The Hi-SAFE Microclimate Module Concept

Above-ground modules in Hi-SAFE

(Tree phenology, tree C allocation, tree light interception, microclimate)

Deliverable D.4.1 (SAFE European Research contract QLK5-CT-2001-00560)

Silvoarable Agroforestry For Europe (SAFE)

Christian Dupraz, Grégoire Vincent, Isabelle Lecomte, François Bussière, Hervé Sinoquet

August 2004

Foreword

Hi-SAFE is the detailed 3D process-based biophysical model of the SAFE project. It includes the main tree functions with regard to major resources (carbon, water, nitrogen) and responses to the major climate variables (light, air temperature and humidity). The present text shows how light and carbon acquisition by the trees has been taken into account in the Hi-SAFE module. Requirements of the Hi-SAFE model include sensitivity to a number of environmental and biological factors, and short computation time.

THE TREE phenology module

This module should trigger other modules to give a fair representation of temperate trees phenology.

Budbreak

We suggest to model budbreak date as a function of accumulated temperatures above a threshold. Accumulation start after a starting date.

Budbreak module
Parameters / acronym / Value suggested for Hybrid Walnut / Unit
Date to start accumulation of temperatures / Ph_Date_Start_Acc_T (Ph_BB_DSAT) / 01 January / DOY
Threshold of effective temperature / Ph_Budburst_Effective_T
(Ph_BB_ET) / 10°C / °C
Threshold of accumulated temperatures to trigger Budburst / Ph_Budburst_Acc_T (Ph_BB_AT) / 210 / °C-day

This module triggers the Tree C photosynthesis, Tree water extraction, Tree C allocation, tree growth modules.

End of leaf area expansion

This is a key phenology stage. According to our field observations, the end of leaf extension is strongly dependent on water/nitrogen stress. Even in non limiting conditions for water, nitrogen light and temperature, most temperate tree species exhibit a limited period of leaf expansion. We describe here the end of the first flush of leaf expansion, including preformated and neoformated phytomeres.

The C allocation module allocates every day some Carbon to the leaf pool, and will apparently never predict the end of leaf expansion. This was already a poor feature of HyPAR. This should be modified by the phenology module.

This phenology module should be able to fairly describe the following situations :

Ø A tree with no stress stops expanding leaves anyway at some date

Ø A tree with some stress stops earlier to expand leaves

It is not possible to use a simple date for stopping leaf expansion (even if this date is predicted from tree stress indexes). Leaf expansion ceases gradually. The phenology module, combined with the C allocation module should result in a sigmoid shape for leaf expansion.

We suggest a module using 3 parameters :

Ø A fixed date of end of the leaf expansion in non limiting conditions (potential expansion with no water stress, no nitrogen stress). This can be documented by monitoring well cared trees (irrigated, fertilised…). Some tree species may never stop (Eucalypts, Paulownia), but most temperate tree species will stop. Some trees, after a pause, will resume a second flush, but this is beyond the scope of HySAFE, as this is very unlikely in real conditions of AF plantations.

Ø A fixed delay between the beginning of the leaf expansion rate decrease until the leaf expansion stop. This is necessary to avoid a sudden stop of leaf expansion.

Ø A threshold for accumulated water and/or nitrogen stress to trigger the slow down of leaf expansion.

End of leaf expansion module
Parameters / acronym / Value suggested for Hybrid Walnut / Unit
Date of end of leaf area expansion in no stress conditions / Ph_Leaf expansion_unstressed (Ph_LE_U) / 30 July / DOY
Delay for leaf expansion slowing / Ph_Leaf expansion_delay (Ph_LE_D) / 15 / days
Threshold of accumulated water stress to trigger leaf expansion slowing down / Ph_Leaf expansion_Threshold (Ph_LE_T) / To be discussed / To be discussed

If no stress occur, at a date given by Ph_LE_D - Ph_LE_T, leaf expansion slows down. The rate of leaf expansion can then be linearly decreasing until Ph_LE_U.

It must be discussed with Marcel van Oijen where in the C allocation module we should include this impact. The sum of all C allocation coefficient must remain 1.

Leaf-fall

A very simple module could be triggered by the average temperature of the last 15 days. When this temperature falls below a threshold, leafall starts. Using an average temperature over a 15 days period is useful to avoid taking into account short periods of cold days. Leafall is assumed to occur during a fixed maximum time lapse, but a faster leafall will occur if some climatic events occur : high winds, frost.

Leafall module (temperature driven)
Parameters / acronym / Value suggested for Hybrid Walnut / Unit
Threshold of temperature (average of air temperature over the last 15 days) / Ph_Leafall_Threshold_T (Ph_LF_TT) / 15 / °C
Usual duration of leafall / Ph_LF_Duration
(Ph_LF_D) / 15 / days
Sensibility to frost or high winds / Ph_LF_Sensibility (Ph_LF_S) / If frost or winds>10 m.s-1 occur, full leaf fall / Y/N switch

Root Phenology

A similar approach could be developed for root phenology. The current C allocation module will allow root growth all over the growing season, and will prevent any root growth when the trees have no leaves (is that right?). This could be done in a similar pattern as for the leaves phenology modules. For trees that display root growth before budburst or after leaf-fall, C should be allocated from the C labile pool?

