Open Access

Niels Böhling

• Die Dynamik der Bodenvegetation in einem frühjahrsfeuchten, kalkreichen Laubmischwald (Querceto-Carpinetum) auf Pelosol wird über einen Zeitraum von 17 bis 22 Jahren untersucht. Die Individuenzahlen von Scilla bifolia L. s.str. nehmen zwischen 1978/79 und 2000 stark ab. Ebenso ist bei weiteren mesophilen Arten ein Rückgang zu verzeichnen. Nur zwei Arten nehmen zu: Mercurialis perennis um im Mittel 80,0%, im Maximum um 171,4%, sowie Allium ursinum um im Mittel 75,0%, im Maximum um 127,3%. Zur Kausalanalyse herangezogene gewichtete Mittelwerte ökologischer Zeigerwerte weisen deutlich auf eine starke Abnahme des Lichteinfalls am Waldboden, eine starke Zunahme der Stickstoff- bzw. Nährstoffversorgung und der Bodenreaktion sowie auf eine Abnahme der klimatischen Kontinentalität und einen Anstieg der Luftemperatur hin. Die phytoindikatorisch angezeigte Abnahme des relativen Lichtgenusses wiederspiegelt 1. die seit der Durchforstung 1978/79 dichter werdenden Strauch- und Baumschichten, 2. außerdem das Vordringen der stark schattenden Buche sowie 3. eine starke Beschattung der Krautschicht durch sich selber (höherwüchsige Geophyten). Der Anstieg der Bodenreaktion und die Erhöhung des Nährstoffangebotes im Boden werden zurückgeführt auf 1. allgemeine atmosphärische, aber auch speziell am Stadtrand wirksame Stickstoff- und Staubeinträge, 2. die Regeneration der Böden nach früherer wenig nachhaltiger Nutzung (mittelalterliche Waldnutzung, Wirksamkeit der Basenpumpe Baum, Entfernen zwischenzeitlich gepflanzter Kiefern) sowie 3. auf eine möglicherweise gestiegene Mineralisationsrate in wärmer und kürzer werdenden Wintern. Die Erhöhung der mittleren Temperaturzahlen entspricht der tatsächlich festgestellten Klimaerwärmung im Untersuchungszeitraum, speziell in den Wintern. Die Reduzierung der Kontinentalitätszahlen reflektiert eine zunehmende Ozeanität des Regional-Klimas, aber auch ein thermisch ausgeglicheneres, luftfeuchteres Bestandesklima. Hypothesen zur lokalen und regionalen Arealbildung von Scilla bifolia und Allium ursinum werden vorgestellt.
• Population dynamics of the forest-floor vegetation of a spring-wet mixed deciduous forest (Querco-Carpinetum) on calcareous Pelosols is studied in four permanent test plots of 100 m2 each. The total population of the Alpine Squill Scilla bifolia L. s.str. decreased by 60.5% between 1978/79 to 2000 (mean of all plots, frequency measured by the number of 4 m2 quadrats inhabited; max. -74.3%). The species has vanished from some 1 m2 quadrats completely. A strong decline between 1978/79 and 1995 is also observed for the Wood Anemone Anemone nemorosa (mean -22.4%, max. -80.0%). Other clearly decreasing species are Arum maculatum, Viola reichenbachiana, Phyteuma spicatum and Polygonatum multiflorum. Lamium galeobdolon and Ficaria verna behave as constant elements in 4 m2 quadrats. Only two species increase: Dog’s Mercury Mercurialis perennis (mean +80%, max. +171.4%) and Ramsons Allium ursinum (mean +75.0%, max. +127.3%). The increase rate and number of 4 m2 quadrats inhabi­ted is highest for the latter. Averaged weighted ecological indicator values (Ellenberg values) indicate a clear, strong decrease in light at the forest floor (summarised weighted means: 2 mL‘ = -1.28), a clear, strong increase of nitrogen and other nutrients (2 mN‘ = +1.07) as well as soil reaction (2 mR‘ = +0.73), and a significant decrease of continentality (£ mK‘ = -0.73) and increase of temperature (J mT‘ = +0.16). The reduction of light results from 1) the increasing coverage of the shrub and tree layer after regular thinning in 1978/79 (“Schonwald” status); 2) the growing up of the most shade-producing Fagus sylvatica; and 3) intensive shading of the herb layer by its own taller geophytes. Increasing soil reaction and nutrient supply is due to: 1) athmospheric and urban nitrogen and dust deposition; 2) soil regeneration after degradation (medieval forest utilisation, transport of nutrients from lower soil strata to upper layer by trees, removal of planted soil - (acidificating pines); and 3) increasing mineralisation by warmer and shorter winters. Increased mean indicator values for temperature correspond to the warming climate, as revealed by tem­perature measurements especially in winters of the last decade. The reduced mean continentality reflects a regional climate that is becoming more Atlantic as well as a micro-climate that is becoming more con­stant in temperature and more humid, due to the closing of the tree layers. Ellenberg values and the method of calculating weighted means proved to be reliable instruments for interpreting ecological conditions. For Anemone nemorosa, a frequent species in the study area, local indicator values for soil reaction and nutrients have been assigned (Ri0k = 5, Ni0k = 5). The species is clas­sified as indifferent in Central Europe. Changing environmental conditions favour highly competitive plants (Allium ursinum, Mercurialis perennis), which replace most of the others by shading. Competition is important in addition to the abiotic factors. Infection with anther rust (Ustilago vaillantii) damages Scilla bifolia populations drama­tically. For the conservation and regeneration of the Scilla bifolia populations, canopy thinning is recom­mended as a first trial. This was done partly by the storm „Lothar“ Weakening Allium ursinum by cut­ting and removing its leaves could be a further measure. Only the tree layer is relatively constant in forest ecosystems. The forest-floor vegetation showed significant quantitative as well as qualitative changes (invasion and extinction in test plots) in just one to two decades. These changes result from climatic change and variation, the transition from coppice-with-standards to high forest since c. 1900, and atmospheric nitrogen deposition. As regards the irregular distribution of Scilla bifolia in Baden-Württemberg, a hypothesis is offered: The species is occurring near its climatic (continentality) boundary, which means that its ecological niche is quite narrow. As a short-lived species that reproduces generatively and is only weakly competi­tive, it is only slightly persistent and is highly vulnerable to (local) extinction. The Alpine Squill is sensi­tive to environmental modifications. The probability of population collapses leading to distribution gaps is high for natural reasons. Its very limited ability to re-colonise plus local introductions by man, contribute additionally to its particular distribution pattern. In Germany the area has to be explained primarily by ecological reasons and population biology, and secondarily by history („tertiary relict“). Allium ursinum can be characterised as a species preferring Atlantic climatic conditions. Its dynamics are probably determined primarily by climatic fluctuations, especially winter temperature.