Effects of different energy cropping systems on plant diversity in Central German agricultural landscapes

The rich biodiversity of the Central European farmland is to a large extent directly dependent on human management decisions. Diverse and dynamic biocoenoses have developed in mutual interactions with farming practices, and have been an integral part of agricultural land ever since the Neolithic. In the middle of the 20th century, when synthetic biocides, mineral fertilisers and more powerful machinery became widely available, an unprecedented, steady and rapid erosion of farmland biodiversity commenced. Despite political declarations of intentions to slow biodiversity loss, pressure on farmland biodiversity has been further increasing in recent years. In the face of climate change and to reduce the dependency on limited fossil fuels, the subsidy-driven bioenergy boom is increasing the demand for arable land to cultivate the required feedstocks. Only few scientific publications have yet addressed the consequences of the rapid expansion of energy cropping on farmland plant diversity. This thesis aimed to contribute to filling this gap, by recording the habitat conditions (field management, light regime, soil chemical properties) and the plant diversity in different energy cropping systems (maize for anaerobic digestion, oilseed rape for biodiesel and poplar/willow short rotation coppices (SRC) for wood chip combustion). Subsequently, the energy cropping systems were compared to conventionally managed food/fodder crops (winter-sown wheat and barley) and to winter cereal fields managed extensively according to an agri-environmental scheme (AES). The results show clearly that the underlying root causes of the strong decline in arable plant diversity extent far beyond energy cropping. We found the arable plant assemblages in the field interior to be extremely impoverished in energy maize and oilseed rape fields as well as in conventionally managed winter-sown wheat or barley (on average 3–6 species 100 m-²). The mean nitrogen fertilisation rate (195 kg N ha-1 yr-1) and herbicide use intensity (HI = 1.8) were shown to be similarly high for maize and winter wheat. Excessive phosphorus fertilisation (on average 96 kg P2O5 ha-1 yr-1) was, however, discerned as a threat to plant diversity which mainly applies to maize production. Maize was also found to be more shading (11% PAR transmissivity vs. 19% in winter cereals) and consequently less species rich at the field margins than winter cereals (11 and 15 species 100 m-2 respectively). Across study regions, the arable plant communities of conventionally managed fields were typically dominated by the same set of only 5–10 common weedy species. We found (summer-sown) maize stands to offer habitats to a slightly different set of arable plant species than fields cultivated with winter-sown crops. A balanced mixture of maize, oilseed rape and winter cereals at the landscape scale consequently offers habitats to a wider range of arable plant species than any of these crops alone. Habitat conditions on extensively managed fields cultivated according to an AES were, however, found to be much more heterogeneous and suitable for a far greater range of species, including rare and threatened taxa (on average 21 species in the interior and 33 species 100 m-2 at the field margins). Our models also demonstrate that moderate reductions in herbicide use intensity or fertilisation while staying within the range of currently practiced conventional farming techniques can hardly be expected to reliably halt the decline in arable plant diversity. Novel, regionally adapted approaches and extensively managed arable habitats are urgently needed. The plant diversity of SRC was found to decline strongly with plantation age. Young, low-input SRCs showed a community composition similar to early successional fallow land. Contrarily, 5–8 year old densely planted SRCs were found to be dominated by a set of few generalist, nitrophilous species. They were more shading (1–4% PAR transmissivity) and only slightly more species rich (8–19 species 75 m-2) than annual energy crops, despite not being fertilised or treated with herbicides for several years. The planting of 5–20 m wide SRC strips dividing larger fields with annual crops, can nevertheless be recommended in structurally impoverished, intensively managed agricultural landscapes, particularly if measures to increase the variability of habitat conditions in the coppices are applied. To address the root causes of plant diversity loss on farmland it seems, however, additionally paramount to create a permanent network of extensively managed field sanctuaries to maintain viable source populations which can potentially disperse to the wider agricultural landscapes in the future. To effectively inform policy makers, future research on energy cropping and farmland plant diversity should broaden the focus from the field to the landscape scale and address remaining open questions with regard to the interactions of arable plant diversity with other taxonomic groups.