Fruit Phenology

It was decided in Clermont-Ferrand to include a simple fruit sink for Carbon and Nitrogen as a forcing variable.

Fruit sink volume is a forcing variable and should be provided as a time-series (one value per year), or as a function of tree growth/vigour. It could include an alternate bearing pattern.

Fruit sink inception date could be fixed in a first approach, or could depending on climate.

Fruit sink end of filling date could also be fixed, or depend on accumulated stress indexes.

Fruit phenology (forcing variable)
Parameters or forcing variable / acronym / Value suggested for Hybrid Walnut / Unit
Fruit sink volume C / Ph_Fruits_C (Ph_F_C) / Depends on tree age / Kg C
Fruit sink volume N / Ph_Fruits_N (Ph_F_N) / Depends on tree age / Kg N
Fruit sink inception date / Ph_Fruits_Start_date (Ph_F_SD) / 150 / DOY
Fruit sink end of filling date / Ph_Fruits_End_date (Ph_F_ED) / 270 / DOY

A high priority for fruits may be assumed, or may not be assumed. This has to be decided within the C allocation module, and may involve other parameters.

Conclusion

Several phenology modules require accumulated stress indexes. Stress indexes are a key component of the HySAFE model, but were not discussed up to now. They are essential tools to introduce controls in the integrated model. They should be now defined and agreed.

TREE Radiation intercEption

1. Objectives:

The radiation interception module is aimed at computing:

· Incident radiation available to the crop canopy: This is the spatial distribution of the transmitted radiation below the tree canopy. The crop canopy is likely to be divided into strips parallel and perpendicular to the row direction, and the radiation model should compute incident radiation above each crop area.

· Radiation intercepted by each individual tree defined in the scene: Note that the scene could include only one tree.

The radiation model provides inputs for the carbon acquisition module, the water consumption and the canopy microclimate modules.

2. The HyPAR solution:

In the HyPAR model, radiation interception is computed from the turbid medium analogy, i.e. the model is based on Beer’s law. For computation of available light to the crop canopy, the trees are modelled as simple shapes filled with leaf area turbid medium, while the canopy is divided in cells (max. number 20 x 20). A transmission coefficient of the tree canopy is computed for each canopy cell. Surprisingly, computation of light interception by the trees is made by assuming that trees make a multilayer canopy, i.e. not a discontinuous canopy.

Input parameters / variables include:

· Incident radiation: The sky is assumed to be overcast, so that computations can be made by using the only daily incident radiation (MJ m-2 day-1). Note however that simulated radiation exchanges are insensitive to row direction, day of year nor latitude.

· Canopy structure: tree dimensions with regard to geometry used to abstract tree shape, tree leaf area, extinction coefficient. Note that the extinction coefficient globally accounts for the effect of leaf angle, foliage clumping and optical properties of leaves. Note also that canopy dimensions must be simulated by the model, namely tree growth and development modules, but I am unsure that any model is able to cope with dynamics of tree dimensions.

3. State of the art:

The only two ways to simulate radiation exchanges in vegetation canopies are the turbid medium analogy – as used in HyPAR – and ray-tracing and/or projection techniques based on simulated 3D plants. None of the tree models proposed in the literature is able to properly simulate dynamics of the 3D architecture in response to environmental factors; and time needed for light computations on simulated 3D plants is incompatible to the time requirements for Hi-SAFE simulations. The only way is thus to adopt a turbid medium model, although projection models for 3D plants could be used to derive parameters of the turbid medium model.

Improvement of the light HyPAR module could be:

To use discontinuous tree canopies for both the computations of available light to the crop, and light interception by the trees.
To improve the flexibility of the scene definition, i.e. number and location of trees on the scene, and discretisation of the crop canopy.
To better take into account the directional components of incident radiation: at least direct radiation coming from the sun direction, and diffuse radiation obeying a classical sky radiance distribution (e.g. Uniform or Standard OverCast distributions).
To better define the extinction coefficient, as a function of leaf angle distribution, foliage clumping and optical properties. The rationale to explicit the extinction coefficient is well known, and Goudriaan’s expression (1977) accounting for both leaf angle and optical properties could be used.

4. Light model in Hi-SAFE:

Courbaud’s light model (MOUNTAIN, 2003) meets most of the above requirements (namely requirements #1, #2, #3). Moreover, it is written in Java, has been incorporated in the CAPSIS system, and the author was available to help us for code adaptations. The model MOUNTAIN has been therefore chosen to be included in the Hi-SAFE model.

As mentioned by its author, this model was developed for spatially heterogeneous coniferous forest canopies. Based on the interception of light rays by parabolic crowns, it calculates simultaneously the energy intercepted by each tree and the distribution of light reaching the ground. Slope and exposure are taken into account as a function of the distribution of incident light rays. An optimisation process that reduces the computing time needed to find trees which intercept a ray and to manage plot boundaries was developed. A detailed description of the model is given in Courbaud et al. (2003).