The effect of weed cutting on Luronium natans

Luronium natans is a rare macrophyte that seems to be disappearing from lakes and rivers throughout its distribution area. In Denmark L. natans is most often found in man-made canals and ditches, and its presence seems to be favoured by regular physical disturbance. Hence, management by cutting presents a possible method of preserving the few remaining populations of L. natans in Denmark and throughout its distribution area. The objective of this study was to elucidate the effect of different weed cutting frequencies on L. natans in order to propose recommendations for management with weed cutting for future conservation of L. natans. Three reaches of a river with L. natans were subjected to the following weed cutting frequencies: no cutting, twice per year, and four times per year, and the cover of L. natans and of other plants was monitored from June 2003 to October 2004. L. natans developed highest cover when subjected to weed cutting four times per year, but the frequency of flowering was then very low, resulting in a reduction in sexual reproduction, which could lead to a lower genetic diversity and possibly fatal effects in the long term. At a cutting frequency of twice per year L. natans was able to maintain healthy and sexually reproducing populations. Future management in canals should consist of weed cutting once or twice per year to prevent a population from being outcompeted by other species and to secure production of flowers and seeds. Alternatively, high frequencies of weed cutting could be performed in order to suppress competitive species, but should then be accompanied by a weed cutting-free zone where flowering can occur.