Whether current hypotheses for geographic patterns of species richness (SR) have a strong explanatory power for the Tibetan Plateau (TP) with extreme climatic conditions remains unclear. In comparison with the classic ‘water–energy dynamics hypothesis', the unique climate factors (e.g. extreme low temperature and low oxygen partial pressure) on the TP likely significantly affect the spatial variation of SR. Here, we investigate geographic patterns and determinants of SR on the TP through a systematic field investigation. We systematically analyzed a total of 2013 plant communities covering 11 different vegetation types on the TP. To compare this SR with that of other sites across the globe, we compiled a global database containing information on 87 forest and 3660 grassland plots. The SR per 400 m2 in the forests and shrubs and that per 1 m2 in alpine grasslands and deserts was 62.76 (± 1.80 SE), 44.53 (± 7.57 SE), 16.84 (± 0.39 SE) and 3.62 (± 0.55 SE), respectively. The SR of forests and shrubs decreased with latitude and altitude, whereas that of alpine grasslands and deserts showed a unimodal pattern along the altitudinal gradient. Unique climate factors, such as extreme low temperature, mean diurnal temperature and oxygen partial pressure, act synergistically with water–energy dynamics and influence the spatial pattern of SR on the TP. Furthermore, the tree SR on the TP was lower than that of global tropical and subtropical broadleaf forests but higher than that of temperate conifer forests. Alpine meadows had a higher SR than other sites; however, the SR in alpine desert grasslands and alpine deserts was lower. Our findings provide novel insights into the mechanisms underlying the spatial variation in plant diversity, especially on plateaus and in high‐latitude regions. Our findings and the SR map with 1 km resolution provide important benchmarks for biodiversity conservation and may help to improve predictions of the effect of climate change on biodiversity.

Nitrogen (N) is an important element for most terrestrial ecosystems; its variation among different plant organs, and allocation mechanisms are the basis for the structural stability and functional optimization of natural plant communities. The nature of spatial variations of N and its allocation mechanisms in plants in the Tibetan Plateau—known as the world’s third pole—have not been reported on a large scale. In this study, we consistently investigated the N content in different organs of plants in 1564 natural community plots in Tibet Plateau, using a standard spatial-grid sampling setup. On average, the N content was estimated to be 19.21, 4.12, 1.14, and 10.86 mg g–1 in the leaf, branch, trunk, and root, respectively, with small spatial variations. Among organs in communities, leaves were the most active, and had the highest N content, independent of the spatial location; as for vegetation type, communities dominated by herbaceous plants had higher N content than those dominated by woody plants. Furthermore, the allocation of N among different plant organs was allometric, and not significantly influenced by vegetation types and environmental factors; the homeostasis of N was also not affected much by the environment, and varied among the plant organs. In addition, the N allocation strategy within Tibet Plateau for different plant organs was observed to be consistent with that in China. Our findings systematically explore for the first time, the spatial variations in N and allometric mechanisms in natural plant communities in Tibet Plateau and establish a spatial-parameters database to optimize N cycle models.

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Aims Sulfur is an essential functional element in leaves, and it plays important roles in regulating plant growth, development, and abiotic stress resistance in natural communities. However, information on the spatial variation of leaf sulfur content (LSC) and adaptive character on a large community scale is limited. Methods Sulfur in the leaves of 2207 plant species from 80 widespread ecosystems (31 forests, 38 grasslands, and 11 deserts) in China were measured. One-way ANOVA with Duncan’s multiple-range tests were used to evaluate the differences in LSC among different plant growth forms (PGFs) and ecosystems. We fitted the relationships of LSC to spatial and climate factors using regression. Structural equation modeling (SEM) analysis and phylogenetic analysis helped us to further explore the main factors of LSC variation. Important Findings LSC ranged from 0.15 to 48.64 g kg –1, with an average of 2.13 ± 0.04 g kg –1 at the community scale in China. We observed significant spatial variation in LSC among different ecosystems and taxa. Overall, LSC was higher in arid areas and herbs. Furthermore, higher LSC was observed under drought environments, low temperatures, and intense ultraviolet radiation. Temperature, precipitation, radiation, soil sulfur content and aridity jointly regulated LSC, explaining 79% of the spatial variation. However, LSC was not significantly related to phylogeny. Our study demonstrated that LSC plays an important role in plant adaptation mechanisms to extreme environments and further expanded our understanding of the biological function of sulfur from the organ to the community level. This highlights the importance of sulfur metabolism for our understanding of the impact of global climate change on plants.