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

Abstract

1. Lakeside Phragmites australis reed beds were mown or burnt in winter as a countermeasure against the die-back of reeds in Lake Constance-Untersee.<br />2. The effect of winter harvesting and burning on the mechanical resistance of reeds against waves and washes was investigated using the PMR model. This model postulates that the percentage of mechanically damaged shoots of a reed stand is a monotonically declining function of the mean stern diameter, the relative thickness of the stern wall, the modulus of elasticity, and the shoot density per square metre.<br />3. Winter harvesting and burning influences the geometric (stern diameter, rel. wall thickness) and the mechanic properties (modulus of elasticity, breaking stress) of culms in each shoot class (primary shoots, secondary shoots, insect infested shoots). The management leads to a shift in stand structure from the dominance of stout primary shoots to the dominance of weaker and thinner secondary shoots. The total shoot density is increased.<br />4. Mown and burnt reeds exhibit thinner culms, on average, with lower moduli of elasticity at higher shoot densities. Their power of mechanical resistance (PMR) is substantially reduced in comparison to the untreated reference. According to this model it is assumed that mown or burnt stands lose significantly more culms and biomass by failing than untreated stands.<br />5. In Lake Constance-Untersee mechanical factors (waves + washes) are of great significance for the development of the lakeside reed border. In this regard, winter harvesting and burning cannot be recommended as a measure against reed decline.