5. Conclusion
By growing poplar SRC on degraded lands and with a minimum of energy input (e.g. use of chemicals, irrigation and fertilization), environmental challenges and competition with food crops can be minimized [8]. From this study, we learnt that the SRC systems on degraded lands can payback the energy invested in their production. Carefully selected plant material and adjusted plantation maintenance may even further increase the energy ratio of poplar SRC on degraded lands. Particularly pure P. nigra and P. trichocarpa clones appeared to be most suitable for growth under suboptimal conditions, i.e. being planted on degraded land and coping with several harvest cycles and with diseases as leaf rust. The initially highly promising D × T and T × D hybrids hardly survived the fourth rotation. Therefore, more long-term research is needed to reveal significant shifts in clonal ranking over the entire lifetime of a poplar SRC and to identify most appropriate clones.