Growth and reproduction and their relationships with soil moisture in artificially established Artemisia sphaerocephala populations of different densities in the Alxa Desert

At the southeastern edge of Tengger Desert in Inner Mongolia of northern China, grassland degradation usually occurs primarily due to improper management regimes in fragile environmental conditions. Over the last several decades, most sandy grasslands in this region have turned into shifting, semi-shifting and semi-fixed sandy lands, corresponding to severe, moderate and light degradation. To curb degradation and alleviate the impacts of degradation on environment and human life in this region, the central and local governments have implemented a variety of measures to restore vegetation on degraded grasslands since the mid 1990s. One of these measures is revegetation by planting indigenous shrubs and semi-shrubs (e.g. Artemisia sphaerocephala) through air seedling on severely degraded areas. As a result, artificial communities of A. sphaerocephala have been established in many parts of the Tengger desert. Little is known, however, about the effects of different densities of artificially established A. sphaerocephala communities on patterns of growth and reproduction of the plants in relation to soil water content. In this paper, we examined the patterns of growth and reproduction of artificially established A. sphaerocephala populations with different densities as well as the relationship between standing biomass per plant and soil water content at different depths over the growing season in 2001. Several important conclusions can be drawn from this study. (1) There were striking differences in the patterns of growth and reproduction of A. sphaerocephala plants among populations of different densities, and above-ground biomass, root biomass, number of flowering shoots and seed yield per plant tended to decrease significantly with increasing density of A. sphaerocephala populations. Among the 5 density treatments, greatest values of the above performance measures were found in population with the lowest density of 1.1 plants/m~(2) and lowest values of these measures occurred in population with the highest density of 5.3 plants/m~(2).(2)There was a very low mortality rate of the plants in both the density treatments of 1.1 and 2.1 plants/m~(2) compared with the other treatments. When density of the A. sphaerocephala populations was more than 3.1 plants/m~(2), however, growth and reproduction of the plants were significantly limited and their mortality rate increased greatly.(3)There were also large differences in soil water contents at different depths among density treatments. This is particularly the case for soil water contents at the depths of 0～20和20～40cm decreasing significantly with increasing density of the population. The optimal density of A. sphaerocephala populations was found to be approximately 2 plants/m~(2), because of greatest growth and reproduction of A. sphaerocephala plants in this density treatment. (4) Regression analyses indicated that standing biomass per plant had a strong positive relationship with soil water content in either the 0～20cm layer or the 20～40cm layer, but the relationship between standing biomass per plant and soil water content at depths below 40cm tended to be much weaker. This suggests that growth and reproduction of A. sphaerocephala plants were strongly related to soil water conditions within the 40cm layer.