WEED SPECIES DIVERSITY IN ORGANIC AND INTEGRATED FARMING SYSTEMS

Phytosociological data were collected in 1994–1996 in plots (relevés) at the Research Station for Organic Farming and Conservation Breeding of the Polish Academy of Sciences in Popielno included in a large-area experiment conducted according to the concept and method proposed by Prof. S. Nawrocki. In a four-field crop rotation (root crops – spring barley undersown with red clover and grasses – red clover/grass mixture – winter triticale), each field was divided into two management units, organic and integrated. Data were collected in relevés by the Braun-Blanquet method, each year at the peak of the growing season. Weed abundance (% cover) in cultivated fields and the number of weed species (species richness) in crops were determined, which provided a basis for calculating the Shannon-Wiener indices of species diversity and evenness, and the Rényi profiles. The qualitative (species) and quantitative structure of weed communities was compared using the Sørensen index. A total of 115 weed taxa (species, subspecies and varieties) were identified in the examined agro-phytocenoses. Echinochloa crus-galli, Chenopodium album, Matricaria maritima subsp. inodora, Capsella bursa-pastoris, Thlaspi arvense and Stellaria media were the most abundant. Weed infestation was slightly higher in the organic farming system than in the integrated system. Organic farming contributed to higher weed species diversity in root crops, red clover/grass mixtures and winter triticale. Weed species richness was reduced in red clover/grass stands, while root crops and – to a lesser degree – spring barley undersown with red clover and grasses decreased weed species diversity. The species composition and in particular the quantitative structure of weeds were affected by crop species and cultivation regime rather than by the farming system. Weed communities of crops grown under organic and integrated farming systems were more similar with regard to species composition than the quantitative structure.


INTRODUCTION
The role of weeds in agricultural ecosystems has been the subject of an ongoing debate in recent years.On the one hand, weeds are pests harmful to crop plants [1], but on the other hand they may contribute to preserving biodiversity [2].Weeds of arable land are a component of biological diversity in agricultural ecosystems, and they play a vital role in supporting diversity within crop fields.Many trophic and paratrophic relationships rely on arable weeds as primary producers [3].
Intensive farming focused on maximizing production efficiency has been a major cause of weed biodiversity decline, adverse changes in the species composition of weed communities, and ecological disturbances in agricultural ecosystems [4].Increased awareness regarding the high environmental costs of agricultural intensification has prompted a search for solutions that promote the preservation and restoration of natural resources [5,6,7].In view of human population growth, reconciling food security and biodiversity conservation is a grand challenge for agriculture [7,8].Organic farming contributes to the preservation and enhancement of biodiversity [9,10], but it is not able to ensure sufficiently high production levels [11].Integrated farming seems to bring harmony between agricultural production and the environment [12,13].As regards weed management, an ideal solution would combine eliminating aggressive species from croplands with maintaining ecologically "desirable" species [4].
Species richness (the number of species in a community) is a common measure of weed biodiversity, but relative abundance indices are also used to assess by Atlantic climate and continental climate [17,18], with a moderating effect of water masses (in particular Lake Śniardwy) surrounding the Peninsula.
Data were collected in relevés each year at the peak of the growing season (five sets per plot, 120 in total).Cover-abundance values were listed for each species by the Braun-Blanquet method.The datasets were used to determine weed species composition and weed abundance (% cover) in cultivated fields.The grades of the quantitative Braun-Blanquet scale were converted as follows: r -weed species cover 0.5%, + -2.5%, 1 -7.5%, 2 -17.5%, 3 -37.5%, 4 -62.5%, 5 -weed species cover 87.5%.The abundance of an individual weed species within the community was measured as the average area covered by this species.The abundance of the entire community was measured as the total area covered by all species.Datasets from each field were synthesized to form eight artificial communities (referred to as communities).The data grouped in this way were further processed.
Weed biodiversity within communities was estimated and compared based on: -species richness (S) -number of species in the community; -Shannon-Wiener diversity index (H'): H' = -Σ (p i × lnp i ), -Shannon-Wiener evenness index (J'): J' = H' × (lnS) -1 , where: p i -relative abundance of the i-th species in the community.The effects of management system and crop species on the abundance and biodiversity of weed communities, measured by species richness and the Shannon-Wiener indices of species diversity and evenness, were determined by one-way ANOVA.The levels of the "farming system" factor were used as replications of the "crop species" factor, and vice versa.The homogeneity of variance within groups was determined by Cochran's C, Hartley's and Barlett's tests.Differences between treatments were estimated by Duncan's test at p=0.05.All calculations were performed using the STATISTICA 7 software package.
The Rényi profiles (H ) were also generated to compare the effects of two cropping systems on weed biodiversity, using the below formula: H = (ln Σp i )(1 -) -1 , where: p i -as in the Shannon-Wiener index; -diversity levels assuming that  0,  1; for = 1, H' values were substituted into the formula.The qualitative (species) and quantitative structure of weed communities was compared using the Sørensen similarity index (SSI): SSI = 2c × 100 × (a + b) -1 , where: c -total number of species shared by the two communities or total abundance of species shared by the two communities a -number of species or total abundance of species in the first community b -number of species or total abundance of species in the second community The scientific (Latin) names of weed species follow M i r e k et al. [19].

