Weed species diversity and cover

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16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/view/projects/conference.html
Weed species diversity and cover-abundance in organic and
conventional winter cereal fields and 15 years ago
Kaar, B. 1, Freyer, B.2
Key words: conventional / organic farming, diversity, weeds, winter cereals
Abstract
In this research, we compared the weed species development in conventional and
organic winter cereals in Upper Austria. The investigations were done in 2003 in 15
paired conventional and organic farms. Following Braun-Blanquet procedure, a total
of 105 weed species were found. 57 of them were found only in organic, four only in
conventional fields and 48 in both. Therefore, there were 105 weed species in organic
fields and 52 in conventional fields. More of the endangered species (Red List
species) were found in organic fields. Low species diversity observed in 2003
compared to that in 1988 in the same field was attributed to higher temperature and
low rainfall in the recent years. The question arises if under increasing temperature
and dryness the diversity of species is regressing and endangered the biodiversity of
weeds and linked with that also insects and other species as well as offers space for
weeds with strong competition to cultivated crops.
Background and objectives
Several investigations have shown, that the diversity of flora and fauna is higher under
organic in contrary to conventional farming (e.g. Rasmussen et al. 2006; Frieben
1998, 1988; Plakolm 1989, Callauch 1981). The abandonment of herbicides and
pesticides as well as the diversified crop rotations are well known as most important
factors influencing this development. Supporting conditions for a high species diversity
are not only limited to the arable fields but reach also the environment of the cultivated
land e.g. hedges or balks. Plakolm (1989) compared the weed populations in arable
fields in organic and conventional fields in the years 1983, 1984, 1985 and 1988 in
Upper Austria. In organic farms the weed diversity was higher (steadiness) as well as
their dominance. The aim of this research was to study the long term development of
weed species on these fields after 15 years.
Material and methods
We analysed the weeds in the field border area (behind the first grain row) as well as
in the centre of a field (minimum of 20m away from field border). Measurements were
done by identification and estimation of abundance following Braun-Blanquet (1964)
procedure. In addition to the field observations, we collected relevant data on arable
techniques for the interpretation of weed data (effects not analysed).
1
2
E-Mail Bernhard.Kaar@gmx.at
Division of Organic Farming; University of Natural Resources and Applied Life Sciences (BOKU),
Gregor-Mendelstr. 33, A-1180 Vienna, E-Mail Bernhard.Freyer@boku.ac.at, Internet
www.nas.boku.ac.at/oekoland.html
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/view/projects/conference.html
In 2003, 15 organic and 15 conventional farms, with mainly two fields (total of 59) of
winter cereals (triticale, winter rye or winter wheat) were analysed in four arable
regions of Austria in the same fields, which were already analysed by Plakolm (1989)
15 years earlier. Whereas the natural conditions of the neighbouring fields were
similar, there were differences in the cereals, the varieties, the sowing intensities, as
well as pre-crops. The size of analysed area was 50m x 2m along the field border and
in the centre of the field following the methodology of Van Elsen (2000). The inventory
method combined species abundance (number of individuals of one species) and the
dominance (cover of one species in percentage) of the whole test field (coverabundance) (see Dierschke 1994; Braun-Blanquet 1964). The analysis followed the
methodology of Fischer (1994). For statistical analysis, we used SPSS. Furthermore
we identified Red List (endangered) species of Austria (see Niklfeld 1999).
Results and discussion
In total, we found 109 weed species, where 57 were only in organic and four only in
conventional fields and 48 in both. Therefore, 105 species were found in organic fields
and 52 in conventional fields, which translated to 100% more different species in
organic than in conventional farms. This high species diversity in organic winter
cereals compared to their conventional counterparts was significant at P < 0.05. In
most cases the median of species frequency in organic fields was 21-25, whereas in
conventional we found only 6-10 weed species. In three of four regions, the weed
species in organic farms was considerably higher, while in one region the differences
between organic and conventional farms were not clear. In the later case two organic
farms has unusually low weed diversity, which we could not explain. Twenty-six
species in the Austrian Red List were counted (Niklfeld 1999), where 18 were found in
the organic, seven in the conventional fields and one only in one of the conventional
fields.
40
Mw Bio
MW Konv
35
Anzahl der Arten
number of species
30
25
20
15
10
5
0
Region 1
Region 2
Region 3
Region 4
Figure 1: Average number of weed species in organic and conventional fields in
four regions in Upper Austria. (Anzahl der Arten means number of weed species;
bars represent standard deviations)
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/view/projects/conference.html
Highest species diversity was found in the border section of the field, regardless of
whether it was conventional or organic. The ratio between species diversity in the
border and in the centre of a field was 4:1, whereas in the organic farms the species
diversity was twofold more in comparison to the conventional farms.
The comparison between investigations done 15 years earlier (see Plakolm 1989) and
the current one (2003) showed a distinctively lower species diversity in the later. The
tremendous dryness in 2003 and considerable higher temperatures can explain this
decline. Temperatures in 2003 were 2.40C higher in the first seven month in contrary
to the period of 1960-1990. Further, the current rainfall was 395mm lower compared to
the 1960-1990 period, amounting to only 125mm in seven months (ZAMG 2003).
Even under these climatic conditions, the median of species diversity was two times
higher in organic as in conventional winter cereals. Another reason for the lower
species diversity in comparison to Plakolm (1989) was the applied methodology.
Whereas Plakolm (ibid) collected weed data from an area ranging from 0.3 to 1.5 ha,
we made our observations from an area of 2x100m -2 per field. Furthermore, he
integrated the species outside the field along the field border.
The analysis of cover-abundance led to the following results: 56 of the 109 species
were only found with a cover-abundance of “r” (1-5 individuals /m2). Most dominant
endangered species in conventional plots was Anthemis arvensis (“2”; cover 16-25%),
all other endangered species were in the rare category (“r”). Most dominant weeds in
organic fields were Anthemis arvensis (“3”; cover 26-49%), Centaurea cyanus (“3”),
Galium aparine (“+”; cover < 1%), Myosotis arvensis and Sherardia arvensis were
mainly rare (“r”). Apera spica-venti, Convolvulus arvensis and Veronica arvensis were
dominant in conventional fields (highest values “4”; cover 50-74%). Fallopia
convolvulus, Viola arvensis were found in both organic and conventional fields with
high dominance (-“1; 1-4% cover /2”). Gnaphalium uliginosum, Kickxia spuria, Stachys
annua, Valerianella dentata identified by Plakolm (ibid), were not found in the present
study. Instead of that, we found Geranium molle („r“), Holosteum umbellatum (“r”),
Rumex stenophyllus (“r”), Trifolium campestre (“r”) and Veronica opaca (“r”) in several
organic fields. Trifolium campestre was only found in one conventional field (“r”).
Conclusions
In organic winter cereal fields, weed species diversity is higher than in conventional
fields. The high weed diversity in the field borders is expected in any investigation on
weed species. Higher survival rates of endangered species under organic farming
conditions are not surprisingly, however in some cases we cannot explain, why some
of these species were only found under conventional farming conditions. The lower
weed species diversity in comparison with previous analysis seems to be an effect of
dryness and higher temperature in 2003. Some species get lost, new ones emerged,
but we cannot explain this development with this survey. Today it is of interest to know
if further climatic differentiation lead to new weed societies with new competition to
and interactions with the cultivated crops. Therefore, investigations over several years
can only explain these developments.
Acknowledgments
Our gratitude is for all farmers who supported us in our field studies as well as Dr.
Gerhard Plakolm, for his methodological advice and the support with reference data.
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/view/projects/conference.html
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