Maurits comments emphasize the importance of being careful about what we are
addressing.! Dereck seemed to be concerned about the initial value of leaf area
per plant ... which in Ceres is embedded in the code, and is essentially a
genotype (at the moment species!!??) characteristic. The leaf area per unit of
ground area obviously  depends on this characteristic plus the plant population
... and changes in this latter aspect could alter the impact of changes in the
initial leaf area per plant. Again emphasizing the need for caution when playing
around with initial LA/plant!
Tony

Maurits Van den Berg wrote:

> 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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