Derek and others:

The initial LAI value is not exlusively a genotype characteristic. You might
also be simulating a different seeding density (i.e. more seed per ha implies
higher initial LAI at the date of emergence). Keeping the other initial values
and parameters unchanged, higher initial LAI would mean an earlier canopy
closure, and problably higher simulated stem mass and canopy height before
flowering (in reality lodging might occur). The number of seeds produced might
also increase, but the  yield in ton/ha would probably be hardly affected.

A very low initial LAI could imply that you never obtain a closed canopy. In
that case, light interception during seed filling and hence final yield would
be considerably lower.

Best regards,

Maurits van den Berg
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Derek .... you have hit on a really fundamental question! Changing the initial
value obviously changes the amount of light intercepted the first day, and
hence (depending on water stress) the leaf area produced the second day, and
so
on .... and thus can have an effect on the rate of leaf area increase and the
rate of dry matter accumulation. The accumulation curves can thus be changed
in
shape, and not only shifted up or down. The impact on yield and production
depends on the duration of the life cycle (less impact for a long season
winter
wheat than for a short season spring wheat .... in accord with classical crop
physiology!) and on the impact of stresses (water and N). In reality, if you
increase the initial value you are in effect changing the genotype ... and
relative to this you may want to recall that barley nearly always produces
initial leaf area faster than wheat, and so generally does better in very
short
season areas. We really need to have the initial value (or the equivalent in
an
expansion growth term) as a cultivar or ecotype specific characteristic ...
and
this we may have in the next version of Dssat.  For now, take care when
changing the value .... it could change everything!!!!!
Tony Hunt

Derek McNamara wrote:

> Hello,
> If one was to change the initial value of LAI used in CERES-Wheat (.4cm2)
> to whatever, would this result in a new growth curve with a different
> magnitude, or would the magnitude be the same but the growth curve would be
> shifted up or down depending on the change of the initial value of LAI.  It
> seems to me that it would just result in the growth curve being shifted up
> or down.  Is this correct, if not why is the magnitude of the growth curve
> affected by the initial value of LAI?  Thank you in advance for any help.
>
> Derek McNamara

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