Population structure of L. rotata
Relative to the mild, moderate and extreme levels for qualitative grading standards, this study considers the P. fruticosa+F. rubra community is a mildly degraded grassland, which is in line with the classification criteria of “more than 95% coverage of grasses and fewer annual plants”. The F. rubra+L. rotata community is a moderately degraded grassland, where the gramineous plants are low due to the high grazing intensity.
It is consistent with the classification standard of “grass plant coverage 80-94%, soil aridification”. Because of the high grazing intensity, gramineous plants are scarce. The bare land is similar to the extremely degraded grassland, which is in line with the classification criteria of “black soil beach, bare land or gravel beach”.
As shown in the Table 2, the L. rotata population structure has different adaptation strategies for grassland degradation. In the F. rubra+L. rotata community, the population density of L. rotata is large and it is one of the grouped species. Its leaf spreading distance is small, and the proportion of plants entering breeding period is small, showed obvious diffusion ability and characteristics of pioneer plants. In the mildly degraded grassland, which is represented by the P. fruticosa+F. rubra community, although the community competition, water, and fertilizer conditions improved, there’s still no competition advantage of L. rotata for its short plant height. The growth of the young plants were inhibited by community competition, plants are expanding towards large-scale development for the ecological position. In the bare land, the seedling growth is inhibited due to the poor habitat of the soil parent layer (lack of fertilizer, severe temperature change, and the long melting period) . This paper speculates that the population depends on the ultralong root system to achieve population reclamation, and through the plant’s large-scale increases in its investment in sexual reproduction.
Morphological structure of leaf
There are several characteristics of morphological structure for the plant of L. rotata to adapt degraded grassland. Firstly, although as a low-growing plant, the mature plants of L. rotata always have a root more than 50cm long, which can penetrate the meadow surface soil and reach the deep earth to overcome the adversity stress in the surface layer of degradation grassland caused by the drought, freezing and thawing, and persistent loss of nutrient. On the other hand, L. rotata seedling can grow root more than 15cm long in the first year in the planting stage, which also showed that L. rotata adapts to the bad habitat through this strategy.
Furthermore, the leaves of L. rotata have thick cuticle layer, developed palisade tissue, regular strip-shape sponge cells, closely arranged, developing gland scales, with functions of heat insulation, water retention, damage resistance, etc. These all are adaptation of plants to severe environment such as strong light, strong ultraviolet radiation, strong airflow changes at high altitude, and physiological drought[25,26]. Meanwhile, leaf stomatal index is closely related to the net photosynthetic rate of plant. Moderate drought will increase the stomatal index, while excessive drought will decrease it[28,29]. In this study, the stomatal index increased with increasing grassland degradation, which means the environment of degraded grassland has not exerted excessive stress on growth of L. rotata and this is the common characteristics of xerophytes like Caragana stenophylla Pojark. L. rotata of the F. rubra+L. rotata community and the bare land can adapt to the harsh habitat by increasing the thickness of the epidermis, the density of glandular scales, the rate of palisade to spongy. At the same time, reduce the thickness of the sponge tissue and the looseness of the leaf tissue structure to adapt to the harsh habitat. However, its structure in the F. rubra+L. rotata community and the bare land did not change in accord with the degree of grassland degradation.
Physiological adaptability of L. rotata
L.rotata showed physiological strategies to adapt degraded grassland as follows: Firstly, the response of enzymatic defense system of L. rotata in the F. rubra+L. rotata community increased significantly, while the PRO and SS content of osmotic adjustment system showed significant decrease and increase respectively, there was a significant negative correlation between them, this regulation mechanism needs further study. The analysis of 7 physiological indicators of stress resistance showed that the average content of MDA, SOD, POD, CAT and SS was the highest in L. rotata of the moderately degraded grassland, and the MDA, SOD, POD and SS were significantly different in leaves from the other habitats. An increase in environmental stress is usually accompanied with per-oxidation action of membrane lipid, produced MDA as final product[31,32]. Which could impair the normal function and structure of the cell membrane. Therefore, the accumulation of MDA can reflects to the degree of membrane lipid per-oxidation and the resistance of plants under stress in some degree. Meanwhile, to prevent damage to the cell membrane system, plants activate the enzymatic defense system to produce protective enzymes to scavenge free radicals generated in the cells, and reduce the degree of membrane lipid per-oxidation, thus resist the effects of stress. SOD, POD and CAT are the 3 most common protective enzymes, and their content can reflect the sensitivity of plants to stress[36,37,38]. SS such as glucose, galactose, fructose, etc., also accumulated under stress. PRO is one of the most effective osmotic adjustment substances. Under stress conditions such as drought, high temperature, saline, and freezing, plant cells may have osmotic stress due to a lack of sufficient water support, and the plants will increase the PRO content to enhance the osmotic adjustment ability of cells.
