A review of plant diversity surveys and monographic studies of the Pan‐Himalaya region

Small mammals as a bioindicator of mercury in a biodiversity hotspot – The Hengduan Mountains, China

A major question in evolution and ecology is why biodiversity is so unevenly distributed across the planet. The most obvious and salient diversity pattern is the order-of-magnitude greater species richness in the tropics compared with the temperate zones. Superimposed on this latitudinal diversity gradient is a much more complex and intricate pattern of regional and more local biodiversity hotspots (1, 2). These are places with unusually high concentrations of species and especially endemic species. Documenting these patterns is of great significance, most obviously for conservation, with ever more sophisticated and higher-resolution biodiversity hotspot maps becoming available as the world’s biota are mapped in greater detail (3). More fundamentally, we need to document hotspots to understand the underlying macroevolutionary and ecological processes shaping the distribution of diversity across the Earth. Most globally significant biodiversity hotspots lie firmly within the tropics or in the Mediterranean climate zones of the world, such as California and the Cape Floristic Region of South Africa. Several coincide with major mountain ranges, including most notably the tropical Andes and the temperate Hengduan Mountains, two of the world’s hottest hotspots (2, 4). It is this Hengduan Mountain hotspot that is the focus of Xing and Ree’s study in PNAS (5), which provides the first integrated analysis of the evolutionary origins and biotic assembly of Hengduan plant diversity. Although formal quantification of biodiversity hotspots really only started 30 y ago (1), the outstanding plant species richness of the Hengduan Mountains has been known for more than 150 y, having been first revealed by the intrepid 19th century plant collectors—Joseph Hooker, Ernest Wilson, George Forrest, Frank Kingdon-Ward, and others. These explorers penetrated into the deeply dissected and remote mountains and river gorges of Yunnan, Sichuan, Sikkim, and eastern Tibet in search of botanical novelties and especially garden plants, … [↵][1]1Email: colin.hughes{at}systbot.uzh.ch. [1]: #xref-corresp-1-1

The geographic distribution of plant diversity matches the gradient of habitat heterogeneity from lowlands to mountain regions. However, little is known about how much this relationship is conserved across scales. Using the World Checklist of Vascular Plants and high-resolution biodiversity maps developed by species distribution models, we investigated the associations between species richness and habitat heterogeneity at the scales of Eurasia and the Hengduan Mountains (HDM) in China. Habitat heterogeneity explains seed plant species richness across Eurasia, but the plant species richness of 41/97 HDM families is even higher than expected from fitted statistical relationships. A habitat heterogeneity index combining growing degree days, site water balance, and bedrock type performs better than heterogeneity based on single variables in explaining species richness. In the HDM, the association between heterogeneity and species richness is stronger at larger scales. Our findings suggest that high environmental heterogeneity provides suitable conditions for the diversification of lineages in the HDM. Nevertheless, habitat heterogeneity alone cannot fully explain the distribution of species richness in the HDM, especially in the western HDM, and complementary mechanisms, such as the complex geological history of the region, may have contributed to shaping this exceptional biodiversity hotspot.

AimsIt has been assumed that montane species will undergo upslope shifts in response to climate warming and their range sizes are therefore predicted to decrease. However, this view has been challenged because a recent study (Elsen & Tingley, ) indicated that land surface area increases with increasing altitude in some mountains. To test this prediction, we used one of the world's biodiversity hotspots as a study system to examine overall patterns of plant distribution shift in response to climate warming.LocationThe Hengduan Mountains and adjacent regions.MethodsBased on distribution data for 151 species at a resolution of 2.5 arc minutes, we employed ecological niche modelling to model their distributions under the climatic conditions of the Last Glacial Maximum, Current (2017), and 2050 separately. We examined the distributional shifts of these species, especially with respect to altitude and range size, in response to two periods of stepwise climate warming.ResultsAll the montane plants sampled shifted upward during the two warming stages, but not only northward, some shifted westward or in other directions. In contrast with the expected consistent loss of range when shifting upward, 63.6% of the plants expanded their range size continuously since the LGM. Only 11.9% of the plants contracted their range size continuously from the LGM to 2050. Estimates of species richness in the regions studied changed greatly, but in an unbalanced manner, from the LGM to the Current and from the Current to 2050.Main conclusionsNumerous montane plants in the Hengduan Mountains are predicted to expand their range sizes as they shift upslope in response to climate warming. Our results highlight the possibility that more available land surface area due to the heterogeneous topography along altitudinal gradients and the adjacent large Qinghai‐Tibet Plateau sensu stricto can mediate the range loss of the montane plants under climate warming. These findings are crucial for estimating the future range sizes of plants and planning biodiversity protection for mountain ecosystems under the anticipated warming of the world's climate.

