| Being one of the important ecosystems on the earth, grassland has important significance for maintaining the ecological balance of the global ecosystem and supports the sustainable development of human society. However, the grassland ecosystems have been destroyed due to unreasonable use by human being for a long time. Fragmentation of grassland landscape has been worst, which becomes one of the most serious threaten to global biodiversity reduction. After being fragmented, grassland vegetation always becomes a heterogeneous ecosystem constituted by discontinuous community patches. The patches and their dynamics determine the spatial pattern and ecological process of the grassland. There is important theoretical significance for us to better understand the change law of the grassland landscape if we could reveal the relationship between the vegetation and environment, and the dynamic mechanism of the spatial pattern of vegetation and environment under disturbance.In order to getting a comprehensive understanding of the succession regulation rule of grassland landscape under positive and negative disturbance, we took the saline-alkali grassland as the research object, and designed two opposite disturbance types which were positive(enclosure) and negative(clipping) ones, then studied the change rule and dynamic mechanism of the grassland landscape on two scales of population and community, under two opposite disturbance, respectively. The goal of our study was to get better understanding of the heterogeneity, stability and persistence of grassland landscape ecosystem. Our study also could provide the theoretical basis for the construction and management of grassland ecosystem, and is beneficial to guided practical activities such as mowing and grazing.In the study, we adopted the method of field investigation and controlled experiment.Under the condition of enclosure, we selected an enclosed pasture which used to be almost occupied by saline-alkalined patches and had been restored for ten years. We investigated the fertile island effect, interior heterogeneity and edge effect of the vegetation patches. In the process of clipping, we had implemented a four year experiment which included light, mild,heavy disturbance gradients to study community characters, species composition, the pattern and complexity of species distribution. We obtained the following results and conclusions:1. The heterognous regularity of grassland landscape under the condition of enclosure(1) Fertile island effect of vegetation patchesSaline-alkalined grassland is a heterogeneous ecosystem which is characterized by vegetation patches and fertile island effect in the process of restoration.① Fertile island effect of soil attribute. In the upper layer(0~30 cm) of the soil, the content of soil organic matter(SOM) was higher, pH and electric conductivity(EC) were lowerer in vegetation patches than in saline-alkalined patches. In vertical direction,distributions of pH and EC in vegetation patches both exhibited hyperbolic curves, and SOM decreased rapidly downward. These results indicated that there were obvious patchy charateristics and fertile island effects in the process of the restoration in grassland. These are results of the reciprocal feedback between vegetation and soil.② Fertile island effect of vegetion charateristics. Biodiversity inside vegetation patches is determined by spatial attributes of patches and soil attributes. Analysis about correlation between species richness and spatial attributes of patches, soil indices showed that species richness was significant positive correlated to area(r=0.67) and perimeter(r=0.65), negative correlated to ration of perimeter to area(P<0.05), not correlated to SOM(r-0.44) and EC(r=-0.33). These results indicated that species richness is determined by spatial morphological indices of patches firstly, and then by soil attributes. Higher biodiversity prefer patches which are larger, circular and then more fertile. The results of CCA showed that SOM and EC were the most relevant environmental factors to CCA axes, which demonstrated that community structure was determined by soil attributes firstly, and then by spatial morphological indices.The forbs prefer patches with more fertile, lower salinity-alkalinity and then larger area.(2) The heterogeous charaters inside vegetation patchesThe heterogeneity of the saline-alkaline grassland is reflected not only by difference of resources between inside and outside patches but also the uneven distribution of resources even inside patches. Analysis through geostatistic indicated that plant richness, SOM, EC present obvious spatial autocorrelation(all values of C0/(C0+C)were less than 25%). The result of Kriging interpolation futher demonstrated richness, SOM and EC exhibited patchy distribution and ring structure. In horizontal direction, the patches enlarge itself from the center of itself by the pattern of the concentric circles, and affect soil attributes also by such pattern. The autocorrelation distance of richness was greater than ones of SOM and EC in0~-10 cm soil layer, which meant the changes in soil attributes were later than that in the vegetation. The result of rang analysis showed that the rang of SOM decreased from upper to lower soil layer, and EC was greatest in-10~-20 cm soil layer, which indicated that in vertical direction, vegetation improves soil conditions gradually downward. However, the saline-alkalined degree was determined by contradiction movement of sodic in vegetation layer and underground saline-alkali layer.