Changes in the stoichiometry of Castanopsis fargesii along an elevation gradient in a Chinese subtropical forest

Introduction

The global forest ecosystem is affected by increasing global temperatures caused by excess greenhouse gases from anthropogenic activities (Wang et al., 2016). Global warming has affected nutrient element cycling in forest ecosystems (Penuelas & Matamala, 1990; Penuelas & Matamala, 1993; Penuelas et al., 2020; Sardans et al., 2015) and its impact on the biogeochemical cycle is difficult to ignore. Global warming may alter the distribution of nutrient elements by impacting plant metabolism, thus affecting nutrient transfer in plant organs (Gavito et al., 2005; Jónsdóttir, Khitun & Stenström, 2005; Yan, Zhu & Yang, 2017). Two the large-scale effects of global warming on element cycling have been studied to date: latitudinal effects over large areas (He et al., 2006; Han et al., 2011; Ordoñez et al., 2009; Zhang et al., 2012; Fang et al., 2019) and altitude effects. These are crucial factors for determining how variations in temperature and relative climatic changes drive ecological processes (Körner, 2007; Normand et al., 2009).

Recently, studies on plant stoichiometry changes along elevation gradients have not shown uniform patterns of change. Changes in plant N has shown an increase (Richardson, Berlyn & Gregoire, 2001; Shi, Körner & Hoch, 2006), decrease (Li & Sun, 2016; Soethe, Lehmann & Engels, 2008; Van De Weg et al., 2009), or no linear correlation (Macek et al., 2012) with increasing elevation. Most studies have shown that plant P decreased with increasing elevation (Soethe, Lehmann & Engels, 2008; Tanner, Vitousek & Cuevas, 1998; Vitousek et al., 1992; Wang et al., 2018). Elevational gradients may lead to significant variations in regional microclimate and soil properties (Zeng et al., 2018; Chang et al., 2016) , which further affects the nutrient cycling of a plant-soil system in forest ecosystems. This suggests that there is closely-coupled element cycling between C, N, and P in the plant-soil system (Elser et al., 2000). Deng et al. (2008) demonstrated that plant P had a significant positive correlation with soil P in the subtropical forest karst area. Plant stoichiometry was also affected by environmental factors. Studying plant stoichiometry and its correlation with environmental factors may provide a theoretical understanding of plants’ nutritional requirements and their mutual feedback with the environment (Li et al., 2019). Reich & Oleksyn (2004) showed that leaf N and P increases from the tropics to mid-latitudes due to temperature-related plant physiological stoichiometry and soil substrate age; these elements decreased at higher latitudes because cold temperatures affect biogeochemistry. Ma et al. (2015) found that root N and P were negatively correlated with annual average temperature and annual precipitation. Many studies of plant stoichiometry have focused on C, N, and P but few have considered the stoichiometry of other crucial elements, including K, Ca, and Mg.

Fagaceae is a vital subtropical plant in China that is important for maintaining and promoting the element cycling of subtropical forest ecosystems. Zheng et al. (2017) showed that there were significant differences between leaf and root C, N, and P in seedlings and young trees of Castanopsis fissa and other plant organs. Chang et al. (2013) showed that P was the most important element for limiting plant productivity in the subtropical forest ecosystem. Castanopsis fargesii is one of the main species of the subtropical forests of China; however, there is little information on its stoichiometry, which affects the regional forest ecosystem and element cycle characteristics. Previous studies have focused on the ecological characteristics of C. fargesii, including its community structure, photosynthetic characteristics, community biomass, and soil organic carbon (Song et al., 2003; Zhao et al., 2005; Qian et al., 2004; Gong et al., 2015; Dai et al., 2018). However, changes C. fargesii ’s stoichiometry along elevation gradients have not been well-studied (Liu et al., 2019; Dai et al., 2018).

Temperatures are known to change with increasing elevation, even over short distances (Tan & Wang, 2016; Paudel et al., 2019). The elevational changes in temperature and soil nutrients may have an effect on plant stoichiometry (Normand et al., 2009; Sundqvist, Sanders & Wardle, 2013; Yu et al., 2013). Thus, we conducted experiments with five elevation gradients as a proxy for the effects of global warming in a subtropical natural forest on Guoyan Mountain in southern China. We sought to investigate the plant (specifically in its leaves and roots) and soil stoichiometry (N, P, K, Ca, and Mg) in C. fargesii populations along elevation gradients and determine the relative importance of elevation to the total variation of ecological stoichiometry.

Material and Methods

Study area description

The study was conducted in the Guoyan Mountain Natural Reserve on Wuyi Mountain (17°29′ ∼118°14′E, 26°38′ ∼27°12′N) in the northwestern Fujian Province. Its peak elevation was 1,383.7 m (Fig. 1). The study area had a moderate-subtropical monsoon climate with an average annual temperature of 19 °C and an average annual rainfall of 2,051 mm. C. fargesii was one of the dominant species along the elevation gradients in the study area, which ranged from 500 to 1,000 m.

