Clones of non-pathological witches' broom and normal crown from the same trees in Pinus sibirica on common and separate rootstocks: how and why David wins over Goliath

Mutational witches' broom (WB) is a local system of tree shoots with abnormally dense branching and slow growth that visually differs from the normal crown and is presumably caused by a dominant somatic mutation. In this study, we compared the growth of the mutant and normal clones from the same trees growing on different rootstocks and on different branches of the same rootstock to determine how a mutational WB affects a normal tree crown if they have the same starting position. We measured graft length, crown width, and stem diameter at the base of the graft in five pairs of 20-old mutant and normal clones originating from the same trees grafted on normal rootstock. The intrinsic capabilities of a witches' broom are very limited. Single WB clones had 2.0-3.8-fold shorter stems, 1.5-2.0-fold lower crown width, and 1.2-2.0-fold lower stem diameter than single normal clones. When the mutant and normal clones grew together on a double-stem rootstock, mutant clones had approximately the same growth rate but easily suppressed normal crown clones. In this case, WB clones had 1.4-1.8-fold taller stem height, 3.0-4.0-fold higher crown width, and 2.5-4.0-fold higher stem diameter than normal crown clones due to the abnormally high attracting capacity of a WB. The more pronounced the specific traits in WB clones, such as reduced growth and increased branching, the more aggressive they behave relative to the normal crown to more quickly and strongly suppress growth. Characteristics of the WB, such as growth abnormality, aggressiveness relative to the normal crown, and reduced level of differentiation, make it more similar to a benign neoplasm.

Automated summary

Mutational witches' broom (WB) is an abnormal phenomenon in tree growth that visually differs from the normal crown. The growth of mutant clones is smaller in stem length, crown width, and stem diameter compared to normal clones. Additionally, when mutant and normal clones grow together, mutant clones have a similar growth rate but easily suppress the growth of normal clones.