N:P ratio reflects the balance of N (nitrogen) and P (phosphorus) for plant growth and development, and it is often used to infer potential nutrient limitations on the primary productivity (Güsewell, 2004; Jiang et al., 2017). Studies conducted during the past two decades confirmed that leaf N:P ratio was significantly impacted by abiotic factors, such as geography, climate, and soil (Reich & Oleksyn, 2004; Tian et al., 2019; Long et al., 2020; Zhang et al., 2020; Luo et al., 2021). Climates are the most important abiotic factor (Reich & Oleksyn, 2004; Yue et al., 2017; Tian et al., 2019; Yang et al., 2019; Long et al., 2020; Zhang et al., 2020), and latitudinal, longitudinal, and altitudinal patterns of tree N:P ratio are clarified based on the temperature, moisture, and heat distribution (Reich & Oleksyn, 2004; Han et al., 2005; Du et al., 2017; Zhang et al., 2018a, c; Hu et al., 2020).
In addition to abiotic factors, the effects of biotic factors on leaf N:P ratio were also widely reported (Chen et al., 2013; Zhang et al., 2016; Hu et al., 2018; Li et al., 2019). These biotic factors included growth forms, taxonomy, species, and organs (Yang et al., 2015; Zhang et al., 2018c; Urbina et al., 2017). However, in most of these studies, age was not considered. Photosynthetic capacity (Zheng et al., 2011) and nutrient requirements (Zhang et al., 2018b) generally vary with plant growth, especially for woody plants or trees. Trees at different growth stages have huge differences in physiological processes and nutrition requirements, resulting in tree nutrient stoichiometry changes along an age sequence (Yan et al., 2018; Zhang et al., 2018b). However, it remains unclear whether age variables should be introduced to analyze the patterns of tree stoichiometry across a large scale.
In the early stage as seedling and saplings, trees grow slowly with small biomass production but have a potent ability to undergo cell division, and thus requires a large amount of protein, resulting in relatively high nitrogen (N) concentration in tree leaves (Gorokhova & Kyle, 2002; Liu et al., 2016). In the middle stage as young adults, trees grow rapidly with high biomass productions, and need more RNA, DNA, and ATP to participate in photosynthetic assimilation (Agren et al., 2012), and produce the high phosphorus (P) concentration (Li et al., 2017). In the stages of maturity and senility, trees degrade gradually due to the change of physiological function from active to passive (Wang et al., 2015), generating the low tree P concentrations (Zhang et al., 2018b). Therefore, it would be expected that changes in physiological process and nutrition requirement would induce the variation in tree stoichiometry along age sequences.
During the past decades, numerous studies have been conducted to examine the patterns of tree (especially for leaves, the metabolically active organ) N:P ratio along age sequences of pure plantations (Liu et al., 2016; Cao & Chen, 2017; Chang et al., 2017; Chen et al., 2018; Zhang et al., 2018b), but the results from these studies are highly variable. For example, leaf N:P ratio significantly increased with age for Larix kaempferi (Yan et al., 2018), but it showed a first downward, then upward trend for Pinus massoniana (Liu et al., 2016). Yet, leaf N:P ratio remained constant from the young to the over-mature stage for Cunninghamia lanceolata (Zhou et al., 2016). These inconsistent results indicate that a comprehensive understanding of leaf N:P pattern over age sequences remains elusive.
Forest plantations now cover about 291 million hectares globally (FAO, 2020). These plantations often encounter the unbalance between nutrient supplies and requirements for tree growth (Peri et al., 2006; Sun et al., 2016). As described above, previous analyses were conducted for individual species, which showed relatively large uncertainties on the trends of leaf N:P ratio with stand age, and a synthesis for global plantations has not yet been attempted. In this study, we explored the general pattern of leaf N:P ratio for global plantations across age gradients through the compilation and analysis of published data from the individual studies. Specifically, we want to determine whether stand age is a nonnegligible factor on leaf N:P ratio for global plantations. By clarifying the effect of age on tree stoichiometry, the study would contribute to improve the plant stoichiometry theory and provide a theoretical direction in the nutrient management of global plantations in the future.