Persistence of four dominant psammophyte species in central Inner Mongolia of China under continual drought

Clarifying the persistence time of seedlings of dominant species under continual drought will help us understand responses of ecosystems to global climate change and improve revegetation efforts. Drought tolerance of four dominant psammophytic shrub species occurring in different environments was studied in the semi-arid areas of Inner Mongolian grasslands. Seedlings of Hedysarum laeve, Caragana korshinskii, Artemisia sphaerocephala and Artemisia ordosica were grown under four air temperature regimes （night/day： 12.5/22.5℃, 15/25℃, 17.5/27.5℃ and 20/30℃） within climate （air temperature and humidity） controlled, naturally lit glasshouses with a night/day relative humidity of 70%/50%. Pots were watered to field capacity for each temperature treatment. Soil water condition was monitored by weighting each pot every day using an electronic balance. Date of seedling death for each treatment was recorded and the dead plants were harvested. Plant dry weights were determined after oven drying at 80℃ for 3 days. Two Artemisia species had higher growth rates than H. laeve and C. korshinskii, and the growth of all four species increased with increasing temperatures. The two Artemisia species had the highest leaf biomass increment, followed by C. korshinskii, and then H. laeve. Shoot biomass increment was higher for A. ordosica and C. korshinskii, intermediate for A. sphaerocephala and lowest for H. laeve. C. korshinskii had the highest root biomass increment. The final soil water content at death for all four species varied from 1% to 2%. C. korshinskii, A. sphaerocephala, H. laeve and A. ordosica survived for 25-43, 24-41, 26-41 and 24-37 days without watering, respectively. C. korshinskii, A. sphaerocephala, H. Laeve, and A. ordosica seedlings survived longer at the lowest temperatures （12.5/22.5℃） than at the highest temperatures （20/30℃） by 18, 17, 15 and 13 days, respectively. Increased climatic temperatures induce the death of seedlings in years with long intervals between rainfall events. The adaptation of seedlings to droughts should be emphasized in revegetation efforts in the Ordos Plateau, Inner Mongolia.

A Comparison of Deterministic and Stochastic Susceptible-Infected-Susceptible (SIS) and Susceptible-Infected-Recovered (SIR) Models

In this paper we build and analyze two stochastic epidemic models with death. The model assumes that only susceptible individuals (S) can get infected (I) and may die from this disease or a recovered individual becomes susceptible again (SIS model) or completely immune (SIR Model) for the remainder of the study period. Moreover, it is assumed there are no births, deaths, immigration or emigration during the study period;the community is said to be closed. In these infection disease models, there are two central questions: first it is the disease extinction or not and the second studies the time elapsed for such extinction, this paper will deal with this second question because the first answer corresponds to the basic reproduction number defined in the bibliography. More concretely, we study the mean-extinction of the diseases and the technique used here first builds the backward Kolmogorov differential equation and then solves it numerically using finite element method with FreeFem++. Our contribution and novelty are the following: however the reproduction number effectively concludes the extinction or not of the disease, it does not help to know its extinction times because example with the same reproduction numbers has very different time. Moreover, the SIS model is slower, a result that is not surprising, but this difference seems to increase in the stochastic models with respect to the deterministic ones, it is reasonable to assume some uncertainly.