The dry valleys of Southwest China specifically refer to a type of azonal landscape distributed in the large river basins of the Hengduan Mountain ranges, and are the outcome of the effects of the monsoon climate, ridge-valley geomorphology, and long-term human perturbations. The dry valley landscape is characterized by prominent seasonal aridity, a vertical climate gradient, rich flora of high endemism, and unique vegetation types well adapted to the distinct environment. The environmental change in the last decades is characterized by consistent climate warming and widespread anthropogenic disturbances, especially land use/land cover changes mainly driven by road building, hydro-power construction, mining, and urbanization. These human disturbances intensified vegetation fragmentation, soil erosion, habitat degradation and biological invasion. Environmental degradation has threatened biodiversity resources and ecosystem security while the conservation measures for this unique ecosystem are lacking. We appeal for attention and timely action of biodiversity conservation in the dry valleys of Southwest China by means of establishing natural reserves. Meanwhile, contingent planning for vertical agriculture, degraded vegetation restoration, utilization program for the unique species resource and ecotourism are critical maintaining the sustainability of the dry valley ecosystems. Moreover, the policy and strategy under the goal of ecological civilization construction are also important tools to be applied in promoting ecosystem services and ecological security in the dry valleys of Southwest China.

Based on plant community data collected from dry valleys of nine major rivers in Gansu, Sichuan and Yunnan provinces of Southwest China, we divided the families and genera of the vascular plants into geographic elements of Chinese flora, and analyzed the spatial patterns. We found the flora of dry valleys of Southwest China revealed a complex composition of geographic elements with wide floristic connections, including 11 family area-types and 15 genera area-types. The ratio of tropical versus temperate floristic elements was 3.06 at the family level, and 1.77 at the genus level, showing a distinctive affinity for tropical floras, and later imprints of temperate floras. A clear floristic linkage existed between the dry valley floras and the Mediterranean, Western- and Central-Asian floras. The percentages of East Asian elements and endemic Chinese elements were below the average value in the flora found for the subtropical region in China. The Sino-Himalayan element had a value higher than that for the Sino-Japan element in the dry valley floras. The floristic affinity to the tropics weakened while affinity to temperate flora intensified moving in the region from the southwest to northeast. Linkages with the Mediterranean-Central Asian floras, and also the percentage of East Asian elements and endemic Chinese elements increased in the same direction. A boundary between the Sino-Japan type and Sino-Himalayan type was detected between the dry valley floras of Nanpanjiang River and Yuanjiang River. With respect to the genus level floristic composition of geographic elements, Nujiang River, Lancangjiang River and Yuanjiang River were separated from the other rivers, which are all upper branches of the Yangtze River. This departure indicated the impacts of the evolution of the Yangtze River, which were characterized by headward erosion, river confluences, on the characteristics of modern flora in dry valleys.

Biological invasion has attracted widespread attention because invasive species threaten native biodiversity and weaken ecosystem services. Based on field investigation of vegetation in Nujiang River valley, Northwest Yunnan, we analyzed the spatial patterns of native and invasive species richness, and the effects of topography, climate, and roadside habitat disturbance on the invasive versus native plant species richness. We recorded 26 exotic invasive plant species that belong to 13 families and 21 genera, and 1,145 native plant species, belonging to 158 families and 628 genera. Along the Nujiang River valley, species richness of invasive plants decreased with increasing latitude and altitude, while species richness of native plants increased with increasing latitude, and showed a hump-shaped pattern with elevation. A generalized linear model was used to estimate the roles of roadside disturbance, climate, topography and soil nutrients on the distribution of both native and invasive species richness. Results of hierarchical variation partitioning revealed that roadside habitat disturbance had primary impact on the distribution of two groups of species. Precipitation was the climatic determinant of invasive species diversity, and small-scale topographic factors, especially aspect, mainly affected native species diversity. It is likely that native species became drought-resistant in the evolutionary process while invasive species failed to adapt themselves to the local arid environments due to the short colonization time. This research supports the hypothesis that resource availability is the main factor limiting plant invasion, and highlights the negative effects of human activity on biodiversity. In addition, results of structural equation modelling revealed that native communities aren’t resistant to plant invasion. The negative relationship between invasive and native species richness reflects the different responses of the two group species to environmental factors.

