Integrating the effects of latitude and altitude on the spatial differentiation of plant community diversity in a mountainous ecosystem in China

Comparisons of the diversity at different levels indicated that the responses of the plant community diversity to the environment were not the same for diverse gradations, and different species exhibited different gradient patterns owing to restrictions from environmental and artificial factors [37–39]. Therefore, the trends of species diversity exhibited differences between the herbs and shrubs. Batunacun et al. [40] discovered that the species diversity of herbs showed a situation of increasing gradually from the north to the south. Sun et al. [41] also determined that the species diversity in the herb layer tended to increase with decreasing latitude. In our research, the distribution patterns of the herb species diversity tended to be the largest at the middle latitude of the Lvliang Mountains (Fig 5). Compared with Guancen Mountain and Guandi Mountain, Wulu Mountain at the lower latitude of the Lvliang Mountains had distinctly reduced altitude and lay in the continental monsoon subhumid climate region of the warm temperate zone, where it was suitable for the growth of secondary forests and shrub vegetation, but the vegetation growth in the herb layer was restricted, making it a diversity centre of the herb layer skewing to Guandi Mountain at the middle latitude in the Lvliang Mountains. This showed that the plant species diversity in the herb layer changed with the latitude, while being affected by complicated habitat conditions such as the altitude and temperature.

The important values of herb in different life forms showed different trends with latitudinal gradients in the Lvliang Mountains in our research; annual herbs were dominant at the relatively middle latitude, while perennial herbs were dominant at the relatively higher latitude; but species diversity indices of perennial herbs were higher than annual herbs at different latitudes (Tables 2 and 3). At present, it is widely believed that the formation of herbs in different life forms was principally impacted by precipitation, whereas in the similar rainfall conditions, water, heat and light conditions need to be considered, which chiefly included average annual precipitation, accumulated temperature and illumination time [42]. From the north to the south in the Lvliang Mountains, the increments of the average annual precipitation increased the number of species and components of the annual herbs, while the hydrothermal matching requirements of Guandi Mountain at the middle latitude were preferred for annual herb growth by comparison with Guancen Mountain at the higher latitude [42]. However, considering whole mountains, the Lvliang Mountains located in the continental monsoon climate region of the warm temperate zone had four distinctive seasons with drought and windiness in the spring and a quick rise of air temperature and had larger diurnal temperature difference. These conditions conformed to the habitat features of perennial herbs. Hence, the hydrothermal distribution status affected by latitude determined the latitudinal distribution patterns of herbs in the Lvliang Mountains.

There were lots of factors influencing community diversity, such as plant community types, altitudes, human disturbance, successional stages, and habitat discrepancy [30]. Herbs in different life forms demonstrated prominent and inconsistent trends at various altitude gradients in the Lvliang Mountains, where perennial herbs tended to grow well at the relatively higher altitude, and annual herbs tended to the relatively lower altitude (Fig 7). This was attributed to plants in the herb layer not only being entirely affected by the altitudinal pattern but also being impacted by canopy density, logging soil, and local microenvironment [19, 27]. The canopy density had distinct effects on the herb distribution by discrepancies from illumination, local humiture, and ultraviolet intensity, where a lower diversity in the tree layer induced good development in the shrub layer and thus limited herb layer growth [19]. We obtained the same results as Qu et al. [27], showing that the altitudinal gradients of the plant species diversity presented an approximately unimodal pattern, with the species diversity being greater at the middle altitude in the Lvliang Mountains. This was because areas of mountainous regions were larger at the relatively lower altitude in the Lvliang Mountains, where their climate included drought and high temperature, so only species that tolerated drought, high temperature, and soil depletion could survive [20, 26].

Serious human disturbances also had adverse impacts on the biodiversity at the lower altitude [26]. For example, human exploitation had a long history in the middle and southern sections of the Lvliang Mountains, which affected the local natural vegetation significantly. Li et al. [30] concluded that the southern section of the Lvliang Mountains had large degrees of crushing in population distribution areas and low population quantity and density owing to stronger human disturbances, so the diversity was seriously threatened in this region. From the northern Guancen Mountain to the southern Wulu Mountain in the Lvliang Mountains, our research obtained that the latitudes and altitudes decreased, with their gradients being not obvious, the vegetation types reduced, and the plant life forms became simple; the tree density increased, but the height increased first and then decreased; the important values of the shrubs and herbs decreased, and their dominant species were distributed at lower locations. Consequently, the responses of the plant species diversity were sensitive to the altitude in the Lvliang Mountains, with the maximal species diversity appearing at the middle altitude. The distribution patterns of the species diversity with the altitude expressed a monotonic function between diversity and resource productivity.