Species richness and relative importance value (RIV) were selected to describe plant variation and quantify species composition. The importance value (Iv; Eq. 1) was calculated based on the relative density (Dr; the ratio of the number of plants of a certain species to the total number of plants of all species), the relative frequency (Fr; the ratio of the frequency of a certain species to the total frequency of all species), and the relative prominence (Pr; the ratio of basal area of a certain species to the total basal area of all species) (Monk, 1966; Song et al., 2014). For the convenience of calculation and comparison, the RIV (Eq. 2) is often used. It is the importance value of each species divided by three (Song, 2016):

It is impractical to apply the distribution type of a genus to the distribution type for each species within that genus (Zhu, 2007). Therefore, the plant family and genus distribution types cited here were based on the Flora of China (Luo et al., 2012; EBCF, 1988), the areal-types of Chinese seed plant genera (Wu, 1991), Chinese Physical Geography and Plant Geography (Volume 1), and other literature sources (CPGC, 1983). All 434 plant species were individually queried, and the trees and shrubs of all three transect lines in the Qinling-Daba Mountains were divided into subtropical, temperate, widespread, and endemic species. The following classification system was used:

(1) Tropical and subtropical distribution genus. The distribution types of “the areal-types of Chinese genera of seed plants” are types 2-7. i) The plant species within genera such as Cinnamomum, Dalbergia, Pittosporum, Citrus, and Camellia are distributed in the subtropical region and strictly limited to the tropical and subtropical regions. These are classified under the subtropical distribution type. ii) The tropical plant genus distribution type is further extended to the northern temperate zone, and the distribution of each plant species within each genus is individually queried. The northernmost extension of the species distribution area into the temperate zone does not reach beyond Henan or Shaanxi Provinces, and the species include evergreen trees/shrubs such as Alangium chinense, Grewia biloba, Ilex chinensis, and Lindera communis. These are classified under the subtropical distribution type. Plant species such as Periploca sepium, Alangium platanifolium, Broussonetia papyrifera, and Albizia julibrissin extending to the temperate zone as far north as Heilongjiang, Jilin, and Liaoning Provinces or across northern and southern China are classified as widespread species, and most of them are deciduous trees/shrubs. Certain plant species distributed mainly in the northern Qinling-Daba Mountains are classified under the temperate distribution type. These include Indigofera bungeana and Ziziphus jujuba var. spinosa and others.

(2) Temperate distribution genus. The distribution types of “the areal-types of Chinese genera of seed plants” are types 8-12 and 14. Most species in these genera are classified under the temperate distribution type, with certain exceptions. i) Certain representative deciduous tree species are distributed in tropical and subtropical forests. These include Castanea seguinii, Sassafras tzumu, Fagus longipetiolata and others; coniferous tree species such as Pinus massoniana, Cupressus funebris and others; and evergreen trees/shrubs distributed in the southern Qinling-Daba Mountains such as Loropetalum chinense, Elaeagnus pungens, Eriobotrya japonica, Trachycarpus fortunei and others. These are classified under the subtropical type. ii) Plant species with distribution areas in the northern and southern provinces of China or that originate across the country are classified as widespread species. These include Fraxinus chinensis, Koelreuteria paniculata, Salix matsudana, and Quercus aliena var. acutiserrata.

(3) Widespread and endemic distribution species. These differ from the globally widespread and Chinese endemic species reported in previous floristic studies. In the present study, “widespread species” refers to those that are widely distributed across northern and southern China, while “endemic species” refers to those indigenous to specific regions of China. i) The genera have global widespread and endemic distribution. The distribution types of “the areal-types of Chinese genera of seed plants” are types 1 and 15. The distribution areas of the plant species contained within these genera were individually queried. Most of them were classified either as widespread or endemic species, with certain exceptions. Though Cunninghamia lanceolata is endemic to China, it is also a subtropical tree species. Hence, it is classified under the subtropical distribution type. ii) The plant genera distribution types are tropical and temperate. The distribution types of “the areal-types of Chinese genera of seed plants” are 2-12 and 14. The distribution areas of the plant species contained within these genera were individually queried. Most species were classified as widespread according to the foregoing criteria. However, some were classified as regional endemic species including Acer cappadocicum, Ilex denticulata, Tilia oliveri, and Prunus szechuanica.

The distribution types with the largest numbers of tree and shrub species on all three transects of the Qinling-Daba Mountains were temperate, followed by subtropical. There were relatively fewer widespread species, and the endemic species were the fewest in number (Table 1). As there were comparatively few widespread and endemic species in the region and the three transects had only minor influences on their distribution patterns, the present study focused mainly on the changes and transition patterns of the plant species with subtropical and temperate distributions.

