Evaluating CAP wildflower strips: high-quality seed mixtures significantly improve plant diversity and related pollen and nectar resources

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In their new study, Schmidt and colleagues investigate the effectiveness of wildflower strips introduced under the European Union’s Agri-Environment Schemes

Flower strips are a fundamental part of agri-environment schemes (AES) introduced by the European Union to counteract the loss of biodiversity and related ecosystem services in agricultural landscapes. At the beginning of every funding period (approx. 5 years), perennial flower strips are sown on sections of arable fields. Farmers receive subsidies to compensate for the yield loss and maintenance costs caused by such flower strips.

Establishing and maintaining a diverse flower strip over the five-year funding period is essential, due to the benefits they provide to most animal groups. Thus, the effectiveness of flower strips for target animal species largely depends on the seed mixture used and the resulting vegetation. Flower strips have been mostly implemented with seed mixtures containing mainly cultivar species, or with species-poor wild plant mixtures. Since many insect species depend on specific native plant species or genera, perennial wildflower strips sown with species-rich native seed mixtures (WFS) are expected to provide the desired diversity of nectar and pollen sources. So far, comprehensive studies analysing the vegetation development on WFS and influencing factors are scarce.

Our study covered the vegetation on 40 WFS spread across Saxony-Anhalt, Germany, and 20 cereal crop fields without WFS as control, from 2017 to 2019. All WFS had been implemented by farmers in 2015 or 2016 as AES with species-rich seed mixtures comprising 30 native forbs from regional seed propagation. Environmental parameters were included in our analyses to evaluate the effects of local site conditions (grass cover, shading, soil fertility), landscape structure (proportion of non-intensively used open habitats, habitat diversity), and WFS proportion in a multi-scale evaluation. Since the primary goal of WFS is to promote faunal diversity, we developed a Pollinator Feeding Index (PFI) to estimate their benefits for pollinators with regard to nectar and pollen resources.

Hairy-legged mining bee (Dasypoda hirtipes) collecting pollen on sown chicory (Cichorium intybus). This eye-catching bee species is specialized on composite plants (Asteraceae) with only ray florets. (Photo: Annika Schmidt)

We found that forb species richness, forb cover and related nectar and pollen supply were much higher on WFS than on controls in all study years. This confirms the effectiveness of WFS to enhance plant diversity in agricultural landscapes. Sown Achillea millefolium, Centaurea jacea, Daucus carota, Lotus corniculatus and Silene vulgaris had the highest frequency as they appeared on most WFS in all years. Although, sown native forbs contributed most to forb cover and high PFI values on WFS, whereas spontaneously established forbs expanded the total range of species considerably, especially in winter and spring.

Yellow swallowtail (Papilio machaon) drinking nectar from the sown brown knapweed (Centaurea jacea). The brown knapweed belongs to the most successful sown forbs as it appeared on most wildflower strips in all years with high cover. (Photo: Annika Schmidt)

Overall, shading and grass cover had the greatest negative effect on the performance of the sown forbs. On heavily shaded areas, however, only a few sown species — such as Achillea millefolium and Daucus carota — established successfully in our study. The cover of grasses increased in parallel to shading. Moreover, WFS with high grass cover in the first observation year retained high grass cover also in the following years, indicating the importance of early established sown forbs to prevent grass dominance. Our results also showed that WFS, if established successfully, provided a high vegetation diversity in agricultural landscapes largely independent of the landscape context.

Carder-bee (Bombus pascuorum) foraging “in” sown viper’s bugloss (Echium vulgare). (Photo: Annika Schmidt)

Altogether, WFS provided a diverse, forb-rich vegetation over the five-year AES funding period, shaped by the good performance of the sown species-rich native wildflower seed mixtures and a high diversity of spontaneously established forbs. We found that sowing a balanced ratio between competitive/weak and high/low growing forbs at a low sowing rate (0.4-0.5 g/m²) left enough gaps for desired spontaneous forbs, whilst also reducing the cost of the seed mixture considerably. By supplying feeding resources for a wide range of pollinators, WFS have significant potential to promote farmland biodiversity and related ecosystem services across various agricultural landscapes. Based on our findings, we recommend the mandatory use of species-rich wildflower mixtures for perennial flower strips in future AES programmes. Grasses should not be included in WFS seed mixtures because they colonise the areas spontaneously anyway. WFS implementation should be avoided in heavily shaded field edges, where grasses often become dominant and AES target species are unlikely to benefit. To prevent failures in implementation and management of WFS and thus improve their ecological effectiveness, advisory services for farmers should be set up.

Read the full article: “Evaluating CAP wildflower strips: high-quality seed mixtures significantly improve plant diversity and related pollen and nectar resources” in Journal of Applied Ecology.