FA6 Field margins and buffer strips

>> Background information

Development

The area of field margins has decreased steeply since the end of the 1950s mainly as a consequence of subsurface ditching. At the moment fields with subsurface ditching cover nearly 60% of the Finnish field area. During 1960?1990 some 25 000?40 000 hectares of field were ditched each year. At the time Finland joined the European Union the rate of subsurface ditching began to decrease, and during the last decade less than 10 000 hectares have been ditched each year.

As a result of the agri-environmental aid, new field margins, buffer strips and buffer zones have been formed. The agri-environmental aid requires that the edges of main ditches must be at least one meter and the buffer strips near water bodies at least three meters wide. For the smaller ditches between two strips of cultivated land there are no actual requirements for the width of the edges. In 2002 there were an estimated 9 000?17 500 hectares of field margins and buffer strips in Finland, and approximately 94 % of farmers had committed to leave unmanaged the edges and buffer strips determined by the environmental aid. Only three percent of the farmers had in addition agreed to form at least 15 meters wide buffer zones on fields which are exceptionally prone to leaching. In the end of 2002 the area covered by agreements and applications for buffer zones was approximately 5 400 hectares.

Impact on biodiversity

Ditching improves cultivation efficiency by increasing the field area. Subsurface ditching also decreases nutrient load to waters in comparison to traditional open ditches. At the same time subsurface ditching lowers farmland biodiversity because ditches between two strips of cultivated land and their edges disappear from the landscape. This is harmful to farmland specialist species, for example many farmland birds.

Field margins and buffer strips have a clear positive effect on biodiversity. Broad field margins support a greater diversity of plant and insect species than do narrower edges. Open margin are important for plant species which suffer from shadowing effect of taller species. Also the species diversities of butterflies and day-active moths have been observed to be greater the wider an unmanaged margin is. The greatest numbers of butterflies are found on sheltered and sunny forest edges. On green fallow lands the butterfly numbers are considerably higher than on cultivated fields but not as high as on margins which are left without any soil management or cultivation. Field margins are a living habitat also for many vertebrates, for example hare and corn crake young. In addition field margins are expected to compensate the loss of meadows by offering a habitat for at least some of the meadow species.

Many important food plants of insect pollinators grow on field margins. Margins are also overwintering places of for example ground beetles, some of which are natural enemies of plant pests. Diverse vegetation is able to maintain a great number of herbivore insect species. They in turn are consumed by predators and parasitoids which in fact regulate the amounts of pest insects. Thus, field margins contribute to the natural biological pest control.