| In recent decades,the problem of grassland degradation in China has become increasingly serious,which not only seriously affects the sustainable development of pastoral economy,but also poses a serious threat to the safety of grassland ecological environment.In this context,in order to effectively curb this situation,the state has successively formulated a series of policies and measures related to grassland ecological protection and restoration and reconstruction of degraded grasslands.The relevant scientific research departments have also increased research investment in the treatment of degraded grasslands.In the existing management theories and technical measures,grazing prohibition and rest are not only widely used as important technical means,but also are preconditions with which almost all other technical measures and the combined technical system can be effectively implemented.At present,the application effect of grazing prohibition and rest have been sufficiently affirmed by large number of studies,but the relevant studies on the response mechanism of vegetation traits under the conditions of grazing prohibition and rest are still inadequate,especially those related to alpine meadow grassland.For this reason,grazing prohibition and grazing rest treatments was conducted to investigate their effects on plant species diversity,plant functional group,plant functional traits,and aboveground productivity in mild,moderate,severe and extreme degraded grassland seperately in Maqu County,Gannan Tibetan Autonomous Prefecture,from 2017 to 2019.The main results were as follow:1.The effects on plant diversity character: Compared with the grazing treatment,the number of plant species,diversity index and evenness index of the grassland were significantly reduced in both grazing prohibition and grazing rest treatments,and these indices in grazing prohibition was lower than in grazing rest.The dominance index,however,increased significantly in both treatments,and there was a higher record in grazing prohibition than grazing rest.2.The effects on plant functional group: Compared with grazing treatment,the proportion of grass group increased by 75% in grazing prohibition treatment,and by 37% in grazing rest treatment.The sedges group increased by 17% in grazing rest treatment,while,there were no significant changes in grazing prohibition treatment.For herbs group,however,it reduced by 26%,and by 18% in grazing prohibition and grazing rest treatment respectively.The proportion of poisonous group reduced by 29% and by 21% in grazing prohibition and grazing rest treatment respectively.3.The effects on plant functional traits: Compared with grazing treatment,the plant height increased by 69% and 55%,the relative growth rate increased by 146% and 55%,the specific leaf area increased by 139% and 96% in grazing prohibition and grazing rest treatment respectively.The leaf dry mass content,however,dropped by 11.9% and 8.5% in grazing rest and grazing prohibition treatment respectively.4.The effects on aboveground net primary productivity: Compared with grazing treatment,the net primary productivity increased by 44% and 23% in grazing rest and grazing prohibition treatment respectively.5.The relationship between treatment time and vegetation characteristics and productivity: For both grazing prohibition and grazing rest,with the prolongation of the treatment time,the height,relative growth rate and specific leaf area of plants increased with the time.Under grazing condition,however,the plant height decreased with the increase of treatment time.The changes in relative growth rate and specific leaf area had no significant correlation with the treatment time.The leaf dry mass content of grazing prohibition and grazing rest decreased with the prolongation of the treatment time,both of them showed a significant negative correlation with the treatment time.While the change of leaf dry matter content in grazing grassland was not significant.The primary productivity of both the grazing prohibition and grazing rest increased with the increase of the treatment time,and there was a significant positive correlation between the two,while there was no significant correlation between the productivity and the treatment time under the grazing condition.6.Comparison of different levels of grassland degradation: No matter in grazing prohibition or in grazing rest,the height,coverage,biomass,species number,diversity index and evenness of vegetation all decreased with the increase of the degree of degradation.Among them,under the condition of grazing prohibition,there was a significant difference between mildly degraded grassland and the three degradation gradients of medium,heavy and extreme,while there was no significant difference between medium,heavy and extreme.Under the condition of grazing rest,there were significant differences between the light and moderate degraded grassland and the heavy and extreme grassland.The variation trend of plant coverage was similar to that of height.Other index only showed significant differences between mild,moderate and severe and extreme degradation,while there was no significant difference between mild and moderate degradation and severe and extreme degradation.In summary,both grazing prohibition and grazing rest can significantly improve the productivity of vegetation and the overall vegetation traits and stabilize them within 2 to 3 years.There was no significant difference in the effect of grazing prohibition and grazing rest on vegetation restoration in mild and moderate degraded grassland.For heavy and extremely degraded grassland,however,the effect of grazing prohibition was significantly better than that of grazing rest.For heavy and extremely degraded grassland,grazing prohibition should be carried out for 1 to 2 years and then be transferred to grazing rest.No matter whether it is grazing prohibition or grazing rest,the implementation period should be controlled within 2 to 3 years in general,and then it can be transferred to reasonable grazing utilization pattern. |
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