Carbon Allocation of Quercus mongolica Fisch. ex Ledeb. across Different Life Stages Differed by Tree and Shrub Growth Forms at the Driest Site of Its Distribution

Abstract

There are less than 10% of woody species that can have both tree and shrub growth forms globally. At the xeric timberline, we observed the tree-to-shrub shift of the Quercus mongolica Fisch. ex Ledeb.. Few studies have explored the underlined mechanism of this morphological transition of tree-to-shrub in arid regions. To examine whether the tree-to-shrub shift affects carbohydrate allocation and to verify the effect of life stage on non-structural carbohydrate (NSC) storage, we measured the concentration of soluble sugar and starch of Q. mongolica in the seedlings, saplings, and adult trees by selecting two sites with either tree or shrub growth forms of Q. mongolica at the driest area of its distribution. Accordingly, there was no significant difference in the radial growth with different growth forms (p > 0.05). The results showed that the effects of growth form on NSC concentrations are significant in the seedling and sapling stages, but become less pronounced as Q. mongolica grows. The results of the linear mixed model showed that life stage has a significant effect on soluble sugar concentration of tree-form (p < 0.05), starch and TNC concentration of shrub-form (p < 0.05). Compared with a shrub form without seedling stage, a tree form needs to accumulate more soluble sugar from seedling stage to adapt to arid environment. Saplings and adult shrubs store more starch, especially in thick roots, in preparation for sprout regeneration. Our study shows that the same species with tree and shrub forms embody differentiated carbohydrate allocation strategies, suggesting that shrub form can better adapt to a drier habitat, and the tree-to-shrub shift can benefit the expansion of woody species distribution in dryland.