The Hengduan Mountains (HDM) of China are a biodiversity hotspot whose temperate flora and fauna are among the world’s richest. However, the origin and evolution of biodiversity in the HDM remain poorly understood, especially in mammals. Given that the HDM shows the highest richness of vole species in the world, we used whole-exome capture sequencing data from the currently most comprehensive sampling of HDM voles to investigate their evolutionary history and diversification patterns. We reconstructed a robust phylogeny and re-estimated divergence times of the HDM voles. We found that all HDM voles could be divided into a western lineage (Volemys, Proedromys, and Neodon) and an eastern lineage (Caryomys and Eothenomys), and the two lineages originated from two migration events from North Eurasia to the HDM approximately 9 Mya. Both vole lineages underwent a significant acceleration of net diversification from 8–5 Mya, which was temporally congruent with the orogeny of the HDM region. We also identified strong intertribal gene flow among the HDM voles and hypothesized that frequent gene flow might have facilitated the speciation burst of the HDM voles. Our study highlights the importance of both environmental and biotic factors in shaping the biodiversity of mammals in mountain ecosystems.

Mountainous regions contain some of the most dynamic landscapes in the world and host a majority of the world's biodiversity hotspots. The Hengduan Mountains, located at the syntax of Eurasia and India, have a complex geological history associated with a hyperdiverse assemblage of species. Tectonic, geomorphic and climatic processes in the Hengduan Mountains may have jointly caused the emergence of high biodiversity. We propose three mechanisms that may promote high-level montane biodiversity: habitat creation, habitat disruption and habitat oscillations. We mapped species richness patterns for seed plants and computed phylogenetic endemism across the entire Hengduan region. We developed a set of geomorphic metrics regarding these mechanisms to link plant richness and phylogenetic endemism patterns. By estimating the component of species richness that is attributable to habitat heterogeneity, we isolate the component that is the result of tectono-geomorphic processes. We found that regions of exceptionally high species richness are explained by a combination of habitat heterogeneity and tectono–geomorphic processes, whereas glaciation and high climate change velocity are associated with low richness, thus contributing to extinctions. We interpret the richness of biodiversity hotspots in the Hengduan Mountains in terms of climatic and tectonic processes.

AimAdaptation, migration and extinction of species is closely associated with climate changes and shape the distribution of biodiversity. The adaptive responses of species in the biodiversity hotspot, the Hengduan Mountains, to climate change remain poorly understood.LocationThe Hengduan Mountains, southeast of the Qinghai‐Tibet Plateau.MethodsThe principles of phylogeography and landscape genomics were applied to the endemic speciesRoscoea humeanain the Hengduan Mountains by genotyping by sequencing data. A total of 5,902 single‐nucleotide polymorphisms were used to analyse the genetic structure/diversity and changes in effective population size over time. Species distribution modelling, principal component analysis and gradient forest analysis were used to explore associations between climate change and genetic variation.ResultsThe population size ofR. humeanarapidly increased after the Last Glacial Maximum. The contribution of climate to genetic variation was greater than that of geography. Precipitation during the warmest season played a pivotal role in the adaptation to climate changes. Loci associated with drought tolerance and anti‐ultraviolet radiation were identified, suggesting local adaptation ofR. humeanato alpine environments.Main conclusionsThese results suggest that the current genetic structure and diversity ofR. humeanawere shaped by Quaternary climate fluctuations and persistence of microrefugia in the Hengduan Mountains. The results further suggest thatR. humeanacan survive in future refugia within the area where the warmest quarter precipitation is higher than 560 mm, and local adaptation to drought tolerance may be beneficial for its acclimation to future climate changes.

The dispersal and diversification of earthworms (Annelida: Oligochaeta) related to paleogeographical events in the Hengduan Mountains

Glaciation-based isolation contributed to speciation in a Palearctic alpine biodiversity hotspot: Evidence from endemic species

Cryptic speciation in the Chinese white pine (Pinus armandii): Implications for the high species diversity of conifers in the Hengduan Mountains, a global biodiversity hotspot

<p>The eastern Tibetan Plateau, particularly the Hengduan Mountains and Sanjiangyuan region, is a biodiversity hotspot also known for its sensitivity to climate change. How these vegetation communities assembled since the Last Glacial Maximum is still unclear. Here we present new results from plant metabarcoding of sediments from Lake Ximencuo (Nianbaoyeze Mountains, Sanjiangyuan region) covering the last 18 ka and compare them with records from Lake Naleng (Hengduan Mountains).  We investigate temporal changes of within-site and between-site alpha and beta diversities. Both sites show the highest richness between 14 and 10 ka when alpine meadows covered the areas while richness was rather low in forested periods during the early Holocene. Ordination results support that the vegetation composition was relatively low between-site beta-diversities indicate that the vegetation composition was relatively similar in the two study areas before the Holocene, particularly during the early late-glacial when alpine steppes dominated. The maximal between-site beta-diversity occurred between 10 and 8 ka when environmental filtering was most relevant, as suggested by the dominating turnover component. The nestedness component of beta-diversity reached a maximum during the middle Holocene indicating that between-site differences during this period possibly originated from e.g., dispersal limitation.</p>

