Nature development at Hondsbossche Zeewering

Now that the flood function in the Hondsbossche Zeewering has expired, there is an opportunity to apply nature-oriented management. Sinus treatment has been chosen for this. Regular assessments showed that both flora and fauna benefit from this new management.

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Written by Richa Nanne (Hoogheemraadschap Hollands Noorderkwartier), Cyril Liebrand (EURECO Ecological Research and Consulting)

When he retired as chairman of the Association of Water Authorities on 15 December 2015, Peter Glas expressed a desire to strive for strong, flourishing dikes in the future. He called the concept the Flower Power Dike. Since then, more and more water boards have strived for a higher species richness and more biodiversity on their dikes. A good example is the Hondsbossche Zeewering, where the rare bio orchid and dogwort appeared after a change in management.

The water retention function has expired
In the past, the Hondsbossche Zeewering had a flood function. The management of the high water protection system was therefore fully focused on maintaining a qualitatively optimal dyke for flood protection. The turfs were kept short so that dike inspections could be carried out. Due to the construction of a beach and a dune area, the seawall’s primary task was no longer necessary. This gave the opportunity to adjust management and focus on increasing biodiversity. The Netherlands’ Noorderkwartier Water Board (HHNK) is increasingly committed to these goals.

Property manager Martien Witte played an important role in this change of course at the water board. At that time he was inspired by a book about the Atalanta walk. This butterfly migrates partly along the coast and has a great need for food due to its high energy consumption. Witte thought it would be great if Hondsbossche Zeewering could contribute to this through customized management.

In addition, the dyke can form a connection zone between Schoorlse Duinen and Pettemerduinen for many plant and animal species. HHNK, in collaboration with the Butterfly Foundation, has therefore decided to use sinus management, a meandering rock pattern, on the dyke’s dam. Since 2016, it has been beaten twice a year. During the first rock round, a winding path, the so-called sinus path, is taken. Rudder vegetation is included as far as possible in this round. During the second mowing round in late summer, the vegetation on the inside of the path is mowed.

Activities
In 2022, the vegetation outside the sinus track will be mowed due to runing, and that within the sinus track will be partially mowed. It is important that the released clipping is removed in time. Nutrients are removed with the clippings, so nectar-producing herbs have less competition from fast-growing vegetation. With each round of mowing, a portion of the vegetation remains in which insects can survive, even in the winter months. The meandering pattern and a diverse vegetation structure create more variation in the microclimate, which benefits insects.

The transition from the original to the current management did not go as desired in the first period. The clipping was not sufficiently cleared and more vegetation was cut than desired. Because clippings partially remained on the site, with a direct result of overgrowth, repair work was carried out in 2020, where some vegetation was removed. In 2021 and the summer of 2022, the work was carried out as desired, with 30% of the vegetation remaining at the end.

Inventory
Before mowing, it is determined which plant and animal species are present, which must be spared during the work. These may be protected and vulnerable species, but desirable species such as legumes are also identified. For example, Kattendoorn grows on the dyke (photo 1), a leguminous plant that is declining in number. On the Hondsbossche Zeewering, Kattendoorn are increasing in number and today cover entire contiguous areas. During the inventory, it was striking that many mouse holes were visible between Kattendoorn. The thorny plant is the perfect hiding place for small mammals.

Figure 1. Catthorn

Figure 2. Bee Orchid

The bee orchid (image 2) was also present in large numbers; more than 90 habitats are mapped, where several specimens often grow together.

That’s reason enough to jump for joy, but then another surprise came in the form of dog food (picture 3). Dogwort is a rare orchid, of which only one growing place is known in the management area of ​​HHNK on Texel.

Figure 3. Dogweed

Due to the current management, which provides great variation in the vegetation structure, there are more chances of survival for, among other things, insects, and the dyke becomes more attractive to breeding birds and therefore also to birds of prey. The Natterjack toad has now also found the dike (picture 4).

Figure 4. The Natterjack toad

Results
In order to examine the effect of the adjusted management, the Sommerfuglefonden conducts insect research, where butterflies and bees in particular are monitored. In addition, Florin carries out vegetation research. In order to map the effects of the sinus management properly, parts of the dyke are managed in the original way, where all vegetation is cut. These parts are thus for reference. As soon as the research is completed, the reference stretches will be canceled and the entire dyke will be used as a connecting zone.

Current management appears to have a positive effect primarily on grassland (day) butterflies, a group under pressure at European level. Good news for Atalanta, who started it all. In 2018, 2019 and 2020, some rare or rare bee species were observed foraging on the leguminous plants. The observed species are the Yellow Clover sand bee, the Little woolly bee, the coastal hanging bee and the silver whistler. These species were not found in 2021. By sparing legumes during the cutting work, the presence of these species is taken into account. Effects on vegetation are not yet measurable, possibly because the adaptations in management did not go smoothly at the start. Furthermore, it takes time to realize a greater diversity of species.

Hondsbossche Sleeperdijk
A nice side effect is that a dyke in the immediate vicinity, the Hondsbossche Slaperdijk, has recently been closed. This dyke is in the immediate vicinity of the Hondsbossche Zeewering, and here, too, the new goal is to increase biodiversity and act as a connecting zone. In the first period, this dyke is cut in its entirety twice a year with the removal of the rock, so that the soil deteriorates somewhat. There is good hope that this site will also develop in the desired direction, because grass species that indicate worse conditions are currently growing, such as sweet grass and comb grass.

In mid-July 2022, a test site 100 meters in length of herbaceous clippings from the Hondsbossche Zeewering was constructed on this dyke by the internal maintenance service (Figure 5). The purpose of this is to introduce plant species from the immediate vicinity of Slaperdijk by introducing seeds. The monitoring takes place here with the help of the Nectar Index, which determines how much nectar is potentially available during the year based on the abundance of flowers and nectar production. The 2022 census showed that the nectar supply was still minimal.

As soon as the vegetation has developed sufficiently (low biomass, high species richness) it is possible to switch to sinus mowing or staged management. The Atalanta, other butterfly species and insects, small mammals, wild plants, reptiles and birds will be able to meet their needs here. HHNK thus makes a small but valuable contribution to increasing regional biodiversity.

Figure 5. Explanation of locally herbaceous clippings from HHNK maintenance service

Conclusion and follow-up
Many dykes seem to be suitable places to work on the (necessary) increase of biodiversity in the Netherlands. The combination of a relatively light top layer and nature-friendly management quickly leads to more plants and animals on the dykes and a strong increase in biodiversity. Because the dikes with a total length of 17,786 kilometers form long bands through the landscape with many interfaces with the environment, thriving dikes can give a significant boost to increasing biodiversity in large parts of the country. [1]. A large amount of information about dykes and specifically about the development of species-rich, flower-rich dykes can be found in the Grass cover guide, an initiative of STOWA [2].

REFERENCES
1. Liebrand, CIJM (2018). ‘Flora and fauna-rich ties in the landscape. Reinforcement of dykes with vegetation rich in species and herbs’. IN Plants from here206-221. Zeist: KNNV Forlag.
2. The Foundation for Applied Research on Water Management (2022). Grass cover guide. www.handreikinggraskleding.nl

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