The effects of short-term rainfall variability on leaf isotopic traits of desert plants in sand-binding ecosystems
Introduction
Desertification is one of the major environmental problems worldwide (Reynolds et al., 2007) and it occurs in almost every habitable region such as North America (e.g., Schlesinger et al., 1990, Li et al., 2008, Ravi et al., 2009), Africa (e.g., Thomas et al., 2008, Wang et al., 2010a) and northern China (e.g., Chen et al., 1996, Mitchell et al., 1998, Liu et al., 2012). The deserts in northern China are expanding at an estimated rate of 2100 km2 year−1 (Chen et al., 1996, Mitchell et al., 1998).
Revegetation of native shrubs, a massive ecological engineering project (Mitsch et al., 1993), has been one of the most effective methods to reduce desertification. Large areas of China's desert regions have been reclaimed through planting of native plants since the 1950s (Shapotou Desert Research and Experiment Station, 1986). For example, the 1000-km long Baotou-Lanzhou railway that passes through Shapotou region has been protected by sand-binding shrubs that were planted in 1956, 1964, 1981 and 1987, respectively (Shapotou Desert Research and Experiment Station, 1986). The success of these efforts suggests that it is an effective approach to control desertification and to restore the ecological environment along transportation corridors in the desert regions of China (Xiao et al., 2003a, Li et al., 2006). Many revegetated species, such as Caragana intermedia, Calligonum arborescens and Tamarix ramosissima, disappeared over the years, whereas some native species such as Artemisia ordosica, Hedysarum scoparium and Caragana korshinskii survived during succession and have become the dominant shrubs (Ma et al., 2002). As a result of long-term succession, the planted vegetation ecosystem is expected to develop into a stable system (Xiao et al., 2003b). The effects of long-term succession and short-term climate variability (e.g., rainfall) on the function of these successful sand-binding species (e.g., A. ordosica, H. scoparium, and C. korshinskii) are not yet fully understood, which hinders our ability to predict future successional pathways of such planted ecosystems under the circumstances of potential warming and drought in this region (Solomon et al., 2007).
In this study, we measured dual isotope (13C and 15N) compositions and nitrogen concentrations ([N]) of three dominant species, i.e., A. ordosica, H. scoparium, and C. korshinskii, and analyzed their interactions to investigate the effects of long-term succession (50 years) and short-term drought (2 years) on plant uptake of water and nitrogen at a desert stabilized by sand-binding plants in northern China. Besides providing valuable isotopic information of these sand-binding species, this study will have both theoretical and applied importance in the restoration of degraded ecosystems and in improving our ability to predict the response of planted sand-binding vegetation to climate change.
Stable isotopes are used in the investigation because they provide integrated information on plant water and nutrient use and serve as powerful tracers in ecosystem studies. The stable carbon isotope ratio (δ13C) of plant tissues reflects the relations between plant carbon and soil water. The effect of water availability on carbon isotope discrimination during photosynthesis of C3 plant is relatively well understood (e.g., Farquhar et al., 1982). For example, photosynthesis depends on water availability, and low water availability can decrease the assimilation of carbon, thereby reducing plant productivity and the corresponding nitrogen requirements or assimilation rates, leading to different patterns of δ13C and δ15N variation (Yoneyama et al., 2001). Foliar δ13C typically increases when water availability is low, as a result of stomatal closure and reduced transpiration (Peuke et al., 2006). And foliar δ13C is higher in the water-limited treatment relative to the well-watered treatment (Grant et al., 2012). Analyses of δ13C have greatly increased our understanding of the relationships between water and carbon use by desert plants (Ehleringer, 1993). Nitrogen isotope ratio (δ15N) provides information on ecosystem nitrogen cycling (Högberg et al., 1995, Roggy et al., 1999, Ometto et al., 2006, Bai et al., 2009) and plant nitrogen isotope discrimination is related to the availability of nutrients and water (Högberg, 1997, Swap et al., 2004, Aranibar et al., 2008, Wang et al., 2010b). Previous studies showed that succession affected N cycling and it was reflected in foliar δ15N (Davidson et al., 2007, Wang et al., 2007), and foliar δ15N signatures decreased as successional age increased at both the plant community and species levels (Wang et al., 2007).
