Directory UMM :Data Elmu:jurnal:A:Agriculture, Ecosystems and Environment:Vol77.Issue3.Feb2000:

(1)

The effects of nitrogen and diaspore availability on the regeneration of

weed communities following extensification

A. Bischoff

a,∗

, E.-G. Mahn

a_{Department of Community Ecology, Centre for Environmental Research Leipzig-Halle, Theodor-Lieser-Str. 4, D-06120 Halle, Germany} b_{Department of Geobotany and Botanical Garden, Martin-Luther-University Halle-Wittenberg, Neuwerk 21,}

D-06108 Halle (Saale), Germany

Received 7 April 1998; received in revised form 15 June 1999; accepted 23 July 1999

Abstract

The regeneration of weed communities after cessation of fertilization and herbicide use was investigated on an arable field representative of the Central German Chernozem Region. As a result of high, but spatially heterogeneous input of farmyard manure and slurry until 1983, there were large differences in soil nitrogen supply within this ‘regeneration field’ during the investigation period (1992–1994). At the same time studies were carried out on a nearby long-term experimental field that reflects the potential species-rich weed community with and without N-fertilization.

Total biomass production was greater in plots of high N-supply and light transmission into the canopy was reduced. Biomass of surviving weed plants was positively related to N-availability but mortality increased. Thus, a large N-supply may delay the regeneration of weed communities because it increases the risk of extinction of small initial populations.

Despite favourable growing conditions in the low N-plots of the regeneration field many typical weeds were absent. As dispersal of these species was too slow in relation to the distance of the closest populations diaspore input was not sufficient for re-establishment. A field experiment showed that Lithospermum arvense seeds were dispersed within 2.5 m distance over 2 years, whereas the next population was found 300 m away from the regeneration field. In species poor agricultural landscapes, regeneration of weed communities following extensification is often limited by dispersal. ©2000 Elsevier Science B.V. All rights reserved.

Keywords: Arable fields; N-fertilization; Plant re-establishment; Population dynamics; Seed dispersal; Germany

1. Introduction

Intensification of crop production has led to se-vere changes in weed communities during the last five decades. Various studies in different European re-gions have shown a rapid decline of most species (e.g.

∗_{Corresponding author. Tel.:}₊_{49-345-5585316; fax:}₊ 49-345-5585329.

E-mail address: bisch@oesa.ufz.de (A. Bischoff).

Erviö and Salonen (1987) for Finland, Albrecht (1995) for Germany, Wilson and Aebischer (1995) for the UK and Andreason et al. (1996) for Denmark). Many weeds have become very rare or totally extinct. Not only did species composition change, but the structure of whole weed communities has undergone alterations (Mahn, 1984a).

Recently, several proposals were made to reduce the intensity of cultivation either for environmental benefits or to reduce overproduction. Since 1992 the

(2)

European Union has supported a change to environ-mentally more appropriate cropping systems through financial compensation (EU-Directive No. 2078/92).

With the exception of field margin programs, which were introduced in Germany during the early eight-ies (Schumacher, 1980; Otte et al., 1988), little is known about the regeneration of weed communities following extensification and the factors influenc-ing this process. The programs were successful at low-yielding sites but often failed in regions with pro-ductive soils, that had been cultivated intensively for a long time (Ritschel-Kandel, 1988; Oesau, 1991). One reason for the failure in re-establishing species-rich weed communities could be a remaining soil nitrogen supply caused by former fertilization. Nitrogen avail-ability distinctly affects weed populations (Haas and Streibig, 1982; Mahn, 1988; Hilbig and Bachthaler, 1992). Another problem is the local extinction of sev-eral weeds even in the soil seed bank (Albrecht, 1989; Oesau, 1991). Diaspore sources are often far away and re-establishment requires long distance dispersal. In the course of an integrated research project (STRAS; see Körschens and Mahn, 1995) investiga-tions were carried out on representative arable fields of the Central German Chernozem Region character-ized by fertile soils and high intensity of agriculture. They focus on two questions to analyse the regenera-tion of weed communities following extensificaregenera-tion:

1. What is the effect of nitrogen?

2. To what extent can dispersal limit the re-establishment?

The effect of nitrogen was investigated on a field (regeneration field) that had been used to deposit large quantities of farmyard manure and slurry from a nearby large-scale livestock farm until 1983. As deposition had not been homogenous there were large spatial differences in N-supply at the start of investigations in 1991. Parts of the field were still heavily loaded with nitrogen, in others soil nitro-gen content had decreased to a level allowing for characteristic arable weeds to regrow. Before 1990, herbicides regularly applied for weed control pre-vented re-establishment. Investigations on a nearby long-term experimental field provided information about the potential weed community that could be expected under extensive cropping. In this field no weed control was carried out within the last 20 years.

Table 1

Mineral soil nitrogen content (kg/ha, 0–20 cm); from Körschens and Mahn (1995) and Pfefferkorna

Date of Regeneration Long-term experimental

sampling field field

G1 G2 N0 N1

05/1986 58.5 112.3

05/1992 15.1 18.2 16.9 57.6

07/1992 30.1 51.2 14.7 20.8

a_{With Dr. A. Pfefferkorn’s permission to use his unpublished}

data.

To compare growing conditions, biomass produc-tion, light transmission and mortality of weeds were analysed. Diaspore availability was estimated by soil seed bank analysis and by recording diaspore sources in the surroundings. Lithospermum arvense, a com-mon species of the long-term experimental field, was artificially introduced on the regeneration field to obtain information about dispersal and establishment.

2. Materials and methods 2.1. Study sites

Study sites were located in the Central German Chernozem Region about 15 km southwest of Halle. They were characterized by low rainfall (average an-nual precipitation: 492 mm) and typical Chernozem soils.

The crop rotation on both experimental fields was: spring barley (1991) – winter wheat (1991/92) – maize (1993) – spring barley (1994).

