The effects of stand age on leaf N:P cannot be neglected: A global synthesis

The effects of abiotic and biotic factors on plant stoichiometry have been widely reported at the global scale. However, stand age has not always been considered and its effect on stoichiometry showed large uncertainties at the global scale. In this study, we examined the role of stand age in leaf N:P, and observed the general pattern of leaf N:P for global planted forests across age gradients through the compilation and analysis of published data from individual studies. We found that stand age, together with life form and climatic variables, strongly affected leaf N:P. This result indicates that stand age is an indispensable underlying mechanism on stoichiometry. Leaf N:P increased with stand age for all planted forests pooled together, revealing the existence of the general pattern in leaf stoichiometry for planted forests across age gradients. Leaf N:P exhibited no trend in evergreen trees, but an increasing trend was obtained in deciduous trees along age sequences. Meanwhile, leaf N:P rose with stand age for planted forests in humid subtropical regions, but it did not vary in dry temperate regions. Our results reveal that the effects of age on tree stoichiometry cannot be neglected. These age-related patterns of stoichiometry indicated that global planted forests changed from N limitation to P limitation with increasing stand age, particularly for deciduous planted forests and in planted forests in humid regions. The study highlights the importance of considering stand age when exploring nutrient patterns in planted forests, which contributes to improving the plant stoichiometry theory and offers guidance for the nutrient management of planted forests at regional and global scales.