The common reed (Phragmites australis) is a plant familiar to almost everyone. It is a tall plant commonly found on riverbanks, lakeshores and in wet soil habitats. They are often perched on by birds and dragonflies. Perhaps less well known is that it is a herbaceous perennial plant taxonomically classified as a grass. As is typical of perennial grasses, the above-ground parts die off each year, storing energy in the root system - rhizomes in the case of common reed. It spreads by seeds, which mature in dense tufted inflorescences, but vegetative dispersal from root and stem parts is also very important. Over the years, the underground system of rhizomes strengthens and many stems grow from the same rhizome, which can form dense stands of reeds.
The common reed is widespread and common, and has no major threat factors. In North America, it is even classified as a highly invasive species. It is often a nuisance and we would prefer to eradicate it by mowing and burning. Why, then, is it actually receiving conservation attention in Lake Cerknica?
The ecological importance of reeds through the seasons
Have you ever set foot in an old reed stand? Not exactly the epicentre of plant diversity. Here and there you might see a blade of grass struggling to climb up to the light on the shaded ground. The ground is covered with years of dead stalks and leaves, creating an impenetrable labyrinth where few animals can be found. The stands are so dense that after a few steps one loses the sense of the direction of the sky. Yet reedbeds are one of the most important habitat types on Lake Cerknica, acting as a refuge, spawning, roosting, nesting and feeding ground for birds and many species. The role of reedbeds depends strongly on the extent and structure of the stand, the season and the water level.
In winter, when the water level is high and the water is cold, the common pike (Esox lucius) and the tench (Tinca tinca) deposit their eggs deep below the water surface in the reeds. Although the reed is dormant, the old woody stems provide cover from other predators and the vegetation also protects them from the water current, which is mild but persistent. In the slightly deeper water of the Stržen riverbed, the pike stops at the edge of the reeds and waits patiently for its prey. Later, at the start of spring, brown frogs make their way out of the woods. The best places to deposit red-backed woodpeckers (Rana temporaria) are shallow open water areas surrounded by dense stands of reeds, which will allow the tadpoles to escape quickly to safety. Such waters could be called lake windows.
In spring, birds also start nesting. The nesting sites of the chestnut-collared warbler (Aythia nyroca) and the brown-rumped warbler (Podiceps grisegena) are preferably lake windows, where the shallow water warms up quickly and there is plenty of food available. Similarly, the coot (Fulica atra), the tufted puffin (Podiceps cristatus) or the greylag goose (Anser anser) and other species are somewhat less picky. Reeds also provide ample nesting material, which they weave into a nest among the dried stalks, and the succulent young shoots of the plants are readily available for feeding. In addition to the birds, the early reed warbler (Brachytron pratense) and numerous juvenile reed warblers (Trichoptera sp.) start flying around in the reeds. In the evening, the chorus of the green reed warbler (Hyla arborea) is joined by the drumming of the drummer warbler (Botaurus stellaris), and the little toucan (Porzana parva), the marsh harrier (Rallus aquaticus), and certainly several other species, are also heard in the reed beds during the day and evening. During the day, the air is filled with the chirping of the reed warbler (Locustella luscinioides) and numerous reed warblers (Acrocephalus sp.).
In summer, the water slowly recedes, increasingly exposing aquatic life to aerial predators. The white-tailed eagle (Haliaeetus albicilla) stops on a large pile of dead reed debris to eat fresh prey in peace, while the otter (Lutra lutra) takes advantage of the calm in the reeds and the abundance of fish in the vicinity. The black-backed nuthatch (Cervus elaphus) also raises its calves in the safety of the reed beds. But when the water finally recedes and the summer flush begins, the moist soil in the shade of the reeds is used by many green frogs (Pelophylax sp.) whose skin would otherwise dry out. In the dried lake windows, bears (Ursus arctos) pick up the remains of dead fish. In August, the short mowing season also begins and landowners rush to harvest biomass from the lake. The first heavy rainfall will soak the ground and tractor access will no longer be possible.
In early autumn, the piercing lowing of a deer's hand rings out at night over the vast plains of Lake Cerknica. Tired from the night's rut, the deer choose what is left of the large reed stands after mowing as their daytime resting places. Wolves (Canis lupus) also take advantage of the abundance of prey in the reeds, while flocks of starlings (Sturnus vulgaris) and migrating barn swallows (Hirundo rustica) flock into the thickets of dry reeds. But when the water floods the karst field again, the terrestrial animals retreat, and after a short period when few animals are visible, many ducks return with the onset of winter, resting in the shelter of the reeds, feeding on seeds or resting on piles of debris.
How to ensure good management?
The following list of species associated with reedbeds in one way or another is just an illustration of the role reedbeds play in the well-being of Lake Cerknica. Of course, there are many other species associated with reeds that are not even mentioned at this time. One of the key questions is therefore how can we ensure that the area is managed in such a way as to maximise its biodiversity potential? What are the key ecosystem functions of reeds - is it peace and shelter, or is it a food source?
It is therefore important to have a good understanding of the distribution of species and their ecology in relation to management practices. It is also important to have a good understanding of the response of reeds to mowing, which is specific to Lake Cerknica due to the challenging habitat conditions. From what? knowledge, it is important to develop an integrated management plan, which in some places provides for no-mowing areas where old reed stands are permanently maintained. Elsewhere, areas with occasional mowing (every 3-5 years, for example), and areas with medium intensity mowing (every 2 years), and areas with regular mowing are envisaged. In addition, it is important to implement low-impact measures, e.g. to maintain lake windows and to ensure connectivity - through riparian vegetation, unmown islands and strips.
As it is almost impossible to draw up a complete management plan in the first attempt, it is important to monitor the status of the populations and the implementation of management on an ongoing basis, and to react accordingly and update it with new knowledge. In addition to mowing, the annual cycle of water level fluctuations and its management, particularly in the light of past interventions and climate change, is also an important factor.
Lessons from abroad
Similar challenges are faced in other areas, many of which have been dealing with such issues for several decades and have accumulated a wealth of experience and developed support systems for good management planning and monitoring. This is why we visited the Ham Wall Nature Reserve in England and Prespa National Park in Greece as part of the LIFE Reed project, which focuses on the reedbeds of Lake Cerknica. We will transfer the experience gained, complemented by knowledge from the literature, and integrate it into the preparation of the management plan for Lake Cerknica. This is a challenging process that will be carried out over the course of several years, together with a wide range of stakeholders.
Author: Rudi Kraševec, conservation biologist for LIFE TRŠCA
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