The genetic contribution (G)
We found heritable variation in branches for both total NSCs and proportion of NSCs in starch (Table 1, Branch H2TNC = 0.19, Branch H2PropStarch = 0.31). As previously reported, this is similar heritability as total NSC storage in P. trichocarpa stems and roots (Table S1; Blumstein et al., 2020). Here we found that the proportion of starch to total stores is also heritable in roots (Root H2PropStarch = 0.37), but not stems (Stem H2PropStarch = 0.01).
Branch genetic variation does not exhibit high degrees of across population variation in either total NSCs or proportion of stores in starch (Qst TNC = 0.07, Qst Proportion = 0.04; Table 1). This stands in contrast to stems and roots, which demonstrate extensive differentiation across populations in total stores (Qst Roots = 0.30, Qst Stems = 0.31) and stem partitioning between sugar and starch (Qst Stems = 0.66) (Table S1). However, the finding of high Qst value for stems in the partitioning between sugar and starch is unlikely biologically meaningful. H2 for stems is only 0.01 for this trait, thus the high Qst value is calculated using just 1% of the phenotypic variation measured.