Variations in the abundance of typical species along the eastern, middle, and western transect lines of the Qinling-Daba Mountains can disclose the transition from subtropical to temperate plants. Here, variations in the abundance of typical species from south to north were analyzed using kite maps. The selection of typical species was determined according to their degree of dominance or the relative basal area of the trees and the abundance of the shrubs. Plant species with the highest degree of dominance at each transect point in the eastern, middle, and western transect lines were selected. After selection, there was 8-12 subtropical, temperate, and widely distributed species per line. No endemic species dominated any plot. Species with subtropical distribution included Platycarya strobilacea, Dalbergia hupeana, Pinus massoniana, Castanea seguinii, Cunninghamia lanceolata, Vernicia fordii, Lindera glauca, Quercus glauca, Lindera communi, Trachycarpus fortuna, and Coriaria nepalensis. Species with temperate distribution included Robinia pseudoacacia, Ziziphus jujuba var. spinosa, Pinus tabuliformis, Populus tomentosa, Morus alba, and Ulmus glaucescens. Species widely distributed in China included Quercus variabilis, Quercus serrata, Platycladus orientalis, Quercus aliena var. acutiserrata, Toona sinensis, Ailanthus altissima, Juglans mandshurica, and Koelreuteria paniculata.

Differences in the RIV between subtropical and temperate species (SND-RIV) were selected to reflect the changes in the degree of plant dominance from south to north. SND-RIV was calculated as shown in Eq. 3:

where$RI{{V}_{\text{subtropical}\ \text{species}}}$is the sum of the RIV for all subtropical species per plot and $RI{{V}_{\text{temperate}\ \text{species}}}$is the sum of the RIV for all temperate species per plot.

Pearson’s correlation analysis (Pearson, 1920) was used to evaluate the associations between the geographic factors (latitude and altitude) and the meteorological factors (mean annual temperature, mean annual precipitation, accumulated temperatures for the periods with average daily temperature ≥ 0℃ and ≥ 10℃, aridity index, and moisture index) and establish the influences of plot latitude and altitude on the above mentioned meteorological factors in the Qinling-Daba Mountains. SND-RIV was the dependent variable, and stepwise regression analysis was used (You and Yan, 2017) to explore the relationships between the degree of plant dominance and the meteorological factors.

The total species richness curve shows is an increase of 5-10 species along the eastern transect from the southern slope of the Qinling Mountains to the northern slope of Shennongjia. The total species richness was stable for the middle transect. Along the western transect, about 15-20 species was recorded on the southern slope of the Qinling Mountains but only ~10-15 species in the southern Longnan region (Figure 2). The subtropical species richness increased from north to south, whereas the temperate species richness decreased from north to south for all three transects. In the eastern transect, the subtropical species richness increased from 3 to 5 species in the north and from 10 to 15 species in the south. In the middle transect, the subtropical species richness sharply increased from the north to the Hanjiang River, increased more gradually thereafter, and stabilized at ~15 species. The subtropical species richness of the western transect increased from north to south in a concave upward curve and sharply increased from ~5 to ~15 species in the southern Longnan region. The species richness in the temperate zone of the eastern transect was relatively high in Funiu Mountain and Shennongjia because of the influence of altitude there. The species richness in the temperate zone of Shennongjia (~15 species) was higher than that in Funiu Mountain (~10 species). The species richness in the temperate zone of the middle transect reached a maximum of ~20-25 species in the Qinling Mountains, was markedly lower in the Hanjiang River (~eight species), and stabilized at 5-10 species south of the Hanjiang River. The temperate species richness of the western transect decreased from 15-20 species to ~2 species in a north-to-south direction.

The north-to-south variation in RIV and the polynomial fitting curve are shown in Figure 3. Subtropical species RIV in the eastern transect increased from 0-10% in the north to 4%-60% in the south. The incline of the fitted curve increased steadily. The temperate species RIV decreased substantially from 90% to 30%-40% from the Qinling Mountains to the Hanjiang River, stabilized at 30%-40% from the Hanjiang River to Shennongjia, and significantly decreased from 40% to ~5% south of Shennongjia. The slope of the fitted curve decreased considerably from north to south, stabilized, and decreased sharply thereafter. The subtropical and temperate species RIV fitting curves were in close proximity between 31.80°N and 33.70°N.

The subtropical species RIV in the middle transect increased from 10% to ~60% from north to south, reached a maximum of 60%-80% in the Daba Mountains, and stabilized thereafter. The fitted curve rose from the north to the south and uplifted in the Daba Mountains. The temperate species RIV decreased from 60%-70% in the Qinling Mountains to ~20% in the Daba Mountains. The fitted curve declined from north to south and stabilized thereafter. The fitted curves for the subtropical and temperate species RIV intersected at 33.30°N.

