| Grassland stability,resilience and productivity were influenced by grazing livestock mainly through trampling,intaking and excrement.However,the effects of these three aspects on grassland were significantly different.The trampling of grazing livestock have longer active time and more permanent effect than intaking and excrement,so the effects of trampling on the grassland was more comprehensive and profound,which plays a leading role in grassland degradation and health maintenance.Livestock trampling lead to plant mechanical damage,changing the plant’s competitive pattern,and then the growth,reproduction and photosynthesis of plants were affected,meanwhile,the effects of livestock trampling on plant was closely related to soil moisture.The Medicago ruthenica var.inschanica is high quality legume forage for cattle and sheep ingestion,and has extensive ecological adaptability and strong stress resistance.In this study,Medicago ruthenica var.inschanica,which naturally distribute in Tianzhu alpine meadow,was used as the research material,a series of field experiments lasted for two years that were conducted at the Tianzhu alpine meadow in Gansu Province,and the effects of simulated yak and Tibetan sheep trampling and simulated precipitation on reproductive and photosynthetic characteristics of M.ruthenica var.inschanica were studied.The main results were as follows.1.The natural height of M.ruthenica var.inschanica decreased with increasing yak and Tibetan sheep trampling intensity and declining of rainfall,and as the trampling time goes on,the natural height increased firstly,then decreased.The plant growth of M.ruthenica var.inschanica was inhibited by lower level of rainfall and heavy trampling by Tibetan sheep and yaks.Simulated rainfall,simulated trampling and the interaction between simulated rainfall and simulated trampling had a very significant effects on the natural height of M.ruthenica var.inschanica.2.The root surface area,root volume and root biomass of M.ruthenica var.inschanica decreased in different soil layers(from 0 to 10 cm,10 to 20 cm,20 to30 cm)with increasing yak and Tibetan sheep trampling intensity and declining rainfall.The root growth was inhibited by lower level rainfall and yaks heavy trampling.The adverse effect of yak trampling on the root growth was more serious than that of Tibetan sheep under the same grazing intensity.In the soil depth from 20 to 30 cm,there were no significant differences in the root surface area between the treatments,there was no significant difference in root volume and root biomass between treatments under average and high level of rainfall,and there were no significant differences in the root surface area,root volume and root biomass between the treatments under natural rainfall,these results indicated that the effects of livestock trampling on the deep root of M.ruthenica var.inschanica(20-30 cm)was small.The results of analysis of variance showed that simulated rainfall had a significant effect on the root surface area,root volume and root biomass of M.ruthenica var.inschanica in different soil layers,and the interaction between simulated rainfall and simulated trampling had no significant effects on the root surface area,root volume and root biomass of M.ruthenica var.inschanica in different soil layers.3.The pod length,number of pods per square meter,seed number per pod,1000-grain weight and seed yield of M.ruthenica var.inschanica decreased with increasing yak and Tibetan sheep trampling intensity and declining rainfall.The adverse effects of Tibetan sheep trampling on the seed reproduction of M.ruthenica var.inschanica were less than that of yak trampling under the same grazing intensity,and the seed reproduction was seriously inhibited under the heavy trampling by yaks and Tibetan sheep.The seed yield of the trampling treatments under lower level of rainfall was significantly lower than that of the high level of rainfall,and the effects of lower level of rainfall on seed yield of M.ruthenica var.inschanica was significant.Compared with CK,the magnitude of decreases in the 1000-grain weight was less than 25% under the trampling of yaks and Tibetan sheep,which showed that yak and Tibetan sheep trampling had less effect on 1000-grain weight of M.ruthenica var.inschanica.The results of analysis of variance showed that simulated rainfall alone or simulated trampling alone significantly affected indexes about seed reproduction,however,the interaction between simulated rainfall and simulated trampling had no significant effect on the reproductive indicators.The results of correlation analysis of M.ruthenica var.inschanica seed yield and yield components showed that the number of pods per square meter had the greatest effect on seed yield,followed by seed number per pod and 1000-grain weight.4.The single leaf area,chlorophyll content(SPAD value),transpiration rate(Tr),stomatal conductance(Gs)and net photosynthetic rate(Pn)decreased with increasing yak and Tibetan sheep trampling intensity and declining rainfall,while the intracellular concentration of CO2(Ci)increased.Under the same rainfall condition,the Tr,Gs and Pn were all higher under light trampling intensity of Tibetan sheep and yaks than in the control(no trampling),while the Ci was lower than that in the control.The magnitude of decreases in the single leaf area,chlorophyll content(SPAD value),Tr,Gs and Pn,and increases in Ci were greater under heavy trampling by Tibetan sheep than under heavy trampling by yaks.Simulated rainfall alone or simulated trampling alone significantly affected photosynthetic tested indicators,and the interaction between simulated rainfall and simulated trampling had a significant effect on the single leaf area,Tr,Gs and Pn of M.ruthenica var.inschanica.These results showed that the photosynthesis of M.ruthenica var.inschanica was promoted by light trampling,but inhibited by heavy trampling by Tibetan sheep and yaks.Compared with Tibetan sheep,yaks had stronger inhibitory effects on the photosythetic activity of M.ruthenica var.inschanica.In summary,the photosynthesis of M.ruthenica var.inschanica was promoted by light trampling.The reproduction and photosynthetic activity of M.ruthenica var.inschanica were inhibited by lower level of rainfall and heavy trampling of livestock,and the inhibition of yaks trampling was greater than Tibetan sheep trampling on the reproduction and photosynthetic activity of M.ruthenica var.inschanica.When the rainfall was less in the plant growing season,the reproduction and photosynthetic activity of M.ruthenica var.inschanica were greatly inhibited by livestock trampling.It is recommended that the suitable grazing intensity should be utilized and appropriate livestock should be chosen in Tianzhu alpine meadow. |
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