Abstract: Caragana microphylla is a primary species that drives shrub encroachment across the Eurasian steppe. Understanding how its resource allocation strategies shift during ontogeny is crucial for understanding its expansion mechanism. Here, we quantified organ-level biomass allocation and the stoichiometric characteristics of carbon (C), nitrogen (N), and phosphorus (P) along an individual size gradient. We sampled 150 shrubs on the shrub-encroached grassland of Inner Mongolia, and classified them into three size classes. For each individual, we measured the biomass and C, N, and P contents of five organs—leaves, current-year twigs, perennial branches, fine roots, and coarse roots. Biomass allocation patterns were analyzed using allometric equations, and resource strategies were evaluated using principal component analysis. Our results revealed that, as shrub size increased, only the coarse-root mass fraction increased, whereas allocation to aboveground organs and fine roots decreased or stabilized. Allometric analyses revealed that underground organs scaled positively with total biomass (α > 1), whereas aboveground organs approached isometry at larger sizes. These patterns indicate that a trade-off occurs between organs across size classes, representing a shift from photosynthetic and absorptive tissues toward structural and hydraulic consolidation as shrubs grow larger. Compared with inactive organs (perennial branches and coarse roots), active organs (leaves, current-year twigs and fine roots) maintained greater amounts of N and P. Overall, as individuals grow larger, C. microphylla shifts from a “conservative” to an “acquisitive” strategy. This ontogenetic shift provides a new physiological perspective on the life history strategies facilitating shrub encroachment in arid environments.

Key words: Biomass allocation, Stoichiometric characteristics, Shrub size, Trade-off strategies, Caragana microphylla