Foliar δ13C values measured in 101 samples from 13 sites in northern Tibet averaged amplitude −26.9‰, and were higher than those of other mountain regions in the world. C4 plants were found at 4161 m, higher than C4 species have been found before. The foliar δ13C values increased with altitude; however, the amplitude of increase was dependent on species. Furthermore, significantly higher δ13C values were found in plants grown in the southern than the northern Tanggula Mountains, a difference ascribed to precipitation.
Located at low latitude, the Tibetan Plateau has an average altitude of over 4000 m. It has been an important area for studying environment and evolution owing to its ecological fragility and climatic sensitivity (Li and Chen, 1998; Wang et al., 2003). The carbon isotope composition of leaves, shown by the value of δ13C, can reflect the physiological and ecological flexibility of the plants during their growth, and it has been recognized as a reliable way to estimate the long-term water-use efficiency of plants (Jiang, 1996). However, knowledge about the characteristics of stable carbon isotope compositions of plant leaves in Tibetan Plateau is limited.
Weather factors, such as temperature and especially precipitation and plant types vary on the two sides of the Tanggula Mountains (Yang et al., 2000, 2002). This study presents the distribution of foliar δ13C values in the northern Tibetan Plateau, the relationship between foliar δ13C values and altitude, and the difference in foliar δ13C values on the northern and southern sides of the Tanggula Mountains. Our goal is to identify the main environmental factors limiting plant distribution or growth and provide some new data for further studies on ecosystem change in a changing environment.
Materials And Methods
Thirteen sites were chosen along the Qinghai-Xizang highway for sample collection. All the samples are leaves whose growth was not affected by overshadowing. By collecting 3 to 18 samples each site, a total of 101 samples were collected. The samples were identified at the School of Life Sciences, Lanzhou University. After the samples were taken back to the laboratory in envelopes, they were dried to a constant weight at 80°C in the laboratory and finely ground to No. 100 mesh. Cellulose was extracted from dry material according to the method of Leavitt (1993). The δ13C values were determined following Qiang et al (2003), using a Finnigan MAT-252 mass spectrometer (Finnigan) supplied by the Cold Arid Region and Environment Engineering Institute, China Academy Science. Measurements were taken in triplicate and the mean was used. PDB was used as a standard and δ13C values were calculated as:
Foliar δ13c Values In Northern Tibet
The foliar δ13C values of C3 plants vary between −30.8‰ and −23.8‰, mainly between −26.0‰ and −27.0‰, with an average of −26.9‰ (Table 1, Fig. 1). This result is not only higher than that in northern China (Han et al., 2002) and eastern Tianshan Mountains (Xu et al., 2002), but is also higher than measured throughout the world (Korner et al., 1988) (Table 1). Additionally, the range of the foliar δ13C values in the northern Tibet is lower than observed elsewhere.
Forb leaf δ13C values in northern Tibetan Plateau and comparison with other regions.
It is generally accepted that the foliar δ13C of C3 plants is between −23‰ and −32‰, and that the foliar δ13C of C4 plants is between −6‰ and −19‰ (Han et al., 2002). Based on foliar δ13C values of −13.8‰ and −13.6‰, respectively, we suggest that Chenopodium album L. and Chenopodium aristatum L. found at 4161 m altitude in Xidatan are C4 plants.
Effects of Altitude on the Foliar δ13c and Difference of Foliar δ13c Values Across the Tanggula Mountains
With increasing elevation, foliar δ13C values increase (Fig. 2). However, the slope of the relationship between foliar δ13C and elevations is species specific. Saussurea nimborum shows the maximum change in the δ13C value, with a δ13C increase of 3.2‰ per km increase in altitude, and Leontopodium nanum shows the lowest sensitivity, with a slope of 0.7‰ km−1.
Foliar δ13C values differed on the northern and southern Tanggula Mountain (Fig. 3): the values in the north are higher. It can be seen from Table 2 that both the average of the foliar δ13C values and the δ13C value of certain species in the Tuotuo River valley located north of the Tanggula Mountains are generally higher than values at Amdo to the south.
Comparison of leaf δ13C values between southern and northern Tanggula Mountains.
