The erodibility of terrace risers in eucalyptus plantations in North-Central Portugal

The construction of bench terraces has become a common practice in north-central Portugal for establishing eucalyptus plantations on steep hillslopes. Terracing is commonly considered as an effective soil conservation technique for steep terrain, with a long tradition in agricultural practices. This method typically involves extensive redistribution of topsoil, potentially causing substantial changes in the soil's physical, chemical, and biological properties, as well as its associated functions. Additionally, modern terrace construction using bulldozers can exacerbate soil instability, leading to soil mobilization through the collapse of risers. Modern forest terracing transforms hillslopes into flat terrain, facilitating the use of machinery for planting, fertilization, vegetation control, logging, and log extraction. However, terracing removes all surface vegetation, leaving the soil exposed to raindrop impact. Moreover, the mechanical forces exerted by bulldozers break, mix, and loosen the soil, detaching particles and compromising soil structure and stability. Historically, terraces were separated by stone walls that provided structural stability and erosion control. Today, however, they are often divided by steep, unsupported sections known as risers. Despite their widespread use in forestry, the impacts of modern terracing on water and soil conservation remain poorly studied. This study aimed to (1) quantify the collapse of modern forest terrace risers during the first year after construction and (2) evaluate the effectiveness of mitigation measures such as anionic polyacrylamide (PAM) and hydromulch. In a eucalyptus plantation in north-central Portugal, nine sediment fences were installed at the base of three different risers (three pairs per riser). One riser was left as a control, while the others were randomly treated with PAM or hydromulch. Preliminary results revealed substantial soil mobilization from the risers, with a median sediment deposition of 258 Mg ha⁻¹ during the first post-terracing year. Landslides affecting risers from top to base were frequently observed, further demonstrating their instability. Both hydromulch and PAM treatments significantly reduced cumulative annual sediment deposition to averages of 113 and 105 Mg ha⁻¹, respectively. However, neither measure completely prevented the collapse of certain riser sections. These findings highlight the urgent need for multidisciplinary approaches to assess and mitigate the adverse effects of modern terracing in forest plantations.