Carbon (C), nitrogen (N), and phosphorus (P) influence the growth of vegetation and the quality of soils in forest ecosystems, while maintaining element cycling and stability (Han et al., 2005; Niklas et al., 2005; Qiu et al., 2019). Ecological stoichiometry has been extensively employed to elucidate plant growth (Elser et al., 2000; Niklas et al., 2005; Elser et al., 2007), reveal plant nutrient allocation strategies and allometric growth (Yang et al., 2014; Zhao et al., 2016), and identify limiting elements (Koerselman and Meuleman, 1996). When the concentrations of external nutrients change, the nutrient absorption and storage strategies of plants will be altered accordingly (Chapin, 1991; Wang et al., 2017; Castellanos et al., 2018). For example, in the context of N deposition on a global scale, the N:P ratios of plants in terrestrial and freshwater ecosystems were observed to increase. However, both the C:N (C:N ratios) of plants and organic soils and the N2 fixation capacities of soil and water decreased (Sardans et al., 2012). Over six consecutive years of research, Mao et al. (2016) proposed that exogenous P inputs induced the C:N, C:P (C:P ratios), and N:P (N:P ratios) of plants to simultaneously decrease, while N and P exhibited synergistic increases. Variable soil nutrient levels and other disturbances are essential for the strategic use of nutrients by forest trees, where higher soil nutrient concentrations translate to their increased acquisition and storage in leaves (Yan et al., 2006). However, relative P deficiencies under nutrient imbalances were reported to reduce the P content of leaves in two alpine coniferous forests (Zhang et al., 2022). Historically, ecologists have proposed several hypotheses to support these phenomena, including the Law of the Minimum, the multiple limitation hypothesis, and the stability of limiting elements hypothesis (Baar, 1994; Han et al., 2011; Agren et al., 2012).
Perennial plants employ a strategy for the redistribution and absorption of nutrients back into living tissues, which is referred to as RE (nutrient resorption) (Niinemets and Tamm, 2005; Hayes et al., 2014). It is generally believed that the resorption rates of N and P in plants are ~ 50%, whereas the resorption efficiency of K is slightly higher. Nevertheless, resorption efficiencies are often impacted by the attributes of plants (Lal et al., 2001), climatic factors (Oleksyn et al., 2003; Yuan and Chen, 2009), soil fertility (Drenovsky et al., 2010; Yuan and Chen, 2015), and the stage of plant development (Brant and Chen, 2015).
In summary, we believe that plants maintain internal stability by regulating the uptake and loss of nutrients and elements through various response strategies. Further, under changing environmental conditions, plants adopt element compensation or absorption mechanisms through a series of responses. The steady state tends to shift in a certain direction to reconfigure the plant element content, stoichiometric ratio, and resorption characteristics.
The soil substrate age hypothesis (Reich and Oleksyn, 2004) proposes that the developmental age of soil affects the nutrient supply capacities of parent soil materials, which impacts plant stoichiometry. Two components appear to be involved in the soil substrate age hypothesis, as follows: 1. The composition and proportion of soil nutrients vary with increasing age 2. Plant nutrient stoichiometry characteristics fluctuate with increasing age. Since age-related changes in forests can be considered as complex disturbances, the soil carbon levels (Ewel et al., 1987; Klopatek and J., 2002), carbon sinks (Zhou et al., 2006; Luyssaert et al., 2008), and litter nutrient contents (Kelliher et al., 2004), etc., which are related to the soil and plants will be affected by aging. However, the age-related changes of arbor soil (e.g., nutrient content and stoichiometry), vegetation (e.g., nutrient content, stoichiometry, nutrient utilization strategy) and their relationships remain unclear.
Prudent and comprehensive management practices are considered as key for the development of healthy and sustainable forest ecosystems (Dixon et al., 1994). Nevertheless, management principles cannot be generalized for forest ecosystems of different origins at various growth and development stages. In contrast to their natural counterparts, plantation forests have erratic resistance against disease and consume additional soil nutrients, albeit with the result of higher productivity (Perry and Maghembe, 1989). Conversely, natural forests are superior to artificial forests in terms of resisting pests and diseases, maintaining soil fertility, and preserving species (Xu, 1991). These differences in plant nutrient utilization strategies and soil nutrient content are likely associated with age (Lambers et al., 2008). Due to the variable growth rates between natural and plantation forests (White et al., 2021), the soil composition (Liao et al., 2012; Cai et al., 2019; Osuri et al., 2020; Parhizkar et al., 2021), diversity of understory vegetation (Tripathi and Singh, 2009; Gong and Tang, 2016; Zhang et al., 2021), and other aspects are significantly different. Single standardized management strategies applied to different types of forests often lead to stunted growth, excessive competitive pressure, and premature tree aging. The reason may be that inappropriate management strategies cause limited nutrient utilization in developing forests and imbalance or deficiency of tree and organ elements. Thus, management principles should be implemented that align with the nutrient utilization strategies and other unique attributes of forests with diverse origins at each growth and developmental stage (Xu et al., 2021). The potential value of the elucidation of age-related nutrient allocation and resorption strategies of vegetation of diverse origins might be manifest as the capacity to address the degradation of soil fertility, nutrient limitations, and constrained growth rates, etc. in plantation forests of various species. Unfortunately, little is known regarding these factors with no in-depth discussions available to date.
Larix principis-rupprechtii is distinct from common evergreen coniferous plants, in that its annual litter generation far exceeds that of common evergreen tree species, and it "frequently interacts" with forest soils (Liu et al., 2021). Further, it is a commercially valuable wood that is extensively planted in the mountainous regions of Northern, Northeastern, and Southwestern China, which comprises the largest area of Larix principis-rupprechtii plantations worldwide (Chen et al., 2016).
This study set its focus on the age-related changes of plants (e.g., nutritional aspects and utilization strategies) and soils (e.g., nutrient content and quantitative attributes) in larix principis-rupprechtii forests, as the relationships between the two are not clear. Consequently, we proposed two hypotheses: 1) Soil (nutrient content and stoichiometry) and vegetation (nutrient content, stoichiometry, and nutrient use strategies) exhibit age-related changes. 2) Plantations and natural forests differ in terms of their nutrient use levels and dependence on soil nutrient resources.