The Qinghai-Tibet Plateau sensu lato (QTP s.l.) harbors an exceptionally high biodiversity, especially at its southeastern margin: this area encompasses the Hengduan Mountains and the eastern Himalayas, which have been listed as biodiversity hotspots. To the contrary, the plateau interior (namely the Qinghai-Tibet Plateau sensu stricto, QTP s.s.) is relatively species-poor because of its particularly harsh climate. With contrasting geological histories and environmental conditions of the Hengduan Mountains, the Himalayas and the QTP s.s., it would be expected that floristic compositions and diversity patterns of these three regions would differ between each other. To compare the floristic diversity of these three regions, we assembled data on seed plant’s distribution in the three regions based on county-level mapping from published monographs and online databases, and we then analyzed their floristic features and species diversity patterns (horizontal and elevational). We found that the Hengduan Mountains hosted the most seed plant species (8,439), as expected. The highest percentage of shrub (22.88%) and tree species (9.80%) were in the Himalayas, whereas herbaceous species (81.50%) were relatively more prominent in the QTP s.s.. The Hengduan Mountains also had the most species-rich genera (10) with more than 50% of their total species diversity in China. Also, temperate genera dominated across these three regions, with a highest percentage (77.61%) within the QTP s.s.. Across the QTP s.l., species diversity gradually decreased from the southeastern part to the northwest, and most of seed plants were distributed in the southern and eastern margin of the Hengduan Mountains and East Himalayas. Along elevational gradients, species richness all demonstrated a hump-shape curve, but the most species-rich elevation zone differed for each type of life-form across the three regions. Our study sets a base for exploring the origin and evolution of mountain taxa, as well as provides a snapshot of the current plant distribution, which will certainly be modified by climate change.

The Hengduan Mountains (HDM), a biodiversity hotspot in Southwest China, harbour numerous endemic taxa whose diversification has been driven by the complex geological history of this region. This study investigates the diversification of the katydid genus Sichuana, endemic to the HDM, integrating mitochondrial genomics, morphology and biogeography. We sequenced complete mitochondrial genomes of all known geographic populations of Sichuana, revealing four major clades diverging during the Late Miocene (5.06-8.37 Ma). Orogeny-driven vicariance fragmented ancestral populations, while unstable barriers may facilitate mitochondrial introgression, as evidenced by paraphyletic lineages contrasting with distinct morphologies. We describe five new species (S. brevicerca sp. nov., S. fortidens sp. nov., S. qiuzhi sp. nov., S. luqiaoensis sp. nov. and S. pseudomagna sp. nov.) and two subspecies (S. pseudomagna pseudomagna subsp. nov. and S. pseudomagna borealis subsp. nov.), highlighting the taxonomic challenges posed by discordance between mitochondrial DNA-based phylogenies and morphological classifications. This study demonstrates that orogeny-driven vicariance dominates speciation in low-dispersal insects, but transient gene flow complicates species delimitation. Our findings advocate for integrative taxonomy in biodiversity hotspots and highlight the HDM as a model system for studying biogeographic complexity.

Origins and evolution of plant diversity in the Hengduan Mountains, China

Understanding the evolutionary and ecological processes driving population differentiation and speciation can provide critical insights into the formation of biodiversity. Here, we examine the link between population genetic processes and biogeographic history underlying the generation of diversity in the Hengduan Mountains (HM), a region harboring a rich and dynamic flora. We used restriction site-associated DNA sequencing to generate 1,907 single-nucleotide polymorphisms (SNPs) and four-kb of plastid sequence in species of the Gentiana hexaphylla complex (Gentianaceae). We performed genetic clustering with spatial and non-spatial models, phylogenetic reconstructions, and ancestral range estimation, with the aim of addressing the processes influencing diversification of G. hexaphylla in the HM. We find the G. hexaphylla complex is characterized by geographic genetic structure with clusters corresponding to the South, North and the central HM. Phylogenetic reconstruction and pairwise FST analyses showed deep differentiation between Southern and Northern populations in the HM. The population in Mount Taibai exhibited the highest genetic similarity to the North HM. Ancestral range estimation indicated that the G. hexaphylla complex originated in the central HM and then diverged in the Pliocene and the Early Pleistocene, before dispersing widely, resulting in the current distinct lineages. Overall, we found deep genomic differentiation in the G. hexaphylla complex corresponds to geographic barriers to dispersal in the HM and highlights a critical role of the uplift of the Daxue Mountains and subsequent climatic fluctuations underlying diversification. The colonization of G. hexaphylla in the Mount Taibai region suggests directional dispersal between the alpine flora of the Qinling Mountains and the HM.