10.18170/DVN/7QIQ6W JingyuDai Peking University Open Research Data Platform 2020 Tree allometry in semi-arid forests is characterized by short height but large canopy. This pattern may be important to carbon sequestration maintaining but still lacks quantification. Here we use terrestrial laser scanning to quantify allometry variations of Quercus mongolica in semi-arid forests. With tree height (Height) declining, canopy area (CA) decreased with substantial variations. The theoretical CA-Height relationships in dynamic global vegetation models (DGVMs) matched only the 5th percentile of our results because of the CA underestimation and Height overestimation by breast height diameter (DBH). Water supply determined Height variation (P = 0.000) but not CA (P = 0.2 in partial correlation). The decoupled functions of stems, hydraulic conductance and leaf spatial arrangement, might explain the inconsistency, which may further balanced tree water consumption and carbon assimilation, avoiding forest dieback. Works on the tree allometry pattern and determinant would effectively supply tree drought tolerance studying and support future DGVM improvements.