The mobile carbon supply to different compartments of a tree is affected by climate but its impact on cell-wall chemistry and mechanics remains unknown. Genet et al. investigate root properties of mature Abies georgei var. smithii growing at different elevations on the Tibet–Qinghai Plateau and find that mechanical resistance, holocellulose and non-structural carbon (NSC) content all decrease with increasing altitude. Root NSC stocks at the treeline may be depleted through over-demand for carbon supply due to increased fine root production or winter root growth.