2.1.1. Regeneration field

Between 1962 and 1983, large amounts of farm-yard manure and slurry from a nearby animal produc-tion unit were deposited on the field. This fertilizaproduc-tion ceased in 1984, whereas herbicide application contin-ued until 1990. Afterwards the field became part of the Bad Lauchstädt experimental station of the UFZ (Centre for Environmental Research Leipzig, Halle) and was divided into 105 plots of 25 m2(5 m×5 m). From the first soil analysis in 1986 until the final assessment, large differences in soil nitrogen content have been recorded within the regeneration field (Table 1). Investigations were carried out in the G1-area, consisting of the six plots with the lowest soil

(3)

nitrogen content, and in the G2-area which included the six plots of highest nitrogen availability.

The soil N-content declined remarkably since 1986. The seasonal dynamics of inorganic soil N-content differed from fields regularly receiving mineral nitro-gen fertilizer (Table 1). In the course of each veg-etation period, the amount of inorganic nitrogen in-creased by mineralization from the organic N-pool. Large amounts of nitrogen were not available until growth of arable plants terminated.

2.1.2. Long-term experimental field

The long-term experimental field was part of the Etzdorf experimental station of the University of Halle. It was used as a study site to investigate herbi-cide and fertilizer effects on arable weeds since 1976. The present paper deals only with the unsprayed plots originally serving as controls. One part of the field received 80–120 kg mineral nitrogen fertilizer (N1) per hectare and year (dependent on the crop species), the other one was not fertilized with nitrogen (N0). Each nitrogen level comprised five plots (10×10 m2) arranged in a block design (cf. Bischoff, 1996). Dur-ing the investigation period (1992–1994) the mineral soil nitrogen content was similar in N1 and G1-plots (Table 1).

2.2. Methods

To determine above-ground biomass production, crop and weeds were harvested three times a year from one circle per plot (∅=50 cm). The mate-rial was separated into the individual species and weighed after drying. In two permanent quadrats per plot (each 0.5 m×0.5 m) the number of weeds was assessed five times a year and the percentage of re-productive plants was estimated. At three locations per plot photosynthetically active radiation (PAR) was measured near soil surface using a line quantum sensor (LI-COR, LI-191SA, length 1 m). Relative transmission was calculated from radiation values in and above canopy (% of the above canopy radiation). To determine soil seed bank, eight soil samples per plot were taken 10 cm deep with a corer of 3.7 cm diameter. Four of them were mixed to provide two composite samples per plot. They were placed in plastic boxes under outdoor conditions and seedling emergence was recorded (Kropac, 1966). Vegetation

analysis of the whole plot area was carried out us-ing the Braun–Blanquet-method (‘vegetation relevé’) modified by Barkman et al. (1964).

50–100 individuals of Chenopodium album and Lithospermum arvense were marked in the different plots for several studies in population biology. C. album was common in both fields. L. arvense being previously absent on the regeneration field was arti-ficially introduced in 1993 by planting 45 juvenile individuals at three leave-stage of maize. The plants were positioned in the western margin of the field (G1-area) to observe dispersal into the crop stand. The number of seeds was assessed and viability was analysed by Tetrazolium-Test. Dispersal was studied by collecting seeds in traps of 10 cm diameter until crop harvest and by recording the following seedling emergence. The traps were arranged in 0.5 m intervals east of the source plants (cf. Fig. 5). The field was ploughed in November.

The distribution of species absent on the regener-ation field (Chaenorhinum minus, Consolida regalis, Euphorbia exigua, Lithospermum arvense, Papaver rhoeas, Silene noctiflora and Veronica polita) was investigated in a 1 km radius.

To analyse nitrogen content one soil sample (depth=0–20 cm) per plot was taken three times a year. Mineral nitrogen was colorimetrically assessed by flow injection analysis. Detailed methods and results are published in Körschens and Mahn (1995). 2.3. Analysis

To compare species composition of the differ-ent plots, two indices of similarity were calculated. The Sørensen-Index is qualitative and considers only the occurrence of species (Goodall, 1973). The Czekanowski-Index, also called quantitative Sørensen-Index, is calculated on the basis of the mean coverage.

Sørensen : 2c

a+b ×100

where a, b is the number of species only occurring in relevé a, b and c is the number of species occurring in both relevés.

Czekanowski : 2ma

(4)

Fig. 1. Total biomass and biomass of weeds (black) on the regener-ation field (G1, G2) and on the long-term experimental field (N0, N1) at the time of maximum vegetation coverage; with 95% con-fidence intervals; a: N1 differs significantly from N0, b: G2 differs significantly from G1, ns: no significant difference; p<0.05.

where ma, mb is the total coverage of all species oc-curring in relevé a, b and mc is the total coverage of species occurring in both relevés (for each species minimum value of relevé a and b).

For calculation of mean coverage values Braun– Blanquet levels were converted into percentage of coverage.

Braun– r + 1 2a 2b 3 4 5

Blanquet-level

% coverage 0.01 0.2 2.5 10 20 37.5 62.5 87.5 Normal distribution and equality of variances were analysed by Kolmogorov–Smirnov- and Levene-test. The results did not necessitate transformation of the presented data before analysis. Differences between the plots of the long-term experimental field were eval-uated by ANOVA (factors: block and fertilization). Differences between G1- and G2-plots of the regener-ation field were analysed by a simple t-test. Chi-square test was performed to compare frequencies.

3. Results

3.1. Biomass production and competition for light Total above-ground biomass production was posi-tively related to nitrogen availability (Fig. 1). On the long-term experimental field the positive effect of fer-tilization was significant in each year. In 1993 and

Fig. 2. Relative transmission (% photosynthetically active radiation at the ground level) on the regeneration field (G1, G2) and on the long-term experimental field (N0, N1) at the time of maximum vegetation coverage; with 95% confidence intervals; a: N1 differs significantly from N0, b: G2 differs significantly from G1, ns: no significant difference; p<0.05.

