Carbon (C), nitrogen (N), phosphorus (P) and potassium (K) are the main nutrient elements widely found in soil, litter and leaves, and their stoichiometric ratios are important indicators of ecosystem functions. However, there is little research on the effects that nutrient cycle and vegetation succession have on leaf, litter and soil nutrients and stoichiometric ratios, especially in the fragile karst areas. To reveal the nutrient cycling characteristics and ecosystem stability mechanism during vegetation succession, leaf, litter and soil samples were collected from the herbaceous community (HC), shrub community (SC), secondary forest community (SF) and primary forest community (PF) in a typical karst area at growth and senescence phases. The results showed that the nutrient contents and stoichiometric ratios of leaf, litter and soil in the main layers of each community were significantly different at the growth and senescence phase. The utilization efficiency of N in the leaves of the main layers of each succession community first decreased and then increased at different growth stages, and the utilization efficiency of P increased but the reabsorption rates of N and P showed a continuous decreasing trend. In addition, there was a significant allometric relationship between N and P contents in plant leaves during the growth phase. More importantly, the internal stability of N content in plant leaves was higher than the P content, suggesting that vegetation succession significantly affected leaf, litter and soil nutrient contents and their stoichiometric ratios in our study region. The strength of the relationship between them reflects the inheritance and co–variation of nutrient content to a certain extent, and the differences in the strategies that different species in the community use to adapt to the fragile karst environment. This study concludes that plants in the karst region mainly improve their P utilization efficiency to adapt to low phosphorus stress in soil and ensure the normal physiological and biochemical responses in the process of vegetation succession.

Southwest China is distributed with the largest area of limestone and dolomite karst landscapes in the world, and its ecological environment is very fragile; the problem of rock desertification is particularly prominent. Karst lithological differences make habitats and soil heterogeneity more complex, and vegetation has developed certain morphological and physiological structural characteristics to adapt to these special environments, which will most likely lead to differentiation in vegetation functional characteristics, such as seed size, wood density, life type and flowering time, thus affecting the number and growth of seedlings and young trees and the future succession potential of forest stands in karst-vegetation restoration. Therefore, it is important to understand the differences in species composition and the functional characteristics of dolomite and limestone karst forests. Over a 2-year period, we investigated a total of 3170 individuals from 123 species and analyzed the relationship between the species compositions and the functional characteristics of two karst-forest areas with different lithology (i.e., limestone and dolomite karst). It was found that the average number of species with medium-diameter seeds and medium wood density in dolomite plots was obviously greater than the average number of species in limestone plots, but this reached no significant difference. However, with the gradual restoration of each forest, the height and carbon sequestration capacity of the trees began to diverge, with the relative abundance of species with tall trees and high wood density being significantly higher in limestone plots than in dolomite plots. The species richness of evergreen trees was significantly higher in the limestone plots than in the dolomite plots. There was no significant difference in the relative species richness of flowering plants in the two karst-forest areas during spring–summer and winter–spring seasons; however, the species richness of flowering plants in the limestone karst forests was significantly higher than that of flowering plants in the dolomite karst forests during summer and autumn seasons. These significant traits were very closely related to the species richness of the understory plants. Further insight into the temporal aspects of karst-forest succession is needed to grasp how functional traits affect the restorations of different lithological karst forests to their primary states.

The technique of stable hydrogen and oxygen isotope tracing has become an important means to study the mechanism of water movement due to its high sensitivity and traceability. In this study, four dominant tree species in the karst forest of Maolan, Guizhou Province, were selected, and their water-use strategies and the mechanism of maintenance of tree species diversity were investigated using the stable hydrogen and oxygen isotope tracing technique. The results show that: (1) The regional precipitation varied evidently with the alternation of seasons, i.e., the values of δD and δ¹⁸O in precipitation had a positive bias in spring and a negative bias in summer and autumn. The value of deuterium excess (d-excess) was between 11.67‰ and 31.02‰, with a mean value of 22.98‰. (2) The soil temperature (ST), soil water content (SWC) and precipitation, which have a significant positive correlation, imposed a joint impact on the dynamics of the soil evaporative fractionation. (3) The line-conditioned excess (LC-excess) varied seasonally in different water bodies, i.e., the relative evaporative fractionation of the rhizosphere soil of deciduous tree species was stronger than that of evergreen tree species, and the evaporative fractionation of hydrogen and oxygen isotopes in the leaf water of evergreen tree species was stronger than that of deciduous tree species in spring and summer. However, that of the latter was stronger than that of the former in autumn. (4) The soil water was the most important potential water source for dominant tree species in karst terrain (71%), followed by epikarstic water, which made up an effective supplement (29%). (5) Finally, trees of different life forms and species varied in capacity and proportion in terms of using the potential water sources in different seasons, i.e., deciduous tree species had a greater capacity for using water from potential sources and variable water-use strategies. This may be a major water-limiting mechanism that maintains photosynthesis in the leaves of evergreen tree species (leaves are evergreen and plants continue to grow via photosynthesis) and constrains photosynthesis in deciduous tree species (leaves fall and plants become dormant and stop growing). These results lead to the conclusion that the dominant tree species in karstic forests resist water stress and adjust water-use strategies towards each potential water source to adapt to the harsh karstic habitat through root plasticity and leaf defoliation.

