T. van Elsen Agriculture, Ecosystems and Environment 77 2000 101–109 103
Fig. 1. Traditional agriculture lead to an increase of biodiversity in the landscapes.
whereas shallow limestone soils become abandoned, supported by set-aside programs. In many cases such
fields were the last reserves for annual field plants — without regular ploughing annual weeds disappear
and perennial plants take over van Elsen and Gün- ther, 1992. A similar development on the grassland
takes place. Through the use of mineral fertilizer and early cutting the spectrum of plants has been reduced
to a few species, while mowing or grazing of grass- land with low yields is not profitable anymore: shrubs
and trees take over, and biotopes for a lot of plants and animals disappear. The effects of today’s agricul-
ture are the opposite of agriculture’s former tendency which allowed a development from the natural land-
scape toward the mosaic of Central European cultural landscapes. Today agriculture is considered to be the
main agent for the decline of plant species in Germany. An analysis of the Red List of extinct, missing and
endagered wild plants by Korneck and Sukopp, 1988 shows that agriculture is responsible for the decline
of 513 out of 711 species that were evaluated. Of all endagered plant species 10.8 are arable field plants,
in addition, 15 ‘weed’ species have already become extinct =25 of all extinct plant species.
3. Effects of organic farming on arable field plants
A small but growing number of farmers try to cul- tivate their land without using artificial fertilizer and
pesticides. They strive for a wider crop rotation and due to idealistic andor economical reasons they have
converted to organic farming. Weed control takes place by using mechanical tools, flame-weeders in
vegetable-fields, undersowing in arable fields and by using field-fodder as a crop for one or two grow-
ing seasons during the rotation. What are the effects of sustainable ways of agriculture on biodiversity,
especially on the diversity of arable field plants?
3.1. The weed flora on organic fields The effects of organic farming on the weed flora
from the viewpoint of nature conservation has been investigated by many researchers. The first papers
on that topic describe positive effects of alternative agriculture on the diversity in arable fields and also
grassland Meisel, 1978, 1979. Callauch 1981 compared the weed communities of neighbouring
fields and found two to three times as many species on the organic fields. Similar results are shown by
Hampl and Herrmann 1987 in Bavaria, Ries 1988 in Luxembourg, Frieben 1988 in Northwest Ger-
many, Wolff-Straub 1989 in the Lower Rhine Plain, Plakolm 1989 in Austria and Hald and Reddersen
1990 in Denmark. In regions with rich soils the number of species on organic and conventional fields
sometimes differs up to 10 times Heinken, 1990. On the other hand, sometimes conversion shows only
104 T. van Elsen Agriculture, Ecosystems and Environment 77 2000 101–109
Fig. 2. Number of weed species in differently managed root crop fields T = Therophytes, H = Hemicryptophytes, G = Geophytes
Graphics-file.
small benefits to species diversity because of mechan- ical weeding and the use of herbicides for a long pe-
riod before conversion Albrecht and Mattheis, 1996. Comparisons by the author of neighbouring biody-
namic and conventional root crop fields van Elsen, 1989 show that the median number of wild plant
species in the biodynamic field margins was 25 aver- age 25.5, compared with 16 average 15.8 in the con-
ventionally managed fields Fig. 2. In the centre of the fields, the median number of weeds in the biodynamic
fields was 18 average 19.5 compared with only two average 3.2 in the conventional fields Fig. 1. The
range of the number of species at the margins of 17 fields is 9–46 biodynamic, 2–25 conventional; and
in the centre of fields is 7–30 biodynamic and 0–11 conventional.
An analysis of the spectrum of species shows that the field margins support a community of arable field
plants described as typical for root crop fields in phy- tocoenological literature. Many of these species also
appear in the conventional field margins, but less fre- quently and often as single specimens. In the middle
of the biodynamic fields most of the species from the margins are also found, whereas almost all species
typical of root crop fields are missing in the centre of the conventional fields. Under highly extensive condi-
tions the vegetation of margins and field centres on or- ganic fields become more and more similar Hofmei-
ster, 1992; van Elsen, 1994a.
The amount of perennial species hemicrypto- phytes, geophytes is higher at the field margins. The
larger number of these species on organic fields is the result of growing perennial field fodder as a crop
during the rotation. This is also the reason for the high frequency of Rumex-species on organic fields.
Some endangered ‘red list’ species Silene noctiflora, Centaurea cyanus, Chrysanthemum segetum were
occasionally found on the biodynamic fields, but were not found on the conventional fields. The important
ecological differences between the margin and cen- tre are the light factor, the different microclimate,
soil compaction, different seed-bank, the ability of plants to invade from neighbouring plots and a lower
intensity of farming compared to the field as a whole.
3.2. Effects of agricultural practices in organic farming on the weed flora
Table 1 summarizes the effects of various agricul- tural practices in organic farming on the weed veg-
etation, based on six years of research in the Lower Rhine Plain and on unpublished data from Hessia and
Thuringia.
Conversion to organic farming leads toward changes within the weed vegetation. After stopping the use
of herbicides nitrophytes like Galium aparine might ‘explode’, normally a gradual decline of species being
in need of a high nitrate level follows and legumes esp. Vicia spp. take over and also might become
problem weeds, Eisele, 1996. Plants that depend on a high amount of light are encouraged by a greater
distance between the rows which is 18 cm on most organic farms instead of 13–14 cm conventionally.
