Phylogeography of an alpine aquatic herb Ranunculus bungei (Ranunculaceae) on the Qinghai–Tibet Plateau

Many phylogeographic studies of terrestrial plant species on the Qinghai–Tibet Plateau (QTP) have been carried out to elucidate the range shifts in response to climatic oscillations in the Quaternary. However, patterns of range shifts for aquatic plants following the climate change on the QTP are poorly understood. Here, we studied the historical range shifts of the aquatic herb Ranunculus bungei Steud. on the QTP using four chloroplast (cpDNA) non-coding spacers. We revealed low within-population genetic diversity (H_S = 0.052) and high interpopulation genetic differentiation (G_ST = 0.914; N_ST = 0.954). But the high population differentiation was not coupled with a distinct phylogeographic structure (N_ST > G_ST, P > 0.05). Phylogenetic analyses revealed two main cpDNA haplotype lineages and the split between these two lineages can be dated back to the late Tertiary (3.84–11.90 Ma). Two independent range expansions within the two intraspecific lineages at approximately 0.15–0.46 and 0.17–0.50 Ma were revealed. Our results suggested that R. bungei survived the Last Glacial Maximum and/or previous glacial periods on the QTP. Colonization or recolonization during the repeated range expansions may have replaced the early haplotypes and the pre-existing genetic structure and could explain the non-significant phylogeographical structure.     

Evolutionary history and underlying adaptation of alpine plants on the Qinghai–Tibet Plateau

As the highest and largest plateau in the world, the Qinghai–Tibet Plateau, with its numerous endemic species, is one of the important alpine biodiversity hotspots. Only recently have the evolutionary histories and underlying adaptations of these alpine plants become clear, through research mostly based on testable experiments and analyses. In this issue, we collected a total of 13 papers related to such aims. In addition, we selected a few published papers to highlight the major findings in the recent past. We also outlined the outlook and direction of future research.     

Survival and reproduction of plant species in the Qinghai–Tibet Plateau

The Qinghai–Tibet Plateau (QTP) is the highest and largest plateau in the world. It covers correspondingly wide geological, topographical, and climatic gradients, and thus hosts greater biodiversity than surrounding lowlands and other high elevation regions. Due to its extreme environmental and biological diversity, the QTP is an ideal region for studying adaptations of plant species under harsh environmental conditions at multiple evolutionary levels. Many recent ecological studies have revealed functions of distinctive morphological features of various plants in the region that improve their reproductive success. Examples include large and showy bracts, hairy inflorescences, and drooping flowers. Numerous other investigations have examined QTP plants' sexual systems, patterns of biomass allocations, and biotic interactions. This paper summarizes recent advances in understanding of morphological adaptations, plant–plant interactions, plant–pollinator interactions, floral color patterns, pollination adaptations, and resource allocation patterns of alpine plants of the QTP. The overall aim is to synthesize current knowledge of the general mechanisms of plant survival and reproduction in this fascinating region.     

Genetic diversity and population structure of Lamiophlomis rotata (Lamiaceae), an endemic species of Qinghai–Tibet Plateau

Lamiophlomis rotata (Lamiaceae), a perennial medicinal herb, is endemic to the Qinghai–Tibet Plateau. A total of 188 individuals from eight natural populations of L. rotata in Qinghai–Tibet Plateau (four from Tibet, two from Yunnan, and two from Qinghai) were analyzed using intersimple sequence repeats (ISSR) and randomly amplified polymorphic DNA (RAPD) techniques. Our results revealed that the level of genetic variation in L. rotata was relatively high (P = 94.85%, I = 0.440 ± 0.220, H _T = 0.289 ± 0.028). Three genetic groups corresponding to the three geographic regions were detected, suggesting significant geographic structure. Our results suggest that the highly structured geographic pattern found in L. rotata might represent diverging evolutionary processes associated with the uplifting of the Qinghai–Tibet Plateau and Quaternary climatic oscillations. These findings imply that as many populations as possible should be preserved in situ for the conservation of this species. Given their genetic variability and peripheral distribution, Qinghai and Yunnan populations should be assigned priority for conservation. Optimal harvesting strategies, domestication and tissue culture of L. rotata should be developed as soon as possible to guarantee its sustainable use.     