The vegetation in dry valleys of Southwest China is characterized by unique vegetation type. Based on plant community sample data (1,339 plots) collected for 9 valleys in Gansu, Sichuan and Yunnan provinces, we used an adaptive affinity propagation method for quantitative classification, and performed an ordination using canonical correspondence analysis. Results showed that the plant communities can be divided into 7 vegetation types (savanna, succulent thicket, evergreen broadleaved thicket, warm deciduous broadleaved thicket, evergreen sclerophyllous forest, deciduous broadleaved forest, warm needle-leaved forest with 24 formations and 31 associations. The most representative vegetation type was the warm deciduous broadleaved thicket, while the most common formations were Form. Bauhinia brachycarpa (50.9%), Form. Heteropogon contortus (11.9%), Form. Bothriochloa pertusa (5.6%), Form. Vitex negundo (4.2%), Form. Eragrostis ferruginea (3.8%), Form. Dodonaea viscosa (3.4%) and Form. Pinus yunnanensis (3.3%). Mean temperature of the coldest month and precipitation seasonality are two dominant climatic factors, which limit the distribution of plant community types in the dry valleys. Savanna and succulent thicket are the dominant vegetation types in typical dry-hot valleys. Dry-warm valley are dominated by warm deciduous broadleaved thicket, evergreen sclerophyllous forest and evergreen broadleaved thicket. Warm needle-leaved forest and deciduous broadleaved forest are more adaptive to lower temperatures.

Beta diversity is the difference in species composition between locations. Recently, much of the published biodiversity research has focused on the assessment of beta diversity because it can be used to investigate community assembly and maintenance of species diversity. In this study, we investigated the impacts of dispersal limitation and habitat on the beta diversity of communities by sampling 41 plots throughout the dry-hot valley of Yuanjiang River. Taxonomic beta diversity and phylogenetic beta diversity were quantified using Jaccard dissimilarity, mean nearest taxon distance (MNTD), and mean pairwise distance (MPD). These indices were then related to the differences in environmental characteristics between sample plots and the geographical distances separating sample plots using multiple regression on distance matrices (MRM) and variance partitioning. The results showed that taxonomic and phylogenetic beta diversity indices significantly increased with the spatial distance and difference in mean annual temperature between communities; that taxonomic beta diversity and MNTD were most strongly influenced by spatial distance, whereas MPD was equally influenced by spatial distance and mean annual temperature difference between communities; and that only 11-13% of the variance in beta diversity was explained by the spatial distance and mean annual temperature difference between communities. These results highlight the effects of niche and dispersal limitations on beta diversity, and suggest that dispersal limitation may play a greater role than habitat differences between plant community locations. Additionally, human disturbances may greatly impact species composition along the dry-hot valley of the Yuanjiang River.

In order to understand the vegetation characteristics of dry valleys in the upstream of Jinsha River, a field survey was conducted by using transect and sampling surveys with 93 plots from September to October of 2011. Results showed that: (1) There were 111 vascular plant species belonging to 95 genera in 51 families, including 6 ferns belonging to 4 genera in 4 families, and 84 species of dicotyledon belonging to 71 genera in 40 families, 21 species of monocotyledon belonging to 20 genera in 7 families. (2) Five main life forms: Nanophanerophyte, Hemicryptophyte, Chamaephyte, Therophyte and Geophyte, can be recognized, of which Hemicryptophyte was the most dominant, occupying 32.43% and Nanophanerophyte accounted for 27.93% of the total. The dominant leaf size belonged to nanophyll and microphyll, accounted for 60% and 22%, respectively. (3) 93 sampling plots can be classified into 5 community types using a two-way Indicators Species Analysis. The species richness of herbs were higher than shrubs, and the richness of the Formation Vitex microphylla+Leptodermis pilosa var. microphylla was the greatest and also distributed most widely. (4) The vegetation in the dry valleys of the upstream of Jinsha River can be regarded as subtropical desert.

