Does phenology play a role in the feedbacks underlying shrub encroachment?

Shrub encroachment has emerged as a global phenomenon over the past century. Multiple drivers have been put forward to explain the increased shrub dominance in various ecosystems around the world. However, the potential role of phenology in regulating shrub encroachment is not well understood. We address this issue using 3-year continuous monitoring of the phenology of coexisting shrubs and grasses combined with observations of ecohydrological processes (water uptake) and soil conditions (root zone soil moisture, soil texture, and soil temperature) at four study sites in Inner Mongolia, China, with shrub coverage of Caragana microphylla ranging from 0%, to 6.8%, 26.8% and 34.2%. Along such an encroachment gradient, shrubs exhibited progressively earlier onsets and later ends of the growing season, with an overall extension in growing season length by 15 days to 22 days in the later stages of shrub encroachment. Conversely, the coexisting grasses showed earlier occurrences both in spring and autumn phenological phases, which resulted in a phenological gap between shrubs and grasses. Thus, a positive feedback could exist between these phenological changes and shrub encroachment. In shrub patches, soils were wetter, with finer texture, and with more suitable temperatures for plant survival and development, which favored the lengthening of growing season of shrubs. The longer growing seasons are associated with longer periods of water use and photosynthesis for shrubs, and better opportunities for water uptake, with the overall effect of facilitating shrub growth and further expansion.

[Seasonal divergence in the responses of vegetation growth to PDO in Tibetan Plateau, China]. / 青藏高原植被生长对PDO响应的季节分异.

Regional-scale normalized difference vegetation index (NDVI) derived from satellite remote sensing observations and gridded climate data were used to study the seasonal responses and underlying mechanisms of vegetation growth over Tibetan Plateau to Pacific Decadal Oscillation (PDO) at period of 1982-2015, by performing Spearman correlation analysis and enhanced multivariate regression model: structural equation model (SEM). The results showed that there was significant negative correlation between PDO index and mean growing-season (April-October) NDVI over Tibetan Plateau; however, marked seasonal divergence in the relationship between PDO and vegetation growth existed among different seasons. It characterized with stronger negative correlation between PDO and NDVI in autumn than in summer, and winter PDO had significant effect on consequent summer vegetation growth. Additionally, it showed great divergence in control processes of PDO on vegetation growth among different seasons, with significant control of PDO on both temperature and precipitation in summer, and significant control of PDO on temperature only in autumn.

[Responses of subalpine Betula albo-sinensis soil invertase activity to elevated atmospheric temperature and CO2 concentration in Western Sichuan].

Taking the soil planted with subalpine Betula albo-sinensis seedlings in Western Sichuan as test object, this paper studied the responses of its invertase activity to elevated temperature (ET), elevated atmospheric CO2 concentration (EC), and their combination (ETC). The results indicated that ET increased the soil invertase activity to a certain extent, being significant in May, June, September and October. EC also had a significant positive effect on the enzyme activity from May to October (P < 0.05), which was higher in rhizosphere than in non-rhizosphere soil, and in those planted with high density B. albo-sinensis. The responses of soil invertase activity to ETC and shading differed with month, planting density of B. albo-sinensis, and distribution pattern of the enzyme.

[Analysis and quality assessment standard of heavy metals and arsenic in Rhizoma et Radix Notopterygii from different localities].

OBJECTIVE: Five heavy metals (Hg, Cu, Cd, Pb, Cr) and arsenic (As) contents in 57 samples of from 19 localities in Sichuan, Qinghai and Gansu of Rhizoma et Radix Notopterygii sourcing from two plants Notoperygium incusum and N. forbesii has been analyzed to evaluate the content standard of heavy metal and arsenic for drafting the herbal medicine quality. METHOD: ICP-AES method was applied to determine As, Cu, Cd, Pb, Cr and CAAS method was applied to determine Hg. RESULT: Hg can not be determined through CAAS, and little Pb was determined except in limited N. forbesii samples from certain sites, while the contents of Cu in most samples are higher than 20 mg x kg(-1). As in N. forbesii samples, and Cd in N. incisum samples are higher or approximate the limitation of related standard of medicinal plants and production. Cr varies from different localities, ranging from 0.48 to 8.36 mg x kg(-1) for N. incisum and 1.44-8.03 mg x kg(-1) for N. forbesii. CONCLUSION: Referring to the related standards of medicine and vegetable, the heavy metals and arsenic contents of Rhizoma et Radix Notopterygii samples from traditional producing areas are confirmed the propositional standard, i. e., Pb < or = 5.0 mg x kg(-1), Cd < or = 0.3 mg x kg(-1), Hg < or = 0.2 mg x kg(-1), Cu < or = 28.0 mg x kg(-1), As < or = 2.0 mg x kg(-1), and Cr < or = 1.0 mg x kg(-1). Of these five heavy metals, Cu is much over the limitation standard, which suggested that GAP base of Rhizoma et Radix Notopterygii must be selected in lower Cu environmental condition in main producing area.