Plant diversity and ecology on the Qinghai–Tibet Plateau

The Qinghai–Tibet Plateau (QTP) comprises a platform (sometimes called the Qinghai–Tibet Plateau sensu stricto), the Himalayas, and Hengduan Mountains (Liang et al.,2018; Mao et al.,2021). The latter two parts and adjacent highlands are also called the Pan-Himalaya. Numerous plants are distributed there with many endemic species, probably because of the high diverse landscapes created by continuous geological and climatic activities (Favre et al.,2015; Mao et al.,2021). As the well known biodiversity hotspot of the alpine plants in the world (Sun et al.,2017), many studies have been conducted on evolutionary origin and ecological adaptation of those species occurring in the QTP (e.g., Wen et al.,2014, 2019; Zhang et al.,2019). In the present special issue, we collected 15 related papers on this topic. Among them, two are invited reviews. Mao et al. (2021) provide a comprehensive review of evolutionary origin of species diversity on the QTP. Especially, they outlined major disputes and likely causes in this research topic, including circumscribing and naming the QTP, the QTP uplifts, dating of molecular phylogenetic trees, non-causal correlations between QTP uplifts and species diversification and the unified ice sheet. The authors also summarized genomic advancements related to high-altitude adaptation of both plants and animals. Tong et al. (2021) reviewed the reproductive strategies of animal-pollinated alpine plants on the QTP, involving pollination system, pollen limitation, self-pollination, and sexual system. In this region, 95.4% of animal-pollinated plants are pollinated by insects (i.e., bees, moths, butterflies, and flies) with only 4% by vertebrates (i.e., bats and birds). Self-pollination through self-compatibility shift from outcrossing has become an effective reproductive strategy to overcome pollen limitation in alpine plants. The other 13 research papers aimed to address origin and adaptation of alpine flora involving three major lines of evidence: genomics, ecology, and paleobotany. We hope that the collection of these papers will increase our understanding of the origin, speciation, and adaptation of alpine species on the QTP.     

Origin of Ostryopsis intermedia (Betulaceae) in the southeast Qinghai–Tibet Plateau through hybrid speciation

It remains unknown how Ostryopsis intermedia B. Tian & J. Q. Liu (Betulaceae) originated in the SE Qinghai–Tibet Plateau (QTP). We examined sequence variations of two maternally inherited chloroplast (cp) DNA fragments and amplified fragment length polymorphisms (AFLP) of 32 populations for this species and two congeners, O. davidiana Decne. and O. nobilis Balf. f. W. W. Sm., distributed in northern China and the SE QTP. Each of the two distinct cpDNA haplotype groups was fixed by O. davidiana or O. intermedia, respectively, whereas both were found in O. nobilis. All analyses of AFLP suggested that O. intermedia is more closely related to O. davidiana than to O. nobilis and Bayesian clustering analysis (K = 2) further suggested that the genomic composition of O. intermedia showed a slightly mosaic origin from the other two species, although mostly from O. davidiana. Analyses of AFLP indicated the highest nuclear genetic diversity for O. davidiana and lowest forO. intermedia. The origin of O. intermedia through hybrid speciation due to southward migration of O. davidiana and the following hybridization with O. nobilis may provide the most reasonable explanation for the inconsistency between phylogenetic relationships of three species and degrees of genetic diversity from cpDNA sequences or AFLP datasets. Ostryopsis intermedia may represent one more diploid hybrid species found in the QTP, although further strong tests are needed based on additional data.     

Genomic insights into speciation history and local adaptation of an alpine aspen in the Qinghai–Tibet Plateau and adjacent highlands

Natural selection serves as an important agent to drive and maintain interspecific divergence. Populus rotundifolia Griff. is an alpine aspen species that mainly occurs in the Qinghai–Tibet Plateau (QTP) and adjacent highlands, whereas its sister species, P. davidiana Dode, is distributed across southwest and central to northeast China in much lower altitude regions. In this study, we collected genome resequencing data of 53 P. rotundifolia and 42 P. davidiana individuals across their natural distribution regions. Our population genomic data suggest that the two species are well delimitated in the allopatric regions, but with hybrid zones in their adjacent region in the eastern QTP. Coalescent simulations suggest that P. rotundifolia diverged from P. davidiana in the middle Pleistocene with following continuous gene flow since divergence. In addition, we found numerous highly diverged genes with outlier signatures that are likely associated with high-altitude adaptation of these alpine aspens. Our finding indicate that Quaternary climatic changes and natural selection have greatly contributed to the origin and distinction maintenance of P. rotundifolia in the QTP.     

