Height–diameter allometric (H-D) relationships play an important role in the light capture and stability of a tree, and it is poorly understood whether the relationships changes with growth stages or functional groups along a large scale environmental gradients. We present a comparative study of H-D relationships of 6810 seedlings and 19,707 trees from about 1000 species in 9 different sites across China. We showed that allometric exponents for seedlings and trees differ between different sites and do not represent a single, constant theoretical value (e.g. 2/3, 1/2 or 1). Seedlings had greater exponents than trees in most sites. The exponents varied between canopy and understory trees in 5 sites, while were indistinguishable from each other in the remaining 4 sites. Canopy-tree seedlings had smaller exponents than understory-tree seedlings in 3 sites but had indistinguishable exponents in the other 6 sites. Gymnosperm trees had exponents greater than or indistinguishable from angiosperm trees. Elevation alone, or together with mean temperature of the warmest month explained variation of tree allometries for canopy-tree and understory-tree seedlings. The exponents of canopy trees decreased with mean annual precipitation. Our results do not support the predictions from metabolic scaling theory or biomechanical models that height–diameter allometries are invariant. Our study provides insight into how ontogeny, adult stature, phyletic affiliations and environmental limitations affect height–diameter allometric relationships at biogeographical scales.