The vegetation of the dry-hot valleys in southwestern China, especially in Yunnan, is unique as the valley bottom in a subtropical plateau and mountainous region, which is characterized by its geographic location, climate, and geomorphology. A long history of biological adaptation to the specific environmental contributes to the rarity and non-zonality rarity rarity of vegetation in these dry-hot valleys. Disturbances including land use through human activities have endangered some vegetation types in the dry-hot valleys. This paper examines the history of vegetation studies in the dry-hot valleys, and summarizes studies on flora, vegetation and land use and cover change, as well as vegetation restoration. Research of plant communities and flora have mainly focused on the classification of the community and vegetation types, the characteristics of the community, the influence of human disturbance, and the characteristics, nature and origin of the flora. The relationship between vegetation and land has mainly focused on the characteristics of soil, land use/cover change, land degradation and soil erosion. The hot spots of vegetation protection and restoration include the study of vegetation restoration objectives, functional zoning of vegetation restoration, the introduction and selection of vegetation restoration research, and evaluation of the effectiveness of vegetation restoration. We propose that more attention to be paid to the study of the prevention of natural disasters, the impacts of hydropower construction on vegetation, and in-depth vegetation monitoring and analysis of the spatiotemporal dynamics at multiple scales.

Pistacia weinmannifolia is a characteristic species of dry valleys in Southwest China. In this study, 165 presence points of P. weinmannifolia were identified through field surveys, along with point data of 22 environmental factors. The suitable habitat model was formulated using the maximum-entropy (Maxent) algorithm and applied to simulate the potential range of the species in Southwest China, and to project the change of species range in past and future climate scenarios. The results indicate that the Maxent model gave a high accuracy in habitat predictions for P. weinmannifolia. Temperature seasonality, minimum temperature and precipitation were the major constraining climatic factors. Contemporarily, the environment suitable for P. weinmannifolia was located in the dry valleys of major rivers in Southwest China, and the regions was characterized by decreased temperature variability, no temperatures below 0°C, and low precipitation. Simulations using climate scenarios of the Last Inter-Glacial (LIG) and Last Glacial Maximum (LGM) periods indicated that the distribution of P. weinmannifolia was centered around the valleys of major rivers in Southwest China, substantially expanded eastward first, and retreated westward following climate change during glacial and inter-glacial periods, supporting the hypothesis of “glacial out-of-Hengduan Mts.”. Under the future climate scenario (2061-2080) with three representative concentration pathways (RCPs), the potential distribution of P. weinmannifolia was projected to migrate eastward to the valleys in the adjacent region of the Yunnan-Guizhou Plateau and Sichuan Basin, and the adjacent region of the Plateau and western Guangxi, reflecting a high possibility of increasing dryness in the river valleys in the future, while its current distribution might disappear. The potential distribution of P. weinmannifolia would decrease by 33% in Southwest China under the future scenario with both RCP2.6 and 4.5, but would increase with RCP8.5.

The relationship between the seed bank and aboveground communities has attracted much interest among plant community ecologists. The objective of this study was to address the relationships between soil seed banks and aboveground plant communities by comparing the species composition in the dry-hot valley of the Yuanjiang River. Typical shrub-grass communities in the dry-hot valley were selected and five plots were established in upper, middle, and lower sections of the Yuanjiang River watershed. A total of 100 sampling quadrats were collected, and plant communities were surveyed. In addition, soil samples were collected in three layers (0-5 cm, 5-10 cm, and 10-15 cm) in each community sample. The seed banks from the soil samples were germinated in the lab and species were identified. The results showed that (1) 76 plant species belonging to 25 families and 64 genera were found in the aboveground plant communities. These species mainly belonged to Fabaceae, Poaceae, Asteraece and Euphorbiaceae. (2) 33 plant species belonging to 14 families and 32 genera were found in the soil seed banks. These species mainly belonged to xeromorphy Gramineae. (3) In three soil layers, the proportion of the surface soil seed banks was the largest, and the numbers of seeds decreased as the depth of soil increased. (4) For each plot, few common species were found between the soil seed banks and plant communities, and thus their similarity is low. In summary, there were no significant correlations between the soil seed banks and aboveground plant communities. Moreover, several invasive species can be found both in the seed banks and aboveground plant communities.