Response of plant functional traits to grazing for three dominant species in alpine steppe habitat of the Qinghai–Tibet Plateau,China

Above-ground biomass (AGB) is an important indicator of grassland ecosystem performance. Easily measured plant functional traits (PFTs) may provide useful predictors of the response of plants to grazing. Understanding the response of PFTs to grazing and the relationship between PFTs and AGB is very important for effectively predicting the response of ecosystems to grazing and rangeland management. A grazing experiment was conducted in Gangcha County, Qinghai Province, in the northeastern part of the Qinghai–Tibet Plateau in 2012 and 2013. We investigated the response of PFTs in three dominant species (Elymus nutans, Kobresia humilis, and Stipa purpurea) to grazing, using six stocking rates. Plant height (PH), plant weight, leaf area, and leaf dry biomass of these three dominant species had significantly negative relationships with stocking rate. Leaf thickness (LT) of these three species usually showed a unimodal response to grazing. Specific leaf area generally showed a quadratic relationship with grazing intensity. No consistent effects of grazing were observed on nitrogen content per unit mass (N _mass) and nitrogen content per unit area (N _area). PH, leaf area, and leaf dry mass (LDM) were positively associated with AGB, but LT, N _mass, and N _area had no statistically significant association with AGB. We thus conclude that PH, leaf area, and LDM best predict the effects of grazing on AGB. Finally, 2.87 sheep/ha is recommended as the optimal stocking rate in this region to maintain the health of this grassland ecosystem and to allow for sustainable development.     

Diversity and distribution of alkaliphilic psychrotolerant bacteria in the Qinghai–Tibet Plateau permafrost region

The Qinghai–Tibet Plateau represents a unique permafrost environment, being a result of high elevation caused by land uplift. And the urgency was that plateau permafrost was degrading rapidly under the current predicted climatic warming scenarios. Hence, the permafrost there was sampled to recover alkaliphilic bacteria populations. The viable bacteria on modified PYGV agar were varied between 10² and 10⁵ CFU/g of dry soil. Forty-eight strains were gained from 18 samples. Through amplified ribosomal DNA restriction analysis (ARDRA) and phylogenetic analyses, these isolates fell into three categories: high G + C gram positive bacteria (82.3%), low G + C gram positive bacteria (7.2%), and gram negative α-proteobacteria (10.5%). The strains could grow at pH values ranging from 6.5 to 10.5 with optimum pH in the range of 9–9.5. Their growth temperatures were below 37°C and the optima ranging from 10 to 15°C. All strains grew well when NaCl concentration was below 15%. These results indicate that there are populations of nonhalophilic alkaliphilic psychrotolerant bacteria within the permafrost of the Qinhai-Tibet plateau. The abilities of many of the strains to produce extracellular protease, amylase and cellulase suggest that they might be of potential value for biotechnological exploitation.     

Nitrous oxide emissions from two alpine meadows in the Qinghai–Tibetan Plateau

Nitrous oxide (N₂O) emission was measured in a Kobresia humilis meadow and a Potentilla fruticosa meadow in the Qinghai–Tibet Plateau from June 2003 to July 2006. Five treatments were setup in the two alpine meadows. Two bare soil treatments were setup in the K. humilis meadow (BSK) and in the P. fruticosa meadow (BSP) by removing the above- and belowground plant biomass. Three plant community treatments were setup with one in the K. humilis meadow (herbaceous community in the K. humilis meadow-HCK) and two in the P. fruticosa meadow (herbaceous community in the P. fruticosa meadow-HCP, and shrub community in the P. fruticosa meadow-SCP). Nitrous oxide emission from BSP was estimated to be 38.1?±?3.6 μg m^?2 h^?1, significantly higher than from BSK (30.2?±?2.8 μg m^?2 h^?1) during the whole experiment period. Rates from the two herbaceous blocks (HCK and HCP) were close to 39.5 μg m^?2 h^?1 during the whole experimental period whereas shrub community (SCP) showed significant high emission rates of N₂O. Annual rate of N₂O emission was estimated to be 356.7?±?8.3 and 295.0?±?11.6 mg m^?2 year^?1 from the alpine P. fruticosa meadow and from the alpine K. humilis meadow, respectively. These results suggest that alpine meadows in the Qinghai–Tibetan Plateau are an important source of N₂O, contributing an average of 0.3 Tg N₂O year^?1. We concluded that N₂O emission will decrease, due to a predicted vegetation shift from shrubs to grasses imposed by overgrazing.     

