a gradient of water supply. It could be shown, that the individual ecotones of the five stands were comparable in terms of site parameters and floristic changes. These five phytosociological classifications were used for a general factor analysis across all stands with the entire data set. The competitive relations between tree species could be analyzed along drought gradients in the submontane and montane zone.
Syntaxonomically, all Downy Oak forests studied can be assigned to the association Quercetum pubescenti-petraeae (Central European Downy Oak-forest). In Southern germany, this forest community can be found as isolated stands surrounded by the more frequent association Carici-Fagetum (dry limestone Beech forest). Ecologically, the forests dominated by Downy Oak can be subdivided into a “pure” Downy Oak-forest, and a “transitional forest”. It could be shown, that the majority of the individuals between Sessile and Downy Oak morphological hybrids. Morphological Downy Oaks (Quercus pubescens) mostly were restricted to drier sites within ”pure” Downy Oak-forest. Hybrids became more frequent with improving water supply and elevation, they dominate the transitional forest to the true Beech forest.
In the sumontane zone, the boundary between Downy Oak- and Beech forest is a narrow andclearly defined ecotone. In the montane zone, the transitional zone is more continuous and less clearly defined.Beech, oak hybrids, few downy oaks, and other tree species cooccur and form a mixed transitional stands.It could be noticed, that in montane ecotones all species showed decreased vitality. Spindly Beeches to grew near their drought limit, and the vitality of  downy oaks was reduced by low temperatures of the higher elevation. The reduced vitality of beech and oaks enabled a number of other tree species to compete, e.g. sessile oak, field maple (Acer campestre), common ash (Fraxinus excelsior).  The analysis of the relationship between soil water storage capacity and climate, represented by radiation , showed that the shallowness of the soil is of paramount importance for the occurrence of downy oak forests. The soils bearing forests with Downy Oak-forests have a soil water storage capacity between 30 and 70 l/m². Within this range, downy oak forest can occur.  The radiation sum itself is determined by slope inclination and exposition. It could be shown, that the term “drought” represents a complex factor, and is determined by soil water storage capacity, climate, and length of vegetation period. For example, a long vegetation period and/or southwest exposed steep slopes induce water stress sufficiently large to prevent beech from dominating the forests. And even west exposed slopes may be occupied by Downy Oak forest when soil is shallow enough.

Funding: DFG
Supervisor: Prof. Dr. Albert Reif
Researcher: Uwe Sayer (Dissertation 1999)




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