Vegetation patterns and species-environment relationships in the Gurbantunggut Desert of China

The Gurbantunggut Desert is the largest stable and semi-stable sand desert in China, yet few data exist on vegetation pattern and species-environment relationships for these diverse desert landscapes. The sand dunes of the survey area are mainly of the longi- tudinal form from north to south, but checkerboard-shaped and honeycomb-shaped forms are also present, with the height of 10-50 m. We measured vegetation and soil data on north-south transects and compared them with vegetation and soil data on east-west transects. Analysis revealed that the varying trend and strength of the species diversity, dominance and cover of the plant communities in the longitudinal and transverse directions across the landscape are significantly different. The results of CCA ordination show that the microhabitats of soil organic matter （OM）, soil salts （TS）, sorting index （ σ ）, topsoil water-content （SM） and pH relate to the differences in vegetation observed as differences in species assemblage from salt-intolerant ephemerals, micro-subshrubs and subshrubs to salt-tolerant shrubs and micro-arbors. The terrain （alt.） and soil texture （the contents of Mz and Ф 1） affect the spatial differentiation of many species. However, this spatial differentiation is not so marked on transects running longitudinally with the landscape, in the same direction as the dunes. The species of the desert vegetation have formed three assemblages under the action of habitat gradients, relating to three sections running transversely across the landscape, at right angles to the direction of the dune crests. In the mid-east section of the study area the topography is higher, with sand-lands or dune-slopes with coarse particles. Here the dominant vegetation comprises shrubs and subshrubs of Seriphidium santofium and Ephedra distachya, with large numbers of ephemeral and ephemeroid plants of Senecio subdentatus, and Carex physodes in spring and summer. On the soil of the dune-slopes in the mid-west of the study area, with coarse particles and abundant TK, the plant assemblage of Haloxylon persicum, Soranthus meyeri and Agriophyllum squarrosum is developed. The species composition in the east marginal belt of the study area has similar characteristics to the mid-west section. There is no corresponding section in the north-south transects （except for the north and south margins）. This is because the habitats of most plants are located in the middle of the microhabitat gradients in the north-south direction in the desert.

Concepts of keystone species and species importance in ecology

This paper discussed the keystone species concept and introduced the typical characteristics of keystone species and their identification in communities or ecosystems. Based on the research of the keystone species, the concept of species importance (SI) was first advanced in this paper. The species importance can be simply understood as the important value of species in the ecosystem, which consists of three indexes: species structural important value (SIV), functional important value (FIV) and dynamical important value (DIV). With the indexes, the evaluation was also made on species importance of arbor trees in the Three-Hardwood forests (Fraxinus mandshurica, Juglans mandshurica, and Phellodendron amurense) ecosystem.

Seed and Wasp Production in the Mutualism of Figs and Fig Wasps

Figs (Moracea: Ficus) and fig wasps (Hymenoptera: Chlocloids: Agaonideae) depend on each other to complete their reproduction. Monoecious fig species and their pollinating wasps are in conflict over the use of fig ovaries which can either produce one seed or one wasp. From observation on Ficus virens Ait., we showed that female flowers with outer layer of ovaries (near to the wall of syconium) had no significant difference from that with inner and interval layer of ovaries (near to the syconium cavity), in which most seeds and wasps were produced. This meant that fig tree provided the same potential resource for seed and wasps production. Observation indicated that there was usually only one foundress in syconium at female flower phase and no com- petition pollinators. Measurement of the style length of female flowers and the ovipositor of pollinators indicated that most ovaries could be reached by pollinator’s ovipositor. However, at the male flower phase, production of seeds was significantly more than that of wasps including non-pollinating wasps but there was no significant difference between seed and pollinating wasp production when without non-pollinating wasps produced. This result indicated that non-pollinating wasps competed ovaries not with seeds but with pollinating wasps for ovipositing. Bagged experiment showed that the sampling fig species was not self-sterile which was important for figs and wasps to survive bad season. Seed production in self-pollinated figs was not significantly different from total wasps in- cluding non-pollinating ones. This might be related with the weaker competition among wasps since bagged figs were not easy to reach by wasps from outside.

Species diversity and interspecific association in development sequence of Hippophae rhamnoides plantations in the Loess Hilly Region, China

Based on our field investigation,we analyzed the characteristics of species diversity and interspecific association at different development stages of Hippohae rhamnoides plantations.The results show that the species diversities of the shrub layer,grass layer and the whole community of the H.rhamnoides plantations all fluctuated in an“S”shape pattern.At different development stages,the species richness and diversity were higher in the grass layer than in the shrub layer.The shrub species diversity was lower on bare land,but increased gradually with development of the community.The shrub evenness index was higher in 13-year forest stand,while the grass diversity index was higher in the 3-year and 25-year plantations and lower in the 8-year plantation.The positive and negative absolute values of interspecific association between H.rhamonides and other principal species changed in a parabolic pattern.The significance level and intensity of interspecific association were lower or weaker in the13-year plantation,indicating that the species substitution rate slowed down,competition became less intense and community composition and its structure reached a relatively stable state.Therefore,to improve the ecological environment,priority should be given to the protection of the H.rhamnoides plantations younger than 13 years,while for those of 25 year plantations,moderate thinning should be made to promote their regeneration.