Better than nothing: Evolution of autonomous selfing under strong inbreeding depression in an alpine annual from the Qinghai–Tibet Plateau

In plants with infrequent pollinator services, the benefits of reproductive assurance could be eroded by severe ovule discounting and inbreeding depression (ID). However, it remains unclear how selfing evolves under complete pollinator failure and strong ID. We examined the mating system and ID under netting and robbing conditions in Comastoma pulmonarium (Turcz.) Toyok. (Gentianaceae), an alpine annual experiencing a high ratio of nectar robbery on the Qinghai–Tibet Plateau.Comastoma pulmonarium produced seeds via selfing at the study site. No pollinator was observed and thus the nectar was consumed exclusively by robbers. Inbreeding depression occurred in the life stages of seed mass and germination, and the cumulative ID was much higher than 0.5 under netting and robbing conditions. Overall, in comparison with netting, the magnitude of ID under robbing conditions tended to decrease. Our results suggested that selfing could assure reproduction for plants under complete pollinator failure and strong ID, supporting the “better than nothing” role of selfing and providing one of the few cases of the evolution of selfing under strong ID.     

Phylogenomics and rapid diversification of the genus Eutrema on the Qinghai–Tibet Plateau and adjacent regions

Both geographic isolation and polyploidization are assumed to play an important role in driving species diversification. However, this is rarely illustrated through phylogenomic analyses. The genus Eutrema (Brassicaceae), which also includes the salt-resistant species, are distributed mainly in Asia with extensive species diversification in the Qinghai–Tibet Plateau (QTP) and adjacent regions. In this study, we revealed almost fully resolved backbone relationships of the genus with genome re-sequencing data for genomes of 168 individuals from 28 species. Phylogenetic analyses of both plastomes and single-copy nuclear genes from the whole genome recovered six well-supported clades with almost consistent relationships. The first two clades are mainly distributed in central China and central Asia, while the other four in the QTP and adjacent regions. All of them diversified within 12 million years. Within each clade, we recovered numerous conflicts in the interspecific relationships between nuclear and plastome phylogenies, likely suggesting hybridization and incomplete lineage sorting during species diversification. Our estimation of genome size and comparison of the number of the single-copy nuclear genes demonstrated frequent occurrences of polyploids in the genus. Except for an establishment of the backbone phylogeny, our phylogenomic analyses suggest that in addition to strong geographic isolation, polyploidization may have played an important role in species diversification of this genus.     

High uncertainties detected in the wetlands distribution of the Qinghai–Tibet Plateau based on multisource data

Landscape and Ecological Engineering - Twenty wetland-related data products (including remote sensing datasets, compilation datasets and model simulation datasets) were collected to evaluate the...     

Shrub and precipitation interactions shape functional diversity of nematode communities on the Qinghai–Tibet Plateau

Land use and climate change alter biodiversity patterns and ecosystem functioning worldwide. Land abandonment with consequent shrub encroachment and changes in precipitation gradients are known factors in global change. Yet, the consequences of interactions between these factors on the functional diversity of belowground communities remain insufficiently explored. Here, we investigated the dominant shrub effects on the functional diversity of soil nematode communities along a precipitation gradient on the Qinghai–Tibet Plateau. We collected three functional traits (life-history C–P value, body mass, and diet) and calculated the functional alpha and beta diversity of nematode communities using kernel density n-dimensional hypervolumes. We found that shrubs did not significantly alter the functional richness and dispersion, but significantly decreased the functional beta diversity of nematode communities in a pattern of functional homogenization. Shrubs benefited nematodes with longer life-history, larger body mass, and higher trophic levels. Moreover, the shrub effects on the functional diversity of nematodes depended strongly on precipitation. Increasing precipitation reversed the effects shrubs have on the functional richness and dispersion from negative to positive but amplified the negative effects shrubs have on functional beta diversity of nematodes. Benefactor shrubs had stronger effects on the functional alpha and beta diversity of nematodes than allelopathic shrubs along a precipitation gradient. A piecewise structural equation model showed that shrubs and its interactions with precipitation indirectly increased the functional richness and dispersion through plant biomass and soil total nitrogen, whereas it directly decreased the functional beta diversity. Our study reveals the expected changes in soil nematode functional diversity following shrub encroachment and precipitation, advancing our understanding of global climate change on nematode communities on the Qinghai–Tibet Plateau.     

