De Levende Natuur nummer 1 van 2007 (English summary)

Nature recovery in grassland by means of clover and potassium fertilization

Eekeren, N. van, G. Iepema & F.W. Smeding

Abandoned agricultural lands that are in conversion to nature conservation areas, often have a disbalance in regard to their chemical soil fertility. A major problem on sandy soils is the high soil phosphate level, inhibiting the development of nature target types. The current management on these abandoned agricultural lands, aiming to impoverish the soil, leads to a depression in dry matter production and consequently inhibits the removal of phosphate. To solve these problems, a grass-clover management is proposed including supplementary potassium fertilisation. In an experiment started in 2002 this management was tested. After 3 years the productivity of the plots that received potassium was about two times higher as compared to plots without potassium. Increase in phosphate removal was on average 15 kg phosphate per year. Calculations using field observations demonstrate that the proposed method of phosphate mining is able to achieve threshold phosphate values for nature target types within 10-20 years.

In the shallow lakes along Flevoland polders nature restoration projects have a negative impact on the Great reed warbler

Foppen, R.P.B. & S. Deuzeman

The Great reed warbler (Acrocephalus arundinacea) is a rare breeding bird in The Netherlands. It occurs in reedbeds and prefers edge situations where early successional stages of reed vegetation are present. The species has declined rapidly the last decades. One of the last strongholds is an area with shallow lakes between the Flevoland polders and the mainland of the provinces of Gelderland and Overijssel. Currently around 50% of the population occurs in this area. Since 1987 the distribution of territorial males has been mapped and this allowed us to analyse changes in densities and distribution on a local scale in response to environmental changes. One of the major changes has been the development of nature restoration areas, mostly small isles with sparse vegetation that are suitable for especially wader species, waterfowl, gulls and terns. In reedbed areas immediately adjacent to these isles some years after the construction the numbers of Great reed warblers decrease, even if the reed vegetation itself has not been damaged. There are several hypotheses as why this might affect the habitat quality for this species. It might be that the reed vegetation is deteriorating because lower water table dynamics increases the organic substrate between the root system and causes anaerobic conditions. This could then lead to decreased food availability and lower reproductive success. A low water level also could lead to an increased nest predation. Unfortunately, we do not have data on reproductive success to test these hypotheses.

Besides demonstrating the effect of nature restoration the data could also be used to test the effect of road traffic on the quality of reedbeds. Reconstruction of a dyke with a road along a lake with a narrow reedbed vegetation clearly showed an increase of the number of breeding Great reed warblers in the two seasons that traffic was absent (fig. 8). The numbers rapidly dropped after re-opening the road for regular traffic.

At the moment the situation for the Great reed warbler is alarming. We recommend nature managers to be aware of possible negative impacts of nature restoration projects on the occurrence of Great reed warblers. They should search for smart solutions and adaptations in restoration plans for the benefit of the Great reed warbler. Not in the least because in the framework of Natura2000 managers have obligations since this is a species of high conservation concern for several sites in this area.

Opportunities for management and (re) development of zinc flora of the Upper-Geul valley

Ent, A. van der

As a follow-up of the International Zinc Flora Congress of January 2006, this article discusses the abiotic and historical background of zinc Flora sites in Belgium and The Netherlands and the requirements for suitable management of the remaining sites. The zinc flora is severely threatened. Therefore, the restoration possibilities of the zinc flora are also discussed. During the flowering-age of the mining-industry in the Upper-Geul valley of Belgium, the zinc flora occurred abundantly. After the fall of this industry the zinc flora declined. Zinc flora species have developed special enzymes or other mechanisms to reduce the incorporation of zinc and other (toxic) heavy metals or to neutralize these within the organism. One can classify the zinc flora and their associated plant communities in three classes: Primary zinc-flora sites: zinc flora occurring on natural exposed ore veins Secondary zinc-flora sites: zinc flora occurring due to excavations or old exploitations of the heavy metal ore bodies, halden and surface excavations. Tertiary zinc-flora sites: zinc flora on the alluvial plains of the Geul inundated by surface water rich in heavy metals. The deterioration of the zinc flora is mainly caused by eutrophication (increased concentration of N and P) and an increase of the Ca2+- concentration of the soil. An increased concentration N and P requires a higher Zn2+concentration to counteract encroachment of the low productive zinc flora vegetation. To counteract the impact of eutrophication removal of the top layer and subsequent hay-making are required to restore the former nutrient-poor vegetation as well as increasing Zn2+ availability in soil by acidification by addition of for instance Iron Sulfide. It is likely that the future of the zinc flora is best ensured by an integral approach, including nature management, ecotourism, education and (re)development of archaeological relics.

Netherlands Court of Audit: improve steering and implementation of policy to realise a National Ecological Network
The Netherlands Court of Audit conducted an audit on government policy introduced in 1990 to create a national ecological network of high-quality, protected nature areas (EHS). This court investigated if the policy is a suitable instrument to protect biodiversity and what has been realised so far: How much nature has been developed, do these areas have a high natural value, do the areas form a coherent network and is the network well protected? Also the underlying causes of policy implementation problems are determined. In order to ensure that the network will be realised as planned eight recommendations have been given to steer the policy process better and to remove obstacles to implementation.

Do threatened breeding bird species benefit from the new Biesbosch?

Nijland, R.