The 2/3 scaling of twig nitrogen to phosphorus in woody plants

Nitrogen (N) and phosphorus (P) are important elements for plant metabolism and growth. Therefore, the quantification of the scaling relationship of N to P in plant organs can help us understand many fundamental ecological processes. The general scaling relationships between N and P in leaves and in fine roots have been reported. As compared to leaves and fine roots, however, little information is available on the N versus P scaling relationship in the stems of woody plants, especially for twigs. In this study, a comprehensive dataset comprising 2,038 ​N=P paired observations for a total of 536 woody species was compiled and analyzed to determine whether a general scaling relationship exists. The global mean values of twig N and P concentrations and N:P ratios were 9.33 ​mg·g−1, 1.12 ​mg·g−1 and 10.16, respectively. Twig N and P concentrations and N:P ratios differed significantly within and across functional groups and biomes. Across all of the species, a uniform 0.67 scaling exponent of twig N to P was observed across different functional groups and biomes, i.e., a 2/3-power scaling relationship was observed. However, this numerical value differed across different sites, albeit converging onto 0.67 with increasing sample sizes. Soil total phosphorus was the largest contributor to the variation in the numerical value of the scaling exponent. These results provide useful parameters for stoichiometric growth models, and advance our understanding of the mechanisms underlying plant nutrient dynamics. This study has potential implications in predicting responses of nutrient cycling in terrestrial ecosystem responses to changes in the environment.