Short rotation woody crops (SRWC) — A model of agriculture in forestry business

The major challenges of shifting forest management goals from traditional forestry to biomass production are sustainability issues, relatively low value of the product in comparison to quality logs and expensive harvesting, being competitive only at a high degree of mechanization in developed nations. As a consequence, rotation periods were shortened and fast growing species are preferred in order to produce woody biomass in an agriculture-like manner. Since the increment is highest at the beginning of stand development and subsequently decreases, only the maximum increment is utilized, ensuring maximum biomass production capacities at a given site. Short-rotation woody crops (SRWC) are hence established, in Europe typically with fast growing willow (Salix) or poplar (Populus) species. However, fast growing hardwood Quercus species are also considered for short rotation [34]. SRWC originated in ancient times, when people coppiced woodlands in order to obtain raw materials, e. g. fuel wood for cooking or heating purposes, but most of the research has been carried out and application of the results has been achieved in the last 50 years [35]. The basic principles of SRWC therefore originate in a coppice land management system, which will be described below. Planting is optimized for maximum biomass production (increment) while minimizing threats of disease and facilitating highly mechanized harvest technologies. Typical rotation periods are between 1 and 15 years [35], and rotations of Salix are shorter (< 5 years) than those of Populus and Quercus. Biomass from short rotations extracts significant amounts of soil nutrients, since a higher share of nutrient rich compartments (bark and thin branches) is extracted from the system. In combination with the short rotation cycles, nutrient extraction rates are larger as compared to conventional forestry. This implies the need of fertilization in most cases and concerns, e. g. about N leaching into groundwater bodies are discussed. However, Aronsson et al.[36] showed that high rates of N fertilization do not necessarily prime leaching, even on sandy soils (Eutric Arenosol) if the demand for N is high. C sequestration in the soil is also primarily controlled by N fertilization and the response of the vegetation [37]. The authors found increasing biomass production and C sequestration in a hybrid poplar plantation following N fertilization. On the other hand, it was argued that short rotations eventually result in the loss of the mineralization phase, thus preventing self-regeneration of the forest ecosystem [38]. Following their argument, a rotation cycle should be long enough to permit the return of autotrophic respiration and high rates of mineralization. In terms of C sequestration potential, it ultimately depends on the land use prior to SRWC if and to what extent additional C is accumulated. Especially sites that were formerly used for agricultural purposes and where organic carbon was depleted are prone to additional sequestration after land conversion [19, 20]. They pointed out that especially, but not only, non-woody Miscanthus plantations, can substantially sequester C with relative high amounts of litter. In a global context, SRWC may interfere with agricultural production if it continues to be a focus and if plantation areas increase since most plantations are not the result of forestland conversion, but rather farmland conversion. One of the reasons for this interference is the varying legal definition of SRWC across nations. While SWRC is considered forest in some countries, it is treated as an agricultural crop in other nations, making comparisons and predictions across borders difficult.