Influence of TPAR on weed assemblages

TPAR was not found to explain a significant part of the variation in weed cover between conventional fields, neither in the field interior nor at the margin. The total weed cover in conventionally managed fields was generally very low with on average 3% (± 2 SE) cover in the field interior and 16% (± 3) at the margin (appendix 3.C). Differences between crops or regions also did not capture a significant part of these subtle weed cover variations.

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In contrast, regression analyses showed that weed cover in the field interior was largely explained by the factors TPAR (34%) and field management (38%), both being highly correlated to each other, when conventional and AES wheat fields were assessed. On the other hand, none of the investigated factors explained the difference in weed cover at the field margin in conventional vs. AES wheat fields. Weed cover was with on average 38% (± 8) by factor 9.5 higher than in conventionally managed wheat fields (mean: 4% ± 4). At the margin, weed cover differed much less between AES fields (mean: 28% ± 8) and conventional wheat fields (mean: 17% ± 4). Variation in weed cover mainly seems to be driven by factors other than the ones included in this study.

3.3.4.2 Influence of TPAR on species richness

Contrary to weed cover, species richness per plot was related to TPAR when management intensity was reduced (i.e. at the margin of conventional fields and when AES fields were included in the analysis, Table 3.3). On conventional field margins, species richness increased with TPAR (correlation coefficient R = 0.32, p = 0.05, n = 40) and TPAR explained 8% of the variation in species richness. The increased variability in light conditions and the reduction of management intensity at the field margins and on AES fields was also found to go along with increased variability in species richness (Fig. 3.2c and 3.3c). When contrasting conventional and AES wheat fields, both TPAR and management intensity were highly significant explanatory factors for species richness. Both factors co-varied tightly and together explained 83% of the variation in species richness in the interior (with 73% being shared between the factors, not being exclusively attributable to either one). At the field margins, both factors together explained 50% of the variation, with 35% being shared between the factors. In contrast to the results from conventionally managed fields, species richness decreased with increasing TPAR on AES field margins where other factors than light seemed to limit plant growth (correlation coefficient R = –0.65, p = 0.04, n = 10). In the interior of conventional fields, differences in species richness between fields were very small and neither TPAR nor crop or region explained this small variation.

Species richness per plot did not differ between the four conventional crops neither for the field margins nor for the field interior (Fig. 3.2c). Weed assemblages of AES wheat fields were, however, much more species-rich than conventionally managed wheat fields (p ≤ 0.05; Fig. 3.3c). Conventional fields (all crops averaged) had on average five species per plot in the interior and 14 species at the field margins, while 21 and 33 species, respectively, were recorded in the AES fields. Apart from few exceptions, we found the most species-rich conventionally managed field margins to still be less species-rich than the poorest AES field margins. There was, however, considerable overlap in species richness between conventionally managed margins and the centres of AES fields.

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We recorded a total of 157 plant species in the 100 plots, of which 155 taxa were found at the field margin and 90 in the field interior (appendix 3.A). 121 species occurred in the ten investigated AES fields and 101 species in the 40 conventionally managed fields. With 24 species restricted to the margin of conventional fields (maize: 8 species; oilseed rape: 2; winter barley: 7; winter wheat: 5) and 31 species restricted to the margin of AES fields, field margins were very important for the regional species pool. Three species were restricted to the interior of AES fields, but no species occurred solely in the interior of conventionally managed fields.

Table 3.3 Fractions of variance (adjusted R², in %) explained by TPAR, management factors (crop;

management intensity: conventional management (conv.) vs. agri-environmental schemes (AES), and region (Lower Saxony uplands vs. Thuringian Basin)) for total weed cover, species richness and the community composition of weed assemblages on arable fields in the central uplands of Germany.

Fractions are based on redundancy analyses (RDA, for weed cover and species richness) or canonical correspondence analyses (CCA, for community composition) and intersections were obtained from variation partitioning.

∩ indicates that the variation explained is shared between the respective components (i. e.

it cannot be attributed exclusively to one of the components). These intersections cannot be tested for significance (Legendre, 2008). Intersections were only assessed when all components explained a significant fraction of the variation in the single factor models.

Models were tested with ANOVA (ns, p > 0.05; *, p ≤ 0.05; **, p ≤ 0.01). FI, field interior; FM, field margin.

3.3.4.3 Influence of TPAR on community composition

A relationship between TPAR and community composition was only found when the most extensively managed cropping systems (margins of conventional and AES wheat fields) were

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assessed. Here, TPAR explained the highest fraction of the variation (8%). The contribution of TPAR was largely independent of the fractions explained by management intensity (7%, 1% shared with TPAR) and region (7%, 1% shared with TPAR). Adonis aestivalis, Euphorbia exigua and Falcaria vulgaris are examples of species which preferred wheat fields with high TPAR, whereas Fumaria officinalis, Lamium purpureum and Myosotis arvensis were less light-demanding and occurred more often when TPAR was low.

In the conventional fields, not TPAR but the choice of crop explained the largest fraction of variation in community composition in the field interior (9%) as well as at the field margin (10%, Table 3.3).

We found Chenopodium hybridum, Echinochloa crus-galli, Persicaria lapathifolia and Solanum nigrum to prefer maize fields over oilseed rape and winter cereals (cp. appendix 3.A). Descurainia sophia, Matricaria recutita, Myosotis arvensis, Papaver rhoeas and Thlaspi arvense are examples of species which were commonly found in oilseed rape, barley and wheat fields, but were absent in maize fields. We could not identify any species which showed a clear preference for any of the three winter-sown crops (oilseed rape, barley and wheat), apart from Capsella bursa-pastoris, which was most commonly found in oilseed rape.

In addition, weed assemblages at the margin of conventional fields differed between the two study regions which explained 2% of the variation in community composition. The influence of region on the composition of the weed assemblages was even larger in AES fields (7 and 6% of variance explained in the field interior and at the margin, respectively) than in conventional cropping systems.

There was a large number of species regularly occurring on AES fields (frequency > 40%) that were completely absent in conventional fields (e. g. Geranium columbinum, Euphorbia exigua, Arenaria serpyllifolia, Valerianella dentata, Medicago lupulina). Rare species, such as Adonis flammea, Bupleurum rotundifolium and Euphorbia falcata (all listed as critically endangered in Ludwig and Schnittler, 1996) were also restricted to AES fields.

3.4 Discussion