Effects of soil stoichiometry of the CaCO_3/available phosphorus ratio on plant density in Artemisia ordosica communities

In soil, CaCO3 can reduce the availability of phosphorus in arid and semi-arid regions, thereby influencing plant growth. However, the effects of soil stoichiometry of the CaCO3/available phosphorus ratio (CaCO3/AP ratio) on plant density are unknown. In this study, we examined 4 Artemisia ordosica communities located in arid and semi-arid regions, and performed a pot experiment with an orthogonal design to examine the relationships between the soil CaCO3/AP ratio and plant density. The results showed that the soil stoichiometry of the CaCO3/AP ratio had a greater effect on growth of A. ordosica than either CaCO3 or AP as single factor. The influence of the soil CaCO3/AP ratio on density of A. ordosica was related to the climatic zone and the types and/or amplitudes of increases in the soil CaCO3/AP ratio. When the soil CaCO3/AP ratio was within the same amplitude and was less than 2.5, the A. ordosica density increased in the semi-arid region, whereas the density decreased with increasing soil CaCO3/AP ratios in the arid region. In the semi-arid region, when the soil CaCO3/AP ratio was within the same amplitude and the increase in the soil CaCO3 content was greater than the increase in soil AP, the A. ordosica density increased with increasing soil CaCO3/AP ratios. This means that the relationship between the soil CaCO3/AP ratio and the A. ordosica density remained the same. In the arid region, an increase in the soil CaCO3/AP ratio as a result of increased soil CaCO3 content and decreased soil AP resulted in increased density of A. ordosica. Therefore, the relationship between soil CaCO3/AP ratios and density of A. ordosica was altered by these changes in soil chemistry. Thus, the soil stoichiometry of the CaCO3/AP ratio is an important factor affecting the density of A. ordosica in arid and semi-arid regions.

Predicting plant traits and functional types response to grazing in an alpine shrub meadow on the Qinghai-Tibet Plateau

The identification of easily measured plant functional types (PFTs) that consistently predict grazing response would be a major advance.The responses to grazing of individual traits and PFTs were analyzed along a grazing gradient in an alpine shrub meadow on the Qinghai-Tibet Plateau,China.Three response types were identified;grazing increaser (GI),grazing decreaser (GD),and neutral (NE) for both traits and PFTs.Seven traits were measured:plant height,economic group,cotyledon type,plant inclination,growth form,life cycle,and vegetative structure.The first five were significantly affected by grazing.Ordinal regressions for grazing response of the seven traits showed that the best single predictors of response were growth form (including the attributes "Scattered","Bunched" or "Closely Bunched"),and plant inclination ("Rosette","Prostrate",or "Erect"),followed by economic group ("Shrub","Grass","Sedge","Legume","Forb",or "Harmful") and plant height ("Tall","Medium",or "Small").Within the four optimal traits,the summed dominance ratio (SDR) of small plants,forbs,rosette and bunched plants,invariably increased,while that of tall plants,shrubs,grasses,and erect plants decreased,when grazing pressure was enhanced.Canonical correspondence analysis (CCA) identified eleven explanatory PFTs based on 195 defined PFTs,by combining the different attributes of the four optimal traits.Among explanatory PFTs,the most valuable in predicting the community response to grazing were Tall×Shrub×Erect×Scattered and Small×Forb×Rosette,as these have the closest connections with grazing disturbance and include fewer species.Species richness,diversity,and community evenness,did not differ among grazing treatments because turnover occurred in component species and their relative abundances along the grazing gradient.We have demonstrated that a minimum set of PFTs resulting from optimal individual traits can provide consistent prediction of community responses to grazing in this region.This approach provides a more accurate indicator of change within a changing environment than do univariate measures of species diversity.We hope to provide a link between management practices and vegetation structure,forming a basis for future,large scale,plant trait comparisons.