Yonghong Su

Seasonal fluctuations and temperature dependence in photosynthetic parameters and stomatal conductance at the leaf scale of Populus euphratica Oliv.

A combined model to simulate CO₂ and H₂O gas exchange at the leaf scale was parameterized using data obtained from in situ leaf-scale observations of diurnal and seasonal changes in CO₂ and H₂O gas exchange. The Farquhar et al.-type model of photosynthesis was parameterized by using the Bayesian approach and the Ball et al.-type stomatal conductance model was optimized using the linear least-squares procedure. The results show that the seasonal physiological changes in photosynthetic parameters (e.g., V(cmax25), J(max25), R(d25) and g(m25)) in the biochemical model of photosynthesis and m in the stomatal conductance model should be counted in estimating long-term CO₂ and H₂O gas exchange. Overall, the coupled model successfully reproduced the observed response in net assimilation and transpiration rates.

Estimation of parameters of a biochemically based model of photosynthesis using a genetic algorithm

Photosynthesis response to carbon dioxide concentration can provide data on a number of important parameters related to leaf physiology. The genetic algorithm (GA), which is a robust stochastic evolutionary computational algorithm inspired by both natural selection and natural genetics, is proposed to simultaneously estimate the parameters [including maximum carboxylation rate allowed by ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylation rate (V(cmax)), potential light-saturated electron transport rate (J(max)), triose-phosphate utilization (TPU), leaf dark respiration in the light (R(d)) and mesophyll conductance (g(m))] of the photosynthesis models presented by Farquhar, von Caemmerer and Berry, and Ethier and Livingston. The results show that by properly constraining the parameter bounds the GA-based estimate methods can effectively and efficiently obtain globally (or, at least near globally) optimal solutions, which are as good as or better than those obtained by non-linear curve fitting methods used in previous studies. More complicated problems such as taking the g(m) variation response to CO(2) into account can be easily formulated and solved by using GA. The influence of the crossover probability (P(c)), mutation probability (P(m)), population size and generation on the performance of GA was also investigated.

Identification and Evolution of Groundwater Chemistry in the Ejin Sub-Basin of the Heihe River, Northwest China

Hydro-chemical characteristics of groundwater and their changes as affected by human activities were studied in the Ejin Sub-Basin of the Inner Mongolia Autonomous Region, China, to understand the groundwater evolution, to identify the predominant geochemical processes taking place along the horizontal groundwater flow path, and to characterize anthropogenic factors affecting the groundwater environment based on previous data. The concentrations of major ions and total dissolved solids (TDS) in the groundwater showed a great variation, with 62.5% of the samples being brackish (TDS≥1000 mg L^(-1)). The groundwater system showed a gradual hydro-chemical zonation composed of Na(superscript +)-HCO(superscript – subscript 3), Na(superscript +)-Mg(superscript 2+)-SO(superscript 2 subscript 4)-Cl(superscript -), and Na(superscript +)-Cl(superscript -). The relationships among the dissolved species allowed identification of the origin of solutes and the processes that generated the observed water compositions. The dissolution of halite, dolomite, and gypsum explained, in part, the presence of Na(superscript +), K(superscript +), Cl(superscript -), SO(superscript 2- subscript 4), and Ca(superscript 2+), but other processes, such as mixing, Na(superscript +) exchange for Ca(superscript 2+) and Mg(superscript 2+), and calcite precipitation also contributed to the composition of water. Human activity, in particular large-scale water resources development associated with dramatic population growth in the last 50 years, has led to tremendous changes in the groundwater regime, which reflected in surface water runoff change, decline of groundwater table and degeneration of surface water and groundwater quality. Solving these largely anthropogenic problems requires concerted, massive and long-term efforts.

