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.
Sulfur (S) plays an important role in plant growth and development. However, due to climatic conditions and limited data availability, the variation and allocation of S are largely unknown at regional or global scales. In this study, we systematically evaluated the S distribution patterns and storage in vegetation on the Tibetan Plateau for the first time, based on consistent field‐measured data of 2,040 plant communities. The mean S contents of leaves, branches, trunks, and roots were 1.68, 0.40, 0.19, and 1.45 g kg−1, respectively; corresponding to S densities of 0.40 × 10−2 (9.57%), 1.18 × 10−2 (28.38%), 1.37 × 10−2 (32.90%) and 1.21 × 10−2 t hm−2 (29.15%), respectively. The mean S densities for forests (5.59 ± 0.26 × 10−2 t hm−2) were higher than that of shrublands (4.54 ± 0.51 × 10−2 t hm−2), grasslands (1.03 ± 0.07 × 10−2 t hm−2), and deserts (1.32 ± 0.28 × 10−2 t hm−2). The S density was generally lower in the northwest and higher in the southeast of the Tibetan Plateau and had divergent allocation patterns between different plant organs and vegetation types. Furthermore, we found that S in leaves and roots was more strongly influenced by environmental factors and was particularly sensitive to radiation and atmospheric pressure. Moreover, the total S storage in the vegetation of the Tibetan Plateau was estimated to be 337.32 × 104 t, with 114.32 × 104 (33.89%), 92.36 × 104 (27.38%), 128.77 × 104 (38.17%), and 1.86 × 104 t (0.55%) in the forests, shrublands, grasslands, and deserts, respectively. Our study clarified the S densities of different plant organs and ecosystems on the Tibetan Plateau and compiled 1 × 1 km vegetation S density data sets, which could help determine the key parameters for regional S cycle models in the future.
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