Species-specific traits determine shrub-annual interactions during a growing season

Stress gradient hypothesis predicted that facilitative interactions usually increase in intensity and are importance with abiotic stress.By contrast,facilitation may be lost in time,when it involves the growth of benefactors or beneficiaries.Less is known about which response pattern is more common in arid desert.We present an empirical study to explore shrub-annual interactions at the community and individual level along the course of a single growing season in a desert steppe in northwest China.Here the severity of drought stress may increase in time due to uneven precipitation during plant growing season.We assessed growth responses of annuals in understory where two dominant shrubs were removed.Annuals responses showed a switch from weakly positive to more strongly positive beneath Calligonum mongolicum,whereas from positive to negative beneath Nitraria sphaerocarpa during the growing season.Additionally,annual species with contrasting functional traits showed distinct growth responses to canopies removal.There was evidence of an increase in soil moisture below the canopy of shrubs,but a decrease in potential evaporation rate and photosynthetically active radiation,which can partly explain these species-specific responses.We conclude that the balance between competitive and facilitative effects in shrub-annual interactions is not only governed by the severity of stress but also determined by plant traits,such as canopy structure of shrubs and functional traits of their understory annuals.

A size-gradient hypothesis for alpine treeline ecotones

Research on the stress gradient hypothesis recognizes that positive(i.e. facilitative) and negative(i.e. competitive) plant interactions change in intensity and effect relative to abiotic stress experienced on a gradient. Motivated by observations of alpine treeline ecotones, we suggest that this switch in interaction could operate along a gradient of relative size of individual plants. We propose that as neighbors increase in size relative to a focal plant they improve the environment for that plant up to a critical point. After this critical point is surpassed, however, increasing relative size of neighbors will degrade the environment such that the net interaction intensity becomes negative. We developed a conceptual(not site or species specific) individual based model to simulate a single species with recruitment, growth, and mortality dependent on the environment mediated by the relative size of neighbors. Growth and size form a feedback. Simulation results show that the size gradient model produces metrics similar to that of a stress gradient model. Visualizations reveal that the size gradient model produces spatial patterns that are similar to the complex ones observed at alpine treelines. Size-mediated interaction could be a mechanism of the stress gradient hypothesis or it could operate independent of abiotic stress.

Renminbi Nominal Effective Exchange Rate for Third Market Competition:An Approach Based on Disaggregated Trade Data

When measuring the nominal effective exchange rate （NEER）, three factors should be considered： direct import competition, direct export competition and third market competuion. The traditional NEER methodology using aggregated export trade data underestimates the competition between countries producing homogeneous goods, so that the weight of the effective exchange rates is too reliant on trade scale. Based on 2002 6-digit items of the Harmonized Commodity Description and Coding System, this paper employs the competitive stress index to adjust the weighting system of the renminbi NEER for third market competition. In the new weighting system, European countries and some emerging economies have higher weights compared with some of the developed countries, including the USA, Japan and resources-dominated eeonomies~ This research will facilitate the understanding of changes in China＇s export competitiveness.