Secondly, the L. rotata also showed adaptation strategies of photosynthesis in the different degraded grasslands. Compared with the P. fruticosa+F. rubra community, the content of ChlA and ChlB in the F. rubra+L. rotata community was significantly lower, which was consistent with the observation of the leaf color of each sampling point in the field. And DAVIDJ.BURRITT and SUSAN MACKENZIE’s study on a shade-loving plant named Begonia×erythrophylla also showed the same result, that the content of ChlA and ChlB decreased in plant leaves when the plant was placed under conditions of full light. It has been found that the activities of different hormones can be influenced to varying degrees by light. The results indicate that strong illumination is a major stress disorder faced by L. rotata in the F. rubra+L. rotata community. However, in P. fruticosa+F. rubra community, L. rotata showed a significant increase in ChlB. This is an important indicator to judge plant shade tolerance for its advantage of use the blue-violet light in low light environment to enhance the ability of supplemental lighting. Such indicate the L. rotata has adapted the depression of the P.fruticosa bush. At the same time, carotenoids are significantly reduced in mildly degraded plots. Carotenoids have the function of protecting chlorophyll under adverse conditions, further indicating that photosynthesis of L. rotata in the P. fruticosa+F. rubra community has not been greatly affected, and the plant has adapted to a low-light environment. In the bare land, the content of three photosynthetic pigments in L. rotata was higher than that in the moderately degraded grassland, while the content of ChlA and ChlB was lower than that of the P. fruticosa+F. rubra community, and the carotenoid content was obviously increased. The result indicates that in the bare land, the L. rotata mainly protects the chlorophyll by increasing the carotenoid content, then adapts to the degraded grassland environment.
In addition, in the submicroscopic structure, the chloroplasts in the leaves of L. rotata in the F. rubra+L. rotata community are more closely arranged and distributed regularly, with no starch. Usually regular chloroplast arrangements could receive more light energy and produce more starch granules around the granules[44,45]. In this study, the unexpected lack of starch granules in the sub-microscopic structure of leaves in the F. rubra+L. rotata community was possibly a result of resisting habitat stress. L. rotata in this community fully utilizes the light energy by adjusting the position of the chloroplast. At the same time, to resist the grassland degradation stress, the energy consumption is higher than in the other 2 degraded grasslands.
Response of L. rotata to climate change
There’s deep impact of climate warming and changes in dryness on grassland ecology, especially in the Sanjiangyuan region. Where large area of swampy meadow and wetland were greatly degraded and reduced, the surrounding grassland was seriously degraded, and desertification is serious. As a result, there is sustained soil erosion, and animal husbandry recession in that region[46,47,48]. These phenomena are bound to have a negative impact on fragile ecosystems. For example, land freezing and thawing is common in highland areas, and water frozen in soil is one of the main sources of water for highland plants. The dry climate will cause the area of frozen soil to expand, the melting layer of the season will be thickened, and even the permafrost under the soil surface will disappear completely, which will directly lead to the reduction of soil moisture in the root layer of the plant, the drying of the topsoil and the reduction of vegetation coverage[50,51,52]. Previous studies have shown that with the increase of grassland degradation, soil fertility (organic matter, nitrogen, phosphorus), water holding capacity, and total porosity continue to decrease, while the soil bulk density, freeze-thaw days, and freeze-thaw cycles continue to increase. This is consistent with the study of Anyuan, Xu Zhu and others. Therefore, with the development of climate warming and drying and the increase in grassland degradation, the stress of drought, fertilizer loss and freezing and thawing stress in grassland communities are expected continue to increase.
The analysis results indicated that due to the special morphological and physiological structure, especially the long root system, which could resist dry and warm stress. The pioneer plant characteristics of L. rotata in degraded grassland indicate that this plant can quickly occupy the bare land after degradation. The author speculates that the trend of dry and warm climate in the main producing areas will result in less competition in the L. rotata community and with no effect on its growth. Thus possibly promote expansion of the L. rotata population in some degree. On the other hand, the rapid expansion of L. rotata is also positively alleviating the changes in soil structure caused by dry and warming. The investigation showed that L.rotata is a good species for soil conservation in the context of dry warming.