166
Carlos A. Gracia, Estlbaliz Tello, Santiago Sabate and Juan Bellot
12.2 Main Features and Initial Hypotheses
The flow chart of the GOTILWA model is shown in Fig. 12.1. The main features and initial hypotheses of the model are:
1. Transpiration is one of the main driving forces of the physiological processes of forest growth.
2. Available soil water is transpired by trees as much as possible. If Eo is
lower than the amount of available water, transpiration reaches the Eo
value. The remaining water can be lost as streamflow according to
Darcy's law.
3. Carbon uptake is calculated from the amount of water transpired using
the water use efficiency (WUE) of the trees. A different value of WUE can
be defined for dominant and suppressed trees.
4. The carbon fixed is transformed to gross primary production (GPP) using the carbon content of dry matter.
5. A part of the GPP is used to compensate the respiratory cost of all the
living tissues. The remaining primary production is used in the formation of new biomass which involves a growth respiratory cost.
6. All the physiological processes such as leaf shedding, leaf formation, etc.
are temperature dependent. GOTILWA uses the value of QIO based on the
monthly average temperature.
7. Translocation of mobile carbon from leaves takes place prior to leaf abscission. The amount of carbon retranslocated depends on the amount of
mobile carbon stored in leaves. Leaf specific mass (LSM) of shed leaves
decreases according to this translocation (Sabate 1993).
8. Leaf area supported by an individual tree is proportional to the sapwood
cross-sectional area in a constant value. Changes in leaf area are translated into changes in the sapwood cross-sectional area.
9. A set of priority rules is used in respiration and carbon allocation processes. If the fixed carbon exceeds the cost of respiration, it is used to increase the pool of mobile carbon stored, first of all, in leaves and, after
that, in woody tissues. The remaining carbon, if any, is used to build new
biomass.
10. During periods in which GPP does not compensate the maintenance respiration, the carbon stored in leaves and in stem sapwood is used to
compensate the leaf and stem respiration while carbon stored in coarse
roots is used to compensate the respiration of fine roots.
11. If the mobile carbon stored does not compensate the respiratory cost, leaf
shedding reduces the amount of leaf tissue and, thus, reduces respiration
until both values, respiration and carbon availability, compensate each
other.
12. If this compensation point cannot be reached, even when all the leaves
are shed, the tree metabolism is unbalanced and it dies.
Carlos A. Gracia, Estlbaliz Tello, Santiago Sabate and Juan Bellot
12.2 Main Features and Initial Hypotheses
The flow chart of the GOTILWA model is shown in Fig. 12.1. The main features and initial hypotheses of the model are:
1. Transpiration is one of the main driving forces of the physiological processes of forest growth.
2. Available soil water is transpired by trees as much as possible. If Eo is
lower than the amount of available water, transpiration reaches the Eo
value. The remaining water can be lost as streamflow according to
Darcy's law.
3. Carbon uptake is calculated from the amount of water transpired using
the water use efficiency (WUE) of the trees. A different value of WUE can
be defined for dominant and suppressed trees.
4. The carbon fixed is transformed to gross primary production (GPP) using the carbon content of dry matter.
5. A part of the GPP is used to compensate the respiratory cost of all the
living tissues. The remaining primary production is used in the formation of new biomass which involves a growth respiratory cost.
6. All the physiological processes such as leaf shedding, leaf formation, etc.
are temperature dependent. GOTILWA uses the value of QIO based on the
monthly average temperature.
7. Translocation of mobile carbon from leaves takes place prior to leaf abscission. The amount of carbon retranslocated depends on the amount of
mobile carbon stored in leaves. Leaf specific mass (LSM) of shed leaves
decreases according to this translocation (Sabate 1993).
8. Leaf area supported by an individual tree is proportional to the sapwood
cross-sectional area in a constant value. Changes in leaf area are translated into changes in the sapwood cross-sectional area.
9. A set of priority rules is used in respiration and carbon allocation processes. If the fixed carbon exceeds the cost of respiration, it is used to increase the pool of mobile carbon stored, first of all, in leaves and, after
that, in woody tissues. The remaining carbon, if any, is used to build new
biomass.
10. During periods in which GPP does not compensate the maintenance respiration, the carbon stored in leaves and in stem sapwood is used to
compensate the leaf and stem respiration while carbon stored in coarse
roots is used to compensate the respiration of fine roots.
11. If the mobile carbon stored does not compensate the respiratory cost, leaf
shedding reduces the amount of leaf tissue and, thus, reduces respiration
until both values, respiration and carbon availability, compensate each
other.
12. If this compensation point cannot be reached, even when all the leaves
are shed, the tree metabolism is unbalanced and it dies.