Abstract:

Objective: The vegetation and shape characteristics and spatial distribution of Nitraria tangutorum Nebkhas in different developmental stages were studied to characterize the spatial point distribution and patterns of variation of N. tangutorum Nebkhas with the increase of size of the sandpiles in order to reveal the characteristics of community formation and the mechanism of maintaining ecological patterns of N. tangutorum Nebkhas during the development process. Methods: Based on field investigation,four typical sample plots of N. tangutorum Nebkhas in 4 different developmental stages,i.e,prototype,developing,stabilizing,and activating,in Minqin desert-oasis ecotone were selected. In light of the two-dimensional coordinates,the N. tangutorum Nebkhas were graded into 3 classes,i.e. small,medium and large,respectively,according to the volumes of sandpiles. Their spatial distribution were analyzed with the univariate O-ring function while the correlation among different grades of the sandpiles at different scales was determined by the bivariate O-ring function under the quantitative statistical methods. Results: With the development of the sandpiles,the overall growth changed from good to poor,and the vegetation coverage tended to increase at the beginning and followed by a decrease,which was 51.4% at the prototype stage,reached to the highest 68.8% at the stabilizing stage,then decreased to 60% at the activating stage,whereas the bush deadwood rate showed an opposite trend,which increased from 15% at the prototype stage to 27% at the activating stage. Different distribution patterns appeared for different grades of sandpiles at different scales. At the prototype stage,they tended to be evenly distributed under the scale of 0-5 m,and randomly distributed on the scale of 2-25 cm,whereas distributed in aggregate under the scale of >25 m; at the developing stage,they tended to be uniformly distributed under the scale of 0-5 m,but were randomly distributed under the scale of 5-30 m; at the stabilizing stage and activating stage,they showed a uniform distribution under small scales(< 5 m),a random distribution under the scales >5 m,respectively. The sandpiles of different sizes at different developmental stages showed different association with changing scales. For the small grade sandpiles(grade 1 and 2),except that there was no correlation between the small sandpiles(grade 1 and 2) at the developing stage,it was positively correlated with each other in small sandpiles(grade 1 and 2) along with the increasing spatial scale in other different stages. For the grades 2 and 3,their spatial correlations with other sandpiles showed as a negative correlation under the scale of < 10 m,whereas there were gradually strengthen negative correlation or even disappearing with increasing scales under the scale of >10 m. Conclusion: The sandpiles of N. tangutorum Nebkhas showed no significant aggregation although the different grades of sandpiles at different development stages have different proportions in each sample plot. With the gradual progress of the sandpile development,large sandpiles tended to be randomly distributed,but,the correlations between the small sandpiles grades showed positive on the small scale,while,the negative correlations appeared strengthen or uncorrelated on the medium and large scales,indicating the composition and structure of the sandpiles of N. tangutorum Nebkhas community become less stabilized as the development progresses.

Key words: Nitraria tangutorum nebkhas, developmental stage, vegetational characteristic, spatial distribution, desert-oasis ecotone