The vegetation of dry valleys is a suit of unique azonal ecosystems embedded in the Hengduan Mountains region of Southwest China, with the difference of plant communities between the valleys and the causal factors still elusive. In this study, we investigated the dry valleys of the Yalong River and the Dadu River and compared plant diversity. Different factors were found to impact the vegetation types between the two rivers. Elevation and topography (slope and aspect) impacted vegetation types of the Yalong River and mean annual precipitation impacted the Dadu River. Plant richness in the valleys of the two rivers decreased with greater mean annual temperature. From a north to south aspect, plant richness of the dry valley communities decreased in the Yalong River, while increased in the Dadu River. Richness of shrubs in both the Yalong River and the Dadu River were greater at increased slopes. The β diversity of plant community in the dry valleys of the Yalong River and the Dadu River was affected more by environmental than geographical distance. The effect of geographical isolation on the two rivers was approximately 5 times greater than the effect of geographical distance. This research fills the knowledge gap about plant diversity of dry valleys vegetation in the Yalong River and the Dadu River, and reference information for vegetation conservation in the dry valleys of the two rivers. In addition, it provides a practical method for the quantitative assessment of geographical isolation effect on regional biodiversity difference.

The structural differentiation of plant communities are associated with species traits, and interspecific interactions in heterogeneous environment. The comprehensive analysis of spatial variation in species assemblages may help infer processes shaping ecological communities. Based on field investigation of 116 sites and 562 sampling points in the dry valley of Jinshajiang River, combined with vegetation classification by adaptive-affinity propagation, we used Moran’s Eigenvector Maps and variation partitioning to quantify the effects of spatial and environmental factors on the community structure. The results showed that: (1) the plant communities were divided into 30 groups by Adaptive-AP, and classified into 7 vegetation types, 23 formations. Savanna (30.0%) and warm deciduous broadleaved thicket (55.7%) were the main vegetation types. (2) Mean annual temperature (MAT) and aridity index (k) are two dominant climate factors limiting the distribution of plant community types in the dry valley of Jinshajiang River. Savanna, succulent thicket and evergreen broadleaved thicket are dominant vegetation types in typical dry-hot valley. Warm deciduous broadleaved thicket and evergreen sclerophyllous forest are dominant in dry-warm valley. Warm needle-leaved forest and deciduous broadleaved forest are more adaptive to lower temperature. (3) The pure environmental fraction can explain 5.5% of the species composition variation, the pure broad-scale spatial fraction can explain 22.5% of the species composition variation, 6.6% can be explained by the fraction corresponding to broad-scale structured environment and the unexplainable part was 65.4%. Among all the factors, MAT and k indicated the critical difference among the community habitats, which has prominent impact on the change of community composition. The broad-scale spatial factors played an important role in shaping the community structure by geographic isolation.

In the typical dry valleys of the Three Parallel Rivers region, northwest Yunnan Province, we investigated vegetation using six sampling transects, each comprised of ten standard plots, along altitudinal gradients on the eastern and the western aspects of Nu River, Langcang River, and Jinsha River. With these data, we compared the elevation-related distribution of plant species richness and species turnover rates along the six transects, and explained the patterns using geography and vegetation variables. The dry-warm vegetation zone was dominated by shrubs and herbs and located below the altitude of 3,000 m a.s.l. At higher elevations, shrub and herbs were replaced with a forest zone. The spatial distribution of plant species richness increased with elevation and latitude, especially for herb and shrub species, and was also related to river, vegetation zones, and longitude. The species richness of shrubs also increased significantly across the region, from west to east. Species richness of herbs and shrubs in the Nu River were higher than those in the Lancang River and the Jinsha River, whereas the difference of species richness among three rivers was not significant for trees. Herb species richness in the forest zone was less than that in the shrub and herb zones. Species turnover rate of different zones presented inconsistent altitudinal gradient patterns, but all peak values appeared in the ecotones between shrub communities, in the lower altitudes of the transect, and forest communities, in the higher altitudes. The forest-shrub ecotone is located at an altitude range of 1,900-2,100 m in the Nu River valley, at an altitude range of 2,300-2,400 m in the Lancang River valley and at an altitude range of 2,700-2,900 m in the Jinsha River valley. The mean species turnover rates between shrub & herb section and forest section within each transect were less than the mean turnover rates of the same vegetation section between different transects within the same basin, and also less than the mean turnover rates for same vegetation section in all six transects. Spatial isolation could explain 34.2% of the variation in species turnover rate among the 12 vegetation sections of the six altitudinal transects, while vegetation differences explained less than 0.5% of the variation. These results show the primary role of environment difference in determining the species richness between vegetation types, whereas geographical isolation between the rivers as a dominant factor in the assembly (e.g. species composition) of plant communities.