Xinxiao Yu

Sensitivity of Land-Use Change to Streamflow in Chaobai River Basin

AbstractWater problems and water crises are of widespread concern around the world against the background of climate change and intense human activity. As the only source of drinking water in Beijing being confronted with severe water shortages, the Chaobai River Basin is extraordinarily important. Land use, being a common and variable influence on streamflow, has been studied widely, whereas spatiotemporal differences and the complex mechanism of land use’s effect on runoff are difficult to explain. On the basis of a series of data about land use and runoff depth in the Chaobai River Basin, the sensitivity of land-use change to streamflow was explored using the principle of elasticity. Results of t-tests and F-tests showed that the difference in stream flow between the two river basins is not explained by precipitation; thus, land-use change is the main cause of stream flow variation. The influence of forests on annual runoff depth was significant and increased gradually from 1978 to 2008, whereas the ef...

Mechanism Underlying the Spatial Pattern Formation of Dominant Tree Species in a Natural Secondary Forest.

Studying the spatial pattern of plant species may provide significant insights into processes and mechanisms that maintain stand stability. To better understand the dynamics of naturally regenerated secondary forests, univariate and bivariate Ripley’s L(r) functions were employed to evaluate intra-/interspecific relationships of four dominant tree species (Populus davidiana, Betula platyphylla, Larix gmelinii and Acer mono) and to distinguish the underlying mechanism of spatial distribution. The results showed that the distribution of soil, water and nutrients was not fragmented but presented clear gradients. An overall aggregated distribution existed at most distances. No correlation was found between the spatial pattern of soil conditions and that of trees. Both positive and negative intra- and interspecific relationships were found between different DBH classes at various distances. Large trees did not show systematic inhibition of the saplings. By contrast, the inhibition intensified as the height differences increased between the compared pairs. Except for Larix, universal inhibition of saplings by upper layer trees occurred among other species, and this reflected the vertical competition for light. Therefore, we believe that competition for light rather than soil nutrients underlies the mechanism driving the formation of stand spatial pattern in the rocky mountainous areas examined.

Rainfall interception by tree crown and leaf litter: An interactive process

Key Laboratory of Soil and Water Conservation and Desertification Combating of State Forestry Administration, Beijing Forestry University, Beijing, China China National Forestry Economics and Development Research Center, State Forestry Administration, Beijing, China Department of Land, Air and Water Resources, University of California, Davis, CA 95616, USA Department of Earth and Environmental Sciences, University of Minnesota Duluth, Duluth, MN 55812, USA USDA Forest Service, Pacific Southwest Research Station, Davis, CA 95618, USA USDA Forest Service, Pacific Southwest Research Station, Albany, CA 94710, USA

Multiple isotopic models using various kinetic fractionation coefficients to estimate δ18O of leaf water

Investigation of δ18O of leaf water may improve our understanding of the evapotranspiration partitioning and material exchange between the inside and outside of leaves. In this study, δ18O of bulk leaf water (δL,b) was estimated by both isotopic–steady–state (ISS) and non–steady–state (NSS) assumptions considering the Péclet effect. Specifically, we carefully modified kinetic fractionation coefficients (αk). The results showed that the Péclet effect is required to predict δL,b. On the diel time scale, both NSS assumption + Péclet effect (NSS + P) and ISS assumption + Péclet effect (ISS + P) using modified αk (αk–modified) for δL,b showed a good agreement with observed δL,b (p > 0.05). When using previously proposed αk, however, both NSS + P and ISS + P were not reliable estimators of δL,b (p < 0.05). On a longer time scale (days), estimates of daily mean δL,b from ISS + P outperformed the estimates from NSS + P when using the same αk values. Also, the employment of αk–modified improved model performance in predicting daily mean δL,b compared to the use of previously proposed αk. Clearly, special care must be taken concerning αk when using isotopic models to estimate δL,b. Highlight For hourly and daily mean data sets, the employment of modified kinetic fractionation coefficients significantly improved model performance for δ18O of bulk leaf water.