1994 biomass was significantly greater in G2- than in G1-plots, but in 1992 there was no significant ni-trogen effect on the regeneration field. Differences between G1 and N1 were small indicating a similar N-availability in both.

The relationship between N-availability and biomass of the weed community was less strong. In 1992 (both fields) and 1993 (only regeneration field) weed biomass was higher in plots with a large N-supply, but differences were only significant in 1992. In 1994 (and 1993 on the long-term experimen-tal field) productivity of the weed community was lower in high N-plots.

High soil nitrogen supply led to a reduced PAR at the soil surface and to an increased competition for light (Fig. 2). Transmission of light was strongly related to total above-ground biomass production. PAR-values at the time of maximum vegetation cov-erage differed extremely between the years. In the maize crop (1993) only 1% of the above canopy ra-diation reached ground level in N1, G1 and G2-plots. Transmission values of winter wheat were about 10% in the same plots.

3.2. Mortality and seed production of weeds

Seedling emergence mostly increased with N-availability (Fig. 3). On the regeneration field num-ber of seedlings was higher in G2- than in G1-plots, but the difference was not significant in 1993. On the long-term experimental field the positive nitrogen

(5)

Fig. 3. Dynamics of the weed density (plants/m2) during three vegetation periods and its dependency on N-supply; significant difference of maximum values: * p<0.05, ** p<0.01, *** p<0.001, ns: not significant.

Table 2

Survival rates and seed production of surviving Chenopodium album and Lithospermum arvense individuals

1992 (Winter 1993 (Maize) 1994 (Spring

wheat) barley)

Survival Seeds/ Survival Seeds/ Survival Seeds/

Plant Plant Plant

Chenopodium album

N0 0.97 81.7 0.36 3660.7 0.63 3.1 N1 0.97 316.0a _0.28 ₉₆₆_.₀a _0.58 _20.0

G1 0.10 3.8 0.40 314.7 0.13 1.2 G2 0.05 21.1 0.05b ₅_.₀b _0.13 _0.1

Lithospermum arvense

N0 1.00 356.1 1.00 278.3 1.00 18.0 N1 0.97 478.1a _0.92 ₉₂_.₈a _0.89 _35.0a

a_{N1 differs significantly from N0.} b_{G2 differs significantly from G1, p}_<_0.05.

effect was significant in 1992; in 1994 the high-est weed density was observed in the unfertilized N0-plots. In N-rich plots a smaller proportion of emerged weeds survived until the end of vegetation period. Detailed demographic results were obtained for C. album and L. arvense. High N-supply increased mortality of C. album and L. arvense populations on both fields (Table 2), but the majority of differences was not significant. Despite a lower survival rate, fe-cundity of the surviving plants was sometimes higher in N1 and G2-plots than in N0 and G1-plots. Seed production of L. arvense was significantly increased by nitrogen in 1992 and 1994, but reduced in 1993.

Nitrogen effect on the seed production of C. album differed largely between the years. In 1992 the rela-tionship between N-availability and number of seeds per plant was positive; in 1993 it was negative and in 1994 results of the long-term experimental field and the regeneration field were contrary.

3.3. Species composition

In Table 3 the mean values of three single years are presented to compare the species composition of above-ground vegetation and soil seed bank. The ef-fect of N-fertilization (N0 versus N1) and of differ-ences in soil N-content (G1 versus G2) differed with species. Nitrophilous weeds with high indicator values for N (Ellenberg, 1992) gained from a high N-supply, particularly Galium aparine, Solanum nigrum, Urtica urens. The majority of species with low indicator val-ues showed a clearly reduced coverage and soil seed bank, such as Euphorbia exigua, Anagallis arvensis and Silene noctiflora. Despite low indicator values coverage of Descurainia sophia and Lithospermum arvense was higher in N-rich plots.

Remarkably, many weeds, frequent in the N1-plots, did not occur in the G1-plots, although soil nitrogen level was nearly the same. Weeds described as char-acteristic for the Central German Chernozem Region (Hilbig, 1973) such as Consolida regalis, Euphorbia exigua, Lithospermum arvense, Papaver rhoeas, Silene noctiflora and Veronica polita were absent or

(6)

Table 3

Species composition of the regeneration (G1, G2) and long-term experimental field (N0, N1); coverage and soil seed bank (10 cm deep) 1992–1994 (average values)a

Indicator Coveragec _{Soil seed bank (/m}2₎

value: Nb N0 N1 G1 G2 N0 N1 G1 G2

Species occurring only or much more frequently on the long-term experimental field

Euphorbia exigua 4 0.9 0.2 . . 52 . . .

Lithospermum arvense 5 1.0 1.6 . . . 5 . .

Consolida regalis 5 0.2 r . . . .

Anagallis arvensis 6 0.7 . . . 31 31 . .

Chaenorhinum minus 5 r . . . .

Veronica polita 7 6.7 6.8 (r) 620 692 . .

Veronica hederifolia 7 7.2 5.0 (r) 145 114 (r)

Papaver rhoeas 6 5.4 4.1 (r) 2294 1671 (r)

Sinapis arvensis 6 0.2 0.1 (r) . 47 . .

Silene noctiflora 5 5.7 2.8 r r 150 72 . .

Viola arvensis – 10.7 7.3 r r 951 558 . .

Euphorbia helioscopia 7 2.3 2.2 r r 31 21 43 .

Polygonum lapathifolium 8 2.6 3.7 0.1 r 31 72 86 .

Galium aparine 8 0.5 5.5 0.1 0.1 16 26 . 26

Lamium amplexicaule 7 3.2 3.2 0.2 r 258 362 52 26

Cirsium arvense 7 11.3 5.2 0.3 . (r) . .

