JCJC SIMI 6 - JCJC - SIMI 6 - Système Terre, environnement, risques

Tree response to wind storm – TWIST

TWIST

Tree response to wind storm

Main issues and general objectives

Recent evidence suggests that wind damage, which currently accounts for more than 50% by volume of forest damage in Europe could double or even quadruple by the end of the century (Gardiner et al., 2010). Wind damage impacts considerably the forest economy and the ecological forest survive and functioning in Europe. Most trees during storms are uprooted. While a large amount of work has been made over the last decade on understanding the aerial tree response to turbulent wind flow, much less is known about the root-soil interface, and the impact of soil moisture and the impact of soil fatiguing on tree uprooting. A key component of all forest wind damage risk modelling is to understand mechanical interactions between the roots and the soil. The role of soil conditions on storm damage is important (soil saturation, soil texture, soil structure). Tree anchorage sharply decreases with water logging, heavy rain and poor drainage. These effects are expected to increase in the future with the higher rainfalls induced by climate change. The project TWIST is focus on tree anchorage mechanism that requires substantial advances in soil-root mechanics. This represents the current missing link for modelling the whole uprooting process. TWIST will provide a key skill needed to embrace the problem of tree stability during windstorms at a larger scale and to suggest practical solutions for reducing the risk of tree damage in a changing climate.

Our methodology is based on several essential choices made in our project. These are the following:
1. Investigations are conducted on individual Pinus pinaster (Pin maritime) trees, anchored in a sandy soil, including the increase in soil water content in winter that is predicted to occur with climate change;
2. Experiments are conducted to quantify the effect of soil conditions on root anchorage and to test the model developed for the root-soil response for static and dynamic solicitations;
3. The deformation of roots and aerial plant parts are calculated using the finite-element software ABAQUS;
4. The turbulent wind flow will be simulated using the LES model developed at ISPA;
5. A long-term experiment (6 months) will be conducted at tree scale to measure tree motion in various wind and soil water content conditions for the pedoclimatic conditions of the Aquitaine region.

TWIST provides the first model of tree anchorage strength for P. pinaster derived from the mechanical strength of individual roots. This new model was developed for root anchorage, including the successive breakage of roots during uprooting. The breakage of individual roots was taken into account using a failure law derived from previous work carried out on fibre metal laminates. Soil mechanical plasticity was considered using the Mohr–Coulomb failure criterion. The mechanical model for roots was implemented in the numerical code ABAQUS using beam elements embedded in a soil block meshed with 3-D solid elements. The model was successfully tested by simulating tree-pulling experiments previously carried out on a tree of P. pinaster. The generic nature of the model permits its further application to other tree species and soil conditions.

By the end of the project, the prospects are divided in three main tasks:
- to measure tree motion in various wind and soil water content conditions for the pedoclimatic conditions of the Aquitaine region during a long-term experiment (6 months);
- to develop a model that calculates the deformation of roots and aerial plant parts using the finite-element software ABAQUS; to test this model against long-term experiment data when calculating the tree deformation under turbulent wind flow using the LES model;
- to propose a simplified model for tree stability to be used at larger scale in wind risk model.

Yang, M., Défossez, P., Danjon, F., Fourcaud, T., 2014. Tree stability under wind: simulating uprooting with root breakage using a finite element method. Annals of Botany 114, 695-709.
Ming Yang, Pauline Défossez, Frédéric Danjon, Thierry Fourcaud. Understanding tree root anchorage with the finite element method : a focus on the impact of root system morphology. In IUFRO 2014 Wind and Trees International Conference, 2014
Défossez P., Cointe A., Coureau J-L., Morel S., Bonnefond J-M., Garrigou D., Lambrot C., Danjon F.2014. Propriétés mécaniques des racines de structure de Pinus pinaster. Les journées scientifiques du GDR 3544. 12-14 nov. 2014. Nancy
Meredieu C, Brunet Y, Danjon F, Défossez P, Dupont S, et al. 2014. Innovations Agronomiques 41: 43-56
Meredieu C, Brunet Y, Danjon F, Défossez P, Dupont S, et al. 2014. CIAg Envrironnement « Intensification durable des systèmes de production forestière » 3 déc.2014. INRA Bordeaux

Wind damage represents more than 50% by volume of forest damage in Europe. Recent evidence suggests that wind damage could double or even quadruple by the end of the century due to climate change with dramatic consequences for the forest economy and the ecological functioning and survival of European forests. Most trees during storms are uprooted. While a large amount of work has been done over the last decade on understanding the aerial tree response to turbulent wind flow, much less is known about the root-soil interface, and the impact of soil moisture and root-soil system fatiguing on tree uprooting.
For this reason, the first objective of TWIST is to estimate at tree scale the effects of soil conditions, such as water saturation during storms, on tree stability. This will use new techniques developed for studying the mechanics of vegetal structure incorporating 3D roots architecture and numerical biomechanics modeling.
The second objective consists of developing a methodology to predict at tree scale the occurrence of tree uprooting as a function of microclimatic conditions (wind flow and soil water content). To this purpose P. Défossez and her colleagues of UR EPHYSE (INRA, Bordeaux) will develop for the first time a complete wind-tree interaction model accounting for tree anchorage and tree motion in wind storm conditions, by combining recent advances in wind-tree interaction modelling and the results obtained in this project on tree anchorage. Modelling the dynamic response of both, root-soil system and tree aerial part, as intended here, is very challenging, especially as root and soil strengths change during storms.
The third objective of TWIST is to deduce a simplified uprooting tree model from our complex mechanistic methodology that could be implemented in wind risk models at regional scale. This will be done in connection with the integrative project on risk management at the landscape scale, conducted by B. Gardiner (EPHYSE, INRA Scientific Package, 2012-2015). The contribution of TWIST in FRAGFOR will improve the mechanical anchorage representation in forestry tools in relation to forestry (methods of plantation…) and soil management (soil drainage, soil tillage…).
TWIST focuses on the maritime pine cultivated on sandy soil, which is representative of the forêt des Landes in the Aquitaine region (France), forest that has been heavily damaged by windstorms over the last 15 years. This forest plays an important role in economic, social and ecologic developments of the region. Aquitaine is a region of intensive research on wood and forests as shown by the development of wind flow and biomechanical models, and the collection of databases on maritime pines.
The research unit UR EPHYSE has developed an international expertise on aerial turbulent flow and its interaction with plants. TWIST will support a new scientific field in EPHYSE, conducted by P. Défossez, on root anchorage in the soil. It will include (i) the development of models describing the tree mechanical strength within the soil (in collaboration with T. Fourcaud, UMR AMAP, CIRAD, Montpellier), (ii) the realization of laboratory tests to measure the mechanical strength of the soil-root system, (iii) the deployment of field experiments to study the effect of soil water conditions and dynamic fluctuations on the mechanical strength of tree (in collaboration with F. Danjon, UMR BIOGECO, INRA, Bordeaux), and (iv) the development of simplified relationships for tree uprooting as function of key microclimatic characteristics to be incorporated in wind risk models at landscape scale. In this favorable research environment, TWIST will give a unique opportunity to bring together peoples modelling airflow, soil scientists, root scientists, and people doing risk modeling for developing a unique mechanistic model including all components of the soil-tree-wind system.

Project coordinator

Madame Pauline DEFOSSEZ (INRA UR 1263 EPHYSE)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

EPHYSE INRA UR 1263 EPHYSE

Help of the ANR 224,981 euros
Beginning and duration of the scientific project: September 2013 - 42 Months

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