Effect of management on the mechanical stability of lakeside reeds in Lake Constance-Untersee

1. Lakeside Phragmites australis reed beds were mown or bumt in winter as a eountermeasure against the die-back of reeds in Lake Constanee-Untersee. 2. The effeet of winter harvesting and buming on the meehanieal resistanee of reeds against waves and washes was investigated using the PMR model. This model postulates that the pereentage of meehanieally damaged shoots of a reed stand is a monotonieally declining funetion of the mean stern diameter. the relative thiekness of the stern wall, the modulus of elastieity, and the shoot density per square metre. 3. Winter harvesting and buming influenees the geometrie (stern diameter, Tel. wall thiekness) and the meehanie properties (modulus of elastieity, breaking stress) of eulms in eaeh shoot class (primary shoots, seeondary shoots, inseet infested shoots). The management leads to a shift in stand strueture from the dominanee of stout primary shoots to the dominanee of weaker and thinner seeondary shoots. The total shoot density is inereased. 4. Mown and bumt reeds exhibit thinner eulms, on average, with lower moduli of elastieity at higher shoot densities. Their power of meehanieal resistanee (PMR) is substantially redueed in eomparison to the untreated referenee. Aeeording to this model it is assumed that mown or bumt stands lose signifieantly more eulms and biomass by failing than untreated stands. 5. In Lake Constanee-Untersee meehanieal faetors (waves + washes) are of great signifieanee for the development of the lakeside reed border. In this regard, winter harvesting and burning eannot be reeommended as a measure against reed decline.