# Field boundary effects on the composition of a species-rich weed community in southeastern France

Thierry Dutoit, Éric Gerbaud, Jean-Marc Ourcival, Bernard Hill, Philip K Roche

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Thierry Dutoit, Éric Gerbaud, Jean-Marc Ourcival, Bernard Hill, Philip K Roche. Field boundary effects on the composition of a species-rich weed community in southeastern France. 41st International Association of Vegetation Science. Vegetation Science in retrospect and perspective, International Association of Vegetation Science, Jul 1998, Uppsala, Sweden. pp.271-274. hal-05533007

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# Field boundary effects on the composition of a species-rich weed community in southeastern France

Dutoit, T.¹*, Gerbaud, E.¹, Ourcival, J.M.², Hill, B.³ & Roche, Ph.³

¹Institut Méditerranéen d'Ecologie et de Paléoécologie, Université de Provence, UPRESA/CNRS 6116 IMEP, FST Saint-Jérôme, case 421 bis, F-13397 Marseilles Cedex 20, France; ²CNRS-CEFE, 1919, route de Mende, F-34293 Montpellier Cedex 05, France; ³Institut Méditerranéen d'Ecologie et de Paléoécologie, Université d'Aix-Marseilles III, UPRESA/CNRS 6116 IMEP, FST Saint-Jérôme, case 461, F-13397 Marseilles Cedex 20, France;

*Fax+33491288707; E-mail thierry.dutoit@bioeco.u-3mrs.fr

**Abstract.** Plant composition and seed bank of a species-rich arable weed community were sampled in an arable wheat field in the Vaucluse (southeastern France). The results showed that seed bank distribution and plant composition are primarily influenced by distance from the field boundary rather than soil characteristics or wheat density. These patterns were true for the boundary species but not for the typical segetal species in Mediterranean fields.

**Keywords:** Arable field; Distribution; Vaucluse.

**Nomenclature:** Tutin et al. (1964-1980).

## Introduction

In the last few decades, species diversity of arable fields in Western Europe has decreased substantially (Aymonin 1962; Holzner 1978; Olivereau 1996; Dalmas 1997). The main reasons for this decrease are likely to be herbicide treatments for weed control and the increased use of fertilisers in recent decades (Dekker 1997). The field boundaries are commonly perceived by land owners to be a haven for weeds, pests and disease (Marshall & Smith 1987). However, informal observations suggest that crop hedges (headlands and corners of arable fields) tend to be areas with high botanical diversity and previous surveys have also supported this view (Marshall 1989; Wilson & Aebischer 1995).

In this study, arable weed distribution in relation to distance from the field boundary was monitored in an arable field which is located in southeastern France. The aims of this survey were primarily to examine the relationship between the distribution of arable weeds and distance from the field boundary as well as to understand the importance of soil conditions and seed banks in the spatial variability of aboveground vegetation in a region where traditional extensive arable management still exists.

## Methods

An arable field was chosen which had been planted with a winter wheat crop in October 1997 but which had not received any herbicide treatments. This field was located near a farm in the Luberon regional park, Vaucluse (5° 26' E, 43° 55' N), southeastern France. The soil was calcareous with a large amount of clay. The field was previously cultivated with cereals (winter wheat in 1995 and barley in 1996) and has never received any herbicide treatments since the first ploughing of an old field several years ago. This management is not a special case and could be compared with the management of most of the arable fields in this study area (Dutoit et al. 1999).

### Analysis of the aboveground vegetation

Five transects were laid out perpendicularly from the field boundary to the middle of the crop (25 m), in an arable field of 50 m × 50 m with sampling points located at 1-m intervals along each transect (125 quadrats). The transects are starting just near the stripe bordering the arable field at the beginning of the crop edge. The direction of ploughing was parallel to the field boundary and the crop edge investigated was not a headland. The transects were positioned away from the corner of the field to avoid any possible variation when compared with the other sides of the field. In May 1998, all dicotyledons and grass weed species seedlings were identified at each point in a 1-m² quadrat and counted in a 400-cm² subquadrat positioned in a corner of the 1-m² quadrat. Wheat density in the 400-cm² quadrats was estimated by counting the individuals.