The subtropical species RIV in the western transect steadily increased from north (10%) to south (40%-60%), and the fitted curve had a nearly linear upward trend. The temperate species RIV decreased from north (~90%) to south (~60%) in the Bailong River, stabilized, and then decreased to ~10%-20% south of Longnan. The fitted curve initially decreased, stabilized, and sharply decreased thereafter. The fitted curves for the subtropical and temperate species RIV intersected at 32.75°N.

The SND-RIV of the eastern, middle, and western transects in the Qinling-Daba Mountains increased with latitude and from north to south. Nevertheless, the change trends differed among transects (Figure 5). The SND-RIV of the eastern transect initially increased, then stabilized, and increased thereafter. The SND-RIV increased from the northern slope of the Qinling Mountains to the southern slope of the Funiu Mountains (33.70°N; 600 m a.s.l. on the southern slope of the Eastern Qinling Mountains) but was < 0. From the southern slope of Funiu Mountain to Shennongjia (31.80°N), the SND-RIV was ~0. Hence, the subtropical and temperate species stably co-dominated there. The SND-RIV in Shennongjia and south of Shennongjia was >0 and increased from north to south. Hence, subtropical species predominated there. In the middle transects, the dominance of temperate species decreased from north to south and the SND-RIV was > 0 on the southern slope of the Qinling Mountains (~33.30°N; 800-1000 m a.s.l.). The SND-RIV increased southward and reached a maximum at Daba Mountain (32.80°N). The SND-RIV of the western transect increased linearly from north to south except at four points near 33.00°N. Temperate species predominated from north to south, whereas subtropical species predominated south of Longnan at 32.75°N.

The correlations among geographic and meteorological factors in the Qinling-Daba Mountains are shown in Table 2. The latitude of the eastern transect was negatively correlated with mean annual precipitation (-0.918**) and the moisture index (-0.756**) but positively correlated with the aridity index (0.292*). Altitude was negatively correlated with mean annual temperature (-0.928**) and accumulated temperature (-0.933**, -0.915**) and positively correlated with the aridity index (0.457**). Moisture decreased with increasing latitude, whereas temperature decreased with increasing altitude.

The latitude and altitude of the middle transect were negatively correlated with mean annual temperature (-0.620**, -0.895**), mean annual precipitation (-0.905**, -0.386**), and accumulated temperature (-0.606**, -0.616**, -0.870**, -0.880**), positively correlated with the aridity index (0.773**, 0.513**), and negatively correlated with the moisture index (-0.792**). Both altitude and latitude significantly affected temperature and precipitation. Precipitation decreased with increasing latitude, whereas temperature decreased with increasing altitude.

Both the latitude and altitude of the western transect markedly influenced temperature. The correlation coefficients between latitude and mean annual temperature, accumulated temperature ≥ 10℃, and accumulated temperature ≥ 0℃ were -0.889**, -0.857** and -0.881**, respectively. The correlation coefficients between altitude and mean annual temperature, accumulated temperature ≥ 10℃, and accumulated temperature ≥ 0℃ were -0.896**, -0.872** and -0.891**, respectively. Latitude and altitude also impacted precipitation.

Overall, the altitudes of all three transects strongly affected temperature. The correlation coefficients between altitude and mean annual temperature, accumulated temperature ≥ 10℃, accumulated temperature ≥ 0℃ were -0.865**, -0.873** and -0.855**, respectively. Latitude strongly affected mean annual precipitation, and the moisture index and the correlation coefficients were -0.705** and -0.495**, respectively.

The R² in the stepwise linear regression model (Table 3) of SND-RIV and the meteorological factors in the eastern, middle, western, and total transect plots in the Qinling-Daba Mountains, were 0.687, 0.702, 0.729, and 0.746, respectively. The contribution rates of the mean annual precipitation and the mean annual temperature in the eastern transect were 0.62 and 0.38, respectively. In the middle transect, the contribution rates of the mean annual temperature and the moisture index were 0.60 and 0.40, respectively. The contribution rates of the mean annual precipitation and the mean annual temperature in the western transect were 0.64 and 0.36, respectively. However, there was collinearity between the accumulated temperature ≥ 0℃ and the other meteorological factors. Hence, the former was excluded from the total linear regression model of the three transects. The contribution rates of the mean annual temperature, the mean annual precipitation, the moisture index, the accumulated temperature ≥ 10℃, and the aridity index were 0.25, 0.26, 0.20, 0.19, and 0.10, respectively.

