Guodong Jia

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.

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.