### Analysis of the soil seed bank

The soil seed bank was sampled in January 1998. Soil cores were taken using a cylindrical steel tube with a 2-cm radius (cross-sectional area 12.5 cm²) to a depth of 30 cm, representing approximate ploughing depth. A total of 125 (5 × 25) soil samples and quadrats was examined.

The number of seeds in the soil samples was determined by allowing viable seeds to germinate (Roberts 1981). Each soil core was broken down, placed on a 2-cm layer of sterile coarse sand in separate trays (45 cm × 30 cm × 8 cm), kept in an unheated greenhouse for six months and watered if necessary. Each tray was stirred at two week intervals and emerging seedlings were counted and removed or replanted for later identification.

### Environmental variables

Spatial variability in the weed seed bank was related to the following physical and chemical characteristics of the soil: 1. Porosity; 2. pH (H₂O) Soil pH was determined in a slurry with water using a glass electrode. 3. Nitrogen and carbon were analysed using respectively Kjeldahl and Anne Methods. For K, P, Mg, and Ca three soil samples were taken in January 1998 for each of the five transects. Analysis of extractable K₂O, P₂O₅, MgO and CaO was performed by atomic absorption spectrophotometry after extraction with ammonium acetate and EDTA (pH = 4,65).
## Statistical analysis

Using ADE software, (1) Principal Component Analysis (PCA) was performed on a [8 variables × 15 samples] matrix to identify the main soil gradients involved in the distribution of arable weed communities. (2) Two Correspondence Analyses (CA) were performed on the [25 quadrats × 62 species] matrix of the above ground vegetation and the [25 quadrats × 40 species] matrix of the soil seed bank to compare the composition and structure of the arable weed community according to distance from the field margin. A pair co-structure analysis was also used to compare the differences between the distribution of soil seed banks and the aboveground vegetation (Torre & Chessel 1995). Species richness, density, diversity (Shannon index) and evenness for the seed bank and above ground vegetation were analysed according to distance from the arable field margin using the ANOVA test. Comparison of these dependent variables with the environmental variables was made using simple linear regressions (STATVIEW software).

## Results

76 species were identified, 62 in aboveground vegetation, 40 in the soil seed bank and 26 species in both. In the Luberon Regional Park 16 species could be considered to have segetal characteristics based on Guende & Olivier (1997). Sørensens index value (Cs = 0.46) and the pair co-structure analysis (similarity: 3.2%) revealed that composition and distribution of the soil seed bank was significantly different from the above ground vegetation.

The PCA performed on the environmental matrix revealed which soil characteristics influence soil fertility from the field boundary to the middle of the field and between the five transects (Fig. 1). The first three axes accounted for almost 78 % of total variation, with 41.7 % for axis 1 (variance = 3.33). Axis 1 is strongly related to pH (contribution 22.6) and soil samples with a high pH (5a, b, c, see legends of Fig. 1 and Table 1). However, axis 1 is also

influenced by K (23,3), C (17,95) and Mg (14,9) concentrations and soil samples 1b, 1c are characterised by high contents of these elements. Axis 1 represents a fertility gradient for soil samples from the first transect, taken from the left-hand side of the crop, to the fifth transect, taken from the right-hand side of the crop. However, no fertility gradient could be identified from the field margin to the middle of the crop (Fig. 1). These results were confirmed by ANOVA tests which showed significant differences only for Phosphorus, whose values increased from the field boundary to the middle of the crop (Table 1). Significant differences were also found for pH and porosity between the five transects (Table 1).

The first axis for the two CAs performed respectively on aboveground vegetation (Fig. 2a) and soil seed bank (Fig. 2b) accounted for 23% and 13.3% of the total variation, respectively. Axis 1 was strongly influenced by species which occurred only in the samples located at 1 m and 2 m from the field boundary. Particularly, *Agrimonia officinalis*, *Geranium dissectum*, *Lolium rigidum*, *Medicago lupulina*, *Lathyrus hirsutus*, *Picris echioides* and *Eringyum campestre* for aboveground vegetation (Fig. 2a) and *Phleum pratense*, *Dipsacus fullonum*, *Carex flacca* and *Hypericum perforatum* for soil seed bank (Fig. 2b).

