The Tanaka Line shaped the phylogeographic pattern of the cotton tree (Bombax ceiba) in southwest China

Climate change impacts the distribution of Quercus section Cyclobalanopsis (Fagaceae), a keystone lineage in East Asian evergreen broadleaved forests

East Asian evergreen broadleaved forests (EBFLs) harbor high species richness, but these ecosystems are severely impacted by global climate change and deforestation. Conserving and managing EBLFs requires understanding dominant tree distribution dynamics. In this study, we used 29 species in Quercus section Cyclobalanopsis-a keystone lineage in East Asian EBLFs-as proxies to predict EBLF distribution dynamics using species distribution models (SDMs). We examined climatic niche overlap, similarity, and equivalency among seven biogeographical regions' species using 'ecospat'. We also estimated the effectiveness of protected areas in the predicted range to elucidate priority conservation regions. Our results showed that the climatic niches of most geographical groups differ. The western species under the Indian summer monsoon regime were mainly impacted by temperature factors, whereas precipitation impacted the eastern species under the East Asian summer monsoon regime. Our simulation predicted a northward range expansion of section Cyclobalanopsis between 2081 and 2100, except for the ranges of the three Himalayan species analyzed, which might shrink significantly. The greatest shift of highly suitable areas was predicted for the species in the South Pacific, with a centroid shift of over 300 km. Remarkably, only 7.56% of suitable habitat is currently inside protected areas, and the percentage is predicted to continue declining in the future. To better conserve Asian EBLFs, establishing nature reserves in their northern distribution ranges, and transplanting the populations with predicted decreasing numbers and degraded habitats to their future highly suitable areas, should be high-priority objectives.

Wind-dispersed seeds blur phylogeographic breaks: The complex evolutionary history of Populus lasiocarpa around the Sichuan Basin

The strength of phylogeographic breaks can vary among species in the same area despite being subject to the same geological and climate history due to differences in biological traits. Several important phylogeographic breaks exist around the Sichuan Basin in Southwest China but few studies have focused on wind-dispersed plants. Here, we investigated the phylogeographic patterns and the evolutionary history of Populus lasiocarpa, a wind-pollinated and wind-dispersed tree species with a circum-Sichuan Basin distribution in southwest China. We sequenced and analyzed three plastid DNA fragments (ptDNA) and eight nuclear microsatellites (nSSRs) of 265 individuals of P. lasiocarpa from 21 populations spanning the entire distribution range. Distribution patterns based on nSSR data revealed that there are three genetic groups in P. lasiocarpa. This is consistent with the three phylogeographic breaks (Sichuan Basin, the Kaiyong Line and the 105°E line), where the Sichuan basin acts as the main barrier to gene flow between western and eastern groups. However, the distribution pattern based on ptDNA haplotypes poorly matched the phylogeographic breaks, and wind-dispersed seeds may be one of the main contributing factors. Species distribution modelling suggested a larger potential distribution in the last glacial maximum with a severe bottleneck during the last interglacial. A DIYABC model also suggested a population contraction and expansion for both western and eastern lineages. These results indicate that biological traits are likely to affect the evolutionary history of plants, and that nuclear molecular markers, which experience higher levels of gene flow, might be better indicators of phylogeographic breaks.

Seed germination schedule and environmental context shaped the population genetic structure of subtropical evergreen oaks on the Yun-Gui Plateau, Southwest China

The evergreen broadleaved forests (EBLFs) of Southwest China have a long-term stable environment and support a diverse flora, thus forming a global biodiversity hotspot. However, the key drivers that have shaped the genetic diversity patterns of species in these EBLFs are not well understood. Quercus delavayi, Q. schottkyana, and Q. kerrii are sympatric oak species with different seed biological traits that are typical for these EBLFs. This study combined multilocus phylogeography and ecological niche modeling to screen 33 Q. delavayi populations. Their population genetic structure was inferred in comparison with previous studies on Q. schottkyana and Q. kerrii. The seed germination traits of all three species were also investigated. cpDNAs showed a significant phylogeographic structure in Q. delavayi, which was not detected in Q. schottkyana or Q. kerrii. Quercus delavayi, Q. kerrii, and Q. schottkyana exhibited different pollen-to-seed migration ratios (r = 219, 117, and 22, respectively), which are linked to the germination schedules of acorns. The distributions of Q. delavayi and Q. schottkyana remained long-term stable since the last glacial maximum (LGM) with a similar nSSR genetic gradient change along latitude. Instead, Q. kerrii experienced a prominent range expansion since the LGM with genetic diversification between the East and the West of the Tanaka line due to environmental heterogeneity. These results identify seed traits and environmental heterogeneity as two key drivers that shape the population genetic structure of EBLF trees in Southwest China. These should be considered in regional forestry conservation and management.

