Study On The Plant Ecological Adaptation Strategy Of Degraded Seriphidium Transiliense Desert Grassland

Seriphidium transiliense desert grassland belongs to the climatic type desert of Central Asia, and distributed only in Xinjiang, the western region of China. This grassland ecosystem is too vulnerable, but still suffering heavy grazing pressure, furthermore, frequent anthropogenic destruction and lack of the public protection awareness which resulting the grassland severely degraded at large. These are seriously threatening the grassland biodiversity, livestock production and oasis ecological security. In this study, the approach for subdivision of degradation succession was explored by using the methodology of spatial sequence instead of temporal sequence, the organizational coordination capacity in terms of plant community, soil seed bank, nutrients, and physiological function during the process of grassland degradation was studied for clarifying its ecological adaptation strategy, assessing the ecosystem risk, and providing theoretical and practical basis on comprehensively understanding the mechanism of grassland degradation, timely conducting ecosystem restoration and sustainable utilization programs. The main results are as follows:(1) The Approach for Subdivision of the Degradation SuccessionBy TWINSPAN, DCA and DCCA, the S. transiliense desert grassland of the study area was subdivided into five successional stages, non-degraded stage (I), medium-degraded stage (II), heavy-degraded stage (III), over-degraded stage (IV), and system-collapse stage (V). The outputs of DCA provided interpretation on the degradation successional sequence from stress injury rate and resistibility, whereas the outputs of DCCA served as an environmental interpretation on degradation successional sequence from the soil physical and chemical properties.(2) The Ecological Adaptation Strategy of the Plant CommunityWith the intensity of grassland degradation, at individual level, the plant autoecology of main species, S. transiliense, Petrosimonia sibirica, Ceratocarpus arenarius, Trigonella arcuata, Tulipa iliensis, Gagea bulbifera, and Geranium transversale, tended to miniaturize for avoidance strategy. At the population level, the plant populations format their individual ecological adaptation strategy in different degraded stages, mainly through adjusting reproductive strategy to adapt the stress. At the community level, community adopts the ecological adaptation strategy of increasing the species number to enhance the competitiveness to filter the dominant species for adapting grazing stress, which possesses high reproductive capacity, grazing tolerance and resistance, but this often leads to main plant species changed and loss of the original vegetation. The plant community had experienced the ecological adaptation process from quantitative to qualitative change.(3) The Ecological Adaptation Strategy of the Soil Seed BankThe soil seed bank had the ecological adaptation strategy by taking advantage of rainfall conditions to germinate rapidly,and had an ecological insurance mechanism of retaining some seeds to resist external interference. The assessment of soil seed bank species diversity showed that its changes were in accordance with"medium disturbance theory", the species diversity of transient seedbank was greater than that of persistent seedbank and with greater fluctuation. The substitution speed of plant species was higher in early and late stages of degradation process, thus accelerated the changes both from non-degraded stage to degraded stages and from degraded stages to system-collapse stage. The similarities of soil seed bank species diversity in different degraded stages were low from 0.22~0.34 respectively, indicating that all the seedbanks were unstable caused by the degraded aboveground vegetation. The density of persistent seedbank was generally decreased as I>IV=V>II>III, whereas the density of transient soil seed bank was generally increased as IV>V>III>II>I. In vertical distribution, the seed density in top layer (0~5cm) was increasing along with the grassland degradation process, indicating that the seeds tend to distribute in shallow soil. The species composition of the two types of seedbank was all dissimilar to that of aboveground vegetation. However, there was a very significant positive correlation between the densities of transient seedbank and aboveground vegetation that could be described by quadratic curve. (4) The Plant Nutritional Ecological Adaptation StrategyIn the process of grassland degradation, the changes of each nutrition component of the main plant species and community fluctuated with individual module, population characteristics and community composition. By PCA the nutrient values of the grassland plant were synthetically evaluated, the results showed that with the intensity of grassland degradation, the nutrient value of dominant species S. transiliense increased gradually, the nutrient value of sub-dominant species P. sibirica in each degraded stage was less than that of non-degraded stage, and nutrient value in latter degraded stages increased slightly compared with that of early degraded stages, the nutrient value of plant community decreased gradually, and the nutrient value of S. transiliense desert grassland became poor.(5) The Plant Physiological Ecological Adaptation StrategyIn the process of grassland degradation, the all physiological resistance indexes of the main grassland species changed, and the dissimilarity coefficient index was high beween non-degraded stage and medium-degraded stage. Therefore, the physiological changes happened prior to the appearance characteristics changes of individual, population and community. And medium-degraded stage was the turning point of plant physiological injury. The plants showed different physiological response mechanisms to grazing stress. The plants showed different physiological response mechanisms to grazing stress. With the intensity of grassland degradation, the physiological adjustment ability as followed, S. transiliense changed from strong to feeble, and died out as the grazing stress exceeding the adjustment limit, so it was the tolerance species, the most physiological resistance indexes of P. sibirica had no significant changes, without prominent injury and adapt to grazing stress, so it was the insusceptible species, the physiological adjustment ability of T. iliensis was relatively poor, and the injury increased gradually, so it was the susceptible species.(6) The Ecological Risk AssessmentWith the intensity of grassland degradation, the ecological risk level of S. transiliense desert grassland became higher and the change rate increased gradually. The risk was subdivided into four risk levels by the ecological risk value (Ri), viz. the first level risk (Ri >0.8030), the second level risk (Ri from 0.0059 to 0.8030), the third level risk (Ri from 0.0034 to 0.0059), and the fourth level risk (Ri <0.0034), with the reverse direction of succession. The specific restoration measures of degraded grassland should be formulated based on risk threshold accordingly.