Species richness, diversity (Shannon index) and evenness of the seed bank and aboveground vegetation showed no significant differences between the plots located near the field boundary (1 and 2) and the others plots (3 to 25). Only the above ground vegetation density of plot 1 was significantly lower than the others.

Simple linear regression analyses were performed for species richness, diversity, density and evenness of the seed bank, above ground vegetation and environmental variables (soil characteristics, PAR Values). Only wheat density showed a significant relationship to above ground vegetation diversity and evenness (Fig. 3a, b), with an error probability of *p* < 0.0001.

<figure><img src="image_2.png" /><figcaption>Fig. 1. PCA performed on the soil data sampled at the beginning (a, 0 m), middle (b, 13 m) and end (c, 25 m) of the five transects (1-5).</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** (A) CA on the main composition of aboveground vegetation plots and (B) on the main composition of soil seed bank data. ○ = species, ♦ = plots. Only species and samples showing a significant correlation with axis 1 have been named. In (A), coordinates of *Geranium dissectum*, *Medicago lupulina* and *Prunus spinosa* are the same as for *Agrimonia eupatoria*.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Simple linear regression between diversity (a), evenness (b) of the above ground vegetation and wheat density.</figcaption></figure>

<table><caption>**Table 1.** Soil characteristics of samples taken at three points: in the beginning (a, 0 m), middle (b, 13 m) and end (c, 25 m) of the transects (n = 5). (Standard error in brackets; significance : *p* < 0.05, Numbers in exponents indicate the sample (I) or transect (II) which showed significant differences).</caption><thead><tr><th></th><th>pH</th><th>Porosity</th><th>C ‰</th><th>N ‰</th><th>P ‰</th><th>K ‰</th><th>Ca ‰</th><th>Mg ‰</th></tr></thead><tbody><tr><td colspan="9">I (n = 5)</td></tr><tr><td>a</td><td>8.25<br>(0.040)</td><td>0.550<br>(0.019)</td><td>28.16<br>(2.142)</td><td>0.838<br>(0.053)</td><td>0.004*<br>(0.001)<sup>b,c</sup></td><td>0.406<br>(0.018)</td><td>10.52<br>(0.129)</td><td>0.220<br>(0.016)</td></tr><tr><td>b</td><td>8.23<br>(0.032)</td><td>0.544<br>(0.019)</td><td>31.97<br>(2.792)</td><td>0.878<br>(0.026)</td><td>0.007<br>(0.001)</td><td>0.440<br>(0.009)</td><td>10.71<br>(0.112)</td><td>0.180<br>(0.009)</td></tr><tr><td>c</td><td>8.27<br>(0.031)</td><td>0.536<br>(0.017)</td><td>27.18<br>(3.858)</td><td>0.816<br>(0.042)</td><td>0.006<br>(0.001)</td><td>0.400<br>(0.017)</td><td>10.61<br>(0.151)</td><td>0.156<br>(0.020)</td></tr><tr><td colspan="9">II (n = 3)</td></tr><tr><td>1</td><td>8.14*<br>(0.028)<sup>2,5</sup></td><td>0.550*<br>(0.021)<sup>5</sup></td><td>35.25*<br>(2.519)<sup>5</sup></td><td>0.847<br>(0.058)</td><td>0.007<br>(0.001)</td><td>0.433<br>(0.018)</td><td>10.59<br>(0.182)</td><td>0.200<br>(0.021)</td></tr><tr><td>2</td><td>8.23*<br>(0.009)<sup>5</sup></td><td>0.533<br>(0.003)</td><td>34.84*<br>(2.365)<sup>4,5</sup></td><td>0.867<br>(0.041)</td><td>0.006<br>(0.001)</td><td>0.437<br>(0.012)</td><td>10.42<br>(0.046)</td><td>0.210<br>(0.020)</td></tr><tr><td>3</td><td>8.26*<br>(0.018)</td><td>0.580*<br>(0.021)<sup>4</sup></td><td>27.46<br>(2.365)</td><td>0.840<br>(0.056)</td><td>0.005<br>(0.001)</td><td>0.427<br>(0.013)</td><td>10.87<br>(0.136)</td><td>0.203<br>(0.009)</td></tr><tr><td>4</td><td>8.29*<br>(0.032)</td><td>0.560*<br>(0.017)<sup>5</sup></td><td>26.64*<br>(1.751)</td><td>0.907<br>(0.055)</td><td>0.006<br>(0.001)</td><td>0.393<br>(0.027)</td><td>10.70<br>(0.127)</td><td>0.160<br>(0.044)</td></tr><tr><td>5</td><td>8.33*<br>(0.012)</td><td>0.493*<br>(0.015)</td><td>21.31*<br>(3.299)</td><td>0.760<br>(0.047)</td><td>0.006<br>(0.002)</td><td>0.387<br>(0.024)</td><td>10.48<br>(0.229)</td><td>0.153<br>(0.003)</td></tr></tbody></table>
## Discussion

