Alley cropping agroforestry systems (ACS) are ascribed to have manifold positive ecological effects; nevertheless their application is still limited due to uncertain productivity of the agricultural crop, especially in the tree-crop competition zone. Therefore, this study investigated the variability of oilseed rape and winter wheat yield, respectively, at different distances from the tree strip edge in 2016 and 2017 in an ACS established in 2008 in northern Germany. The ACS consisted of strips of fast-growing poplars alternating with narrow (48 m) and wide (96 m) crop alleys, each with a crop rotation including winter oilseed rape and winter wheat. Each tree strip contained 6 rows of poplars with a density of 10,000 trees per ha. Moreover, multi-year (2009-2016) crop yield data of oilseed rape and winter wheat in the narrow and wide crop alleys were compared with those of a corresponding non-agroforestry control field. In general, crop yields observed in 2016 and 2017 in the narrow crop alleys at 1 m from the tree strip edges were on average 77% (oilseed rape) and 55% (winter wheat) lower than in the middle of the crop alley. One reason for low yield close to the tree strips might be the leaf litter coverage of the seedlings in autumn. Leaf litter deposition was highest at 1 m on the windward and the leeward side of the tree strips in 2015 and on the leeward side in 2016, respectively. However, the average long-term crop yields of the narrow crop alley, the wide crop alley and the control field did not differ substantially among each other. Although oilseed rape and winter wheat yields were lower close to the tree strips, this yield reduction did not negatively influence the average long-term crop yields of the ACS. Keywords Alley cropping Á Crop yields Á Winter wheat Á Oilseed rape Á Yield variability Á Competition zone Á Long-term yield Á Leaf litter

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The most reliable and practicable measure in assuring coexistence in respect to pollen‐mediated gene flow from genetically modified (GM) to conventional maize (Zea mays L.) is an isolation distance separating GM and non‐GM fields. Therefore, we tested distances between 24 and 102 m at three sites in northern Germany using a field orientation representing a worst case scenario concerning wind direction. During the 3 yr of field trials the highest levels of gene flow occurred at the site and year with the longest flowering synchrony and the strongest wind blowing constantly from the GM to the non‐GM field. It was shown that the GM content of a neighboring non‐GM maize field is mainly determined by wind speed and direction as well as by non‐GM maize field depth. Based on the maximum outcrossing data obtained it can be concluded that for non‐GM maize fields being 200 m deep or more an isolation distance of 50 m is sufficient to keep the GM content of the total fields grain harvest below the European Union (EU) labeling threshold of 0.9%. However, non‐GM grain maize fields with smaller field depth require larger isolation distances or additional coexistence measures. In most cases discarding 6 m of the GM maize facing non‐GM maize field edge has proven to be such a valuable measure. In silage maize production 50 m isolation distance is adequate even for non‐GM maize field depths down to 50 m. We recommend flexible separation distances in dependence on non‐GM maize field depth to comply with EU coexistence requirements.

One approach to ensuring coexistence of genetically modified (GM) and conventional maize (Zea mays L.) is reducing pollen‐mediated gene flow. Field experiments were conducted in 2005 at four sites in Germany to compare a tall sunflower crop (Helianthus annuus L.) vs. a short clover–grass crop (Trifolium pratense L. and Lolium spp.) with regard to their ability to reduce outcrossing when grown as buffer between pollen donor and recipient maize plots. Three different maize test systems were used: (i) quantification of a donor transgene via real‐time polymerase chain reaction (rt PCR), (ii) a nontransgenic test system based on a dominant kernel color trait, and (iii) a molecular marker test system based on rt PCR quantification of a cultivar‐specific nontransgenic DNA sequence. We found that the three test systems yielded comparable results concerning buffer‐crop effectiveness and edge effects. There was no difference in outcrossing rates when comparing the sunflower vs. clover–grass buffer crop. Outcrossing rates downwind beyond 12 m sunflower as buffer crop within adjacent 12‐m‐wide recipient maize were 4.2, 11.7, and 3.8% for the GM maize, the kernel color, and the molecular marker test system compared with clover–grass with 4.3, 9.6, and 3.6%. Pronounced edge effects were detected at the edges of recipient maize fields. Based on the present study, growing sunflower as a tall crop between GM and non‐GM maize cannot be recommended as an appropriate coexistence measure.

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