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Background and Aims Bioenergy is central for the future energy mix to mitigate climate change impacts; however, its intricate link with the water cycle calls for an evaluation of the carbon–water nexus in biomass production. The great challenge is to optimize trade-offs between carbon harvest and water use by choosing cultivars that combine low water use with high productivity. Methods Regional scenarios were simulated over a range of willow genotype × environment interactions for the major UK soil × climate variations with the process-based model LUCASS. Soil available water capacity (SAWC) ranged from 51 to 251 mm and weather represented the north-west (wet, cool), north-east (dry, cool), south-west (wet, warm) and south-east (dry, warm) of the UK. Scenario simulations were evaluated for small/open narrow-leaf (NL) versus large/closed broad-leaf (BL) willow canopy phenotypes using baseline (1965–89) and warmer recent (1990–2014) weather data. Key Results The low productivity under baseline climate in the north could be compensated by choosing BL cultivars (e.g. 'Endurance'). Recent warmer climate increased average productivity by 0.5–2.5 t ha−1, especially in the north. The modern NL cultivar 'Resolution' had the smallest and most efficient water use. On marginal soils (SAWC <100 mm), yields remained below an economic threshold of 9 t ha−1 more frequently under baseline than recent climate. In the drought-prone south-east, 'Endurance' yielded less than 'Resolution', which consumed on average 17 mm year−1 less water. Assuming a planting area of 10 000 ha, in droughty years between 1.3 and 4.5 × 106 m3 of water could be saved, with a small yield penalty, for 'Resolution'. Conclusions With an increase in air temperature and occasional water scarcities expected with climate change, high-yielding NL cultivars should be the preferred choice for sustainable use of marginal lands and reduced competition with agricultural food crops. [ABSTRACT FROM AUTHOR]

Abstract: Fast growing short rotation coppice willows are being developed as biomass crops for bioenergy and biofuels. The UK Perennial Bioenergy Crops programme (BSBEC-BioMASS) aims to optimise biomass yield and wood composition of willow. Towards this goal, new trials were established in 2009 of contrasting genotypes (Endurance, Resolution, Terra Nova and Tora) in two different environments (wet and dry) for intensive studies and to increase our process understanding. From data of the first growth cycle we developed a model for light interception, representing the canopy structure, and accounting for both the vertical and horizontal heterogeneity of willow canopies. The key components of light interception were considered as: leaf inclination distribution and its effects on the extinction coefficient; clumping index; and leaf area index as a function of canopy height. The model realistically represents the varietal differences observed in the field and predicts well light interception for willow (r 2 ranges from 0.91 to 0.99) when compared with measured data. The extinction coefficient proved to be well represented using a standard ellipsoidal distribution function (r 2 ranges from 0.81 to 0.97). Vertical leaf area distribution and clumping index were found to be the most important traits for light interception and, unlike the extinction coefficient, the clumping index can be expressed as a function of the leaf area. We conclude that the new model provides a useful and simple approach to characterise the efficiency of light interception in willow genotypes and can help to identify target traits for the selection of varieties with an improved architecture for light interception. [Copyright &y& Elsevier]