EGU General Assembly 2020
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Modelling competition for water between tree and crop roots in an agroforestry system

Florian Heinlein, Xiaohong Duan, and Eckart Priesack
Florian Heinlein et al.
  • Helmholtz Zentrum München, Institute of Biochemical Plant Pathology, Neuherberg, Germany (

In times of climate change, many regions of the world suffer from heat waves and drought periods, which can lead to failure of crops. To a certain extent, irrigation can help to overcome these extreme events. However, in a sustainable agricultural system the water and nutrient applications should be minimized in order to avoid the waste of valuable resources.

Another method to use water more efficiently is the introduction of agroforestry systems, e.g. planting tree strips within a field. On the one hand, these tree strips reduce the evapotranspiration of the crop-soil-system due to shading and reduction of wind speed. On the other hand, temperatures tend to be higher near the trees and the tree roots may deplete available water and nutrient resources for crops.

Recently, an agroforestry sub-model has been implemented into the modular model system Expert-N to simultaneously simulate tree and crop growth. In principle, trees and crops are simulated separately at different grid points next to each other. However, the agroforestry sub-model allows for the exchange of water and matter between the different grid points to simulate mutual influences of trees and crops. Up to now the following processes are considered: shading, distribution of dead tree biomass to the crop area, and changed water distribution as tree roots grow into the crop area.

Depending on the simulated tree root length density at the crop grid points, the tree roots can uptake a certain amount of water from neighbouring grid points. If the total water demand of trees and crops cannot be fulfilled, the water uptake at the respective grid point is reduced for both, trees and crops.

Expert-N is used to simulate the plant production and the water cycle within an agroforestry system. The results comprise plant biomasses, leaf area indices, evapotranspiration, and soil water contents. To show the impact of the agroforestry sub-model on the simulation results, the differences between two simulations, which only vary in the activation of the agroforestry sub-model, are presented and discussed.

How to cite: Heinlein, F., Duan, X., and Priesack, E.: Modelling competition for water between tree and crop roots in an agroforestry system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8581,, 2020

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Presentation version 2 – uploaded on 05 May 2020
Corrected author list
  • CC1: Comment on EGU2020-8581, Sarah Garré, 05 May 2020

    Cool to see that there is an agroforestry module in expert-N now. Might be something nice to try out on the data of our root shield experiment where we put a plastic between tree row and field soil in a trench to compare to the unshielded condition (poplar). We should get in touch!

    • AC1: Reply to CC1, Florian Heinlein, 06 May 2020

      Thank you very much for your comment. Actually, our agroforestry sub-model is not completed yet as some processes are not integrated, this is an ongoing process. Nevertheless, I'm also in the meantime very happy to test the model against various data sets (depending on my workload).

      Actually, my current research is embedded into the project FarmImpact ( dealing with agroforestry in South Africa. As far as I know, at some of the local farms, there are also such barriers in the soil to prevent root growth from the tree strip into the crop.

  • CC2: Comment on EGU2020-8581, Horst H. Gerke, 06 May 2020

    Very interesting! Maybe I missed it but I did not understand what is the main driver controlling the competition - is it the soil water potential?

    • AC2: Reply to CC2, Florian Heinlein, 07 May 2020

      Thank you very much for your question. However, to answer it I think, I'll have to go a bit further into details. I hope that this is OK.
      First of all, a rough description of the "standard" water uptake simulation in our models:
      1. Calculate potential water uptake, this takes into consideration the plant availability of soil water (soil water content - wilting point) and the root length density within a specific soil layer. This value's unit is mm/mm, i.e potential water uptake per root length; we also check for absolute maximum values here, so there is an upper limit
      2. Calculate potential uptake from the soil layer: here the unit changes to mm/day multiplying (1) with root length densities in and the thickness of the soil layer
      3. Calculate potential transpiration depending on the leaf area index and potential evapotranspiration
      4. Compare potential transpiration and total root water uptake (sum of all layers) to get the actual transpiration (= actual root water uptake) including some “corrections” when looping over the layers, e.g. when there is not enough water in one soil layer, more water can be taken from another soil layer
      Now to the agroforestry module:
      - Transpiration and water uptake of trees and crops are calculated simultaneously, even if both are situated at  different “grid points”, which normally are computed separately.
      - for the calculation of (1) we consider the total amounts of root in the soil volume, i.e. roots of crops and trees together
      - for (2): (1) is “separated” by use of individual maximum uptake rates for trees and crops, and by multiplication with separate root length densities of trees and crops within the specific soil volume
      - (3) stays the same
      - total root water uptake (4) is not just calculated by summing up over the soil layers, but for the trees also by summing over the different grid points;
      - actual uptake is the sum of the actual uptakes by the trees and the crop at the specific gridpoints
      I hope that this gives you a rough idea about the simulated root competition and its drivers. I’m also very happy to answer further questions.

      • CC3: Reply to AC2, Horst H. Gerke, 07 May 2020

        Dear Florian,

        many thanks for the comprehensive answer!

        I see it is a model based on the available soil moisture (water content minus PWP) and certain rules to distribute the water.

        All the best and hope to meet you sometimes




Presentation version 1 – uploaded on 04 May 2020 , no comments