Die Initialphase der Vegetationsentwicklung nach Windwurf in Buchen-Wäldern auf Zechstein- und Buntsandstein-Standorten des südwestlichen Harzvorlandes

Abstract

Vegetation development following a local summer storm in 1997 was studied in windthrown beech (Fagus sylvatica) forests on base-rich Zechstein sites (gypsum karst and dolomitic limestone) as well as on acidic Bunter Sandstone in the foothills of the Harz Mountains (southern Lower Saxony) from 1998 through 2001. Plant succession and natural reforestation of these stands are discussed relative to site conditions (base-rich vs. acidic and deep vs. shallow soils), intensity of initial disturbance (unthrown stands, gaps, extensively thrown stands), previous forest management (managed stands vs. natural forests) and current treatment or lack thereof (abandoned windthrown vs. artificially afforested plots). After four years, all extensively windthrown stands were characterised by steadily increasing numbers of species. Base-rich stands showed a higher species diversity than acidic, artificially afforested plots were more diverse than abandoned, and stands on shallow soils contained more species than those on deep. All stands except those on shallow gypsum karst showed a strong expansion of the shrub layer while the field layer stagnated or slightly decreased. The following trisection could be worked out by analysing functional species groups on extensively thrown stands: 1. Base-rich, deep soils on dolomitic limestone (e.g. Hordelymo-Fagetum): dominance of forest floor species and a very individual-rich natural regeneration of the climax trees (mostly Fraxinus excelsior, further more Fagus sylvatica and Acer pseudoplatanus). 2. Base-rich, shallow soils on gypsum karst (e.g. Carici-Fagetum): stands are covered by increasing pioneer vegetation (especially herbs of the Artemisietea and Epilobietea); the population density of the natural regeneration is medium-ranged and includes Fraxinus excelsior, Fagus sylvatica, Acer pseudoplatanus, A. platanoides, Ulmus glabra and Tilia platyphyllos. 3. (More or less) Acidic soils on Bunter Sandstone (e.g. Galio odorati-, Luzulo-Fagetum): pioneer shrubs dominate (mostly Rubus idaeus, further more Sambucus racemosa, S. nigra and Rubus fruticosus agg.); the population density of natural regeneration is relatively low and dominated by Fagus sylvatica. Pioneer trees such as Betula pendula or Salix caprea hardly occured until after four years following the windthrow. Typical Querco-Fagetea species were nowhere outcompeted. Even on gypsum karst and Bunter Sandstone the forest floor species perennate under different types of pioneer vegetation and keep constant or partly increasing absolute coverages. Especially on acidic soils the course of natural vegetation depends upon the severity of disturbance. The larger the disturbed area, the more the pace of natural reforestation toward a climax community is retarded and pioneer vegetation becomes dominant. Beech sapling number and percentual share of the climax species Fagus sylvatica, strongly decrease along a gradient from unthrown stands over gaps to large disturbed areas. In small windthrown gaps, natural regeneration of beech seems to be secure, but in extensively windthrown stands the initially established shrubs outcompete the beech. In opposition to this, natural reforestation on base-rich, deep soils proceeds very rapidly and seems to be independently from the severity of disturbance. Here, natural regeneration of the climax trees and the old forest floor vegetation dominate even under conditions of large scale windthrow. The vegetation development on shallow gypsum karst stands probably represents an intermediate successional pathway. Plant succession on root plates of uprooted trees caused a (especially quantitative) floristic adjustment between these microsites and the surrounding plot which proceeded faster on shallow than on deep soils. On deep (productive) soils root plates were preferred by light indicating species and pioneer vegetation that increased their coverages stronger here than on the surrounding plot. On shallow (less productive) soils a nearly identical development on both subsites could be observed. Although one third of the whole species pool did not occur on root plates, an analysis of functional species groups showed no qualitative differences between these microsites and their surrounding and further more stable spectra within the first four years of succession on both subsites. A site-dependent gradient of natural regeneration from base-rich to acidic conditions can be shown. The more base-rich the substrate, the faster and more independently from severe extrorse disturbance natural reforestation proceeds. Among all investigated stands, beech forests on base-rich, deep soils show the most resilient response to natural disturbance. A complete species-change can´t be confirmed for Central European beech forests growing in their optimal phase. Succession in these forest communities doesn´t involve major species changes but rather temporal changes of dominance of residual species. A replacement of the typical beech forest communities caused by windthtrow can´t be expected - neither in the investigated nor in comparable areas.