| Anabasis aphylla in south Junggar Basin was taken as the research object by plot invesitigation, anchor point measure and laboratory experiment in the different regions. Studies on seeds dispersal and germination, effects of sand burial depth and seed size on seedlings regeneration, responses of xylem anatomy and assimilating branch physiology characteristics to drought, population characteristics, spatial pattern and spatial association in diluvial fan and desert-oasis area, and effects of sand burial on growth and physiology of plant were conducted in the desert-oasis ecotoneto provide a theoretical basis for the conservation and sustainable management of A. aphylla population in Junggar Basin. The results are as follows:(1) The cumulative annual seed rain intensity reached 853 seeds /m2 and 751 seeds /m2 respectively in the diluvial fan and sand area, although in some small local environments it reached as high as 2118 seeds /m2. The peak period of seed rain was from late October to early November, producing 82.42% and 76.03% of the total seed rain respectively in the diluvial fan and sand area. Subsequently seed rain density decreased gradually with time. The seed rain density stabilized in mid-November and the process of seed rain essentially ended by late November. Semivariogram analysis of exponential model revealed significant spatial seed rain patterns within an effective range of 8.97 m and 0.42 m respectively in the diluvial fan and sand areas. 45.0% of the spatial heterogeneity was caused by spatial autocorrelation and 55.0% was caused by random factors in the diluvial fan. But, the spatial heterogeneity almost was caused by random factors in the sand area.(2) Germination percentage of A. aphylla decreased with an increase in temperature and Na Cl concentration. The higher percentages were obtained at 15℃ in seeds exposed to 100 m M Na Cl. Recovery percentage increased over 16% in the higher concentrations, whereas in the 100 Na Cl m M concentration it increased by 5.5%. Thus, A. aphylla was moderately salt tolerant at germination stage, and the tolerance was affected by the interaction of temperature and Na Cl concentration. Finally, attached winged perianths and the presence of detached winged perianths significantly inhibited germination, removal enhanced germination. The winged perianth was a mechanical barrier and inhibitor for seeds emergence; hence, it inhibited germination.(3) Sand burial significantly affected seed germination, seedling emergence, survival and biomass of A. aphylla. Percentages of seed germination and seedling emergence, survival were higher at burial depths of 0.2 and 0.5 cm. Germination and emergence rates of A. aphylla were significant differences among different sizes seeds or different burial depths. In the same sand burial depth, large seeds seedling emergence was significantly higher than medium seeds and small seeds. In 0.2–2 cm depth of sand, seedling survival rates of big seeds were higher than medium seeds and small seeds.(4) Young A. aphylla among treatments showed a high survival rate and no significant differences were observed during the test period. However, the older branch mortality rate was greatly increased in the 12-cm and 18-cm sand burial. Shallow burial had no negative effects on the growth and physiological response of A. aphylla. An increase in sand burial depth had a serious negative impact on growth and physiological response of A. aphylla, decreasing its assimilating branch number and chlorophyll content, declining seeds production and leading to physiological disorders. In addition, mechanical friction and high temperature of sand surfaces after deep sand burial may play important roles in determining growth and physiological response of A. aphylla.(5) Compared to the secondary xylem vessel of the artificially building plants, the natural desert plants have a smaller single vessel diameter, a smaller hydraulic diameter, a bigger vessel density, a bigger thickness of single vessel and size vessel coexist in the distribution of vessel diameter, but less wide vessel, water conduit rate is smaller, but for the safety of the water transport is stronger, the mechanical strength is bigger, too. The natural desert plants, Tamarix ramosissima, Haloxylon ammodendron and A. aphylla, have similar vessel characteristics, comparing among the three, the vessel length becomes shorter in turn, the single vessel diameter becomes smaller, the hydraulic diameter becomes smaller, to the vessel density, the largest of H. ammodendron and A. aphylla and the smallest of T. ramosissima, size vessel coexist, but the wide vessel of T. ramosissima is more and the narrow vessel of H. ammodendron and A. aphylla is more, showing that the hydraulic conductivity rate of natural desert plants is progressively smaller, besides, H. ammodendron and A. aphylla have a bigger vessel density and more narrow vessel, which can improve the safety of water transport. These differences are the results to niche environment of the three desert plants. Under the drought conditions, chlorophyll content, Soluble sugar content and peroxidase content of H. ammodendron, T. ramosissima, A. aphylla were reduce while MDA content was increased.(6) A. aphylla population has smaller distribution area in China. Patch number of A. aphylla population is 107, and its mean area is 287 hm2. In Junggar basin, number structure of A. aphylla is a peak pattern in diameter and height class. The number structure of A. aphylla likes “J” model variation in the canopy class. In the field, age structure of A. aphylla population is “convex” type. Due to the microenvironment, A. aphylla populations have some differences in diameter, age class, height class, canopy class structures performance in different study areas. The survival rates of A. aphylla population decreased with increasing age classes, and the disappearance rates and mortality rates increased with largening age classes. The survival curve was close to DeeveyⅠ. That performance showed A. aphylla was a declining population in the study area. Individual number of A. aphylla population was uneven distribution in different habitats, and the spatial distribution was a random type.(7) Different types of A. aphylla population in three slopes showed significantly aggregated distributions at relatively smaller scales and the degree of aggregation decreased with increasing spatial scales, and tended to uniform distributions at larger scales in the gentle slope. Seedlings were more likely to follow aggregated distribution than those big ones and the aggregated intensity of the larger individuals was weaker. At relatively smaller scales, positive association was showed significantly among different developmental stages in the three slopes. Negative or uncorrelated association was showed between seedling and the other two stages at larger scales, but positive or uncorrelated association was showed in juvenile and mature plant. However, at larger scales, pattern association among different developmental stages tended to negative association in the gentle slope.(8) Although an aggregation of A. aphylla and H. ammodendron plants was found in each plot, the cause of spatial aggregation and association was different in the three sandy desertification areas. In the diluvial plot, the spatial association of A. aphylla and H. ammodendron seems to reflect a tense negative woody plant interaction. On the other hand, the random labeling null model showed that plant–plant interactions were largely determined by the combined effects of interspecific competition and harsh environments in the diluvial–sandy and sandy plots, contrary to the competitive effect of the early stage of sandy desertification expansion. In addition, we also revealed the recruitment potential of H. ammodendron and therefore its potential for encroachment with the desertification environment.(9) The complete spatial randomness null model(CSR) showed four species exhibited significant aggregations at small scales(< 20 m). A. aphylla and N. roborowskii, H. ammodendron and R. songarica were spatially positive associations at small scales with the independent null model, while A. aphylla and H. ammodendron, A. aphylla and R. songarica, R. songarica and N. roborowskii species pairs exhibited negative associations at small or moderate scales(20–60 m) in our study. The random labeling null model showed that dead standing plants of A. aphylla were largely determined by the combined effects of intra- and interspecific competition. In addition, the results also indicated that the two main factors of habitat heterogeneity and sandy desertification play important roles in determining spatial distribution patterns and associations of woody species in the desert-oasis ecotone of South Junggar basin. |
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