*3.2. The Rootstock Factor*

It has been widely studied that the selection of an appropriate rootstock is a key factor in fruit trees, as rootstocks can confer better adaptability to climatic and edaphic conditions and have influence on the plant mineral uptakes, the yield properties and efficiency, and the vigor of the grafted variety [54]. Figure 2 presents the statistical comparison of rootstocks by means of variance analysis. The rootstock with the highest gross calorific value was GF 305 (18850.70 J·g<sup>−</sup>1) and the lowest gross calorific value was presented by rootstock GN Garnem (18148.20 J·g<sup>−</sup>1). These differences between rootstocks cannot be attributed in our study to differences in carbon content, as we can see in Figure 2. This result would not correspond to those published by Demirba¸s [55], in which the heating power was linked to the oxidation phase of the fuels, and in which carbon commonly dominated and overshadowed small changes of the hydrogen content. Although the contents of nitrogen in almond trees are low compared to other species [50], we can see that the rootstock factor affects nitrogen levels. In this study, the rootstock with the lowest nitrogen content is GN Garnem (0.84%) followed by GF 677 (0.95%). Nox and N2O gases are generated when obtaining energy through biomass and are highly

polluting [56]. These emissions directly depend on the nitrogen content of the biomass and must be as limited as possible.

**Figure 2.** LSD intervals at 90% confidence level in the influence of the rootstock factor in residual biomass characteristics. (*n* = 108).