Fallopia convolvulus 6 29.3 27.8 0.4 0.5 98 134 (r)

Species occurring only or much more frequently on the regeneration field

Rumex crispus 6 . . 0.6 3.6 . . 26 95

Atriplex patula 7 . . 0.8 r . . . .

Malva neglecta 9 . . 0.3 r . . . .

Urtica dioica 9 . . 0.3 . . . 164 .

Ballota nigra 8 . . 0.2 r . . . .

Arctium tomentosum 9 (r) 0.2 0.1 . . (r)

Artemisia vulgaris 8 r r 2.8 2.3 . . 611 327

Urtica urens 8 r r 0.3 1.2 (r) 78 629

Thlaspi arvense 6 r r 4.1 6.2 (r) 284 586

Matricaria maritima 8 0.1 r 1.5 0.4 5 198 155

Sonchus oleraceus 8 0.2 r 1.8 1.1 5 5 95 69

Solanum nigrum 8 0.9 7.2 14.0 28.8 0 62 698 2093

Species occurring on both fields

Chenopodium album 7 15.3 12.7 4.5 3.1 1116 1183 629 560

Descurainia sophia 6 5.0 8.0 0.8 6.1 171 207 586 1567

Chenopodium ficifolium 7 4.7 6.8 4.4 3.4 868 1721 1679 2110

Amaranthus retroflexus 7 0.5 2.2 1.8 0.5 10 52 379 215

Stellaria media 8 0.1 1.0 0.6 0.6 196 227 52 26

Fumaria officinalis 7 0.7 1.9 0.1 r . . 17.2 .

Polygonum aviculare 6 0.8 0.7 0.4 0.7 10 36 . .

Lactuca serriola 4 0.9 0.7 0.2 0.1 (r) . .

Sonchus arvensis – 1.4 r r r 10 . . .

Capsella bursa-pastoris 6 0.1 r 0.1 0.1 5 . 26 103

Mercurialis annua 8 0.2 r r r . . . .

Chenopodium hybridum 8 r 0.2 r r . . . .

Echinochloa crus-galli 8 r r r 0.2 (r) . 26

Taraxacum officinale 8 0.2 r r . . . . .

Hyoscyamus niger 9 0.1 . r r (r) . .

a_{Additional species occurring sporadically (without crops): On both fields: Achillea millefolium, Agropyron repens, Arctium minus,}

Atriplex nitens, Avena fatua, Conyza canadensis, Epilobium adnatum, Galium spurium, Galinsoga ciliata, Galinsoga parviflora, Lamium purpureum, Poa annua, Poa pratensis, Rumex obtusifolius, Senecio vulgaris, Setaria viridis, Sisymbrium altissimum, Sisymbrium loeselii, Sonchus asper, Trifolium repens, Trifolium pratense, Veronica persica; Only on the long-term experimental field: Anethum graveolens, Apera spica-venti, Diplotaxis tenuifolia, Euphorbia peplus, Myosotis arvensis, Papaver dubium, Salsola kali, Senecio vernalis, Veronica agrestis; Only on the regeneration field: Anthriscus caucalis, Atriplex tatarica, Brassica napus, Cirsium vulgare, Datura stramonium, Geranium pusillum, Lepidium ruderale, Medicago sativa, Plantago major, Sambucus nigra, Sisymbrium officinale.

b_{See Ellenberg (1992): Scale from 1 to 9 (indicator of sites from extremely poor to rich in available nitrogen).}

(7)

Table 4

Species composition of the different plots compared by qualitative (Sørensen) and quantitative (Czekanowski) similarity indices; N: average value of long-term experimental field, G: average value of regeneration field

Sørensen-Index Czekanowski-Index 1992 1993 1994 1992 1993 1994 N0–N1 79.5 86.5 73.0 82.4 74.7 49.7 G1–G2 74.6 80.0 70.8 62.5 63.4 76.0 N–G 64.4 66.7 66.7 18.0 27.3 15.9 N1–G1 57.0 71.1 53.3 10.8 39.9 23.7

extremely rare on the regeneration field. Instead, ni-trophilous weeds dominated both G1 and G2 plots. Similarity indices, especially the quantitative one (Czekanowski), showed a low similarity between N1 and G1 plots compared with N0 and N1 or G1 and G2 (Table 4). Species composition depended much more on the experimental field than on nitrogen supply. 3.4. Availability and dispersal of seeds

Differences in nitrogen cannot entirely explain dif-ferences in species composition. Repeated sampling of the seed bank and detailed above-ground vegetation analysis between 1992 and 1994 indicated that many characteristic weeds of Central German Chernozem Region had no soil seed bank at all on the regenera-tion field (e.g. Consolida regalis, Euphorbia exigua, Lithospermum arvense), or seed number in the soil was too small to establish stable populations (e.g. Papaver rhoeas, Silene noctiflora, Veronica polita, see Table 3).

Fig. 4 shows the distribution of seed sources of ‘missing’ species in the neighbourhood of the regen-eration field. Findings were divided into small groups of 10 or less individuals and larger ones of 50 or more plants. Populations of intermediate size were recorded as several small ones. All species common on the long-term experimental field but absent from the re-generation field were found in the surroundings. The populations were often small and confined to the field margins. Single individuals of Silene noctiflora, Con-solida regalis and Lithospermum arvense were found in a distance of 50–200 m. Papaver rhoeas was quite common around the regeneration field. The species was not plotted in Fig. 4 because it was recorded at

36 locations. Larger populations of all investigated species were not found within 300 m.

