A traction effect refers to the mechanical effect of lateral to horizontal roots, normally in shallow soil. to enhance the in-plane tensile strength of soil in the rooted soil zone. It is one way in which plant roots can contribute to lateral reinforcement of a shallow soil mass. To verify whether or not a traction effect exists in the root system of a pine forest (Pinus yunnanesis French) in the Hutiaoxia Gorge, southwest China, and evaluate the magnitude of any such effect, an experiment-based modelling and a direct in situ test were conducted at the Erdui site in the gorge. The modelling prediction indicates that the lateral roots produce tremendous tractive resistance in the upper soil (0-60 cm below the surface). with a magnitude of 4169 N. on a vertical cross-section area of 105 mm2 at the top depth interval of soil (0-20 cm). The direct in situ test shows that the tractive resistance exerted by the lateral roots averages 561 N in the top depth interval and for the same vertical area. or an increase of sliding-pulling resistance by 38%. Under the influence of this tractive resistance. the tensile strength of the upper rooted soil was increased by at least 5.7 kPa. Together with the strong vertical anchorage of the taproot of the pine and sinker roots. the lateral roots are able to stabilise the shallow soil mass to a certain degree.

研究发现滇西北高山峡谷区云南松也具有通过林冠截流减少有效雨量和降低雨滴对地表的滴溅能量，减少降雨对土壤的侵蚀程度。产生较大直径的叶滴并引起林冠滴溅效应。云南松林冠在整个雨季截留率平均达36.29%，使林下有效降雨量仅为降雨量的63.71%。松林在整个雨季的林下溅蚀累积总量为4.94kg/m2，棵地为6.12kg/m2，表明松林减少了19.28%的溅蚀全量。但林地在雨季中后期的各阶段溅蚀全量却高于裸地。特别在后期高出裸地2.6倍。

植物的侧根对周围土壤具有机械牵引作用。一般来讲，垂直根可以通过增加根际土层的抗剪强度来加固土体，斜向侧根则能够在对周围土体进行牵引固持的过程中，通过提高土体的水平抗张强度实现对土体的增强加固，这一作用即侧根的水平牵引效应。为了检验高山峡谷区云南松林是否也有这种效应．并调查其量值，我们在金沙江中上游的虎跳峡地区对云南松根际土层进行了直接剪土测试。结果表明，在表臣土体中(0～20cm)松树侧根能在1OOOcm2的垂直横截面上提供421.19～561.05N的水平牵引力，使土层的抗滑力提高33.2%～37.58%、受该牵引力的影响，何根把根际土层的抗张强度提高了4.2～5.6kPa：测试结果预示．只要云南松垂直根可以把浅层土体锚固到较深层稳定的土体上，通过水平牵引效应，云南松侧根可在一定程度上与垂直根共同克服林地浅层坡面的不稳定性，如浅层滑动和蠕移。

以滇中地区的松林为例，初步定量探讨了乔木对顺坡下竹的浅层土壤的斜向支撑作用。研究表明，树根可能提供的锚固力可达0.69kN,在有效的锚固作用下．单株树干的支撑力（抗滑力）最高为0.42kN．平均为0.17kN;受斜向支撑作用的影响，本来不稳定的浅层松散物质的稳定性系数F值．由0.54提高到1.10．使坡面成为稳定斜坡。如果在特定条件下，树根的锚固力全部转化为支撑力，坡面的F值可以达到2.82。

Cegetation provides potential erosion control and slope protection, athrough its hydrological and mechanical interaction of plants eith local climate and soil. The tehoretical understanding of these vetetyative effects is the base of slope eco-engineering. In the past few decades, the importance of vegetation in protecting slopes has been stressed by many researcher. The greater awareness that vegetation can play a range of roles in slope protection has made it increasingly a valuable tool in natural environmental protection and vivil engineering. and theis knowledge has led to a invreasing application of vegetation types world-wide. Though it is still a kind of young techniques, an in creasing amout of experience of slope eco-engineering of vegetation is gathering from the past practoces, and the lager number of suocessful examples of its application is showing its great future.