Divergent nitrogen-phosphorus allocation strategies across plant organs in Tibetan shrubs

The nitrogen (N)-phosphorus (P) scaling exponent reflects key trade-offs in plant nutrient allocation and is critical for understanding plant growth and ecosystem productivity. However, whether N-P scaling differs among organs and its associations with phylogeny, plant functional types and environmental conditions remain unclear, particularly in Tibetan shrublands.

We estimated organ-specific N-P scaling patterns at individual, species and phylogenetic levels using 580 leaf, branch and root samples from 23 shrub species across 77 sites in Tibet. We further tested how exponents differed between functional types and varied along climatic and soil gradients using bootstrapping methods, and quantified the relative importance of functional composition and environmental factors.

At the individual level, the exponents of leaves (1.36), branches (1.13) and roots (1.35) were higher than most reported global and shrubland estimates, with leaf and root exponents significantly higher than the branch exponent. These relationships remained robust at the species and phylogenetic levels. Deciduous shrubs had higher branch and root exponents than evergreen shrubs, whereas legumes had a lower leaf exponent than non-legumes. Leaf and branch exponents decreased with increasing mean annual temperature, whereas the root exponent increased; all three organ exponents decreased with increasing moisture index and soil total N. Functional-type composition showed the greatest relative importance for leaf and branch exponents, whereas soil total N was most important for roots.

Our results demonstrate contrasting nutrient-use strategies between above- and belowground organs and highlight the need to incorporate organ-specific variation into stoichiometric models to better predict nutrient allocation under environmental change.