Multivariate analysis (AFC) showed that some species were restricted to the outer 3 m of the field margin for aboveground vegetation and 2 m for the soil seed bank. These species included mostly those typical of calcareous grasslands situated near the crop, e.g. *Agrimonia officinalis*, *Carex flacca*, *Dipsacus fullonum*, *Eryngium campestre*, *Lathyrus hirsutus* and *Picris echoides*. It is possible that seedlings of grassland species may originate from parent plants in the field boundary. Nevertheless, a previous study of seed spread from a weedy area into an adjacent cultivated field indicated that seed dispersal for most species was limited (Hume & Archibold 1986).

Other reasons must therefore be found to account for the distribution of arable weeds. Farming machinery often turns on the headland, resulting in lower crop yields. The presence of typical grassland species and high seedling density in the headland could also be explained by the effects of crop competition, which increase across the arable field, creating more opportunities for these species to develop near the field boundary than in sites located further away (Firbank 1991).

Our study showed that there was no significant increase in species richness, diversity and evenness in the seed banks and the above ground vegetation in plots sampled near the field boundary. No distinct differences between the environmental relationships in the soil seed bank and the above ground vegetation could be observed using the pair co-structure analysis and the Sørensen index. In arable soils, the spatial pattern that exists immediately after seed rain is modified by agricultural practices and because farm machinery can transport weed seeds.

An EEC project for the conservation of species-rich headland in the Vaucluse has recently been initiated. This project includes reducing wheat seedling density on crop edges to increase arable weed richness. However, our results showed that this type of action could be more beneficial to the typical species of calcareous grasslands or field boundaries than to the typical segetal species of southeastern France. Consequently, it is essential that the effects of such practices on arable weed populations are understood before they are applied on a large scale.

**Acknowledgements.** We would like to thank Prof. Maurice Roux and Dr. Frank Torre for statistical analyses, Dr. Christiane Rolando and Delphine Auvré for technical support, and the Luberon Regional Park and André Armand for granting us access to the sites. This work was supported by a grant from the French Foundation (n°97008577).

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Dalmas, J.P. (eds.) 1997. *Faut-il sauver les mauvaises herbes? Conservatoire Botanique de Gap-Charance, Gap, France.*

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Dutoit, T., Hill, B., Mahieu, P. & Gerbaud, E. 1999. Restauration et conservation de communautés d'espèces ségétales *in situ* : des coteaux de la basse vallée de Seine aux cultures extensives du Parc Naturel Régional du Luberon. *Bull. Soc. Bot. Centre-Ouest* 19: 303-316.

Firbank, L.G. 1991. Interactions between weeds and crops. In: Firbank, L.G., Carter, N., Darbyshire, J.F. & Potts, G.R. (eds.) *The ecology of temperate cereal fields*, pp. 209-231. Blackwell Scientific Publications, Oxford.

Guende, G. & Olivier, L. 1997. Les mesures de sauvegarde et de gestion des plantes messicoles du Parc naturel régional du Luberon. In: Dalmas, J.P. (eds.) *Faut-il sauver les mauvaises herbes?*, pp. 179-187. Conservatoire Botanique National de Gap-Charance, Gap.

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