Michael Dienst

Disturbance and rehabilitation of lakeside Phragmites reeds following an extreme flood in Lake Constance (Germany)

After an extreme flood in Lake Constance in 1999 the Phragmites australisbelt showed a severe decline in area and vitality. A three year monitoring project was installed in 2000 to document the die-back and rehabilitation process immediately afterwards, to identify the co-factors of the damage, and to find out significant stressors that may impede the recovery of the reeds. The monitoring is based on CIR aerial photo interpretation, quantitative GIS analyses and field investigations on shoot density, stand structure and biomass production in 50 monitoring plots, grouped in five degrees of damage. In result we found that c. 0.306xa0km2 of aquatic reed bed area died back at Lake Constance-Untersee (i.e. 23% of the former area in 1998). Among the stands which had survived the severely damaged stands were mainly composed of secondary shoots, whereas primary and insect infested shoots dominated in less damaged stands. The development from 2000 to 2001 was characterized by an overall decrease in shoot density, a change in the composition of the shoot population in favour of primary shoots, and in a recovery in culm stature. All variables depended on the degree of initial damage by the extreme flood. A conceptual model is proposed to assess the future development of Lake Constance reeds.

Auswirkungen des Extremhochwassers 1999 auf die Flächendynamik und Bestandsstruktur der Uferröhrichte des Bodensees

Abstract In consequence of the extreme flood at Lake Constance in 1999 the reed belts showed a severe loss in vitality so that a permanent damage could be expected. The monitoring project presented here focuses the documentation of the damage following the disturbance event, the identification of co-factors, and is designed to find out significant stressors that may impede the recovery of the reeds. The monitoring is based on CIR aerial photo interpretation, quantitative GIS-analyses and field data. The design of the study implies the definition of five degrees of damage using the interpretation of CIR aerial photo series from July 2000. The aquatic reed beds were digitised and areal sums were calculated. These data were compared with the corresponding data from 1999 and 1993. Stand structural and biomass attributes were investigated in 50 randomly distributed permanent plots covering all site types and damage degrees. The results showed a loss of about 24 hectares of aquatic reeds at Lake Constance Untersee alone. The stands which had survived showed a great patchiness in terms of culm density, shoot population composition, culm architecture, and aboveground biomass production. Severely damaged stands were mainly composed of secondary shoots, whereas primary and insect infested shoots dominated in less damaged stands. One co-factor which explained the patchiness was the vertical position of the stand, i.e. the probability of being severely damaged increased with increasing distance below mean water level. The monitoring programme will be continued till 2003.

Dynamik der Schilfröhrichte am Bodensee unter dem Einfluss von Wasserstandsvariationen

Abstract On the basis of different sets of aerial photos the dynamics of the reed bed areas of Lake Constance were investigated in relation to the dynamics of the water levels. The objectives of the study were to quantify the changes of reed areas due to different flood events in the last decades and their recovery in the time periods between these events. The results should given information of the relevance of water level variations on reed bed dynamics and the regeneration times of reed beds after extreme disturbance events. Following the extreme flood at Lake Constance in 1999 the reed belts of Lake Constance lost approximately 30 ha (24%) of the lakeside reed beds. The loss is comparable to the situation in the late 1960s, when approximately 40 ha died back due to the extreme flood in 1965 and the high spring water levels in the subsequent years. In the time period between the extreme floods of 1965 and 1999, the reed areas expanded to nearly 85% of the area before 1965. The expansion rates increased with increasing distance to the flood event of 1965. Especially in periods with series of years of low spring water level the expansion rates were high. The damage degrees of the reed areas in the years 2000 and 2002 showed a clear relation to the elevation (i.e. average water level) of the stands. The damage degree increased with decreasing elevation. Furthermore the regeneration process of severely damaged stands was related to the elevation level of the stands. Whereas stands at high elevation regenerate fast, those at low elevation died off completely in the years after the extreme flood. This supports the hypothesis that the water level flutuations play a major role in the reed dynamics of Lake Constance. As a consequence of the climate change an increase in the frequency of high spring water levels is expected. Thus, it seems unlikely that reed stands will ever expand again to the same area as before 1965.

Die Auswirkungen der Wasserstände am Bodensee auf das Deschampsietum rhenanae zwischen 1989 und 2003

Abstract The Deschampsietum rhenanae is an endemic and endangered lakeshore community growing in the upper littoral of Lake Constance, the occurrence of which has been reduced dramatically during the last 100 years. The existence of this community depends on the seasonal water level changes of the unregulated, prealpine Lake Constance. Long-term monitoring indicates that the typical species of the Deschampsietum rhenanae and their competitors may be strongly affected by extreme water levels. An earlier onset of the water level increase in spring could potentially pose a threat to the Deschampsietum rhenanae, especially if the water level rises before seed ripening in the Deschampsietum rhenanae.

Hydromorphologische Übersichtserfassung, Klassifikation und Bewertung von Seeufern

Viele der mehr als 20 000 größeren euroxad päischen Seen sind nicht nur durch stoffxad liche Einflüsse (eutrophierende Nährstofxad fe, Xenobiotika, saure Depositionen), sonxad dern auch durch hydrologische und morxad phologische Eingriffe des Menschen bexad einträchtigt [4]. Insbesondere die Uferzoxad ne ist häufig durch Auffüllungen, Uferverxad bau, intensive landwirtschaftliche Nutxad zung, Urbanisierung, Verkehr und Freixad zeitnutzung erheblich verändert. Um die strukturellen Beeinträchtigungen zu idenxad tifizieren, ihre mutmaßlichen Auswirxad kungen auf die Biozönosen zu charakterixad sieren und im räumlichen Zusammenxad hang darzustellen, bedarf es eines geeigxad neten Erfassungsxad und Klassifikationsverxad fahrens. In den vergangenen Jahren wurxad den international drei unterschiedliche Verfahren vorgestellt [1], [2], [5], [6], [7], die jedoch mit spezifischen Schwächen bexad haftet sind [3]. Vor diesem Hintergrund wurde ein deskxad topxadorientiertes Übersichtsverfahren zur raschen Erfassung und Klassifikation von strukturellen Beeinträchtigungen der Seexad uferzone beiderseits der Mittelwasserlinie entwickelt. Seine wesentlichen Merkmale liegen in einer strengen Trennung zwixad schen Erfassungsxad und Bewertungsschritxad ten, der Nutzung bereits vorhandener Daxad tenquellen, einer raschen Durchführbarxad keit mit höchstens kursorischer Geländexad erkundung, der weitgehenden Revisionsxad freundlichkeit sowie in seiner Anwendbarxad keit auf die meisten natürlichen und künstxad lichen Stillgewässertypen Europas. Das Verfahren ist dazu geeignet, einige Aufgaben bei der Umsetzung der WRRL und der FFHxadRichtlinie mit dem Schutzxad gebietssystem Natura 2000 zu erfüllen [3]. So fordert die WRRL in bestimmten Fälxad len neben der Erfassung biologischer Quaxad litätskomponenten auch eine Erfassung der hydromorphologischen Qualitätskomxad ponenten, u. a. des Zustands der Seeufer (Anh. V, 1.1.2 WRRL). In der FFHxadRL ist den EUxadMitgliedsstaaten aufgegeben, die FFHxadLebensräume in ausreichendem fläxad chenmäßigen Umfang und in günstigem Erhaltungszustand zu bewahren oder wiexad der herzustellen.