Adjusting cotton planting density under the climatic conditions of Henan Province, China

The growth and development of cotton are closely related to climatic variables such as temperature and solar radiation. Adjusting planting density is one of the most effective measures for maximizing cotton yield under certain climatic conditions. The objectives of this study were (1) to determine the optimum planting density and the corresponding leaf area index (LAI) and yield under the climatic conditions of Henan Province, China, and (2) to learn how climatic conditions influence cotton growth, yield, and yield components. A three-year (2013-2015) field experiment was conducted in Anyang, Henan Province, using cultivar SCRC28 across six planting density treatments: 15,000, 33,000, 51,000, 69,000, 87,000, and 105,000 plants ha-1. The data showed that the yield attributes, including seed cotton yield, lint yield, dry matter accumulation, and the LAI, increased as planting density increased. Consequently, the treatment of the maximum density with 105,000 plants ha-1 was the highest-yielding over three years, with the LAIs averaged across the three years being 0.37 at the bud stage, 2.36 at the flower and boll-forming stage, and 1.37 at the boll-opening stage. Furthermore, the correlation between the cotton yield attributes and meteorological conditions indicated that light interception (LI) and the diurnal temperature range were the climatic factors that most strongly influenced cotton seed yield. Moreover, the influence of the number of growing degree days (GDD) on cotton was different at different growth stages. These observations will be useful for determining best management practices for cotton production under the climatic conditions of Henan Province, China.

Effects of the Tanaka Line on the genetic structure of Bombax ceiba (Malvaceae) in dry-hot valley areas of southwest China

Southwest China is an important biodiversity hotspot. The interactions among the complex topography, climate change, and ecological factors in the dry‐hot valley areas in southwest China may have profoundly affected the genetic structure of plant species in this region. In this study, we determined the effects of the Tanaka Line on genetic variation in the wild Bombax ceiba tree in southwest China. We sampled 224 individuals from 17 populations throughout the dry‐hot valley regions. Six polymorphic expressed sequence tag–simple sequence repeat primers were employed to sequence the PCR products using the first‐generation Sanger technique. The analysis based on population genetics suggested that B. ceiba exhibited a high level of gene diversity (H_E: 0.2377–0.4775; I: 0.3997–0.7848). The 17 populations were divided into two groups by cluster analysis, which corresponded to geographic characters on each side of the Tanaka Line. In addition, a Mantel test indicated that the phylogeographic structure among the populations could be fitted to the isolation‐by‐distance model (r² = .2553, p < .001). A barrier test indicated that there were obstacles among populations and between the two groups due to complex terrain isolation and geographic heterogeneity. We inferred that the Tanaka Line might have promoted the intraspecific phylogeographic subdivision and divergence of B. ceiba. These results provide new insights into the effects of the Tanaka Line on genetic isolation and population differentiation of plant species in southwest China.

Ecological and Genetic Divergences with Gene Flow of Two Sister Species (Leucomeris decora and Nouelia insignis) Driving by Climatic Transition in Southwest China

Understanding of the processes of divergence and speciation is a major task for biodiversity researches and may offer clearer insight into mechanisms generating biological diversity. Here, we employ an integrative approach to explore genetic and ecological differentiation of Leucomeris decora and Nouelia insignis distributed allopatrically along the two sides of the biogeographic boundary 'Tanaka Line' in Southwest China. We addressed these questions using ten low-copy nuclear genes and nine plastid DNA regions sequenced among individuals sampled from 28 populations across their geographic ranges in China. Phylogenetic, coalescent-based population genetic analyses, approximate Bayesian computation (ABC) framework and ecological niche models (ENMs) were conducted. We identified a closer phylogenetic relationship in maternal lineage of L. decora with N. insignis than that between L. decora and congeneric Leucomeris spectabilis. A deep divergence between the two species was observed and occurred at the boundary between later Pliocene and early Pleistocene. However, the evidence of significant chloroplast DNA gene flow was also detected between the marginal populations of L. decora and N. insignis. Niche models and statistical analyses showed significant ecological differentiation, and two nuclear loci among the ten nuclear genes may be under divergent selection. These integrative results imply that the role of climatic shift from Pliocene to Pleistocene may be the prominent factor for the divergence of L. decora and N. insignis, and population expansion after divergence may have given rise to chloroplast DNA introgression. The divergence was maintained by differential selection despite in the face of gene flow.

Geographical isolation and environmental heterogeneity contribute to the spatial genetic patterns of Quercus kerrii (Fagaceae)

Southwest China is one of the major global biodiversity hotspots. The Tanaka line, extending within southwestern China from its northwest to its southeast, is an important biogeographical boundary between the Sino-Japanese and Sino-Himalayan floristic regions. Understanding the evolutionary history of the regional keystone species would assist with both reconstructing historical vegetation dynamics and ongoing biodiversity management. In this research, we combined phylogeographic methodologies and species distribution models (SDMs) to investigate the spatial genetic patterns and distribution dynamics of Quercus kerrii, a dominant evergreen oak inhabiting southwest China lowland evergreen-broadleaved forests (EBLFs). A total of 403 individuals were sampled from 44 populations throughout southwest China. SDMs and mismatch distribution analysis indicated that Q. kerrii has undergone northward expansion since the Last Glacial Maximum (LGM). Quantitative analysis revealed that the range expansion of Q. kerrii since the LGM exceeded that of the sympatric mid-elevation species Quercus schottkyana, likely owing to their contrasting distribution elevations and habitat availabilities. The historical climate change since the LGM and the latitude gradient of the region played an important role in shaping the genetic diversity of Q. kerrii. The genetic differentiation index and genetic distance surface of Q. kerrii populations east of the Tanaka line exceeded those to its west. The long-term geographic isolation and environmental heterogeneity between the two sides of the Tanaka line might increase species divergence patterns and local adaptation. This study provides new insights into the historical dynamics of subtropical EBLFs and the changing biota of southwest China.