Changes Of Leaf Anatomical Structure Of Two Dominant Plants In Desert Steppe Under Different Grazing Intensity Over A Long Period Of Time

In grassland grazing ecosystem,plant leaf anatomical traits can effectively reflect plant responses to different grazing intensities,but there are relatively few studies on the effects of different grazing intensities on plant anatomical traits in desert steppe.In this study,two dominant species of Stipa breviflora（C₃）and Cleistogenes songorica（C₄）in desert steppe were used as research subjects in a long-term（19 years）fixed monitoring grazing plot.The leaf anatomical structure indexes were measured under different grazing intensities（control,lightly grazing,moderately grazing,heavy grazing）over a long period of time,and the response of leaf anatomical structure of Stipa breviflora and Cleistogenes songorica to long-term grazing disturbance was discussed.The main research results are as follows:（1）As a C₃plant in desert steppe,Stipa breviflora had an appropriate response to grazing disturbance in leaf cross section area,transport tissue area,mechanical tissue area and assimilative tissue area of leaves.With the increase of grazing intensity,the area of cross section of Stipa breviflora leaves,the area of vessel,the area of sclerenchyma tissue and the area of mesophyll tissue all showed an increasing trend,and the four indexes were significantly increased in the heavily grazed area compared with the control area（p<0.05）.This indicates that grazing intensity leads to plant dwarfing,which is caused by the strategies of avoiding grazing and tolerating grazing,and this strategy can achieve compensatory growth of plant productivity by increasing the mass per unit area.Under the interference of long-term grazing,Stipa breviflora leaves formed well-developed and compact vascular tissues,which enhanced the ability to transport water and organic matter.The increase of sclerenchyma tissue area was to reduce palatability and avoid grazing,while the increase of mesophyll tissue area could reduce water transpiration dissipation and burn caused by strong light.In addition,there was a strong correlation between the indexes of Stipa breviflora.（2）As a C₄plant in desert steppe,the thickness of protective tissue,vascular tissue area and Kranz structure of Cleistogenes songorica were appropriately responsive to grazing disturbance.With the increase of grazing intensity,the thickness of motor cells,the proportion of xylem to vascular bundle area,and the ratio of xylem to phloem are showed an increasing trend,and the ratio of xylem to phloem area was significantly increased in the heavily grazed area compared with the control area（p<0.05）.The increase of vascular tissue area can promote the transport of water,inorganic salts and organic matter and improve the mechanical strength of leaves.This change is not only an adaptation to the change of community environment,but also an adaptation to the high intensity trampling of livestock.It is worth mentioning that,as an important photosynthetic structure of C₄plants,compared with the control area,the area of Kranz structure in grazing area was significantly increased（p<0.05）,The area of Kranz structure area was positively correlated with the area of vascular bundle and the area of main vessel tube.These results indicated that in order to maintain normal photosynthesis under long-term grazing interference of different intensities,the Kranz area should be increased to improve photosynthesis and water use efficiency.（3）Cuticle thickness and main vessel tube area had significant interaction effects on species,while vascular bundle area,xylem area and phloem area had no significant effect on species,grazing intensity and interaction between species and grazing intensity.In conclusion,the study of plant anatomical structure in desert steppe under different grazing conditions over a long period of time will not only enrich the research content of grazing ecology in desert steppe,but also provide scientific basis for rational utilization and protection of fragile ecosystem.