Biomass partitioning pattern of Rheum tanguticum on the Qinghai–Tibet Plateau was affected by water-related factors

To identify the patterns of aboveground biomass (M_A) and belowground biomass (M_B) partitioning of a tall perennial herbal plant, Rheum tanguticum Maxim. ex Balf. (R. tanguticum), we determined M_A, M_B, total biomass (M_T), and below- to aboveground biomass ratio (M_B/M_A) through three consecutive sampling campaigns during 2016–2018 on the Qinghai-Tibetan plateau. We then documented M_A, M_B, M_T, and M_B/M_A, and their log–log relationship with environmental factors using data from 47 sites. M_B/M_A showed a significant negative relationship with mean annual precipitation (MAP) and altitude but a positive relationship with mean annual temperature (MAT), soil total nitrogen content (TN), and soil humidity (SH). While M_T increased with altitude and decreased with MAT, TN, and SH, but the relationship between MAP and M_T was not significant. Reduced major axis analysis revealed a slope of the log–log relationship between M_A and M_B to be 1.16, supporting an allometric partitioning pattern of R. tanguticum. Furthermore, the scaling exponent revealed different changes to different environmental factors. Specifically, scaling exponent was sensitive to the gradient of MAP and SH, but not to the gradient of altitude, MAT, or TN. This indicated that the scaling exponent was affected by water availability.

     

Patterns of grassland community composition and structure along an elevational gradient on the Qinghai–Tibet Plateau

青藏高原草地群落组成和结构的海拔梯度格局 青藏高原高寒草地是维持区域生态安全的天然屏障,也在一定程度上造就了该区域较高的生物多样性。然而,我们对青藏高原高寒草地植物群落组成和结构的海拔分布格局及其自身维持机制仍知之甚少。本研究在青藏高原东北部沿公路形成的海拔梯度设置了39个实验样地(海拔跨度为2800–5100m),每个样地设5个调查样方进行群落调查,包括物种组成、高度、盖度,评估青藏高原高寒草地植物群落的α和β多样性的海拔梯度格局及其影响因素。研究结果发现草地群落高度随着海拔的增加而显著降低,而群落盖度变化却不显著。随着海拔的增加,植物物种丰富度(α多样性)显著增加,而群落变异性(β多样性)显著降低。约束聚类分析表明,随海拔增加草地群落结构逐渐发生变化,基于此,在这种变化过程中,我们监测到3个渐变的海拔间断点,分别在海拔3640、4252和4333 m处。结构方程模型(SEM)表明,降水增加和温度降低对α多样性有显著的正向作用,但植物群落α多样性的变化显著改变群落变异性。以上结果表明,青藏高原的群落组成和结构沿海拔梯度发生了从量变到质变的过程。     

Range expansion,genetic differentiation,and phenotypic adaption of Hippophaë neurocarpa (Elaeagnaceae) on the Qinghai–Tibet Plateau

Plant species shift ranges in response to climate fluctuation over time. However, differentiation related to heterogeneity in space has been illustrated only rarely. Here we selected Hippophaë neurocarpa S. W. Liu & T. N. Ho (Elaeagnaceae), a shrub endemic to the Qinghai–Tibet Plateau, to exemplify this process. We characterized the genotypic, phenotypic, and climatic variations among 27 populations of this species sampled across its entire distribution. Genotype analyses revealed six highly differentiated groups that may have resulted from expansions in spatial range. Despite recent fluctuation in size, it is likely that most groups survived the last glacial maximum in situ. Instead of two previously described subspecies, we identified four morphotypes (stellate, peltate, and two additional types) that can be characterized based on leaf trichomes. This phenotypic subdivision is consistent with a climate gradient defined by temperature and precipitation, although similar phenotypes may derive from distinct genotypes. Collectively, we propose that the demographic history of H. neurocarpa is characteristic of an early spatial range expansion combined with a recent bottleneck, subsequently subdivided into multiple morphotypes following local adaption to heterogeneous climates.