Soil microbes are a crucial component of karst ecosystems, and exploring their community changes during succession can help to elucidate the mechanisms driving succession dynamics. However, the variation of soil microbial communities during vegetation succession in karst ecosystems is still poorly understood. We studied the variations in community structure and potential functions of soil microbes within the four successional stages of grassland (GL), shrubland (SL), secondary forest (SF), and primary forest (PF) for the topsoil (0–10 cm) and subsoil (10–20 cm) in a karst area using high-throughput sequencing. The research findings showed that the bacterial and fungal community diversity and composition changed more obviously in the topsoil than in the subsoil across the succession. With vegetation succession, the structural and functional characteristics of soil bacterial and fungal communities show different trends, with soil fungal communities having a greater response to successional stage changes. Actinobacteria and Acidobacteria were dominant in secondary and primary forests, respectively, while Bacteroidetes was prevalent in grassland. However, the change in Proteobacteria was not significant at both soil depths. Ascomycota was the dominant phylum of soil fungi throughout the succession. The function of soil bacteria was mainly carbohydrate metabolism, which had the highest proportion in the shrubland at different soil depths. The dominant fungal functional groups were saprotroph, pathotroph, and pathotroph–saprotroph. The soil bacterial communities were observably affected by soil organic carbon, total nitrogen, total potassium, ammonia nitrogen, nitrate nitrogen, and leucine aminopeptidase, among which soil organic carbon, ammonia nitrogen, and leucine aminopeptidase mainly influenced the bacterial community in the topsoil, while nitrate nitrogen chiefly influenced the bacterial community in the subsoil. The soil fungal community was only significantly affected by soil organic carbon. Collectively, these results indicate that the effects of vegetation succession on soil microbial communities are largely driven by successional stage and soil properties, with soil fungi being more susceptible to the vegetation successional stage and soil bacteria being more sensitive to the soil properties. During this process, soil bacterial and fungal communities follow different succession patterns.

The main dominant tree species of karst forest at the microtopography and the microhabitat scale were taken as the research object in this study, and the stoichiometric characteristics of different components and their influencing factors were analyzed in order to reveal the survival strategy of karst forest plants in harsh habitats and their mechanism of adaptation to complex terrain. The results showed that the nutrient distribution among different components of the plant was closely related to its organizational structure and functional attributes. The microtopography had a significant effect on plant nutrient accumulation. However, the effect of the microhabitat on plant stoichiometric characteristics was relatively small. Different ecological factors had various regulatory effects on the stoichiometric characteristics of plant components, among which the specific leaf area (SLA) was the most critical biological factor affecting the stoichiometric characteristics of new leaves. Leaf dry matter content (LDMC) had the greatest effect on mature leaves, litter, and branches, and the trunks were mainly affected by plant species. There are synergistic tradeoffs between different plant components, and the interaction between each element mainly shows antagonistic and synergistic effects. Plants adapt to the changes in the karst microtopography and microhabitat by adjusting resource allocation and structural and functional traits. In the upslope, shady slope, and semi-shady slope regions and slopes above 25°, the plants adopted a conservative strategy. In depressions, on sunny slopes, and on flat land, as well as on slopes below 25°, the resource acquisition strategy was adopted. This is the result of the interaction of biological and abiotic factors, which reflects the resource acquisition and nutrient allocation strategies of plants in different habitats, and it is also an mechanism of adaptation to a complex and changeable environment.

Twenty-four plants were studied in Guizhou province, China. Based on various microhabitats (soil surface, stone gully, and stone surface) and microtopographies (slope aspect, slope position, and slope degree), the photosynthetic characteristics of the plants were measured, including the net photosynthetic rate (P_n), carboxylation efficiency (CE), stomatal conductance (G_s), water use efficiency (WUE), transpiration rate (T_r), intercellular CO₂ concentration (C_i), and light use efficiency (LUE). The results demonstrated remarkable variations in the WUE of four types of plants in three microhabitats, showing that stone gully > soil surface > stone surface; there were no remarkable variations in the P_n, T_r, C_i, G_s, CE, and LUE in the three microhabitats. The P_n, T_r, G_s, and LUE of deciduous trees exhibited remarkably higher values compared to the other species studied in the three microhabitats. The evergreen trees’ WUE was much higher than that of the other plants when they were growing on stone surfaces or in a stone gully. There were remarkable variations in the plants’ photosynthetic characteristics dependent on the microtopography. In terms of slope steepness, the P_n, CE, and LUE were the highest in plants on slopes ≤ 5°. Meanwhile, in terms of slope position, the P_n, T_r, CE, and LUE were highest for plants growing in depressions. Concerning slope aspect, the P_n, G_s, CE, and LUE reached the largest values in plants growing on flat land. Principal component analysis (PCA) indicated that the P_n, LUE, and WUE were the key photosynthetic parameters reflecting the adaptation of plants to karst environments. Correlation analysis revealed that the P_n and LUE displayed a striking positive correlation with the T_r, C_i, G_s, CE, and WUE. The T_r displayed a striking positive correlation with the C_i, G_s, CE, and LUE, while a striking negative correlation was observed with WUE. This study indicated that evergreen trees exhibit conservative strategies, while deciduous trees use strategies associated with high growth rates. This study provides insights important for the restoration and reconstruction of vegetation in karst regions.