|
plus 2șC |
>350 ”mol mol-1 CO2 |
|
Empirical |
Mechanistic |
Empirical |
Mechanistic |
| Leaf Photosynthesis |
Arrhenius or Q10 function is
applied to the photosynthetic rate calculated from a non rectangular hyperbola. (3,4,5,6,7,8,9,10) |
Arrhenius or Q10 function is
applied to the photosynthetic rate calculated from the Farquhar or Collatz models. |
Parameters for the non
rectangular hyperbola which gives photosynthetic assimilation may be changed using an
empirical relationship. |
Farquhar C3, Collatz C4.
Biochemical models respond directly to CO2 as part of their core function. (1,2) |
| Stomatal Conductance |
A relationship between
temperature and stomatal conductance is not usually defined |
Via indirect effects on
assimilation. Iterative calculation of stomatal conductance and assimilation
|
Either not defined, or poor
empirical expression primarily used to give some response of transpiration (via stomatal
conductance) to enhanced CO2. (3 - 10) |
Direct effects on Ball &
Berry model and from stomatal conductance onto transpiration and water loss. (1,2) |
| Leaf [N] |
No relationship between leaf [N]
and temperature is usually included |
Optimum leaf [N] may change due
to increased maximum assimilation rates. Models will respond if optimum leaf [N]’s
are calculated. (1,2) |
Empirical relationships between
leaf [N] and the parameters of the non rectangular hyperbola exist but these are not
modified by enhanced CO2. (3 - 10) |
Direct effects
on the Vcmax/Jmax parameters used in the leaf A/Ci and
light response of the Farquhar model. Linear models from Harley and Field are consistent.
Less certain is how Vcmax/Jmax change in response to CO2.
(1,2) |
| CHO
allocation, partitioning |
Partitioning
coefficients used in conjunction with thermal time calendars. Do the coefficients and
thermal time duration’s still apply? |
Current
physiologically based models of partitioning may respond correctly but are often not
generalist enough to be useful. |
Usually no
direct effect of CO2 on partitioning. Possible effect on C availability from
photosynthesis. But root:shoot will stay the same in most models. (2 - 10) |
A few
physiologically based models respond to different C loadings arising from modified
atmospheric CO2. (1) |
| Organic matter turnover,
decomposition |
Empirical temperature response
function. Many models relate tissue C:N and lignin content of the litter to decomposition
rate using empirical observations. |
Temperature response functions
based on enzyme kinetics and metabolism. The Century model provides a good compromise
between model simplicity and ability to respond correctly. |
Broad overlap in the
methodologies of empirical and mechanistic models resulting from the current lack of
knowledge Potential effects through tissue composition changes, lignin content, C:N ratio.
NPP and litter quality. (1 - 10) |
| Soil C:N status |
Litter formation,
decomposition and plant N uptake rates may all be effected and a number of empirical
models for this exist. Plant N uptake/availability changes not clear. Will N uptake remain
constant? (1,3,4,5,6,7) |
Effects of CO2
remain uncertain. No process based models currently available for direct effects. Some
evidence of indirect effects via N uptake, litter quality and decomposition used in some
models. (1,3,6,7) |
| Soil water status |
Evapo/transpiration via
Penman/Monteith or Priestly Taylor. Often potential water loss is calculated in which
stomatal resistance is not considered. (3,4,5,6,7,9,10) |
Evapo/transpiration via
Penman/Monteith. Actual water loss in which stomatal resistance is considered. Temperature
effects on energy balance. (1,2,8) |
No direct
effect of CO2 on soil water evaporation. Some models use an empirical
relationship between stomatal conductance and plant evapo/transpiration which gives some
response of soil water status to CO2. Others ignore the potential effect
altogether. (3,4,6,7) |
Effect via stomatal conductance
at the leaf level scaled to canopy level with physical models of light interception/energy
budget and resulting in a physical model of evapo/transpiration. Soil evaporation from
energy budget of the soil layers. (1,2) |
| Respiration |
Most models use a fixed
respiration rate per g dry weight of each different structural material in the plant
(maintenance) and a fixed fraction of assimilation (growth) and modify these amounts using
an Arrhenius or Q10 function. |
Experimental measurements of
plant material N content and direct respiration measurements are used to parameterise the
empirical approaches to respiration suggested by McCree and Penning de Vries (1,2).
Few mechanistic models of respiration are used. |
Direct effect of CO2
on respiration is not usually considered. Indirect effects on sink sizes and concomitant
effect on maintenance respiration is usually considered. (1-10) |
Few models use a process based
approach to calculating respiration and opt instead for the empirical McCree and Penning
de Vries approach of considering growth and maintenance respiration. (1-10) |
| Ageing & Senescence |
Elapsed thermal time since tissue
formation will be affected by temperature increase. Will tissues age more quickly under
new conditions? (3,4,5,6,7) |
Few mechanistic models of ageing
and senescence exist and none are used in the group of plant models examined here. |
Direct effect of CO2
on ageing/senescence is not usually considered. May be effects in C:N content of tissues.(1-10)
|