RESULTS
In a four-field crop rotation, weed infestation was slightly higher in the organic cropping system than in the integrated system, which was particularly noticeable in root crops and spring barley undersown with red clover and grasses (Table 1).However, the observed differences were statistically non-significant.Regardless of the farming system, the lowest weed abundance was observed in dense stands of red clover and grasses.Weed infestation was higher in grain crops than in red clover/grass mixtures, but a significant difference was only noted with respect to spring barley undersown with red clover and grasses.Weed infestation levels were highest in root crops.A total of 115 weed taxa (species, subspecies and varieties, including self-sown crop plants; Tab. 2) were identified in the examined agro-phytocenoses.The following weed taxa were most abundant in root crops: Echinochloa crus-galli (average cover of 42.5% and 33.5% in organic and integrated farming systems, respectively), Chenopodium album (17.5% and 10.8%, respectively), Thlaspi arvense (2.5% and 6.2%), Sonchus arvensis (5.8% and 0.5%), Matricaria maritima subsp.inodora (2.5% and 4.2%), Agropyron repens (3.5% and 1.2%), Sonchus asper (3.5% and 1.2%), Sinapis arvensis (3.3% and 0.8%).The majority of weed taxa identified in root crops did not reach 1% cover (79.8% of species in the organic system and 77.4% of species in the integrated system).In the phytocenosis of spring barley undersown with red clover and grasses, apart from the predominant species Echinochloa crus--galli (average cover of 21.2% in the organic system and 12.5% in the integrated system) and Chenopodium album (7.5% and 6.8%, respectively), the following taxa were characterized by relatively high abundance: Agropyron repens (2.8% and 3.5%), Matricaria maritima subsp.inodora (2.5% and 3.5%), Stellaria media (2.7% and 1.0%), Veronica arvensis (2.8% and 0.5%); the majority of taxa (77.4% in the organic system and 88.0% in the integrated system) were accessory and did not reach 1% cover.In dense stands of red clover and grasses, the following taxa reached 1.5% cover: Stellaria media (in both cropping systems), Cirsium arvense (in the organic system) and Artemisia vulgaris (in the integrated system), while the other taxa did not exceed the 0.5% threshold.In winter triticale fields, the highest abundance levels were reported for Matricaria maritima subsp.inodora (average cover of 3.5% and 7.5%, respectively) and Vicia villosa (6.0% and 2.8%, respectively), followed by Capsella bursa-pastoris (2.8% in both systems), Stellaria media (1.2% and 3.3%, respectively), Agropyron repens (3.5% and 0.5%), Galeopsis tetrahit (2.8% and 0.5%), and self-sown Secale cereale (2.7% and 2.5%).The average cover of the remaining species (80.2% in the organic system and 86.4% in the integrated system) did not exceed 1%.In general, more weed species were identified in the organic farming system (except in spring barley fields with undersown red clover and grasses; Tab.3), but a statistical analysis did not confirm differences in weed species richness between the studied management systems.The phytocenosis of red clover and grasses was characterized by a significantly lower number of weed species than the other communities which formed a homogeneous group.
Table 4 data show that the species composition and in particular the quantitative structure of weeds were affected by crop species and cultivation regime rather than by the management system.Weed communities of crops grown under organic and integrated farming systems were more similar with regard to species composition (similarity coefficient of 74.7-90.2%)than abundance (66.9-72.9%).A comparison of pairs of crop plant communities in each system revealed substantially higher similarity with respect to floristic composition (47.3-75.2%)than quantitative structure (17.8-62.6%); in most cases, the communities in fields under organic farming were more similar than the corresponding pairs in the integrated system.

