Abstract： Ammopiptanthus mongolicus, the sole evergreen broad-leaved shrub and a Tertiary relict rare plant in the arid northwest desert region of China, has developed adaptive resource trade-off strategies across precipitation gradients through plastic adjustment of functional traits. In extreme arid areas, the A. mongolicus community adopted a 'conservative resource use' strategy, characterized by smaller, thickened leaves and low growth rates to enhance stress resistance for survival. In typical desert areas, the community balanced growth and defense mechanisms to cope with intensifying intra-specific and inter-specific competition. Conversely, in the relatively precipitation-rich desert steppe area, the community shifted to a 'rapid resource acquisition' strategy, featuring expanded, thinner leaves and high growth rates to improve resource uptake capacity. This strategic divergence drove distinct gradient responses in community species diversity (Gleason and Margalef indices) and functional diversity (quadratic entropy, RaoQ, and functional richness index, FRic). All metrics showed significant increases along precipitation gradients, while the functional divergence index (FDiv) gradually decreased. Notably, the herbaceous layer exhibited higher sensitivity to precipitation variation than the shrub layer, with more pronounced diversity responses. Stable functional traits (leaf C content and N:P ratio) enhanced overall community stress resistance. Community assembly in extreme arid areas was predominantly governed by environmental filtering, peak FDiv in low-precipitation areas indicates that species need extreme traits to survive. In contrast, functional complementarity (increasing yet decelerating RaoQ trends) became crucial for community stability in moderate-to-high precipitation area (typical desert and desert steppe) due to intensified interspecific competition. Through coordinated regulation of leaf traits (leaf area, leaf thickness, specific leaf area, leaf relative water content, and leaf nitrogen and phosphorus content) and vertical structural stratification (high shrub-layer coverage vs. sparse herbaceous layer), the A. mongolicus community achieved optimized resource allocation under water constraints. These findings unveiled key ecological strategies of desert plants adapting to precipitation variation. This study provided critical theoretical foundations for vegetation restoration and biodiversity conservation in arid regions.