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Additional file 1: Table S1. Statistical tests used to analyze all traits measured over the course of the field trial. Differences among genotypes were identified using post hoc Tukey HSD test for ANOVA analyses, and pairwise comparisons with the Wilcoxon test for Kruskal–Wallis analyses.

Additional file 3: Fig. S2. Growth-related characteristics of the RAV1-engineered poplars in the field. Scatterplots showing the distributions of individual values per block for heights and diameters (a) of the main stem (first cultivation cycle, years 2012 and 2013) and (b) of the dominant shoot (second cultivation cycle, years 2014 and 2015) of wild-type (WT) and CsRAV1-overexpressing and PtaRAV1&2-knockdown transgenic poplars. Horizontal lines represent median values per block.

Additional file 2: Fig. S1. Syllepsis and shoot resprouting performance of the RAV1-engineered poplars in the field. Scatterplots showing the distribution of individual values per block (a) for densities of sylleptic branches on the main stem (first cultivation cycle, upper graph) and on the dominant shoot (second cultivation cycle, lower graph); and (b) for the number of shoots resprouting from the remaining 10 cm-long stumps. Counting of sylleptic branches was made in December 2012 and 2014, and shoots in December 2015, respectively. Horizontal lines represent median values per block.

The possibility of using mixtures of species in an attempt to increase biomass production in plantations managed in short rotation coppice is an avenue of research which has been little explored in scientific literature. These kinds of plantations have gained importance in recent times because they allow biomass to be produced over a short time span and in relatively small areas, while the biomass produced is highly suitable for different energy uses and for obtaining other bioproducts, all of which contribute to the development of the bioeconomy. Unlike monocultures, in mixed-stands the phenomena of competition, complementarity or facilitation can arise between the species comprising the stands, leading to changes in the availability of water, nutrients and light, thus modifying the carbon allocation of the component species in comparison to a pure stand. The objective of the experiment is to identify whether or not there is any facilitation and/or complementarity leading to an increase in biomass production in mixed plantations of Populus alba L. and Robinia pseudoacacia L.. A mixed plantation was set up in the center of Spain comprising Populus alba and Robinia pseudoacacia, at a density of 10,000 trees per hectare, which included pure as well as different percentage mixtures of the species (100:0; 75:25; 50:50; 25:75 and 0:100) following a random block design with 36 trees per block and mixture. Highly productive hybrids of the Populus genus have been shown to be suitable for short rotation plantations. Among the species of this genus, the suitability of P.alba for use in Mediterranean environments is currently being assessed given its high adaptive potential. Robinia pseudoacacia is a nitrogen fixing species capable of sprouting from the stump and which exhibits moderate water needs. Hence the suitability of R. pseudoacacia for use in biomass production is also being explored. The results presented correspond to the biomass production at the end of the first rotation (3 years). The most suitable monoculture plantations for biomass production were found to be P.alba versus R.pseudoacacia. Similarly, an improvement in biomass production is achieved when P.alba is grown in mixture with 25% of R.pseudoacacia. In parallel, the individual production of black locust appears to improve with increasing presence of P.alba in the plantation. The results point to the presence of both complementarity and facilitation as the mechanisms for increased biomass production under certain mixture ratios., Proceedings of the 23rd European Biomass Conference and Exhibition, 1-4 June 2015, Vienna, Austria, pp. 210-213

Additional file 2: Fig. S1. Syllepsis and shoot resprouting performance of the RAV1-engineered poplars in the field. Scatterplots showing the distribution of individual values per block (a) for densities of sylleptic branches on the main stem (first cultivation cycle, upper graph) and on the dominant shoot (second cultivation cycle, lower graph); and (b) for the number of shoots resprouting from the remaining 10 cm-long stumps. Counting of sylleptic branches was made in December 2012 and 2014, and shoots in December 2015, respectively. Horizontal lines represent median values per block.

Additional file 4: Fig. S3. Aboveground biomass yields of the RAV1-engineered poplars after two cultivation cycles. Scatterplots showing the distributions of individual values per block, for the aerial biomass production of wild-type (WT) and CsRAV1-overexpression and PtaRAV1&2-knockdown transgenics. Trees were coppiced in December 2013 (first cultivation cycle, upper graph) and December 2015 (second cultivation cycle, lower graph). Horizontal lines represent median values per block.

Additional file 5: Fig. S4. Stem volume and basal area of the RAV1-engineered poplars after two cultivation cycles. Scatterplots showing the distributions of individual values per block, for the stem volume and basal area of wild-type (WT) and CsRAV1-overexpression and PtaRAV1&2-knockdown transgenics. Trees were coppiced in December 2013 (first cultivation cycle, upper graph) (a) and December 2015 (second cultivation cycle, lower graph) (b). Horizontal lines represent median values per block.

Additional file 5: Fig. S4. Stem volume and basal area of the RAV1-engineered poplars after two cultivation cycles. Scatterplots showing the distributions of individual values per block, for the stem volume and basal area of wild-type (WT) and CsRAV1-overexpression and PtaRAV1&2-knockdown transgenics. Trees were coppiced in December 2013 (first cultivation cycle, upper graph) (a) and December 2015 (second cultivation cycle, lower graph) (b). Horizontal lines represent median values per block.

Additional file 4: Fig. S3. Aboveground biomass yields of the RAV1-engineered poplars after two cultivation cycles. Scatterplots showing the distributions of individual values per block, for the aerial biomass production of wild-type (WT) and CsRAV1-overexpression and PtaRAV1&2-knockdown transgenics. Trees were coppiced in December 2013 (first cultivation cycle, upper graph) and December 2015 (second cultivation cycle, lower graph). Horizontal lines represent median values per block.

Additional file 3: Fig. S2. Growth-related characteristics of the RAV1-engineered poplars in the field. Scatterplots showing the distributions of individual values per block for heights and diameters (a) of the main stem (first cultivation cycle, years 2012 and 2013) and (b) of the dominant shoot (second cultivation cycle, years 2014 and 2015) of wild-type (WT) and CsRAV1-overexpressing and PtaRAV1&2-knockdown transgenic poplars. Horizontal lines represent median values per block.

The sustainable management of shrub formations is a challenging aspect of forest management. Such management aims not only to generate biomass resources for multiple uses, creating employment in rural area, but also reduce the risk of fires in fire-prone areas. However, the current methodologies for estimating the potential of shrub biomass, either through direct or indirect approaches, have been developed without taking into consideration the legal, technical, economic and environmental restrictions often found in Mediterranean environments. Moreover, the mechanized harvesting of shrub biomass requires environmental impact assessments to be carried out to identify both the positive and negative impacts in the short-term, which is especially important in the Mediterranean given its specific climatic conditions. This study provides a new methodological approach which considers real scenarios and is based on a GIS for estimating shrub biomass availability in the Mediterranean. Furthermore, it also provides an appropriate methodology for monitoring the environmental impacts of shrub harvesting on biodiversity and the physical and chemical properties of the soil, as well as on erosion and fire risks. The results of this study show that realistic restrictions lead to only part of the overall biomass fraction being considered as available for harvesting. Our results also reveal that the environmental impacts of shrub biomass harvesting, which leads to changes in biodiversity, soil properties, erosion and fire risk, can be easily assessed through a monitoring approach. All of these approaches have been tested in specific, real cases and the results presented and discussed appropriately., Proceedings of the 25th European Biomass Conference and Exhibition, 12-15 June 2017, Stockholm, Sweden, pp. 133-142