Besides long-term changes of the vegetation caused by conversion, the fluctuation of weed vegetation dur-
ing the crop rotation is typical for the plant commu- nities on fields. After each soil treatment a stand of
plants is built anew out of the seed bank — a com- bination of plants that varies from year to year van
Elsen, 1994a. Many factors influence that variation. The time of tilling is very important; it leads to dif-
ferent plant communities in winter and summer crops.
T. van Elsen Agriculture, Ecosystems and Environment 77 2000 101–109 105
Table 1 Effects of agricultural practices in organic farming on the weed vegetation
Agricultural measures Effects
During the period of conversion cessation of chemical weeding
Pronounced appearance of resistant ‘problem weeds’ before cessation of mineral fertilizer application
Gradual decline of weeds like Galium aparine which need a high nitrate level; increase of legumes esp. Vicia spp.
Mechanical weeding Decline of long-lived winter annuals and support of short-lived summer annuals;
therefore danger of static and impoverished weed vegetation Undersowing in winter grains
Due to weather weeds may be shaded out Growing perennial field-fodder
Long-lived annuals cannot develop; increase of perennial species like Rumex crispus, R. obtusifolius; few weed problems after next ploughing
Turning of the stubble immediately after harvest Decline of late-flowering weed species that need stubble fields to develop Kickxia
spp., Stachys arvensis Use of own seeds
Possibly spreading of seeds of other cultivated and wild plants Composting manure
Spreading of nitrophytic plants esp. Chenopodiaceae on fields Frequent driving on fields
Increase of plants appearing on compressed soils
Investigations on fields during a period of six years show that most annual weed species occur mainly
on winter-crop fields. Examples are Apera spica-venti and Veronica hederifolia, which germinate in autumn
and overwinter. Species like Thlaspi arvense, Fallopia convolvulus and Chenopodium album occur primarily
on summercrop fields; the latter is a good example of the extent to which the growth form of weeds can dif-
fer depending on different times of germination.
Often Trifolium, Lolium and Medicagospecies are used for undersowing with grain. Due to weather fac-
tors it can happen that the stands produce too much shade for most of the arable field weeds to grow
Fig. 3. After harvesting the undersown legumes and grasses are used as field-fodder for one or two years;
the fields get mown or grazed several times each year. During that period some additional wild species in-
crease, i.e., Taraxacum officinale, Ranunculus repens and Plantago major. On the one hand, the cultivation
of legumes is used for the enrichment of nitrogen, but they also provide indirect weed control because annual
weed species in particular are reduced. The weed veg- etation under field fodder is different from the weed
vegetation under grain and root crops. Many thero- phytes with a rhythm of development similar to winter
grains i.e., Centaurea cyanus, Papaver rhoeas, Vicia spp. are unable to flower and complete their life cy-
cle. If they do germinate, they remain vegetative. In addition, the frequency of short-lived annuals like Stel-
laria media and Veronica persica is strongly reduced, but at least some plants are able to develop on small
Fig. 3. Undersowing can lead to a decline of weed species.
open patches within the field fodder stands between the repeated mowing or grazing and can produce flow-
ers and seeds. Species that develop in the second half of the year like Polygonum aviculare are scarcely to
be found.
106 T. van Elsen Agriculture, Ecosystems and Environment 77 2000 101–109
Table 2 Measures to encourage the species-richness of segetal plants on organic fields
Measure Aim
Reduce mechanical weeding at field margins and discontinue Increase in annual weeds of winter grain fields that have become
undersowing rare
Discontinue immediate ploughing of stubble-fields after harvest if Encouragement of late-flowering weed species that need
rare weed species appear that are late-flowering stubble fields to survive Kickxia spp., Stachys arvensis
Division of fields by structural elements hedges, field-banks; Creation of species-rich margin biotopes; Increasing the number
Reduction of field size of different treatments per area
In organic winter grain fields it is quite usual to turn the stubble immediately after harvesting. This causes
a problem for late-flowering arable field plants like Kickxia spp. and Stachys arvensis that need stubble
fields to complete their life cycle. The use of seeds collected on the farm might lead to a spreading of
some weed species. Also, composting the manure can cause a spreading of some species over the farm. The
high frequency of species like Poa annua and Plantago major might be caused by frequent driving on the fields
with heavy machinery leading to compacted soils.
3.3. Measures to encourage the diversity of arable field plants on organic farms
Many investigations show the positive effects of organic farming on the diversity of arable field plants,
but today economic pressure leads to an improvement of mechanical weed control and undersowing. These
measures threaten sensitive winter annuals with a similar life cycle to the grains in particular, while
short-lived summer annuals may grow even better after mechanical weeding due to weather factors. The
floristic differences between fields with grain and root crops become less and less distinct.
The aim of preserving, encouraging and developing a diverse arable field vegetation cannot be reached au-
tomatically by converting to organic farming. An inte- gration with the aims of organic agriculture is needed.
One step in this direction is to consider arable field plants as hosts and food for beneficial insects Rup-
pert, 1993; Lys et al., 1994; Raskin, 1994 that they have a positive effect of weeds on the soil. Preserving
weed species with their flowers for utilitarian purposes would be a first step toward a more conscious approach
to arable field plants which goes beyond merely sup- pressing and destroying unwanted weeds. Some pos-
sible measures to encourage the species diversity of segetal plants on organic fields are shown in Table 2.
They of course need to be adapted to local conditions.
4. The integration of species diversity into organically farmed land — a task ignored by
