Population Ecological Characteristics Of Thymus On The Loess Plateau

Thyme, a general designation of Thymus(Labiatae), is native to the Mediterranean coast, and most of the thyme grows in the temperate grassland located in the north of Qinling-Huaihe. In fragile habitats, especially in the area of serious soil degradation, thyme usually forms a natural dominant species or mono-dominant community, and plays an important ecological functions in the process of vegetation restoration and ecological succession. In the present study, we determined the distribution of thyme in the Loess Plateau as entry point. We tested whether the soil types and depths affect soil root biomass and root parameters in the Loess Plateau. We also revealed the species composition, thyme and other dominant species dynamics and succession niche status during the vegetation restoration process of typical steppe. The results are as follows:(1) The distribution of thyme was revealed by a systematic approach with synthesis analysis and field surveys. Digital maps of the thyme distribution were generated by ArcMAP(ArcGIS 10.2). The results showed that thyme mainly distributed in the north of the Qinling-Huaihe, concentrated in the Loess Plateau, Taihang Mountains, Altai and Tianshan Mountains, Altay area, Tacheng, and Urumqi regions in Sinkiang, Alxa League, Erdos, Xilin Gol League, and Hulun Buir League in Inner Mongolia, also discovered in Liaoning, Jilin and Heilongjiang provinces.(2) The thyme distribution pattern related to climatic conditions, vegetation type, habitat conditions, biotic and abiotic factors. Mainly distributed in 28°~52°N and 74°~135°E, and the distribution belonged to temperate monsoon climate and temperate continental climate. High altitude and alpine climate zone mainly distributed in northern Sinkiang. No thyme was found in tropical, sub-tropical and cold temperate zone. In terms of vegetation zone, thyme were distributed in the temperate steppe zone, temperate deciduous broad-leaved forest, cool temperate zone mixed conifer, broadleaf forests, and temperate desert zone. However, the temperate grassland is the main distribution area. In Loess Plateau, thyme mainly distributed between 35°~41°N, and 106°~112°E, the range of altitude is 900~1950 m, where located in mid temperate and warm temperate zone. Temperate typical steppe subzone and temperate sylvosteppe subzone were its main distribution area.(3) The effect of the three soil types: loessal soil(LS), aeolian sandy soil(AS) and other soil(OS) which were widely distributed in the loess plateau, and depth on thyme root biomass and morphological characteristics were analyzed. The results demonstrated that there were significant effect of soil types on thyme root biomass, root length density, root surface area, and root area index, but no significant difference was detected in specific root length of the 0~40 cm soil depth. Multiple comparisons showed that the thyme root biomass, root length density, root surface area under OS were significantly higher than LS and AS. Both specific root length and specific surface area showed a trend of LS> OS> AS, but no significant difference was found. Root area index under LS significantly lower than AS and OS.(4) The effect of the soil depth(0~10 cm, 10~20 cm, 20~30 cm, 30~40 cm) on thyme root biomass and morphological characteristics were analyzed. The results demonstrated that thyme root biomass was decreased with increasing soil depth except AS. Both 0~10 cm and 30~40 cm root biomass under OS were significantly higher than AS and LS. Root length density, root surface area, and root area index of LS and OS were decreased with the deepening of soil depth, but these parameters at 10~20 cm of AS were higher than 0~10 cm. Correlation analysis showed that there were significant negative correlations between soil moisture and root biomass, root length density, root surface area, root area index, respectively, but no significant correlations with specific root length and specific surface area.(5) Levin’s and Pianka’s indices were used to analyze the main dominant species niche of natural grassland with different enclosure years(5 years, 9 years, 14 years, 18 years and 30 years). The results showed that: during the recovery process, thyme has high status in the region. The species composition and community types differed among the grasslands of different enclosure years. Heteropappus altaicus, Stipa grandis, Potentilla bifurca and Thymus mongolicus, which appeared in five different enclosure-year grasslands, have larger niche breadth than other species. With the increase of enclosure years, the niche breadth of dominant species fluctuated. The niche breadth of Stipa grandis and Thymus mongolicus showed "up-down-up" trend along the increasing of enclosure years. The trend of Stipa bungeana appeared to be a decline in initial, followed by an increase up to 18-year enclosure. Stipa przewalskyi, which deserves further study appearing only in 30-year enclosure plots, took up a niche breadth second only to Stipa grandis. There is no linear relationship between the niche overlap and niche breadth of each species in different enclosure-year grassland. The average of niche overlap index got the largest in 9-year enclosure grassland. The grassland ecosystem gradually restored with long time enclosure, competition among populations intensified at the same time.