DISCUSSION
According to numerous authors [20][21][22][23][24][25][26][27], organic farming promotes weed biodiversity.Both weed abundance and the number of weed species are higher in the organic system than in the conventional system [20,22,24].The above is associated with the absence of herbicides and earlier application of fertilizers, which leads to lower stand density thus creating niches for weed growth [28].Less attention has been paid to comparing organic farming with integrated farming or integrated and conventional farming [22,[28][29][30], most probably due to differences in the definitions and classifications of agricultural management systems [31].In some approaches, integrated farming is considered as part of the conventional system [32].Due to differences in classification as well as in the scale and protocol of experiments, weed species diversity in fields under the integrated cropping system cannot be unambiguously placed in between the biodiversity values determined in conventional and organic systems [33,34].According to some authors [30], weed species diversity in integrated and conventional systems can be considered comparable, while other studies [35] point to higher similarity between integrated and organic systems in this respect.In a study by F e l e d y n -S z e w c z y k et al. [29], the Shannon-Wiener (H') diversity index calculated for segetal flora was higher in the integrated system than in the organic system, whereas a reverse trend was noted in species richness.Our findings suggest that organic farming had a protective effect on weed diversity, as compared with integrated farming.However, a different response of weed communities of spring barley with undersown red clover and grasses was noted: weeds in fields under the integrated system were characterized by higher biodiversity, as most clearly shown by the Rényi profiles.F e l e d y n --S z e w c z y k et al. [29] also reported higher weed diversity (determined using the Shannon-Wiener index) in spring wheat grown under the integrated system, as compared with the organic system; an opposite trend was observed in potato and winter wheat fields, which is consistent with our findings.
According to popular belief, more diverse weed communities are less harmful to crops and easier to control [36,37].In the exact sense, species diversity is defined as a combination of the number of species and their relative abundance [38].Popular diversity indices, such as the Simpson index and the Shannon--Wiener index, are measures of diversity defined above, as opposed to species richness that refers to the number of species in a community, with each species given the same rank.High species richness is expected to be positively correlated with high diversity and low dominance.However, a comparison of the number of species and species diversity in weed communities does not always give consistent results [37] and this was also found in our study.Diversity profiles (Rényi entropy) quantify diversity as a multidimensional concept with the use of the parameter [39].A group of indices for measuring species diversity at different levels of forms a "family", where = 0 represents the number of species, = 1 represents the Shannon-Wiener index (H'), = 2 represents the Simpson index (C), and infinity represents the Berger-Parker index [16,40].The indices may be presented graphically by plotting a curve (a diversity profile).Assemblages can be ordered with respect to biodiversity if their profiles do not intersect.One assemblage is more diverse than another if its diversity has been confirmed at all levels of If their profiles intersect, assemblages are incomparable and cannot be ordered with respect to biodiversity, because one of them is more diverse due to the presence of rare species (characterized by low abundance) than the other, and vice versa with regard to dominant species [41].The Rényi profiles have been successfully used to assess the biodiversity of plant communities, while only a few authors have applied this method to compare weed communities [14,37,41] in different farming systems [42].In a study conducted in Hungary, Z a l a i [42] used the Rényi diversity profiles to compare weed flora of organic and conventional maize and wheat fields.An assessment carried out by the cited author in May revealed higher weed species diversity in organic fields.
The hypothesis proposed in our study that the qualitative and quantitative structure of weed communities is affected by crop species and cultivation regime rather than by the management system corroborates the findings of H y v ö n e n and S a l o n e n [43].The cited authors compared low-input and conventional systems, but their general conclusions are identical to ours.

CONCLUSIONS
1. Weed infestation was slightly higher in the organic cropping system than in the integrated system.2. Organic farming contributed to higher weed species diversity in root crops, red clover/grass mixtures and winter triticale.3. Weed species richness was reduced in red clover/ grass stands, while root crops and -to a lesser degree -spring barley undersown with red clover and grasses decreased weed species diversity.4. The species composition and in particular the quantitative structure of weeds were affected by crop species and cultivation regime rather than by the farming system.
5. Weed communities of crops grown under organic and integrated farming systems were more similar with regard to species composition than quantitative structure.

Table 2
Species composition (total for three years) and average abundance of weeds (means for three years)

Table 3
Species richness (S) of weed communities (total for three years)

Table 5
Species diversity (H') of weed communities (total for three years) * -values followed by the same letters within columns and rows are not significantly different at p=0.05