Plant species richness on the Tibetan Plateau: patterns and determinants

Abstract

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.

Spatial assembly of grassland communities and interrelationships with productivity

“…Moreover, we found that the mechanism determining the spatial aggregation of the Tibetan Plateau differed from that of the Loess Plateau and Tibetan Plateau (Figure 4). Richness and MAP × richness had stronger effects than those of RA, indicating a strong environment‐dependent effect on spatial aggregation, as species richness variation at large scales is mainly dominated by climate change (Cheng et al, 2023), which confirms our first hypothesis. In addition, neighbourhood microenvironments are pertinent for the adaptation of alpine plant communities to extreme environments (Callway, 2002); hence, we also observed that environmental factors strongly constrain spatial aggregation variation more than the Loess Plateau and Mongolian Plateau (Figure S4).…”

“…Additionally, RA and species richness were considered fixed effect variables. When the three species diversity parameters were all included in the model, their variance inflation coefficients all exceeded five (Cheng et al, 2023). Therefore in order to circumvent the effect of multicollinearity on model accuracy, we only retained species richness as it yielded the greatest effect on ANPP (Figure S3).…”

“…Since extreme environmental conditions on the Tibetan Plateau have a strong constraining effect on community dynamics. For example, high intensity environmental filtering effects can lead to species-poor communities (Cheng et al, 2023), and lower species numbers mean a lower probability of interspecific encounters within the community. (II) Above-ground net primary productivity (ANPP) will increase with the degree of interspecific segregation, and spatial aggregation better explains ANPP than species and phylogenetic diversity because of its stronger link to community dynamics and complementary effects (Wiegand et al, 2021).…”

“…However, it is unclear whether and how exogenous variables (refers to those factors that independent from the self‐organization of community members and likely include climate and soil factors and changes in species diversity due to climate and soil variations) affect spatial aggregation at large scales and the relative importance of this in relation to self‐organizing processes. We argue that exogenous variables may influence spatial aggregation through the following two processes: (I) Species diversity is strongly influenced by the environment, which in turn affects the chance of intra‐ or inter‐species encounters within the community (Cheng et al, 2023; Francis & Currie, 2003). (II) Climate and soil can indirectly affect spatial aggregation by influencing self‐organizing processes.…”

Decoupling of uptake‐ and transport‐related traits in absorptive roots across coexisting herbaceous species in alpine meadows

“…On the other hand, the unique environmental conditions on the Tibetan Plateau may shape a unique variation in root anatomical traits and nutrient foraging strategies across coexisting species (Ding, Ge, et al., 2023; Ding, Yin, et al., 2023; Weber & Iversen, 2023). For example, the Tibetan Plateau is characterized by low temperature, low atmospheric pressure and low oxygen concentration due to high altitudes (Chen, Zeng, et al., 2013; Cheng et al., 2023; He et al., 2020). These harsh conditions, especially low temperature, tend to suppress soil nutrient mineralization and plant photosynthetic processes (Lambers & Oliveira, 2019; Sorensen et al., 2008) and consequently affect root tissue construction (Kong et al., 2021).…”