Water stress and CO2 concentration interactions affect carbon isotope signatures of leaf and phloem organic matter

Investigation of δ13C of leaf and twig phloem water-soluble organic material (WSOM) is a promising approach for analysis of the effects of environmental factors on plant performance. In this study, orthogonal treatments of three CO2 concentrations (Ca) × five soil water contents (SWC) were conducted using Platycladus orientalis saplings to investigate the interaction of water stress and CO2 concentration on δ13C of leaf and twig phloem WSOM. Under the lowest SWC, the δ13C of leaf and twig phloem WSOM had the most positive values at any Ca and their values decreased as Ca increased. However, at improved soil water conditions, the greatest values of δ13C of leaf and twig phloem WSOM were mostly observed at C600. In addition, a more significant relationship between SWC and δ13C of twig phloem WSOM than that between SWC and δ13C of leaf WSOM demonstrated that δ13C of twig phloem WSOM is a more sensitive indicator of SWC. Twig phloem WSOM was generally 13C-depleted compared with leaf WSOM for potential post-photosynthetic fractionation, and the 13C discrimination from leaves to twig phloem was insensitive to the interaction between SWC and Ca. Clearly, interacting effects play a more important role in photosynthetic fractionation than in post-photosynthetic fractionation. Highlight The δ13C of leaf and twig phloem WSOM exhibited the most positive values at C400×35%–45% FC. Post-photosynthetic fractionation from leaf to twig was not be impacted by the interacting effects.

Comparison of the partitioning of evapotranspiration – numerical modeling with different isotopic models using various kinetic fractionation coefficients

BackgroundIn the context of a warming climate and dry conditions, aggravating water shortages, research on partitioning total evapotranspiration (ET) into soil evaporation (E) and plant transpiration (T) is needed.Methods and aimsRecently, using the oxygen isotope ratio as a tracer has proved to be a valuable way to better partition ET. In this study, we carefully considered the process of heavy water fractionation during the transpiration process, and specifically, we modified the kinetic fractionation coefficient (αk2) of transpiration, based on previous formulations used to estimate it.ResultsOur results show that, for the hourly and daily mean data set, both the isotopic–steady–state (ISS) and non–steady–state (NSS) assumptions for δ18O of leaf water (δL,b) provided a good fit with observed δL,b when using the modified αk2. In contrast, using αk2 values traditionally assigned led to significant deviations from observed δL,b (p < 0.05), potentially influencing ET partitioning results. On diurnal time scales, the percent contribution of T to total ET (FT) is sensitive to different model assumptions and different formulations to estimate αk2. The modeled FT, assuming NSS conditions and using the modified αk2 value, led to the best agreement with observed values. In contrast, on longer time scales (days), using the ISS assumption to partition ET is adequate, as the NSS assumption could introduce more complexities and uncertainties.ConclusionsOur study demonstrates that the stable isotope technique is a promising utility for quantitatively partitioning ET. To more accurately estimate FT, we also call on a better description of the nature of αk2 of transpiration.

Variations in δ13C of different plant organs: implications for post-photosynthetic fractionation

Compared to photosynthetic fractionation, the mechanism of post-photosynthetic carbon isotope fractionation is not well understood. The aim of this study was to investigate post-photosynthetic fractionation in both above and below ground tissues and to evaluate potential hypotheses explaining differences in carbon isotope composition (δ13C) among different plant organs, which can provide valuable insights into plant physiology. The results revealed that there is no significant day-night difference in δ13C of twig phloem water soluble organic materials (WSOM), which could be explained by the unrestricted exchange of triose-phosphates between the chloroplast and cytoplasm and a time lag for carbohydrate exportation. Further, we found that δ13C of twig phloem WSOM is more sensitive to plant water status than leaf WSOM. Analysis of δ13C in different plant organs showed that the greatest 13C enrichment was recorded in stem phloem. Divergences in δ13C of phloem WSOM among different plant organs were not likely to be explained by respiratory fractionation or time lag and were ascribed to transport of carbohydrates across organ boundaries and metabolic processes. Our study demonstrated that post-photosynthesis fractionation could not be ascribed to a single, unifying hypotheses; instead, it is the result of multiple processes. Highlight δ13C of twig phloem water soluble organic materials varied no clear diel pattern. In the leaf-twig-stem-root sequence, the greatest 13C enrichment was recorded in stem phloem.