放牧家畜的践踏作用研究评述

A review on trampling by grazed livestock

放牧家畜的践踏作用是家畜作用于草地的 3个途径之一 ,相对于采食和排泄物的作用具有作用时间长、直接作用的草地组分多和效果持久的特点。家畜践踏损伤牧草、埋实种子、促进萌发 ,减少凋落物的现存量 ,增加土壤的紧实度、容重 ,降低土壤孔隙度、水稳性团聚体、透水性和透气性 ,导致雨后水涝和植物根区缺氧 ,引发水土流失。国内外使用的 4种践踏强度指标分别存在不确定性大、忽视践踏强度的时间依赖性和畜种间差异、没有考虑放牧行为及其时间分配模式等问题 ,分析认为践踏强度应为家畜放牧行为、不同行为的持续时间、家畜体重、放牧地面积和坡度的函数。家畜的践踏作用因草地的健康状况差异而与之存在正或负的反馈机制以及践踏强度阈值 ,可能在草地退化和健康维护中起主导作用。今后需建立严格的研究方法和科学的践踏强度指标 ,重视草地植被对践踏的耐受阈限、践踏对牧草更新的调控机理及其对凋落物产生和分解的影响 ,以及土壤水分与践踏作用的关系 ,确定放牧系统的践踏强度阈值。

Trampling is one of three ways for livestock to impact on grazing land is recognized to have longer active time and more permanent effect than intaking and excrement. It is therefore important to study the mechanism of trampling on grazing land in order to discover the mechanism of grassland degeneration and properly manage grazing ecosystems.(1) Trampling on vegetationWhile the trampling of livestock physically damaged aboveground herbage and buried the seeds, it was determined to be beneficial to the germination and emergence of seedlings. Further effort and consideration should be given to the critical point of vegetation resistance to trampling and the basic understanding of feedback mechanisms between vegetation and trampling.(2) Effects of trampling on litterOnly two studies showed that trampling aided litter breakdown, by facilitating the infiltration of litter into the soil profile, thereby reducing litter accumulation. The effects of trampling on output and litter decomposition was however still unclear, indicating that further study may be required.(3) Effects of trampling on soilThe majority of studies focused on this phenomenon. There was the most studies on effects of trampling on soil. Trampling increased the compaction and bulk density of topsoil, while decreasing the porosity and water stable aggregates, air permeability and hydraulic conductivity. Within the studies, there was two disputes; one was that some studies showed that trampling resulted in a decrease in the compaction and bulk density of topsoil or no change, while another studies showed that trampling also changed the physical properties in deeper soil horizons. It is our opinion dispute has resulted from the difference in water content of topsoil and or the difference in response mechanism between topsoil and deeper soil respectively, a factor not considered in any studies. Further study should determine the critical value of soil water content while the physical properties of soil began to change because of trampling.(4) Effects of trampling on run-off and soil erosionTrampling caused low infiltration of topsoil and resulted in water logging and O_2 deficiency in the root zone after rainfall. When combined with the reduced coverage of vegetation and litter on soil surface, runoff and soil erosion increased. In our view, wind was also playing an important role in degeneration of grazing land of northern China, but there few studies on the effects of wind on soil erosion after trampling of grazed livestock. Furthermore, all studies had not paid attention to the protection of vegetation and litter from the damage of trampling on soil.(5) Index of trampling intensityCurrent studies have used 4 kinds of indices of trampling intensity. (a) Changes were noted in soil physical properties such as soil compaction and bulk density etc, but were not definite in different soil conditions. (b) Number or body weight of grazed livestock per hectare, but did not distinguish the trampling intensity of different grazing animals and their subsequent effects of grazing behavior on trampling.(c) Pass of human trampling on grassland. We consider the simulated trampling was fundamentally different from that of the grazed livestock. Moreover, the above 3 kinds of indices neglected the time-dependant dynamics of trampling intensity. (d) Body weight of livestock×grazing time Livestock per hectare. This did not take the grazing behavior of livestock and its time distribution pattern into consideration. We suggest that a proper index of trampling intensity should be the function of grazing behavior, persistence of different behaviors, body weight of livestock, area and slope of grazing land, however this will need be developed in future studies.(6) Interrelationship between grazing intensity, the health of grazing land and trampling intensityThe feedback between grazing intensity and trampling intensity was stronger than a between grazing intensity and intaking. There was a positive feedback between trampling intensity and degeneration process in the degraded grassland