Non-growing season soil CO2 efflux and its changes in an alpine meadow ecosystem of the Qilian Mountains, Northwest China

Most soil respiration measurements are conducted during the growing season. In tundra and boreal forest ecosystems, cumulative, non-growing season soil CO2 fluxes are reported to be a significant component of these systems’ annual carbon budgets. However, little information exists on soil CO2 efflux during the non-growing season from alpine ecosystems. Therefore, comparing measurements of soil respiration taken annually versus during the growing season will improve the accuracy of estimating ecosystem carbon budgets, as well as predicting the response of soil CO2 efflux to climate changes. In this study, we measured soil CO2 efflux and its spatial and temporal changes for different altitudes during the non-growing season in an alpine meadow located in the Qilian Mountains, Northwest China. Field experiments on the soil CO2 efflux of alpine meadow from the Qilian Mountains were conducted along an elevation gradient from October 2010 to April 2011. We measured the soil CO2 efflux, and analyzed the effects of soil water content and soil temperature on this measure. The results show that soil CO2 efflux gradually decreased along the elevation gradient during the non-growing season. The daily variation of soil CO2 efflux appeared as a single-peak curve. The soil CO2 efflux was low at night, with the lowest value occurring between 02:00–06:00. Then, values started to rise rapidly between 07:00–08:30, and then descend again between 16:00–18:30. The peak soil CO2 efflux appeared from 11:00 to 16:00. The soil CO2 efflux values gradually decreased from October to February of the next year and started to increase in March. Non-growing season Q10 (the multiplier to the respiration rate for a 10°C increase in temperature) was increased with raising altitude and average Q10 of the Qilian Mountains was generally higher than the average growing season Q10 of the Heihe River Basin. Seasonally, non-growing season soil CO2 efflux was relatively high in October and early spring and low in the winter. The soil CO2 efflux was positively correlated with soil temperature and soil water content. Our results indicate that in alpine ecosystems, soil CO2 efflux continues throughout the non-growing season, and soil respiration is an important component of annual soil CO2 efflux.

The influences of canopy species and topographic variables on understory species diversity and composition in coniferous forests.

Understanding the factors that influence the distribution of understory vegetation is important for biological conservation and forest management. We compared understory species composition by multi-response permutation procedure and indicator species analysis between plots dominated by Qinghai spruce (Picea crassifolia Kom.) and Qilian juniper (Sabina przewalskii Kom.) in coniferous forests of the Qilian Mountains, northwestern China. Understory species composition differed markedly between the forest types. Many heliophilous species were significantly associated with juniper forest, while only one species was indicative of spruce forest. Using constrained ordination and the variation partitioning model, we quantitatively assessed the relative effects of two sets of explanatory variables on understory species composition. The results showed that topographic variables had higher explanatory power than did site conditions for understory plant distributions. However, a large amount of the variation in understory species composition remained unexplained. Forward selection revealed that understory species distributions were primarily affected by elevation and aspect. Juniper forest had higher species richness and α-diversity and lower β-diversity in the herb layer of the understory plant community than spruce forest, suggesting that the former may be more important in maintaining understory biodiversity and community stability in alpine coniferous forest ecosystems.

Relationships between foliar carbon isotope discrimination with potassium concentration and ash content of the riparian plants in the extreme arid region of China

Carbon isotope discrimination (Δ) has been proposed as an indirect estimation criterion for water use efficiency in C3 plants. Because of the higher cost for Δ analysis, ash content or K concentration has been proposed as an alternative criterion for Δ in many species. In five typical habitats of the extreme arid Ejina desert oasis in northwest of China, the seasonal variations of foliar δ, ash content, and potassium (K) concentration were researched in two constructive desert riparian plants (Populus euphratica Olivier, Tamarix ramosissima Ledeb). The correlations of foliar Δ with ash content and K concentration in both species were also examined to evaluate the feasibility of the foliar ash content and K concentration as surrogates of Δ in P. euphratica and T. ramosissima. Results showed that there were significant effects of plant species, habitats and growth season on foliar Δ, ash content, and K concentration. Foliar Δ and K concentration in P. euphratica were significantly higher than those in T. ramosissima, whereas, the ash content was reverse. Among habitats, the trends of δ signatures in both P. euphratica and T. ramosissima were similar, δ values and ash content in both species were the lowest in the dune. Both in the Gobi and dune sites, K concentration in P. euphratica and T. ramosissima was different. In the whole growth period, foliar Δ values and ash content in both species were gradually increased, but K concentration was decreased. Ash content was significantly and positively related to δ in both P. euphratica and T. ramosissima. However, significantly negative correlations between foliar δ and K concentration as well as between ash content and K in P. euphratica were found. In T. ramosissima, the relationship was positive but very weak.