The artificial introduction of Lithospermum arvense seedlings on the regeneration field in 1993 provides information about seed dispersal (Fig. 5). The plants were competitive and produced 490 seeds per indi-vidual. Altogether 7500 viable seeds reached the soil. Two months after release, diaspores were only found in the nearest seed traps (maximum distance 0.5 m). Har-vest of the crop terminated the dissemination analysis by seed traps. Until 1994 only one seedling emerged. Another 26 seedlings were recorded in the spring of 1995, all within a maximum distance of 2.5 m from the source. The seedlings did not attain the reproduc-tive stage because the subsequent tillage for sowing maize destroyed them. No further seedlings appeared until autumn 1997.

4. Discussion

Nitrogen is often a factor limiting plant growth in terrestrial ecosystems. However, higher N-availability and larger biomass production increases competition for light (Schulze and Chapin III, 1986; Tilman, 1987; Bischoff and Mahn, 1995). Svensson and Wigren (1982) showed that many weeds gain little advantage from fertilization if growing together with crops. De-spite light competition, in most cases nitrogen had a slightly positive effect on weed growth (e.g. Alkäm-per et al., 1979; Wells, 1979). Mahn, (1984b) found that the number of individuals was lower in fertilized plots, but biomass of single plants was higher and compensated for the lower density. In the present study, the effect of nitrogen on weed density, biomass and coverage depended on the crop and was species specific. Mortality was often greater in fertilized plots even if more biomass and more seeds were produced by the surviving plants. High mortality enhanced the risk of extinction of small initial populations. Espe-cially low-growing species were shaded out in dense canopies, and even weeds that normally benefited from higher N-availability had a minor chance to es-tablish. Thus, a large N-supply had a negative effect on the regeneration of weed populations.

If fertilization is stopped and N-supply reduced, di-aspore availability becomes more important for the re-generation process. In the agricultural landscapes of

(8)

Fig. 4. Distribution of some characteristic weeds in the surroundings of the regeneration field.

Europe, formerly typical weeds are now very rare (Albrecht, 1995; Andreasen et al., 1996). Control mea-sures lead to a disappearance of many species in the soil seed bank. Arable weeds are often confined to the field margins (Wilson and Aebischer, 1995; De Snoo, 1997).

A small number of seed sources in the surround-ings hampers the regeneration of weed communities because the likelihood of seed input is very low. The introduction experiment with Lithospermum arvense illustrates that a high input of seeds may be neces-sary to re-establish an extinct population. Primack and

(9)

Fig. 5. Dispersal of the planted Lithospermum arvense population on the regeneration field recorded by seed traps and seedling emergence in the following years; direction of harvest and soil cultivation: west⇔east.

Miao (1992) found that new populations of annual species introduced by sowing established only at three out of 34 suitable natural sites. In agroecosystems ploughing moves the seeds deep into the soil (Cousens and Moss, 1990). An emergence is often not possible in the following season resulting in an increased risk of extinction by seed predation and unsuccessful ger-mination.

The re-establishment of weeds is also delayed by slow seed dispersal. In the present investigation Lithos-permum arvense seeds only dispersed a few metres within 2 years. In a dispersal experiment with Papaver rhoeas Blattner and Kadereit (1991) observed the most seeds within 3 m of the mother plant. Rew et al. (1996) recorded 46% of Bromus sterilis seeds±1 m from the source with a maximum dispersal distance of 53 m. Kothe-Heinrich (1991) found that Valerianella rimosa needed 4 years to pass a small field-path between two arable fields.

In conclusion, regeneration of weed communi-ties following extensification is often limited by dispersal. In species poor agricultural landscapes re-establishment mainly depends on dispersability, frequency and distance of weed populations.

Acknowledgements

The crop management was carried out by members of the Centre for Environmental Research (UFZ), experimental station Bad Lauchstädt and of the Martin-Luther-University Halle, experimental station of the Agricultural Faculty in Etzdorf. Thanks are expressed to the leaders of the stations Dr. A. Pfeffer-korn and H.-S. Pentschew for excellent cooperation and to the team of the Institute of Geobotany for as-sistance in determining biomass production. We also thank two anonymous referees for improvements to the original version of this paper. The research was supported by the German Federal Ministry of Educa-tion, Science, Research and Technology (BMBF).

References

Albrecht, H., 1989. Untersuchungen zur Veränderung der Segetalflora an sieben bayerischen Ackerstandorten zwischen den Erhebungszeiträumen 1951/68 und 1986/88. Diss. Bot. 141, 1–201.

Albrecht, H., 1995. Changes in the arable weed flora of Germany during the last five decades. In: Proc. EWRS Sympos.

(10)

Challenges for Weed Science in a Changing Europe (Budapest), pp. 41–48.

Alkämper, J., Pessios, E., Do van Long, 1979. Einflub der Düngung auf die Entwicklung und Nährstoffaufnahme verschiedener Unkräuter in Mais. Proc. EWRS Sympos. The influence of different factors on the development and control of weeds (Mainz), pp. 181–192.

Andreasen, C., Stryhn, H., Streibig, J.C., 1996. Decline of the flora in Danish arable fields. J. Appl. Ecol. 33, 619–626. Barkman, J.J., Doing, H., Segal, S., 1964. Kritische Bemerkungen

und Vorschläge zur quantitativen Vegetations analyse. Acta Bot. Neerl. 13, 394–419.

Bischoff, A., 1996. Vegetations- und Populationsdynamik in N-belasteten Agrarökosystemen nach dem Übergang zu einer extensivierten Nutzung. Diss. Bot. 268, 1–184.

Bischoff, A., Mahn, E.-G., 1995. Zur Regeneration hochbelasteter Agrarökosysteme bei extensivierter Nutzung – Populationsbiologische Untersuchungen an ausgewählten Segetalpflanzenarten. Verh. Ges. Ökol. 24, 99–104.

Blattner, F., Kadereit, J.W., 1991. Patterns of seed dispersal in two species of Papaver L. under natural conditions. Flora 185, 55–64.