Correlation and stepwise regression analyses revealed that latitude and altitude influenced temperature and precipitation and, by extension, plant species and their quantitative characteristics in the eastern, middle, and western transects. Temperature decreased with increasing altitude in the eastern, middle, and western Qinling-Daba Mountains. Precipitation decreased with increasing latitude in the eastern and middle Qinling-Daba Mountains. Temperature and, to a certain extent, precipitation also decreased with increasing latitude in the western Qinling-Daba Mountains. Low temperatures, high wind velocity, and drought can limit plant growth (Troll, 1973). The decrease in precipitation with increasing latitude explained the latitudinal transition pattern observed in the plants of the eastern transect. Hence, the abundances of subtropical and temperate plants decreased and increased, respectively, from south to north. Shennongjia is located in the southern section of the eastern transect and has adequate precipitation. However, the proportion of temperate plants increased with rising altitudes and falling temperatures. The combined influences of latitude and altitude widened the plant ecotone zone in the eastern Qinling-Daba Mountains. Increases in latitude from south to north in the middle and western Qinling-Daba Mountains were accompanied by increases in altitude and corresponding decreases in temperature and precipitation. In that region, the dominance of subtropical plants decreased while that of temperate plants increased, and the ecotone was clear and narrow.

In this study, we obtained continuous data for the latitudinal variation in the plant species of the Qinling-Daba Mountains and analyzed the Qinling-Daba Mountains as a complete geographic and geomorphic unit. We obtained the north-to-south plant ecotone zone ranges for the eastern, middle, and western Qinling-Daba Mountains (Figure 6). The results of the present study partially validated those of previous investigations and provided a scientific basis for determining the North-South boundary in China. Analysis of the latitudinal variation in plant species on three north-south transects verified that the North-South Boundary of China is not a simple geometric line but rather a transition region of a certain defined width. Moreover, it was established that the latitudinal spans of the transition region differed among the eastern, middle, and western Qinling-Daba Mountains. The eastern ecotone was the widest and was situated between the southern slope of Funiu Mountain (33.70°N) and Shennongjia (31.80°N). The middle ecotone zone was located between the southern slope of the Qinling Mountains (33.70°N) and the northern slope of the Daba Mountains (32.80°N). The western ecotone was situated between 32.75°N and 33.40°N. The northern boundary of the distribution of evergreen broadleaf woody plants or southern species corresponded to the northern boundary of the subtropical zone. Previous studies delineated the boundary on the southern slope or at the southern slope altitude of 800-1000 m of the Qinling Mountains (Zhang and Zhang, 1979; Kang and Zhu, 2007; Zhang et al., 2021). The contour of the Qinling Mountains at 1000 m (Figure 6). Zhu (1958) used climate indexes such as isoline of accumulated temperature ≥ 10℃ and isotherm of the mean temperature of the coldest month 0℃ to define the boundary between the warm temperate and subtropical zones in China (Figure 6). The eastern and middle parts of this boundary are within the ecotone zone; only the western part is located on the south side of the ecotone zone in our study (Zhu, 1958). Zhao et al. (2020) reported that the transition between the north subtropical and the warm temperate zone was reflected in the base zone of the mountain altitudinal belt spectrum from the southern slope of Funiu Mountain to the northern slope of Shennongjia in the eastern Qinling-Daba Mountains, from the southern slope of Taibai Mountain to the northern slope of the Micang Mountain in the middle, and from south of the Baishui River Nature Reserve in the west (Zhao et al., 2020). Kou et al. (2020) used several climate indicators to locate the northern boundary of the subtropical zone with reference to vegetation and soil data (Kou et al., 2020). Here, the northern boundary of the ecotone did not markedly differ from that of the northern subtropical zone proposed by a previous study. However, the ecotone was further extended in the present study. This discovery provides additional data supporting the boundary between the warm temperate and subtropical zones as well as comprehensive geographic regionalization in China.

Prior studies investigating the boundary used different indices, subjects, research scales, and differentiation criteria, and the resulting position of the boundary differed considerably. Another key reason is that the local differentiation of climate and vegetation in the interior of the, on average, 300-km wide Qinling-Daba Mountains was poorly understood. This study only used community survey data along three north-south transects and got a deeper understanding of the inner complexity of the Qinling-Daba Mountains. Considering the species data of the approximately 40 nature reserves in the Qinling-Daba Mountains will provide a more solid basis for delineating the North-South boundary. Future research should also incorporate high-resolution climatic data, intensive plant species data, and even soil data to more precisely explain the warm temperate-subtropical boundary of China.