**4. Discussion**

The results here partially supported our hypothesis that the dynamical patterns of N and P resorption and C:N:P stoichiometric ratios of in leaves vary significantly with forest types and are regulated by seasonal climate changes. We found that N and P resorption efficiencies in the *Castanopsis carlesii* plantation were higher in the middle of the growing season than in the *Cunninghamia lanceolata* plantation, and lower in the early and late growing season. Moreover, the P-limited *Castanopsis carlesii* plantation exhibited better P resorption, while the *Cunninghamia lanceolata* plantation limited by both N and P showed similar levels of N and P resorption. These findings revealed that plant growth limitation could have a dominant role in controlling nutrient resorption [23,24]. We also found that temperature and precipitation were significantly associated with N and P resorption, but this relationship differed between forest types, implying that adaptive nutrient use strategies respond to the change of the external environment and ultimately to plant development and growth [16,22].

Resorption is one of the important nutrient conservation strategies to improve plant nutrient utilization, playing a key role in several ecosystem processes such as species competition, nutrient return, and decomposition of leaf litter [1,18]. We found that the N and P resorption characteristics in the two plantations were significantly different (Figure 4). Compared to the *Castanopsis carlesii* plantation, the *Cunninghamia lanceolata* plantation showed higher *NRE* and *NRP* but lower *PRE* and *PRP* (Figures 2 and 4). These results indicated that the *Castanopsis carlesii* plantation could be generally more efficient and proficient in resorbing P, while the *Cunninghamia lanceolata* plantation could be better at resorbing N. This is in agreemen<sup>t</sup> with findings on coniferous and broadleaf trees in subtropical karst regions [24]. The higher soil N content in the *Cunninghamia lanceolata* plantation may also be due to the fact that the needles require less N and that most of the N is obtained through resorption rather than root uptake, hence the higher N retained in the soil. In addition, *PRE* was higher than *NRE* in both plantations. This result is consistent with the average *NRE* (49.1%) and *PRE* (51.0%) of plants in Eastern China [23]. The possible reasons are the general lack of P in soils in subtropical regions and the differences in the relative amounts of N and P leached from leaves [45,46]. Therefore, subtropical trees might survive in P-deficient ecosystems by increasing P resorption.

Plant nutrient resorption may exhibit seasonal dynamics in response to environmental changes [13–15]. The dynamical patterns of N and P resorption efficiencies in response to seasonal climate changes differed significantly between the two plantations (Figure 4). In the *Castanopsis carlesii* plantation, N and P resorption efficiencies were significantly negatively correlated with precipitation, and P resorption efficiency was significantly positively correlated with temperature (Figure 6). The possible reason is that as a broad-leaved evergreen species, *Castanopsis carlesii* has a large leaf area [47]. Higher precipitation may lead to abnormal senescence of *Castanopsis carlesii* leaves and leaching of nutrients from the leaves, which results in lower nutrient resorption efficiencies [48]. In addition, it has been suggested that elevated temperatures may delay the leaf senescence in broadleaf trees, which may prolong the time to retransfer P in leaves, resulting in higher P resorption efficiency [49–51]. In the *Cunninghamia lanceolata* plantation, N and P resorption efficiencies were significantly negatively correlated with temperature, but not with precipitation (Figure 7). This indicates that the nutrient resorption of *Cunninghamia lanceolata* is more regulated by temperature than precipitation. In some studies, a higher temperature may aggravate the degree of membrane lipid peroxidation in leaf cells, thereby accelerating leaf senescence, leading to a shorter time for nutrient retransfer and lower nutrient resorption [52,53]. This may explain the negative relationship between nutrient resorption efficiencies and temperature in the *Cunninghamia lanceolata* plantation. Differences in the responses of N and P resorption efficiencies to temperature and precipitation between plantations also supports our hypothesis.

Ecological stoichiometric ratios can usually reflect the growth and nutrient utilization status of plants [25,54]. In this study, mature leaf C:N was significantly higher in the

*Cunninghamia lanceolata* plantation than in the *Castanopsis carlesii* plantation. This might be attributed to the higher C content as well as the lower N content in the *Cunninghamia lanceolata* plantation. Previous studies have observed that coniferous trees tend to have high amounts of resins, waxes, and tannins. These substances are rich in C, which help to increase the C content and C:N in leaves [16,24]. In addition, we found that the C:P in mature leaves varied with time, and these variation patterns varied significantly between plantations (Figure 5). Previous studies have shown that plants need more rRNA to synthesize the required proteins during high growth periods and that increased rRNA leads to an increase in P content [6,55,56]. The Mature leaf C:P in *Cunninghamia lanceolata* plantation was significantly lower in April, May, and September than in *Castanopsis carlesii* plantation, and higher in other months (Figure 5). This indicated that the growth rate of the *Cunninghamia lanceolata* plantation may be higher in the early and late growing season and lower in the middle of the growing season compared to that of the *Castanopsis carlesii* plantation. In addition, resorption causes changes in leaf nutrient contents by transferring nutrients to other active tissues before leaf drop, thus it may be an important process affecting the seasonal dynamics of leaf stoichiometric ratios [24]. In the present study, *NRE* and *PRE* were higher in the early and late growing season and lower in the middle of the growing season in the *Cunninghamia lanceolata* plantation compared to the *Castanopsis carlesii* plantation. This suggested that trees in subtropical forests might meet the nutrient demands of a high growth rate by increasing nutrient resorption.

The availability of N and P tends to limit plant growth and community composition in terrestrial ecosystems, so leaf N:P is widely used to assess the nutrient limitation of plants [28]. In this study, the leaf N:P ratios differed significantly between two plantations, but changed less over time (Figure 5), which did not support our hypothesis. The theory of dynamic equilibrium believes that organisms can control many of their characteristics, especially nutrient balance so that the internal environment will not change drastically with changes in the external environment [55]. The leaf N:P in the two plantations showed a certain degree of homeostasis, which might be the result of the long-term adaptation of different subtropical plants to the environment, or, might be related to its strong ability to maintain homeostasis [57,58]. Therefore, it is speculated that the difference in leaf N:P of trees in different plantations is more determined by the genetic characteristics of the species itself. In addition, a study proposed that the plant growth of terrestrial plants was N-limited when mature leaf N:P is less than 10, and P-limited when N:P is more than 20 [28]. Our results suggested that the *Castanopsis carlesii* plantation was strongly P-limited (average N:P of 27.23), whereas the *Cunninghamia lanceolata* plantation was both N and P-limited (average N:P of 16.24). The stronger P limitation in the *Castanopsis carlesii* plantation compared to the *Cunninghamia lanceolata* plantation may explain its more efficient P resorption. These findings implied that N and P resorption might be adapted to the nutrient limitation status of plants. In subtropical low-nutrient habitats, *Castanopsis carlesii* and *lanceolata* plantations may maintain leaf N:P relative stability as much as possible by regulating nutrient resorption. This is consistent with previous findings [58,59].
