One of the strategies to ensure energy security and to mitigate climate change in the European Union (EU) is the establishment and the use of short rotation woody crops (SRWCs) for the production of renewable energy. SRWCs are cultivated in the EU under different management systems. Addressing the energy security problems through SRWCs requires management systems that maximize the net energy yield per unit land area. We assembled and evaluated on-farm data from within the EU, (i) to understand the relationship between the SRWC yields and spatial distribution of precipitation, as well as the relationship between SRWC yield and the planting density, and (ii) to investigate whether extensively managed SRWC systems are more energy efficient than their intensively managed counterparts. We found that SRWC yield ranged from 1.3 to 24tha−1y−1 (mean 9.3±4.2tha−1y−1) across sites. We looked for, but did not find a relationship between yield and annual precipitation as well as between yield and planting density. The energy inputs of extensively managed SRWC systems ranged from 3 to 8GJha−1y−1 whereas the energy ratio (i.e. energy output to energy input ratio) varied from 9 to 29. Although energy inputs (3–16GJha−1y−1) were larger in most cases than those of extensively managed SRWC systems, intensively managed SRWC systems in the EU had higher energy ratios, i.e. between 15 and 62. The low energy ratio of extensively managed SRWC systems reflected their lower biomass yield per unit area. Switching from intensively managed SRWC systems to extensively managed ones thus creates an energy gap, and will require more arable land to be brought into production to compensate for the yield loss. Consequently, extensification is not the most appropriate path to the success of the wide scale deployment of SRWC for bioenergy production in the EU

Current global temperature increases resulting from human activity threaten many ecosystems and societies, and have led to international and national policy commitments that aim to reduce greenhouse gas emissions. Bioenergy crops provide one means of reducing greenhouse gas emissions from energy production and two novel crops that could be used for this purpose are Sida hermaphrodita (L.) Rusby and Silphium perfoliatum L. This research examined the existing scientific literature available on both crops through a systematic review. The data were collated according to the agronomy, uses, and environmental benefits of each crop. Possible challenges were associated with high initial planting costs, low yields in low rainfall areas, and for Sida hermaphrodita, vulnerability to Sclerotinia sclerotiorum. However, under appropriate environmental conditions, both crops were found to provide large yields over sustained periods of time with relatively low levels of management and could be used to produce large energy surpluses, either through direct combustion or biogas production. Other potential uses included fodder, fibre, and pharmaceutical uses. Environmental benefits included the potential for phytoremediation, and improvements to soil health, biodiversity, and pollination. The review also demonstrated that environmental benefits, such as pollination, soil health, and water quality benefits could be obtained from the use of Sida hermaphrodita and Silphium perfoliatum relative to existing bioenergy crops such as maize, whilst at the same time reducing the greenhouse gas emissions associated with energy production. Future research should examine the long-term implications of using Sida hermaphrodita and Silphium perfoliatum as well as improve knowledge on how to integrate them successfully within existing farming systems and supply chains.