The foliar δ13C values in northern Tibetan Plateau are relatively high and cover a relatively narrow range, probably due to the high elevation (the average elevation of Tibetan Plateau is over 4000 m). Because of the elevation, it is difficult for the airflow from the sea to arrive at the top of the plateau, and consequently the precipitation there is relatively low and a large numbers of cold and desert ecosystems, arid grassland and semiarid grassland develop there. Research shows that foliar δ13C value is a reliable index of water-use efficiency: the higher the foliar δ13C value, the higher the water-use efficiency (Marshall and Zhang, 1994). Therefore, plants with high foliar δ13C values have a stronger ability to acclimate themselves to drought. The narrow range of the foliar δ13C values in northern Tibetan Plateau is probably related to the similar environmental conditions of the sampling sites with a cold and dry climate.
It is generally accepted that C4 plants are rarely found over 3000 m in other regions of the world (Korner et al., 1988; Tieszen et al., 1979). In Ohio, U.S.A., the proportion of species and biomass of C4 plants is 26% and 88 to 85% at low altitudes, such as 1400 m; when the altitude reaches 2650 m, all species are C3. In Kenya, the transition zone between C3 and C4 plants is also rather abrupt and occurs between 2000 and 3000 m (Tieszen et al., 1979; Luo, 1985). In Tianshan Mountains, China, at approximately 2100 m, no C4 plants are found. Even in select areas at high altitudes where a few C4 grasses are found, these species disappear above 4000 m. However, when the altitude reaches 4161 m in Tibetan Plateau, C4 species can still be found. In the future, it is necessary to further investigate the distribution of plants there.
In the present study, it is significant that altitude exerts influence on the foliar δ13C values in northern Tibetan Plateau. The average slope of the seven species investigated is 1.1‰ km−1, which is very close to the result of Korner et al. (1988) of 1.2‰ km−1. The foliar δ13C values of different species have different sensitivities to the changes of elevation, which is probably related to the adaptive strategies of plants to different environments. For example, some plants may acclimate with the environment by changing their morphologic structures, while others by changing their metabolism. It is reported that the leaf thickness increased with the increasing of elevation (Smith et al., 1984). The photosynthetic speed of Korbresia humilis increases with elevation. Both low stomatal conductance and high photosynthetic rate lead to high δ13C values (Han et al., 1998).
There are many environmental factors influencing foliar δ13C values, including temperature, precipitation, light intensity and relative humidity etc. The difference between the foliar δ13C values in northern and southern Tanggula Mountain is perhaps an integrated response to these factors (Lin and Ke, 1995). From Table 2, the difference seems not to depend on the species, because Stipa purpurea and Potentilla bifurca, found on both sides of Tanggula Mountains, has higher foliar δ13C values in southern Tanggula Mountains (Amdo) than the north (Tuotuo River). Since during sampling, shaded leaves were avoided and annual radiation time reaches above 2300 h, we believe that light is not important in interpreting the difference between the foliar δ13C values in northern and southern Tanggula Mountains. Temperature, atmospheric pressure, and relative humidity were similar at the samplings sites, but precipitation and elevation are rather different (Table 2). From the present result, the effect of altitude can be discounted, for difference in foliar δ13C between Amdo and Tuotuo River shows the opposite trend expected for the altitude difference. Consequently, the difference of the foliar δ13C values between the two regions is an effect of the precipitation. Precipitation is higher on the southern, windward side of the mountains than on the northern, leeward side (Wang et al., 2003). Plants close some stoma to reduce the evaporation when precipitation is insufficient or the air humidity and soil water decrease. When the stoma are closed, which causes a decline of the CO2 density in the leaves, and the photosynthesis rate remains normal, the discrimination of 13CO2 will decline and increase foliar δ13C values (Farquhar et al., 1982).
The Tibetan Plateau is gradually warming and drying (Zheng et al., 2002). Therefore, to maintain a high water-use efficiency will be important for plants to survive in the future. The relatively high foliar δ13C values on the Tibetan Plateau are probably a result of a long-term acclimatization of the plants. Similarly, the fact that the foliar δ13C values to the north of the Tanggula Mountains are higher than those to the south probably reflects of the plant acclimatization strategies.
This research was supported by the Centurial Program of Chinese Academy of Sciences (Grant No. 2004401). Special thanks to editor Suzanne Anderson for helpful suggestions for improving of the manuscript.