Confronting Allometric and Optimal Partitioning Theories in Biomass Allocation: Evidence from Artemisia Species along an Elevational Gradient in Southwest of China

The allometric partitioning theory (APT) and optimal partitioning theory (OPT) emphasize the roles of plant size and resource constraints in shaping the adaptive strategies of plants growing under diverse environmental conditions. However, the relative importance of APT and OPT in explaining adaptive strategies is still uncertain. This study selected a region within the same latitudinal zone but spanning a large elevational gradient (2350 m). We conducted systematic sampling of Artemisia species across eight distinct slopes. Biomass data for roots, stems, leaves, and whole plants were measured from 620 individual specimens. Using closely related and widely distributed species is feasible for validating the APT and OPT theories, as it not only minimizes the significant influence of evolutionary differences but also ensures a broad environmental gradient. We found that there was a significant allometric relationship between the biomass allocation and the whole plant size of Artemisia species. However, after accounting for size effects, organ allocation ratios still showed significant responses to elevation and environmental principal components. In particular, stem-to-root ratios consistently decreased with increasing elevation across life forms and most species. Structural equation modeling further revealed that elevation influenced biomass allocation indirectly by altering environmental variables (PC1 and PC2). Our findings provide rare empirical support for the simultaneous operation of APT and OPT. Allometric rules offer a baseline. This baseline is adaptively fine-tuned in response to environmental gradients. We stress the importance of integrating both theoretical frameworks. It can help improve predictions of plant biomass allocation strategies under changing environments.