Cousens, G.W., Moss, S.R., 1990. A model of the effects of cultivation on the vertical distribution of weed seeds within the soil. Weed Res. 30, 61–70.

De Snoo, G.R., 1997. Arable flora in sprayed and unsprayed crop edges. Agric. Ecosyst. Environ. 66, 223–230.

Ellenberg, H., 1992. Zeigerwerte der Gefäßpflanzen (ohne Rubus). Scripta Geobot. 18, 9–166.

Erviö, L.R., Salonen, J., 1987. Changes in the weed population of spring cereals in Finland. Ann. Agricult. Fenn. 26, 201–226. Goodall, D.W., 1973. Sample similarity and species correlation.

In: Handbook of Vegetation Science 5 (Ordination and classification of communities), pp. 105–156.

Haas, H., Streibig J.C., 1982. Changing patterns of weed distribution as a result of herbicide and agronomic factors. In: Lebaron, H.M., Gressel, J. (Eds.), Herbicide Resistance in Plants. Wiley, New York, pp. 57–79.

Hilbig, W., 1973. Übersicht über die Pflanzengesellschaften des südlichen Teils der DDR. VII: Die Unkrautvegetation der Äcker, Gärten und Weinberge. Hercynia N.F. 10, 394–428.

Hilbig, W., Bachthaler, G., 1992. Wirtschaftsbedingte Veränderungen der Segetalvegetation in Deutschland im Zeitraum von 1950–1990 (two parts). Angewandte Botanik 66, 192–209.

Körschens, M., Mahn, E.-G., 1995. Strategien zur Regeneration belasteter Agrarökosysteme des mitteldeutschen Schwarz-erdegebietes. B.G. Teubner, Stuttgart-Leipzig, 568 pp.

Kothe-Heinrich, G., 1991. 5 Jahre Feldflora-Reservat Hielöcher im östlichen Meibnervorland. Verh. Ges. Ökol. 19, 69–75. Kropac, Z., 1966. Estimation of weed seeds in arable soil.

Pedobiologia 6, 105–128.

Mahn, E.-G., 1984a. Structural changes of weed communities and populations. Vegetatio 58, 79–85.

Mahn, E.-G., 1984b. The influence of different nitrogen levels on the productivity and structural changes of weed communities in agro-ecosystems. In: 7th Int. Symp. Weed Biol., Ecol. and System, Paris, pp. 421–429.

Mahn, E.-G., 1988. Changes in the structure of weed communities affected by agro-chemicals – what role does nitrogen play?. Ecol. Bull. 39, 71–73.

Oesau, A., 1991. Auswirkungen intensiver Bewirtschaftung-smaßnahmen auf die Zusammensetzung der Getreidewild-krautflora im Rheinhessischen Tafel- und Hügelland. Fauna Flora Rheinland-Pfalz 6 (2), 299–334.

Otte, A., Zwingel, W., Naab, M., Pfadenhauer, J., 1988. Ergebnisse der Erfolgskontrollen zum ‘Ackerrandstreifenprogramm’ aus den Regierungsbezirken Ostbayern und Schwaben (Jahre 1986 und 1987). Schriftenreihe des Bayerischen Landesamtes für Umweltschutz 84, 161–205.

Primack, R.B., Miao, S.L., 1992. Dispersal can limit local plant distribution. Conserv. Biol. 6, 513–518.

Rew, L.J., Froud-Williams, R.J., Boatman, N.D., 1996. Dispersal of Bromus sterilis and Anthriscus sylvestris seed within arable field margins. Agric. Ecosyst. Environ. 59, 107–114. Ritschel-Kandel, G., 1988. Die Bedeutung der extensiven

Ackernutzung für den Arten- und Biotopschutz in Unterfranken. Schriftenr. Bayer. Landesamt f. Umweltschutz 84, 207–218. Schulze, E.D., Chapin III, F.S., 1986. Plant specialisation to

environments of different ressource availability. Ecological studies 61, 120–148.

Schumacher, W., 1980. Schutz und Erhaltung gefährdeter Ackerwildkräuter durch Integration von landwirtschaftlicher Nutzung und Naturschutz. Natur und Landschaft 55, 447–453. Svensson, R., Wigren, M., 1982. Nagra ogräsarters tillbakagang belyst genom konkurrens-, gödslings-, och herbicidförsök. Svensk Bot. Tidskr. 76, 241–258.

Tilman, D., 1987. Secondary succession and the pattern of plant dominance along experimental nitrogen gradients. Ecological Monographs 57, 189–214.

Wells, G.J., 1979. Annual weed competition in wheat crops – the effect of weed density and applied nitrogen. Weed Res. 19, 185–191.

Wilson, P.J., Aebischer, N.J., 1995. The distribution of dicotyledonous weeds in relation to distance from the field edge. J. Appl. Ecol. 32, 295–310.

(1)

Table 2

Survival rates and seed production of surviving Chenopodium

album and Lithospermum arvense individuals

1992 (Winter 1993 (Maize) 1994 (Spring

wheat) barley)

Survival Seeds/ Survival Seeds/ Survival Seeds/

Plant Plant Plant

Chenopodium album

N0 0.97 81.7 0.36 3660.7 0.63 3.1 N1 0.97 316.0a _0.28 _966.0a _0.58 _20.0

G1 0.10 3.8 0.40 314.7 0.13 1.2

G2 0.05 21.1 0.05b _5.0b _0.13 _0.1 Lithospermum arvense

N0 1.00 356.1 1.00 278.3 1.00 18.0 N1 0.97 478.1a _0.92 _92.8a _0.89 _35.0a

a_{N1 differs significantly from N0.} b_{G2 differs significantly from G1, p}_<_0.05.

effect was significant in 1992; in 1994 the

high-est weed density was observed in the unfertilized

N0-plots. In N-rich plots a smaller proportion of

emerged weeds survived until the end of vegetation

period. Detailed demographic results were obtained

for C. album and L. arvense. High N-supply increased

mortality of C. album and L. arvense populations on

both fields (Table 2), but the majority of differences

was not significant. Despite a lower survival rate,

fe-cundity of the surviving plants was sometimes higher

in N1 and G2-plots than in N0 and G1-plots. Seed

production of L. arvense was significantly increased

by nitrogen in 1992 and 1994, but reduced in 1993.

