Patterns of leaf C, N, and P stoichiometry at Quercus-level were partly consistent with those at community-level
Geographical variations of leaf traits especially those related to C and N cycles are helpful to predict the adaptation and the fate of wide distribution plant species under global change (Blonder et al. 2017; Martínez-Sancho et al. 2018). Our results showed that leaf [N], [P] and N/P of the studied deciduous Quercus species were lower than those of 1,900 plant species in China (Han et al. 2011) and 1,280 plant species across the globe (Reich and Oleksyn 2004) (Table 1). The decreasing leaf [C], [N], C/P, N/P the increasing leaf [P] and C/N with increasing latitude (Figure 2) indicated that the Quercus species might alter nutrient allocation strategies to adapt to environmental change. The patterns of leaf C, N, and P stoichiometry at genus-level (Quercus) across China in this study were partly consistent with those general biogeographic ones that leaf [N] and [P] increase and leaf N/P decrease with increasing latitude (or decreasing MAT) at multispecies- and community-level (e.g., Reich and Oleksyn 2004; Han et al. 2005, 2011; Tian et al. 2018, 2019), which tested our hypothesis 1.
The increase of leaf C/N and the decrease of leaf [N] along the latitudinal gradient in this study (Figure 2) were consistent with the patterns of 109 dominant species across China’s forests (Tang et al. 2021). The results identified the reduction of N use efficiency (Wang et al. 2014) and N availability (Tang et al 2021) from tropical to temperate forests. Leaf C/P and N/P are vital for plant growth, since the distribution and the variation of P-rich ribosomal RNA occur at different growth rates (Elser et al. 2003; Lambers and Poorter 2004; Makino et al. 2003). The significant increases in leaf C/P and N/P of the studied deciduous Quercus species with latitude coincided with a field investigation from temperate to tropical forests (Zhang et al. 2018). The results implied that the deciduous Quercus species maintain higher growth rate in temperate than in tropical forests, since deciduous species growing at temperate tended to be more resource acquisitive and grew faster within short growing season than at tropic (Ramírez-Valiente et al. 2017; Qi et al. 2020). Furthermore, the higher leaf [N] and N/P but the lower leaf [P] at low than high latitudes (Figure 2) suggested intensified P limitation (Li et al. 2016; Zhang et al. 2018) from northern to southern China. This pattern can be explained by the soil substrate age hypothesis (SSAH) which suggests that P limitation occurs in tropical forests (low latitude) due to soil aging and leaching (Reich and Oleksyn 2004).
Climatic variables determine the spatial patterns of leaf C, N, P of Quercus at a large geographical scale
The geographical variations in plant nutrients are associated with climate gradations and likely reflect the responses and adaptations of plants to climate change (Hedin 2003; Wright et al. 2004). The north-to-south and west-to-east transects across China both reflect shifts from cold, dry to warm, humid conditions, although the temperature gradient is more obvious in the former and the moisture gradient is more pronounced in the latter (Figure S1, S2). In this study, the significant relationships between MAT, Tmax, TS, MAP, PET, AI and leaf C, N, P stoichiometry implied that the nutrient status of the studied deciduous Quercus species was more likely affected by the co-regulation of temperature- and moisture-related factors, due to the slowdown of litter decomposition and soil N mineralization at low precipitation and temperature (Finger et al. 2016; Li et al. 2020). The results were partly identified by the temperature-plant physiology hypothesis (Reich and Oleksyn 2004), as reported that the stoichiometry of Quercus variabilis leaves was driven mostly by the variations of temperature and aridity rather than soil conditions and leaf structure (Sun et al. 2015).
Soil is the primary source for most plant nutrients at standing and plotting levels (Asner and Martin 2016; Feng et al. 2021; Liu et al. 2017). The roles of soils on plant nutrients can not be ignored despite we did not find the dominance of edaphic variables on leaf C, N, P stoichiometry of the studied deciduous Quercus at broad geographic scales (Figure 3, 5). On large spatial scales, the roles of climatic variables may conceal or be more significant than those of edaphic factors on plant elemental stoichiometry (Ordoñez et al. 2009; Sun et al. 2012), because the physiological and metabolic processes of plants were more sensitive to climate change than to soil conditions, which were identified at regional (Feng et al. 2021; Liu et al. 2017) and global scales (e.g. Hartmann 2011; He et al. 2020). Soils provide the main nutrient elements to maintain plant growth, but have a weak capacity to alter species composition or vegetation types (Zhang et al. 2018). The alterations of hydrothermal conditions caused by the changes in climate might have resulted in the different patterns of leaf C, N, and P stoichiometry of the studied deciduous Quercus species.
Possible mechanisms of climatic variables controlling leaf C, N, P stoichiometry
Plants form different nutrient strategies to adapt to changes in soil and climate (McGroddy et al. 2004; Freschet et al. 2015). Results from SEM in this study (Figure 5) suggested that the studied deciduous Quercus species regulate nutrient strategy under different hydrothermal conditions, that is, directly alter leaf [P] and then indirectly alter leaf C/N, C/P and N/P under the changes in climate variables across the geographic scale. The increasing temperature (Han et al. 2005; Kang et al. 2011) as well as the increasing precipitation (Du et al. 2020) had negative effects on leaf [P], which supported our results that the increasing Tmax and MAP led to low leaf [P] (Figure 5), and further decreased the photosynthesis P utilization, photosynthesis rate (leaf [C]) and N use efficiency, as indicated by the negative effects on stoichiometric ratios such as C/N, C/P and N/P (Figure 5). The identified strong correlations between leaf [N] and photosynthetic rate, N use efficiency (Guo et al. 2016) coincidentally confirmed our results (Figure 5). Although local variations in leaf stoichiometry were affected largely by the heterogeneity of soil nutrients and soil ages (Reich and Oleksyn 2004), our results indicated that regional variations of leaf C, N and P stoichiometry were primarily affected by climatic factors. This finding coincided with Zhang et al. (2018), who demonstrated that the spatial patterns of leaf stoichiometry from temperate to tropical forests were more affected by climate factors than soil factors. Specifically, the low precipitation and temperature in northern China might inhibit the decomposition and mineralization of soil organic matter and reduce N inputs of the ecosystem (Finger et al. 2016; Li et al. 2020), leading to low leaf [N] and N/P of the studied Quercus species (Figure 4). The results revealed how climatic variables influence leaf C, N, P stoichiometry of the deciduous Quercus species, which provided evidence for predicting nutrient strategies, which have considerable impacts on plant growth and survival, and further leading to potential distribution shifts of the eurytopic species, under the ongoing climate change.