Microclimatic characteristics of the Heihe oasis in the hyperarid zone of China

The microclimate of a desert oasis in hyperarid zone of China was monitored using micrometeorological methods and compared with those of areas adjacent to forested land. Differences in ground-level photosynthetically active radiation (PAR) on clear, cloudy and dust storm days and their subtending causes are analysed and discussed. Desert oases serve the ecological functions of altering solar radiation, adjusting near-ground and land surface temperatures, reducing soil temperature differences, lowering wind velocity, and increasing soil and atmospheric humidity. The total solar radiation in the interior of the oasis was roughly half of that outside a forest canopy. During the growing season, air temperatures in Populus euphratica Oliv. (poplar) and Tamarix ramosissima Ledeb. (tamarisk) forests were 1.62 °C and 0.83 °C lower respectively than those in the areas around the forests. Furthermore, the taller the forest cover, the greater the temperature drops; air temperatures in the upper storey were greater than those in the lower storey, i.e., air temperature rose with increasing height. Over the growing season, the relative humidities of the air in the poplar and tamarisk forests were 8.5% and 4.2% higher respectively than those in areas around the forests. Mean wind velocity in poplar-forested lands was 0.33 m s−1, 2.31 m s−1 lower than that in the surrounding area. During dust storm days the PAR was significantly lower than that on cloudy or clear days, when it was high and varied in an irregular manner.

Shrub communities and environmental variables responsible for species distribution patterns in an alpine zone of the Qilian Mountains, northwest China

Understanding the factors that drive variation in species distribution is a central theme of ecological research. Although several studies focused on alpine vegetation, few efforts have been made to identify the environmental factors that are responsible for the variations in species composition and richness of alpine shrublands using numerical methods. In the present study, we investigated vegetation and associated environmental variables from 45 sample plots in the middle Qilian Mountains of the northwestern China to classify different community types and to elucidate the species-environment relationships. We also estimated the relative contributions of topography and site conditions to spatial distribution patterns of the shrub communities using the variation partitioning. The results showed that four shrub community types were identified and striking differences in floristic composition were found among them. Species composition greatly depended on elevation, slope, shrub cover, soil pH and organic carbon. The important determinants of species richness were soil bulk density and slope. No significant differences in species richness were detected among the community types. Topography and site conditions had almost equal effects on compositional variation. Nonetheless, a large amount of the variation in species composition remained unexplained.

A hierarchical Bayesian approach for multi-site optimization of a satellite-based evapotranspiration model

Key Laboratory of Eco‐hydrology of Inland River Basin (CAS), Northwest Institute of Eco‐ Environment and Resources, CAS, Lanzhou, China Laboratory of Western Chinas Environmental Systems (Ministry of Education), Lanzhou University, Lanzhou, China School of Environmental Studies, China University of Geosciences, Wuhan, China Correspondence Gaofeng Zhu, Laboratory of Western Chinas Environmental Systems (Ministry of Education), Lanzhou University, Lanzhou 730000, China. Email: zhugf@lzu.edu.cn Funding information National Key R & D Program of China, Grant/ Award Number: 2016YFC0501002; National Natural Science Foundation of China, Grant/ Award Numbers: 41571016 and 41871078

Forest Evapotranspiration and Energy Flux Partitioning Based on Eddy Covariance Methods in an Arid Desert Region of Northwest China

In this study, the characteristics of energy flux partitioning and evapotranspiration of P. euphratica forests were examined in the extreme arid region of Northwest China. Energy balance closure of the ecosystem was approximately 72% (