Plant δ13C and δ15N signatures are often regulated by two or more factors (BassiriRad et al., 2003, Murphy and Bowman, 2009). For example, Schulze et al. (1991) point out that the significant correlation between δ15N and δ13C values of nitrogen-fixing African trees is caused by decreased water use efficiency, compared with that of the non-nitrogen-fixers. The decreased water use efficiency in nitrogen fixer usually is a result of the extra cost of carbohydrate supply to the nitrogen-fixing diazotroph (Farquhar and Richards, 1984).
Previous research also shows that photosynthetic capacity is strongly correlated with C3 plant foliar [N] because photosynthetic enzymes, such as RuBP carboxylase, contain large amounts of nitrogen (Reich et al., 1998, Tognetti and Penuelas, 2003). It is also found that higher photosynthetic activity leads to enriched foliar δ13C values (O’Leary, 1981). Because of these two reasons, positive correlations between foliar δ13C and [N] have been found in many cases (e.g., Sparks and Ehleringer, 1997, Wang et al., 2008). Such relationships together with the positive correlation between foliar δ15N and [N] in non-nitrogen-fixing plants (Hobbie et al., 2000, Wang et al., 2007) suggest that foliar δ13C, δ15N, and [N] levels are closely linked.
This study is to investigate the interactions of plant nitrogen and water use in planted sand-binding vegetation after long-term succession and to test whether there are functional group (potential nitrogen-fixer and non nitrogen-fixer) dependent adaptation strategies to water and nitrogen variations in the harsh environments. Specifically, the objectives of this study are to investigate (1) the long-term effect (up to 50 years) of successional age on the isotopic signatures (δ13C and δ15N) and [N] of three dominant shrubs in planted sand-binding vegetation communities; (2) the effects of seasonality and short-term drought on variations in isotopic signatures and [N] of each species; and (3) the relationships between δ13C, δ15N, and [N] of the three shrub species. The results will provide important implications on how plant species adapt to harsh environments in terms of water use efficiency and nutrient use during succession and how plant species respond to environmental changes.
Section snippets
Study area
Shapotou region is located at the southeastern border of the Tengger Desert in China (Fig. 1). The region receives high solar radiation and is low in relative humidity. Average annual precipitation is 180.2 mm, with 80% of the rainfall occurring from May to September. The mean annual temperature is 10.0 °C, with a mean January temperature of −6.9 °C and a mean July temperature of 24.3 °C. The water table is more than 80 m below ground, thus rainfall is usually the only source of water for plants.
The
Foliar δ15N, δ13C, and [N] of each species
Foliar δ15N values of A. ordosica, H. scoparium, and C. korshinskii differed significantly (p < 0.05), with the most negative value in A. ordosica (−3.1 ± 0.1‰ (mean ± SE)) and the most positive value in C. korshinskii (−1.5 ± 0.1‰) (Table 3). Foliar δ13C values of A. ordosica (−24.4 ± 0.1‰) and H. scoparium (−24.5 ± 0.1‰) did not show significant difference, but both values were significantly more negative than that of C. korshinskii (−23.2 ± 0.1‰, p < 0.05) (Table 3). Foliar [N] of C. korshinskii (16.1 ± 0.98 g
Variations in three plant species
Foliar δ13C is an indicator of long-term plant water use efficiency (Farquhar et al., 1989). Our results showed that the δ13C values (ranging from −21.1 to −26.6‰) of the dominant shrubs in the Tengger Desert (a cold desert) were more positive than the values reported for other desert C3 plants, which ranged from −21.0 to −28.0‰ in the northern Namaqualand Desert of South Africa (Rundel et al., 1999), from −22.0 to −29.0‰ in the Arizona deserts of the United States (Ehleringer and Cooper, 1988)
Summary
To better understand the long-term succession and short-term climate variability effects on the functions of three major sand-binding species (A. ordosica, H. scoparium and C. korshinskii), we examined the effects of succession, rainfall variability, and seasonality on foliar δ13C, δ15N and [N]. The study has both theoretical and applied importance in the restoration of degraded lands and in improving our ability to predict the response of planted sand-binding vegetation to climate change. The
Acknowledgments
This research was supported by the National Natural Science Foundation of China (91125025, 91025016), the National Science & Technology Pillar Program during the Twelfth Five-Year Plan Period (2011BAC07B05), and the Ministry of Forestry Welfare Special Project (201004010-05).
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