Abstract: Biomass allocation is crucial for predicting ecosystem responses to global change, and yet, whether patterns follow the plastic optimal partitioning theory (OPT) or the constrained allometric partitioning theory (APT) remains contentious across different biomes. A key uncertainty is whether vast, functionally distinct ecosystems, such as temperate and alpine drylands, show different allocation strategies. Here, we investigated root:shoot ratio (R/S) patterns across 120 sites spanning temperate and alpine drylands in northern China. Significant differences in allocation were observed, with temperate drylands showing lower R/S than alpine regions. In temperate drylands, R/S scaled allometrically with plant community size, consistent with APT, with key soil factors exerting only an indirect influence through their effects on plant community size. Conversely, in alpine drylands, R/S was insensitive to plant community size and instead responded directly to the mean annual temperature, a pattern indicative of OPT. We propose that this strategic divergence is linked to their underlying community functional structures. Communities with greater functional dissimilarity may achieve higher niche complementarity, providing the necessary capacity to optimize allocation in response to environmental constraints. Our findings demonstrate that climatic regimes drive alternative biomass allocation strategies, providing both a predictive framework for vegetation responses and a theoretical basis for dryland ecosystem restoration under climate change.

Key words: allometric partitioning theory, biomass allocation, climate change, optimal partitioning theory, root:shoot ratio