Testing the biogeochemical niche hypothesis using leaves, stems and roots of 62 Artemisia species across China

The biogeochemical niche (BN) hypothesis is based on the concentrations of the predominant elements of a given organism to be stoichiometrically matched in order for it to function adequately. However, it is unknown how BN is represented by different plant organs and to what extent environment or evolution affects BN. We measured C, N, P, K, Ca and Mg concentrations in leaves, stems and roots of 1022 individuals of 62 Artemisia species collected across China to quantify BNs of the three organs. The BN of leaves was offset from and smaller in volume than that of stems and roots. BNs of the three organs differed in their sensitivities to environmental gradients, and leaves were less responsive to environmental variation than stems and roots in both BN volumes and positions. Environmental gradients had larger effects on BN positions than on BN volumes of all three organs. The BN volumes and positions of leaves and roots had no phylogenetic signal, while stem BN had a weak signal, that is, repeated species divergences from various Artemisia branches explained most of the BN variation of the three organs. The BN hypothesis cannot be fully tested using the elemental composition of a single organ owing to different physiological mechanisms and diverse responses of BN among organs. At least in Artemisia, leaves are strongly constrained in a limited elemental niche space to support a relatively stable supply of elements for leaf functioning, especially photosynthesis. In contrast, stems and roots develop larger elemental hypervolumes also representing nutrient storage and other functions. The BNs of Artemisia showed different environmental responses between volumes and positions, allowing these species to adjust elemental concentrations while maintaining a stable overall elemental composition under different environmental conditions. Synthesis. In conclusion, BNs of extant Artemisia populations are determined mostly by short‐term phenotypic responses to current environmental conditions and/or genotypic variation, while the recently evolved species diversity results mostly from species‐specific and organ‐specific use of nutrients and little by early divergence in the phylogeny.