Slope aspect has effects on productivity and species composition of hilly grassland in the Xilin River Basin, Inner Mongolia, China

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Abstract

In order to investigate the productivity of hilly grassland of the Xilin River Basin as affected by slope aspect (North versus South), aboveground green biomass (AGB) and species composition were studied at four hill sites in 2005. Additionally, a detailed investigation combining vegetation parameters, physical and chemical soil characteristics was carried out at different slope positions (base, middle, and top) of north and south slope aspect on one of the hills in 2004 and 2005. Northern slopes exhibited a higher productivity and species diversity compared to southern slopes. Plant available soil K, P and N were not limiting the productivity at south- relative to north-oriented slopes, whereas soil available water was the primarily limiting factor for plant productivity, which was supported by results of leaf carbon isotope discrimination of main species. A negative correlation between productivity and relative abundance of C4 plants indicates that the relative biomass of C4 plants can be used as a water-stress indicator.

Introduction

Xilingole grassland (∼19 million ha) is a typical semi-arid temperate steppe ecosystem in northern China and of great ecological and economic importance. During the last decades, the fragmentation and degradation of this environment has rapidly advanced and the total area of degraded steppe within the Xilin River Basin has increased up to 72% from 1985 to 1999 (Tong et al., 2002).

The intensity of degradation differs among landform types, which are subject to different land use practices (Tong et al., 2002, Tong et al., 2004). Hilly grassland is an important landform, which constitutes approximately 40% of total Xilin River Basin (Chen, 1988). Compared to flat regions, grazing intensity was relatively low in the past, but due to the rising number of livestock an accelerated degradation of hilly grassland has occurred during the last two decades and more attention should be paid to the effects of land use on productivity and stability of hilly grassland ecosystems. Slope aspect, as the main topographic factor of hilly grassland, may have pronounced effects on species richness and productivity, as well as on nutrient dynamics.

Partially due to complementary resource use (Hooper and Vitousek, 1997), the functional group composition and species richness of natural grassland are considered as prominent factors of productivity and stability (Hooper, 1998; Tilman, 1996; Tilman et al., 1994). For instance, Tilman (2000) summarized the results of field experiments in North America and at eight European sites, showing that each halving of the number of plant species within a plot leads to a 10–20% loss of productivity. This postulated relationship between species richness and grassland productivity is supported by results in Mediterranean areas, in which different productivity and diversity were illustrated on equatorial- and polar-facing slopes. These slope differences in species composition and productivity were explained by resource availability, especially water (Armesto and Martínez, 1978; Badano et al., 2005; Martínez and Armesto, 1983; Sternberg and Shoshany, 2001). By monitoring long-term vegetation change, Bennie et al. (2006) showed that, due to the edaphic factors, south-facing slopes maintained more stress-tolerant and light-demanding flora in British chalk grasslands.

Next to the difference of plant species composition and productivity, long-term operating soil weathering and erosion processes are usually accelerated on south-facing slopes (Rech et al., 2001) resulting in different soil properties of north- and south-directed slopes (Bochet and García-Fayos, 2004; Gupta and Chera, 1996). It has been reported that north-facing slopes developed a thicker humic soil layer than south-facing slopes in hilly grassland of Inner Mongolia (Wang and Cai, 1988); however, no information is available with regard to the significance of these differences in soil layer depth for the availability of nutrients and water during the vegetation period and species composition and productivity.

In order to assess the relevance of the above-mentioned parameters on productivity of hilly steppe grassland in this semi-arid environment, we surveyed vegetation parameters and soil chemical and physical characteristics of northern and southern slopes of four hills under local management practices in 2005. On one of the hills, more detailed transect sampling was done at base, middle and top positions on both northern and southern slopes in 2004 and 2005. Our objectives were to identify (i) differences in productivity, diversity, and functional group composition between slope aspects and (ii) the limiting factors (nutrients or water) of plant growth in hilly grassland.

Section snippets

Study area

The Xilin River Basin (43°26′– 44°29′N, 115°32′– 117°12′E) has a semi-arid, continental climate, an annual mean temperature ranging from 0.5 °C in the Southeast to 2.1 °C in the Northwest and a total annual precipitation ranging from 500 mm (Southeast) to 250 mm (Northwest) (Chen, 1988). More than 70% of the annual precipitation occurs from May to August with a clear maximum in July. According to the meteorological data (from 1988 to 2003) monitored by the Inner Mongolia Grassland Ecosystem

Aboveground biomass

Based on a comparison of AGB at peak biomass time in 2005 at four hill sites, the productivity was affected by slope aspect (Table 2). North-oriented slopes had a significantly higher AGB than south-oriented slopes on all hills. The most pronounced difference between slope aspects was found on H1. This difference in productivity was confirmed by a more detailed sampling of aboveground biomass at H1 in 2004 and 2005 (Fig. 1). In both years, AGB at the base and middle sampling positions of the

Discussion

In general, the results indicate a strong effect of slope aspect on grassland productivity and species composition as well as on seasonal soil water and temperature. As indicated by the differences in AGB (Table 2, Fig. 1), slope aspect had a strong effect on plant productivity. Contrary to environments where primarily the solar radiation and heat balance determines plant productivity (e.g., in mountain grassland of the northern hemisphere), in this hilly grassland system under semi-arid

Acknowledgements

We would like to thank the National Nature Science Foundation of China (NSFC 40471077) and the Deutsche Forschungsgemeinschaft (DFG, project Sa-359/30-1) for their financial support and the Inner Mongolia Grassland Ecosystem Research Station of Botany Institute, Chinese Academy of Science for providing working facility.

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