Jiamao Han

Climate as the dominant control on C3 and C4 plant abundance in the Loess Plateau: Organic carbon isotope evidence from the last glacial-interglacial loess-soil sequences

Abundance of C3 and C4 photosynthesis plants can be inferred relatively from stable carbon isotopic composition of organic matter in soils. The samples from five sequences of the last glacialinterglacial loesssoil in the Chinese Loess Plateau have been measured for organic carbon isotopic ratios (δ13 Corg). The organic carbon isotope data show that relative abundance (or biomass) of C4 plants was increased ca. 40% for each sampling site from the last glacial maximum (LGM) to Holocene optimum, and increased southeastward on the Loess Plateau during both periods of LGM and Holocene. Statistic analyses on the steady maximum δ13 Corg values of Holocene soils and modern climatic data from the Loess Plateau and Inner Mongolia indicate that the C4 plant abundance increases with increasing temperature and decreasing precipitation. The C4 plant abundance is related much closer with mean April temperature and precipitation than annual. These results lead us to deduce following conclusions. First, temperature is the major factor for control on variations in C4 plant abundance in the Loess Plateau from the last glacial to interglacial. In the absence of favorable temperature condition, both of low moisture and low atmospheric CO2 concentration are insufficient to drive an expansion of the C4 plants in the plateau. Second, δ13 Corg in the loess-paleosol sequences, as a proxy of the relative abundance of C4 plants in the Loess Plateau, could not be used as an indicator of changes in the summer monsoon intensity unless the temperature had changed without great amplitude. Since all C4 plants are grasses, finally, the increase of the C4 plants supports that forest has not been dominant in the ecosystem on the Loess Plateau during Holocene although precipitation and atmospheric CO2 were largely increased relative to those during LGM.

Discovery of C4 species at high altitude in Qinghai-Tibetan Plateau

Plant specimens are collected from the areas between latitude 27 ° 42′N and 40 ° 57′N, and longitude 88 ° 93′E and 103 ° 24′E, with an altitudinal range from 2210 to 5050 m above the sea level in Qinghai-Tibetan Plateau. The stable carbon isotope analysis indicates that two of Chenopodiaceae and six of Poaceae in the samples are C4 plants. Four of the C4 plants are found in 11 spots with altitudes above 3800 m, and Pennisetum centrasiaticum, Arundinella yunnanensis and Orinus thoroldii are present in six spots above 4000 m, even up to 4520 m. At low CO2 partial pressure, that sufficient energy of high light improving C4 plant’s tolerance of low temperature and precipitations concentrating in growing season probably are favorable for C4 plants growing at high altitude in Qinghai-Tibetan Plateau.

Altitudinal trends of leaf δ13C follow different patterns across a mountainous terrain in north China characterized by a temperate semi‐humid climate

Many studies have documented that the delta(13)C values of plants increase with altitude both on a global scale and locally in humid climates, while in semi-arid areas the opposite trend has been found. The study reported herein was conducted in a mountainous area of China characterized by a temperate semi-humid climate. The delta(13)C values of C(3) species do not exhibit a consistent variation along an altitudinal gradient and the observations suggest that the pattern of increasing delta(13)C with altitude cannot be generalized. In the study area, in addition to environmental factors such as changing air pressure and light, the interaction between temperature and plant water balance determines the delta(13)C-altitude variations in C(3) plants. The delta(13)C of the leaves of C(4) plants is found to increase with altitude with a mean gradient of 0.9 per thousand/km. The altitudinal trend of C(4) plants is attributed to the combined influences of water availability and other factors rather than temperature.

Effect of burning C3 and C4 plants on the magnetic susceptibility signal in soils

To understand the origin of the ultrafine pedogenic components responsible for the magnetic susceptibility (MS) enhancement remains a major challenging problem, in linking the magnetic signal with paleoclimatic conditions. Here we examine the influence of natural fires on the MS signal of both plants and modern soils and in particular the MS difference between C3 and C4 plant ashes and their influence possibly on soil magnetic susceptibility. We demonstrate that burning of C3 and C4 plants can enhance MS signal of modern soils. We show that C4 plants have greater potential to enhance the MS signal. The average MS value of C4 plant ashes is (532±61)×10−8m³ kg−1, much higher than the average MS value of (120±65)×10−8m³ kg−1 of C3 plant ashes. The Fe2O3 concentration in C4 plant is two to four times higher than that in C3 plant. One burning of the grassland, mainly consisting of C4 plants, can enhance MS value of the surface soil up to about 30–40%.

Experimental measurements of leaf carbon isotope discrimination and gas exchange in the progenies of Plantago depressa and Setaria viridis collected from a wide altitudinal range

Significant correlations between leaf carbon isotope discrimination (Delta) and altitude and between gas exchange and altitude have been reported in previous studies, raising the question of whether the altitudinal variations in discrimination and gas exchange can be attributed to genetic differences among populations from different altitudes. Studies that focus on in situ analysis cannot distinguish the effects of genetic variation from environmental variation. This article describes an experiment in which seeds of Plantago depressa (C3 species) and Setaria viridis (C4 species) collected from a wide altitudinal range were grown in the same environment. Carbon isotopic ratios (delta(13)C) and gas exchange of the seedlings were measured. The progenies of P. depressa and S. viridis no longer display any significant Delta decreases with the altitude of origin as seen in situ. Furthermore, photosynthetic rate, stomatal conductance, the ratio of intercellular to ambient CO(2) and intrinsic water use efficiency for P. depressa and S. viridis grown in the greenhouse are also not significantly related to the altitude of origin. The observations suggest that altitudinal variations in Delta and gas exchange are not because of genotypic differences, independent of photosynthetic type.