(3) The charateristics of boundaries between dominant species stands and saline-alkali patchesThere were obvious boundaries between dominant species stands and saline-alkali patches, and there are abrupt changes in soil indicators and vegetation indices around boundaries. The succession started from the inhabiting of perennials as pioneer species in saline-alkali boundary to improve environment, and followed by forbs invading, and then the dominant species stand extending continuously. There were differences between boundaries locations determined by vegetation indices and soil indicators, which meant the changes in vegetation and soils were not synchronous. Soil indicators were more stable than vegetation ones. Therefore, although the vegetation boundaries looked like more obvious, the soil indicators were more suitable to determine the boundary location.2. Spatial distribution regularity of vegetation population under clipping distubance(1) Being clipped, the population regulatory mechanism of Leymus chinensis which had threshold would function and contended with the disturbance, which lead to density and maximum density intensity represent fluctuating curves of M pattern. The influences exertedby disturbance with different intensity on competition abilities of L. chinensis were different.The competitive advantage would be suppressed or even destroyed by heavy disturbance,restrained by moderate disturbance, affected but maintained by light disturbance. The competition between populations would be reflected by spatial pattern. Not only dominant species- L. chinensis but also invading species- Setaria viridis and Artemisia scoparia represented contagious distribution which included aggregated distribution(negative binominal distribution) and kernel distribution(Neyman distribution). Negative binominal distribution represented population had superiority in competition, maturity in spatial pattern,stabilizing in development and stationary in structure. Neyman distribution meant that under unstable situation, populations adopted the spatial strategies of aggregated inhabiting at large scale and scattered settling at small scale in order to win the spatial competition.(2) Cutting could lessen the heterogeneity of L. chinensis population. While, after being cut, there es a restoration process for L. chinensis population which would improve heterogeneity. Cutting also could affect the spatial autocorrelation of L. chinensis population.Under moderate disturbance, there was most obvious patchy aggregation for L. chinensis population. Under situation of heavy and light cutting, the shapes of patches were complicated. Changes in pattern intensity were great. Distribution of individuals was not uniform. Community structure was complex.(3) Under situation of disturbance, in the process of succession, L. chinensis was dominant species. A. scoparia was invading species and S. viridis was transition species. S.viridis requested more strict circumstance than A. scoparia, and its competitive advantage would also be restricted and quit from the habitat quickly under disturbance. Just like L.chinensis, A. scoparia had regulatory mechanism corresponding to cutting. S. viridis and A.scoparia had great heterogeneity and spatial autocorrelation, and they were aggregated patchily. From competitive superiority reflected by fractal dimension, L. chinensis had little advantage when heavy disturbance restricted competitive strength of all populations; S. viridis and A. scoparia were more capable to occupy space than L. chinensis under moderate disturbance; L. chinensis was still dominant species while S. viridis and A. scoparia were accompanying species in community under light disturbance.In view of the two opposite disturbance which were enclosure and clipping, at population and community scales, we had studied grassland micro landscape including the pattern characteristics, complexity, dynamic as well as the interaction between biology and biology,biology and environment, biology and interference, environment and interference in or between patches. We aimed to explain the differences of pattern and process of grassland ecosystem to reveal the mechanism of grassland degradation. Our study broke through the natural and balanced thought of traditional ecology and the method of taking patches as the smallest units of heterogeneity in previous research. Studies about landscape by experiment at small scale could not only solve problems about pattern of traditional landscape ecology, but also reveal the mechanisms of the pattern by breaking through the limitation of research scale and the object of study. Our research could provide the theoretical basis for forecasting the dynamic change of salinization meadow steppe ecosystem, diagnosing the grassland ecosystem health, strengthening the management of grassland ecosystem, improving thesalinization of grassland, as well as preventing the land degradation in ecologically fragile area. |
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