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Shrubs are important plant species in grassland ecosystems worldwide, and their density and cover have been gradually increasing globally. However, the interaction effect of grazing and aridity on population recruitment and population growth of shrub species in grasslands has not been examined explicitly. We examined sapling establishment, sexual recruitment, population mortality, and population growth of Caragana stenophyllaalong a climatic aridity gradient and a grazing intensity gradient in the Inner Mongolia Steppe, using manipulative field experiments. Sapling establishment, sapling height, and sexual recruitment of C. stenophylladecreased as climatic aridity and grazing intensity increased. The negative effects of grazing on sapling establishment and sexual recruitment gradually increased as climatic aridity increased. The effect of climatic aridity and grazing on population mortality was influenced by sexual recruitment. In the combined treatments of climatic aridity and grazing, population mortality was relatively high when sexual recruitment was relatively high, while population mortality increased as climatic aridity and grazing increased when sexual recruitment was relatively low. C. stenophyllapopulation increased under relatively low drought stress and mild grazing but declined under strong drought stress and/or severe grazing. Our results suggested that to maintain viable Caraganapopulations, appropriate grazing policies must be made according to climate aridity gradient.

AbstractA study on the effects of silicon supply on the resistance to drought in cucumber plants was conducted in pot experiments. The results suggested that in the absence of stress, silicon slightly enhanced the net photosynthetic rate, but significantly decreased the transpiration rate and stomatal conductance in cucumber plants. Silicon enhanced the net photosynthetic rate of cucumber plants under drought stress. Since silicon decreased the stomatal conductance, enhanced the capacity of holding water, and kept the transpiration rate at a relatively steady rate during drought stress, the photosynthesis of the cucumber plants was sustained. And under drought stress, silicon increased the biomass and water content of leaves in cucumber plants. Silicon decreased the decomposition of chlorophyll in cucumber plants under drought stress, limited the increase of the plasma membrane permeability and malondialdehyde (MDA) content in leaves, alleviated the physiological response of peroxidase (POD) to drought stress, maintained the superoxide dismutase (SOD) normal adaptation, and increased the activity of catalase (CAT). Under severe stress, these physiological biochemical reactions showed positive correlations with the amount of silicon supply. These findings demonstrated that silicon enhanced the resistance of the cucumber plants to drought. Statistical analysis indicated that under drought stress the cumulative value of biomass showed a highly significant correlation with the cumulative value of diurnal photosynthesis (r = 0.9812, p < 0.01), and was significantly correlated with the water content of leaves (r = 0.8650, p < 0.05). These results demonstrated that under drought stress the first factor responsible for the effects of silicon application on the cumulative value of biomass was the increase of photosynthesis, and the second factor was the enhancement of the water holding capacity. Based on these facts, it was concluded that silicon enhanced the resistance to drought mainly by taking part in the metabolism of plants.