When planning and undertaking logging operations in the plains, French forest companies are often confronted with multiple challenges related to soil bearing capacity and respective sensitiveness towards compaction and rutting. Risk mitigation measures are already in place both in the forest management plan and the pre-harvest planning but some knowledge gaps still hinder efficiency.

French mineral soils make it a hard job because forest soils are heterogeneous and sensitive situations are numerous with high proportions of silt and clay, and frequent hydromorphy. Knowledge-based risk prediction is hindered by the limited understanding of the complex phenomena are at play below the surface (spatial and vertical heterogeneity, rockiness, abundant root systems from the perennial forest stand…). And traffic by forest machines is frequent, whereas soil recovery is limited at best between operations (7-10 years, Figure 1).

Figure 1: Frequent forest machine interventions over the 150years rotation in oak dominated forest in French plains


As an example, the cooperative Forêts et Bois de l’Est (F&BE) is typically confronted to sensitive soils in 25% of the cases when working in Franche Comté. Weather conditions strongly influence logging operations as trafficability-ban (red area in Figure 2) can last for months and situations requiring extra-cautiousness (yellow area in Figure 2) apply half of the year in “usual dry years”. Hence, capacity building for practitioners to organize multi-site and seasonal distribution of mechanized logging operations is one of the EFFORTE objectives.

Figure 2: challenging weather conditions to be taken into account when F&BE is planning its logging operations in Franche Comté.


Aiming for better understanding for soil moisture

Research is also on-going to better understand water transfer phenomena at the stand level. Soil moisture is indeed a key determinant when assessing bearing capacity and practitioners usually observe that logging trails (on which traffic is concentrated in order not to burden the rest of the soil) require more time to dry up than the undisturbed forest soil in-between the perennial logging trails.
Four sites (including different disturbance modalities) are instrumented for that purpose in the North-East. The design of the scientific protocol is a result of the fruitful cooperation between EFFORTE experts from France, Finland and Sweden. Intermediate outputs, including drainage model, will be discussed in summer 2018 and shall be improved as site monitoring continues through the project duration.

Figure 3: instrumentation on one of the French sites located in a forest managed by ONF. Active monitoring started in autumn 2017 with PhD candidate Manon Martin as a driving force

Water infiltration tests included in the protocol contribute to better understanding soil behavior after rainy episodes and during changing weather conditions. A typical example is provided in Figure 4 where soil under the logging trail drains up much more slowly than its undisturbed equivalent.

Figure 4: Water transfer monitoring on 1 on the test-sites, 0 and 15cm deep below the soil surface, in logging trails and undisturbed forest soil conditions

Focus also on soil deformations and risk prediction

In addition to this focus on soil moisture behavior and prediction, soil deformations and the capacity to predict those risks are also studied. “Wheeling experiments” are starting in November 2017 together with French partners (ONF, F&BE, IP-CBB, SEFE) and local logging contractors working with different types of forwarders and skidders.
New knowledge expected from this scientific part of the EFFORTE work program will help maintain the long term trafficability of logging trails and improve seasonal scheduling during pre-harvest planning.