Results

Variations in the plant stoichiometry of C. fargesii along the elevation gradients

The mean N, P, K, Ca, and Mg concentrations in the leaves were 14.76 ± 1.60 mg/g, 2.00 ± 0.16 mg/kg, 41.59 ± 8.71 mg/kg, 20.68 ± 7.14 mg/kg, and 8.83 ± 1.51 mg/kg, respectively. The mean N, P, K, Ca, and Mg concentrations in the roots were 12.53 ± 1.91 mg/g, 1.96 ± 0.43 mg/kg, 18.56 ± 4.29 mg/kg, 10.89 ± 4.29 mg/kg, and 5.30 ± 1.56 mg/kg, respectively. Significant differences (P <0.05) in the leaf (Fig. 2) and root (Fig. 3) ecological stoichiometry were found among different elevation gradients. The linear regressions between the ecological stoichiometry and the elevation gradients are shown in Fig. S1 and Fig. S2, respectively. The greatest leaf N concentration was found at the lowest elevation. The mean leaf N:P along the elevation was 7.50 ± 0.63. A significantly positive correlation was found between leaf K and elevation. The leaf and root Ca increased significantly at higher elevations.

Correlations between AGB and plant stoichiometry

Variations in C. fargesii’s AGB at different elevations are shown in Fig. 4. The greatest AGB was found at 600 m. The AGB showed a significantly negative correlation with leaf Ca and no correlation with other leaf stoichiometry. The AGB also showed a significantly positive correlation with root K and root P. There was a significantly negative correlation between the AGB and root Ca (Fig. 5).

Relationships between soil stoichiometry and plant stoichiometry in the C. fargesii population

There were significantly positive correlations between soil N, leaf Ca, and root Ca. There were significantly negative correlations between soil N, leaf N, and leaf Mg. However, there was no correlation between soil P and leaf and root nutrients. Soil K was found to be positively correlated with root P and negatively correlated with leaf Ca, leaf Mg, and root Ca. Soil Ca was found positively correlated with leaf N and leaf P. There was a significantly positive correlation between soil Mg and leaf K, and a negative correlation between soil and leaf Mg, root N, and root K (Table 2). The changes in the soil stoichiometry of the C. fargesii community are shown in Fig. 6.

Table 2:

Correlation analysis between soil stoichiometry and plant stoichiometry of C. fargesii.

Soil stoichiometry	Leaf stoichiometry					Root stoichiometry
	Leaf N	Leaf P	Leaf K	Leaf Ca	Leaf Mg	Root N	Root P	Root K	Root Ca	Root Mg
Soil N	−0.46***	−0.16	0.17	0.44***	−0.21*	−0.14	−0.056	0.028	0.4***	0.012
Soil P	0.059	0.11	−0.048	0.2	0.026	−0.085	0.14	0.081	0.064	−0.0035
Soil K	−0.29*	−0.051	−0.011	−0.34**	−0.35**	−0.22*	0.66***	0.059	−0.48***	−0.00094
Soil Ca	0.36**	0.37**	−0.036	0.065	0.03	−0.25*	0.15	−0.024	−0.24*	−0.12
Soil Mg	−0.068	0.2	0.62***	−0.037	−0.52***	−0.35**	−0.0079	−0.49***	0.21*	−0.091

DOI: 10.7717/peerj.11034/table-2

Notes:

*p < 0.05.

**p < 0.01.

***p < 0.001

No asterisk denotes no significant correlation coefficient was found.

Relationships between environmental factors and leaf and root nutrients in the C. fargesii population

RDA analysis identified two main axes of environmental variation that together accounted for 58.73% of the total variance of ecological stoichiometry (Fig. 7, Tables 3 and 4). The first RDA axis contributed 38.99% of total variance which was explained by soil pH and elevation. The second RDA axis contributed 19.74% of total variance and was mainly explained by soil moisture and soil temperature. We show the ranking of environmental variables in the order of contribution in Table 3. Elevation, soil moisture, soil temperature, and soil pH contributed 46.4%, 33.8%, 16%, and 3.9%, respectively.

Discussion

Conclusions

Leaf N, K, and Ca levels within C. fargesii were significantly related to elevation, while leaf P and Mg showed no relationship with elevation. The leaf N:P indicated that C. fargesii was limited by N in this subtropic forest ecosystem in China. Soil P showed no relationship with leaf stoichiometry. The elevation explained nearly half of the variation of ecological stoichiometry in C. fargesii. Our results may improve our understanding of the biogeochemical cycle for nutrients in the subtropical forests of China.

Supplemental Information