Holozäne Landschaftsentwicklung und aktuelle Vegetation im Fimbertal (Val Fenga, Tirol/Graubünden)
[Reconstruction of Holocene landscape development and present vegetation of the Val Fenga (Tyrol/Grisons)]
Bauerochse, A.; Katenhusen, O.
The Val Fenga in the Central Alps has been selected as the study area of a project comprising palaeoecological and phytosociological investigations, of which the results have been connected. The valley is situated in the border district between Austria and Switzerland. Its geological-geomorphological situation is complicated due to its location in the transition zone of two different geological nappes. The eastern side of the valley is part of the Lower Engadine Window and it is composed by flysch of the Penninic nappes while the western part belongs to the Austroalpine nappes, dominated by the crystalline rocks of the Silvretta massif. As a consequence of the comparatively high degree of hygric continentality the Val Fenga belongs to the climatic region of the Central Alps. To reconstruct the holocene climate and vegetation development palaeobotanical investigations of peat profiles, and in addition, analysis of pollen dispersal have been conducted. The investigations of the actual pollen deposition are made of moss cushions out of the entire investigation area. Different groups of pollen types ("indicator units") which characterize the representation of different vegetation units and human impact in surface samples are determined by the results of the pollen analysis. The suitability of these units for the judgement of fossile pollen profiles is discussed. The main emphasis of the investigations about the climate and vegetation history was put on the palaeobotanical analysis made of five peat profiles which are situated in a transect from the lower subalpine up to the alpine belt. A survey of the investigated mires is given in Tab. 3. By a synoptic reflection the palynological results of these peat profiles are discussed, and the synchronous effects of climatic change and human impact on the landscape in different altitudes are shown. From the Boreal to the beginning of the Atlantic period a section of high climatic dynamics is shown by the pollen diagrams. During this time, the timberline, which reached almost the altitude of today's alpine belt, was depressed several times. Approximately at the beginning of the older Atlantic period Picea abies immigrated into the region during a section, which is characterized by humid climatic conditions. Sometime later, about 7000 BP, the spruce spread out at the same time the Larch-Arolla pine wood reached its highest extension. Findings of fossile Pinus cembra prove the rise of the Arolla pine to an altitude of about 2400 m asl. During the following time Alnus viridis immigrated into the region as well as Picea abies under more oceanic conditions. But because of the continental climate of the Central Alps, the species only occurs in small amounts since then. Climatic oscillations with only small effects on the timberline characterize the section of the younger Atlantic period. Only in the second half of the younger Atlantic and the following Subboreal period the intensity of the climatic changes increased and also had effects on the subalpine spruce forest. At that time the timberline reached an altitude of today's upper subalpine belt. At the end of the Atlantic period first palynological references of human impact appear, which are intensified during the following section of the Subboreal period, and which were connected with fortified settlements in the Lower Engadin. During the second part of the Subboreal period the pollen diagrams show clearings of the mountain forest downvalley to the lower subalpine belt. By the beginning of the Subatlantic period the human impact became more intensive. Sections of different human impact characterize the diagrams from the Early Middle Age. Since then the lower areas of the Val Fenga also were influenced by this impact. About 1400 BP, 1000 BP and during the period from the 15th to the 19th cent. the human impact culminated. During this time, the mountain forest was extensively destroyed. The area investigated phytosociologically in the middle and upper section of the Val Fenga contains plant communities of fourteen different classes. Basing on their vertical occurrences a separation of the subalpine and alpine belt into three resp. into two zones has been carried out. The description of the various plant communities focusses on rare syntaxa and those which are characterized by their insufficient state of investigation in the Alps up to now. This group includes the relictual Caricion atrofusco-saxatilis which occurs on alluvions, alluvial terraces and fens. In the Val Fenga the alliance is represented by five character species (Carex atrofusca, Carex microglochin, Juncus arcticus, Kobresia simpliciuscula und Tofieldia pusilla) and three associations of different ecological habitats. The Juncetum arctici appears at comparatively dry sites whilst the sparse stands of the Caricetum atrofusco-vaginatae occur on alluvions irrigated by quickly flowing water. The Caricetum microglochinis can be separated into two subassociations: the kobresietosum simpliciusculae on drier habitats and the very wet caricetosum microglochinis. Often a close relationship between the latter unit and stands of Eleocharis quinqueflora can be observed. Due to floristic, syndynamic and habitual reasons the community of Eleocharis has been adjoined as an initial unit to the Caricion atrofusco-saxatilis. As a result of their rarity, threat and the high rate of species mentioned in the Red Lists the communities of the Caricion atrofusco-saxatilis need the uppermost level of protection and conservation. According to investigations about the stratigraphy of peat layers the bryophyte Calliergon trifarium has attained a considerable importance concerning the foundation of the mire communities and the formation of peat in the higher elevated fens of the upper Val Fenga. In small pools becoming temporary dry shallow-water swards constituted by Alopecurus aequalis, Callitriche palustris, Ranunculus trichophyllus ssp. lutulentus und Rorippa islandica attract special attention. Similar vegetation types from the Alps are rarely mentioned, yet. According to the current knowledge these stands can be classified as Veronico scutellatae-Alopecuretum aequalis within the Littorelletea uniflorae. The very heterogenous composition of species of a special kind of meadows called Hypochoeris uniflora-Nardion strictae-community can be regarded as a consequence of their former irrigation by melted snow water enriched with dung and the decrease of mowing. In the subalpine belt the poorly investigated Chaerophyllo villarsii-Agrostietum schraderianae colonizes scree slopes composed of large rocks of gneisses of the Silvretta crystalline. A subtype of this association including tall herbs and named after Valeriana versifolia occurs on slippery and moist slate slopes. On stable scree slopes the Chaerophyllo villarsii-Agrostietum schraderianae develops to the heath community of the Junipero-Arctostaphyletum. A Chaerophyllum villarsii-subtype of the latter association with Laserpitium halleri, Ranunculus nemorosus s. str., Hieracium hoppeanum und Poa chaixii represents a conspicuous stage of this succession.
Central Alps • pollen analysis • vegetation history • timberline • human impact • Caricion atrofusco-saxatilis • Veronico scutellatae-Alopecuretum aequalis • Chaerophyllo villarsii-Agrostietum schraderianae