Nitrogen effect on the seed production of C. album

differed largely between the years. In 1992 the

rela-tionship between N-availability and number of seeds

per plant was positive; in 1993 it was negative and in

1994 results of the long-term experimental field and

the regeneration field were contrary.

3.3. Species composition

In Table 3 the mean values of three single years

are presented to compare the species composition of

above-ground vegetation and soil seed bank. The

ef-fect of N-fertilization (N0 versus N1) and of

differ-ences in soil N-content (G1 versus G2) differed with

species. Nitrophilous weeds with high indicator values

for N (Ellenberg, 1992) gained from a high N-supply,

particularly Galium aparine, Solanum nigrum, Urtica

urens. The majority of species with low indicator

val-ues showed a clearly reduced coverage and soil seed

bank, such as Euphorbia exigua, Anagallis arvensis

and Silene noctiflora. Despite low indicator values

coverage of Descurainia sophia and Lithospermum

arvense was higher in N-rich plots.

Remarkably, many weeds, frequent in the N1-plots,

did not occur in the G1-plots, although soil nitrogen

level was nearly the same. Weeds described as

char-acteristic for the Central German Chernozem Region

(Hilbig, 1973) such as Consolida regalis, Euphorbia

exigua, Lithospermum arvense, Papaver rhoeas,

Silene noctiflora and Veronica polita were absent or

(2)

Table 3

Species composition of the regeneration (G1, G2) and long-term experimental field (N0, N1); coverage and soil seed bank (10 cm deep) 1992–1994 (average values)a

Indicator Coveragec _{Soil seed bank (/m}2₎

value: Nb N0 N1 G1 G2 N0 N1 G1 G2

Species occurring only or much more frequently on the long-term experimental field

Euphorbia exigua 4 0.9 0.2 . . 52 . . .

Lithospermum arvense 5 1.0 1.6 . . . 5 . .

Consolida regalis 5 0.2 r . . . .

Anagallis arvensis 6 0.7 . . . 31 31 . .

Chaenorhinum minus 5 r . . . .

Veronica polita 7 6.7 6.8 (r) 620 692 . .

Veronica hederifolia 7 7.2 5.0 (r) 145 114 (r)

Papaver rhoeas 6 5.4 4.1 (r) 2294 1671 (r)

Sinapis arvensis 6 0.2 0.1 (r) . 47 . .

Silene noctiflora 5 5.7 2.8 r r 150 72 . .

Viola arvensis – 10.7 7.3 r r 951 558 . .

Euphorbia helioscopia 7 2.3 2.2 r r 31 21 43 .

Polygonum lapathifolium 8 2.6 3.7 0.1 r 31 72 86 .

Galium aparine 8 0.5 5.5 0.1 0.1 16 26 . 26

Lamium amplexicaule 7 3.2 3.2 0.2 r 258 362 52 26

Cirsium arvense 7 11.3 5.2 0.3 . (r) . .

Fallopia convolvulus 6 29.3 27.8 0.4 0.5 98 134 (r)

Species occurring only or much more frequently on the regeneration field

Rumex crispus 6 . . 0.6 3.6 . . 26 95

Atriplex patula 7 . . 0.8 r . . . .

Malva neglecta 9 . . 0.3 r . . . .

Urtica dioica 9 . . 0.3 . . . 164 .

Ballota nigra 8 . . 0.2 r . . . .

Arctium tomentosum 9 (r) 0.2 0.1 . . (r)

Artemisia vulgaris 8 r r 2.8 2.3 . . 611 327

Urtica urens 8 r r 0.3 1.2 (r) 78 629

Thlaspi arvense 6 r r 4.1 6.2 (r) 284 586

Matricaria maritima 8 0.1 r 1.5 0.4 5 198 155

Sonchus oleraceus 8 0.2 r 1.8 1.1 5 5 95 69

Solanum nigrum 8 0.9 7.2 14.0 28.8 0 62 698 2093

Species occurring on both fields

Chenopodium album 7 15.3 12.7 4.5 3.1 1116 1183 629 560

Descurainia sophia 6 5.0 8.0 0.8 6.1 171 207 586 1567

Chenopodium ficifolium 7 4.7 6.8 4.4 3.4 868 1721 1679 2110

Amaranthus retroflexus 7 0.5 2.2 1.8 0.5 10 52 379 215

Stellaria media 8 0.1 1.0 0.6 0.6 196 227 52 26

Fumaria officinalis 7 0.7 1.9 0.1 r . . 17.2 .

Polygonum aviculare 6 0.8 0.7 0.4 0.7 10 36 . .

Lactuca serriola 4 0.9 0.7 0.2 0.1 (r) . .

Sonchus arvensis – 1.4 r r r 10 . . .

Capsella bursa-pastoris 6 0.1 r 0.1 0.1 5 . 26 103

Mercurialis annua 8 0.2 r r r . . . .

Chenopodium hybridum 8 r 0.2 r r . . . .

Echinochloa crus-galli 8 r r r 0.2 (r) . 26

Taraxacum officinale 8 0.2 r r . . . . .

Hyoscyamus niger 9 0.1 . r r (r) . .

a_{Additional species occurring sporadically (without crops): On both fields: Achillea millefolium, Agropyron repens, Arctium minus,}

b_{See Ellenberg (1992): Scale from 1 to 9 (indicator of sites from extremely poor to rich in available nitrogen).}

(3)

Table 4

extremely rare on the regeneration field. Instead,

ni-trophilous weeds dominated both G1 and G2 plots.

