6 Perspectives

Buffer zones for biodiversity of plants and arthropods: is there a compromise on width?

The results of the project and the conclusions, which can be drawn, provide a very important possibility. Instead of prolonging a discussion about buffer width too much, it is now possible to proceed meaningfully with other buffer aspects linked to a fairly narrow 6 m buffer zone.

6.1 Perspectives for management

The recommendation of a 6 m buffer zone does not ignore that fact that a very wide buffer zone will certainly add to habitat development. However, the question for buffer zones wider than for example 24 m is, whether the debate is not about buffers any more, but rather about replacement of field area with another habitat or simply set aside. Irrespective of the width and location of bufferzones, the close interactions between buffer area and treated field calls for ongoing efforts of the farmers to use as small amounts of pesticides as possible.

The possibility of increasing/protecting biodiversity by the establishment of 6 m buffer zones along a number of existing hedges should also be attractive in terms of management and political decision making.

The above indications about buffer zone dimensions are of interest for growers and policy makers discussing measures to reduce the negative impacts of modern intensive crop production. In particular, a 6 m buffer zone ought to be fairly acceptable, as this to a great extent includes the “low-yield-zone” along any large hedgerow. Furthermore, a certain limited amount of money for directional subsidies may have a much larger impact on landscape heterogeneity if stretched into 6 m buffers instead of for example 12 m or even 24 m buffers.

The result (6 m buffer zones) offers important possibilities which seem within a practically acceptable frame for farm practice. Furthermore, the crop yields close to hedges are often influenced by hedge competition, and a buffer zone may potentially increase income from hunting. Hence such a solution may be considered in a future discussion about requests and subsidies to farmers.

In summary the perspectives for management are:

For farmers, 6 m extensive managed zones along one hedge-side of a few fields ought to be fairly acceptable as the yield always is depressed by hedge competition within the first few metres of the field.
A 6 m buffer zone is a practically manageable width, as the outermost 6 m of the spraying boom often is a separate unit, which can easily be shut down during spraying along the field edges without slowing down field operations.
For policy makers, the relatively narrow 6 m buffer zone may be an attractive choice for biodiversity protection using subsidies. However, the size of the subsidy is vital, especially for relatively small areas, in order to offset the cost of additional paper work for farmers.
In order to avoid development of problematic weeds in the crop edge, it can be recommended, on the basis of results from the UK, to separate the buffer zone from the field by a 1 m barrier strip of bare soil.

In a slightly more distant future, and after answering some remaining questions (see section 6.2), 6 m buffer zones might become an element of a more thorough planning of an arable landscape with increased support for the remaining biodiversity.

6.2 Perspectives for future research

That a 6 m wide buffer zone efficiently supports biodiversity leads to a few very important follow-up questions requiring research:

Which vegetation development and management is relevant under Danish conditions in order to sustain and improve the positive effects of a buffer zone along a hedgerow for a longer period?
Which further development of biodiversity can be obtained over 3 years, 5 years, 10 years or even longer?
Can buffer zones serve as a source of beneficial arthropods, and thereby enhance natural regulation of crop pests in the field?
Which corridor improvement (anti-fragmentation) can be obtained if a network of connected buffer zones are created in the landscape?
Is there a certain minimum amount of buffer area per hectare field which is necessary in order to obtain a reasonably biodiversity increase on a larger scale?

Ad 1. Vegetation development and management

A first test suggestion might be one annual mowing of the vegetation which will remove nutrients, thus supporting the development of a diverse flora while reducing problematic weed species (Hovd & Skogen 2005). More than one annual mowing can be detrimental, as shown for Coleoptera (Woodcock et al. 2007).

Our chosen experimental design revealed effects of the buffer zone along hedges, where the buffer zone was not treated with fertilizer and pesticides. With this treatment being a combined fertilizer, herbicide, fungicide and insecticide treatment, we cannot distinguish if the significant effects result from either one or a combination of the applied chemicals. From a scientific view point it might be desirable to obtain knowledge of the effects of the single components and their combination. Taking into account, however, the biodiversity benefits found by a 75% reduction of herbicide and insecticide dosages (Esbjerg et al. 2002) and the further improvements seen if such areas are converted into organic farming (Navntoft et al. 2003) it seems irrelevant for an applied approach with the aim of improving biodiversity, to do anything but avoiding all chemical treatments. Furthermore, with the general lack of nutrient poor areas for plants it seems also less challenging to put too much effort into the fertilizer aspect unless it has an important, overlooked agricultural angle.

The type of crop (cereal) in the present investigation may be important to wild plants and arthropod species, and therefore the conclusions drawn may not be general across crops such as for example winter rape, corn, potato etc. The hedges in the present investigation were tall, old, managed and with a herbaceous hedge bottom. This biotope represents a type of hedge wide spread in the southern UK (Petit et al. 2003), but many more types of hedges are found in Denmark, e.g. tall-trees together with other types of field margins, such as dry stone walls, ditches and trenches. For a full investigation on effects of buffer zones in the Danish landscape, inclusion of these other types of hedges and field margins should be considered.

Ad 2. Development of biodiversity over time

A strong improvement, e.g. of the herbaceous flora and its flowering within the hedge bottom, may be expected already after 3-5 years with buffer zones. Sowing a seed mixture of wild flora can speed up the process, but over time it will converge with the natural established flora (De-Cauwer et al. 2005). The speed of the recovery using different strategies of buffer establishment remains to be studied in more detail. In particular, studies are not available with respect to the delay in the biodiversity recovery of arthropods in relation to habitat improvements.

The results obtained in the present project reflect the development within one growing season. With continued exclusion of pesticides and fertilizers together with regular vegetation management, developments in plant occurrence and continuous immigration of species belonging to later succession stages will occur. This will further improve the ecological benefit of buffer zones. This succession, however, requires in the case of plant life several decades to approach equilibrium.

Ad 3. Enhanced natural pest regulation

Natural enemy activity is usually associated with herbaceous habitats such as buffer zones. Buffer zone-driven pest suppression into the field may result in lower yield losses, although this has yet to be documented.

Ad 4 and 5. The role of landscape connectivity and heterogeneity

Many plants do not disperse easily, and also many animals, for example lizards and some of the threatened bumblebees and butterflies, are very reluctant to cross even short distances of “hostile” crop area. Increased isolation of habitats leads to less pollination, less seed formation, inbreeding and risk of loss of many plant species (Matthies et al. 1995, Fischer & Matthies 1997, Goverde et al. 2002, Steffan-Dewenter & Tscharntke 1997).

It is possible to increase landscape heterogeneity and connectivity for example by planting hedgerows. Though hedgerows are often put forward as corridors for organisms, more documentation is needed in arable landscapes. In order to distinguish the value of connected habitats, experiments on the establishment of new habitats being either connected or not connected with old vegetation may be a possibility (Serholt & Heller 1997). Criteria for connectivity (i.e. at what scale do different species respond positively to newly connected habitats?) require also more studies.

The rate of immigration to the buffer zone of late successional plant species depends on the proximity to old, dry grassland with high biodiversity. This implies that the distribution and pattern of non-agricultural biotopes within the landscape is of great importance for the succession process (Bruun & Ejrnæs 1998). Establishment of buffer zones may also greatly improve exchange and dispersal of plants and animals between the different habitats of the landscape and thus reduce the present impoverishment of habitat quality due to isolation of species and populations between large monotonous agricultural fields.