Abstract:

Increases in the world population combined with accelerated rates of industrialization has led to increases in green-house gases in the atmosphere, particularly CO₂ concentrations, resulting in an increase in global temperatures. Water use efficiency (WUE) is a measure of a plant’s performance growing under different environmental conditions, and is an important index that reflects a plant’s ability to adapt to climate change. Carbon isotopes provide a long-term, integrative measure of WUE with high δ¹³C indicating high WUE. Stable carbon isotopes have become an important technique in ecophysiology studies in recent years for analyzing the long-term WUE of plants growing in different habits and to study the response of plant to different environmental variables, such as light intensity, precipitation, temperature, CO₂, and mineral nutrient availability. The Loess Plateau is a critical ecosystem in China but suffers from serious soil erosion problems. Water is the most important limiting environmental factor in this region and research on plant WUE responses to climate change is of particular relevance. However, only a few studies have examined foliar δ ¹³C value in response to climate change in this ecosystem. Four C₃ plants that are typical of the Loess Plateau were chosen as materials: Quercus liaotungensis, a tree, and three species of shrubs, Ostryopsis davidiana, Zizyphus jujuba var. spinosa and Sophora viciifolia. Dry leaves were collected from herbarium specimens collected over a 70 year period from the 1930’s to 2002. A total of 25 samples from about 160 plant specimens were analyzed that were collected from the hilly and gully region of the Plateau ranging from the northern Tongchuan to the southern Yanan. The carbon isotope composition (δ¹³C) of leaves was analyzed using a MAT-251 mass-spectra photometer. The results showed that the variation in the δ ¹³C values differed among the species tested: :Ostryopsis davidiana (-25.05‰ to -29.75‰) > Quercus liaotungensis (-25.51‰ to -29.20‰) > Sophora viciifolia (-25.12‰ to -28.80‰) > Zizyphus jujuba var. spinosa (-26.69‰ to -28.69‰). The mean δ13C value of four plants was -27.04‰ ranging from -25.05‰ to -29.75‰. Comparing the mean δ¹³C value among four species in the age of 30, 50, 70, 80 and the year 2002 indicated that the following order of Sophora viciifolia (-26.54‰) > Ostryopsis davidiana (-26.99‰) > Quercus liaotungensis (-27.14‰) > Zizyphus jujuba var. spinosa (-27.49‰) is obtained. A decrease in the foliar δ¹³C value with time was found in all four species, indicating that the WUE of the four species all declined over time. However, the decrease in foliar δ ¹³C values varied among the four species with significant decreases measured in two species, Sophora viciifolia and Quercus liaotungensis, a strong but not significant decrease in the leaves of Ostryopsis davidiana, and only a slight decrease in Zizyphus jujuba var. spinosa. The decrease in the δ ¹³C values in the four species were: 14.65‰, 14.46‰, 11.99‰ and 2.44‰, respectively. It was shown that different species have different sensitivities to climate change, and Zizyphus jujuba var. spinosa was the most drought-tolerant of the four species with a high WUE.

Key words: Carbon isotope composition, C₃ plant, Loess Plateau, Climate changes