Similarity indices, especially the quantitative one

(Czekanowski), showed a low similarity between N1

and G1 plots compared with N0 and N1 or G1 and G2

(Table 4). Species composition depended much more

on the experimental field than on nitrogen supply.

3.4. Availability and dispersal of seeds

Differences in nitrogen cannot entirely explain

dif-ferences in species composition. Repeated sampling

of the seed bank and detailed above-ground vegetation

analysis between 1992 and 1994 indicated that many

characteristic weeds of Central German Chernozem

Region had no soil seed bank at all on the

regenera-tion field (e.g. Consolida regalis, Euphorbia exigua,

Lithospermum arvense), or seed number in the soil

was too small to establish stable populations (e.g.

Papaver rhoeas, Silene noctiflora, Veronica polita, see

Table 3).

Fig. 4 shows the distribution of seed sources of

‘missing’ species in the neighbourhood of the

regen-eration field. Findings were divided into small groups

of 10 or less individuals and larger ones of 50 or more

plants. Populations of intermediate size were recorded

as several small ones. All species common on the

long-term experimental field but absent from the

re-generation field were found in the surroundings. The

populations were often small and confined to the field

margins. Single individuals of Silene noctiflora,

Con-solida regalis and Lithospermum arvense were found

in a distance of 50–200 m. Papaver rhoeas was quite

common around the regeneration field. The species

was not plotted in Fig. 4 because it was recorded at

36 locations. Larger populations of all investigated

species were not found within 300 m.

The artificial introduction of Lithospermum arvense

seedlings on the regeneration field in 1993 provides

information about seed dispersal (Fig. 5). The plants

were competitive and produced 490 seeds per

indi-vidual. Altogether 7500 viable seeds reached the soil.

Two months after release, diaspores were only found in

the nearest seed traps (maximum distance 0.5 m).

Har-vest of the crop terminated the dissemination analysis

by seed traps. Until 1994 only one seedling emerged.

Another 26 seedlings were recorded in the spring of

1995, all within a maximum distance of 2.5 m from

the source. The seedlings did not attain the

reproduc-tive stage because the subsequent tillage for sowing

maize destroyed them. No further seedlings appeared

until autumn 1997.

4. Discussion

Nitrogen is often a factor limiting plant growth in

terrestrial ecosystems. However, higher N-availability

and larger biomass production increases competition

for light (Schulze and Chapin III, 1986; Tilman, 1987;

Bischoff and Mahn, 1995). Svensson and Wigren

(1982) showed that many weeds gain little advantage

from fertilization if growing together with crops.

De-spite light competition, in most cases nitrogen had a

slightly positive effect on weed growth (e.g.

Alkäm-per et al., 1979; Wells, 1979). Mahn, (1984b) found

that the number of individuals was lower in fertilized

plots, but biomass of single plants was higher and

compensated for the lower density. In the present

study, the effect of nitrogen on weed density, biomass

and coverage depended on the crop and was species

specific. Mortality was often greater in fertilized plots

even if more biomass and more seeds were produced

by the surviving plants. High mortality enhanced the

risk of extinction of small initial populations.

Espe-cially low-growing species were shaded out in dense

canopies, and even weeds that normally benefited

from higher N-availability had a minor chance to

es-tablish. Thus, a large N-supply had a negative effect

on the regeneration of weed populations.

If fertilization is stopped and N-supply reduced,

di-aspore availability becomes more important for the

re-generation process. In the agricultural landscapes of

(4)

Fig. 4. Distribution of some characteristic weeds in the surroundings of the regeneration field.

Europe, formerly typical weeds are now very rare

(Albrecht, 1995; Andreasen et al., 1996). Control

mea-sures lead to a disappearance of many species in the

soil seed bank. Arable weeds are often confined to the

field margins (Wilson and Aebischer, 1995; De Snoo,

1997).

A small number of seed sources in the

surround-ings hampers the regeneration of weed communities

because the likelihood of seed input is very low. The

introduction experiment with Lithospermum arvense

illustrates that a high input of seeds may be

neces-sary to re-establish an extinct population. Primack and

(5)

Miao (1992) found that new populations of annual

species introduced by sowing established only at three

out of 34 suitable natural sites. In agroecosystems

ploughing moves the seeds deep into the soil (Cousens

and Moss, 1990). An emergence is often not possible

in the following season resulting in an increased risk

of extinction by seed predation and unsuccessful

ger-mination.

The re-establishment of weeds is also delayed by

slow seed dispersal. In the present investigation

Lithos-permum arvense seeds only dispersed a few metres

within 2 years. In a dispersal experiment with Papaver

rhoeas Blattner and Kadereit (1991) observed the most

seeds within 3 m of the mother plant. Rew et al. (1996)

recorded 46% of Bromus sterilis seeds

±

1 m from the

source with a maximum dispersal distance of 53 m.

Kothe-Heinrich (1991) found that Valerianella rimosa

needed 4 years to pass a small field-path between two

arable fields.

In conclusion, regeneration of weed

communi-ties following extensification is often limited by

dispersal. In species poor agricultural landscapes

re-establishment mainly depends on dispersability,

frequency and distance of weed populations.

Acknowledgements

The crop management was carried out by members

of the Centre for Environmental Research (UFZ),

experimental station Bad Lauchstädt and of the

Martin-Luther-University Halle, experimental station

of the Agricultural Faculty in Etzdorf. Thanks are

expressed to the leaders of the stations Dr. A.

Pfeffer-korn and H.-S. Pentschew for excellent cooperation

and to the team of the Institute of Geobotany for

as-sistance in determining biomass production. We also

thank two anonymous referees for improvements to

the original version of this paper. The research was

supported by the German Federal Ministry of

Educa-tion, Science, Research and Technology (BMBF).

References