In this special issue, we publish a total of 20 papers arising from a conference held at the University of Hohenheim in September 2015, which brought together five European Framework Programme 7 research consortia to report the results of their work over the previous five years. The research programme was initiated in recognition of Europe's need for sustainably produced biomass to support European strategic objectives for the bioeconomy.The conference title was 'Perennial Biomass Crops for a Resource-Constrained World', and we have retained this for the title of the special issue. The focus of the papers is on the growth and utilization of secondgeneration, nonfood bioenergy crops, such as perennial rhizomatous grasses (PRGs), for example Miscanthus and Arundo donax, and short rotation coppice (SRC) species, like poplar and willow. Both of these types of perennial biomass crops (PBCs) are established only once in a plantation's lifetime and can then be harvested regularly over a lifespan of at least 20-30 years. The overarching questions are related to the suitability of perennial crops for feedstocks for a European bioeconomy and in particular the need to exploit environments for biomass crops which do not compete with food crops. We focus on two major issues relating to the future use of biomass energy: the identification of the most suitable second-generation biomass crops, and the need to utilize land not used for intensive agricultural production (broadly referred to as 'marginal' land) so that we avoid the potential conflict between food and fuel production.The papers have been grouped into three broad categories. In the first category, the papers review the potential of bioenergy resources from perennial crops grown in largely 'marginal areas' across Europe. Miscanthus is probably the most widely planted PRG in Europe, but establishment costs are currently high because the crop is established from pieces of rhizome. In their paper, Clifton-Brown et al. (2017) review progress in upscaling Miscanthus production with seedbased hybrids. One of the advantageous features of Miscanthus is that it possesses the potentially more productive form of photosynthesis (C 4 as opposed to the more common C 3 photosynthesis) which appears to be unusually well adapted to cool temperate climates. However, Jiao et al. (2017) cautions plant breeders screening for low temperature tolerance that variation in leaf-level photosynthesis among switchgrass genotypes exposed to low temperatures does not scale with the final biomass yield.In the second category, there are six papers that focus on examples of end-uses of perennial biomass crops. The papers illustrate how end-uses of biomass can be novel and lead to new applications and products like using specific plant fragments for biorefining. Three of the papers consider uses for Miscanthus feedstock.
