FLASH! - New Report says plants love SPS, why not environmentalists

> Yes, this is what I have been told by a botanist. I was thinking
> linearly also, being an engineer and physical scientists, and that
> doubling the light intensity would double the yield, and was told
> that would not work, plants didn't need and could not use most of the
> light they got. Obviously at some point reduced light intensity
> will reduce the yield, but at what level of reduction? How much do
> clouds reduce the light intensity? Do farmers get different yields
> in deserts (with water) over land where it rains but is covered with
> clouds most days? I don't have specific answers, but would think
> relying on what Zubrin suspects is dangerous. Jay Huebner

Ah, this question brings back memories... And I should preface this with
IANAA (I am not an agronomist).

Assumptions related to sunlight and yield are probably buried somewhere
in the EPIC code some of the models in Garden With Insight simulation
were derived from.
http://www.gardenwithinsight.com/help100/00000281.htm
http://www.gardenwithinsight.com/help100/00000584.htm
http://www.gardenwithinsight.com/help100/00000530.htm
http://www.gardenwithinsight.com/help100/00000424.htm
http://www.gardenwithinsight.com/help100/00000283.htm
[This last one has the equations referenced below]
Frankly, I have trouble understanding them all myself. :-)

In general, looking at plant growth as a system, no matter what the
maximum growth may be (described below), a plant's real growth is
typically constrained by some nutrient (which includes available water).
So, if water of nitrogen constrain growth to a level below the maximum
possible for available radiation, even if there is less solar radiation,
the restraining effect of lower light may not be as large as one might
otherwise expect (or perhaps even noticeable at all).

Basically, from looking at some of those equatiosn just now (it's been a
long time) it seems the EPIC model assumes a linear correlation with
solar radiation (RA).
Equation 255 (Photosynthetically active radiation):
PAR = 0.5 * RA * (1.0 - exp(-0.65 * LAI))

However, there are other factors such leaf area index (LAI) which it
would seem to me might be affected differently by significantly lower
light (outside the range the model expects and have been tested for by
the USDA ARS) depending on the transmission to successive plant layers.
Obviously, lower light levels might effect temperature which can have
positive and negative effects on various plant characteristics,
especially depending on availabilty of water for transpiration
(otherwise the plants wilt etc.).
Equation 258 (Potential Increase):
deltaB(p) = 0.001 * BE' * PAR
BE* = 100 * CO2 / (CO2 + exp(bc(1) - bc(2) * CO2))
BE' = BE* - bc(3) * (VPD - 1.0), VPD > 0.5

I haven't followed through the connection if any of RA to vapor pressure
[related to humidity] which might make the relation other than linear.
Also, the weather simulation has some empirical correlation assumptions
built into it which might also effect this.

There are much more detailed models of plant growth than the one we used
in GWI. For example, there is the CERES models. See for example:
http://www.wiz.uni-kassel.de/model_db/mdb/ceres-maize.html
Other models can be found here in either The Register of Ecological
Models (REM) or ECOBAS:
http://mars.wiz.uni-kassel.de/model_db/
I'm not saying looking at the equations in these sorts of models is a
definitive answer compared to looking at the actual science; just that
they are a place to start in understanding the issues.

In general, to maximize productivity, you want to grow plants with a
physical structure and preferences for temperature and humidity and
water and CO2 etc. for the actual conditions, thus I agree with the
earlier suggestion of choosing shade loving plants. In the GWI model,
some of these preferences are reflected in parameters that vary by
plant, but to be frank they are probably not all that accurate and
haven't been refined enough for each specific plant (we drew mostly from
EPIC which focuses on common field crops, augmented some by SPUR,
another USDA ARS model focusing on rangeland and grasses).

When we started poking around in this we discovered how much basic
research in understanding plant growth in terms of geometry and complete
lifecycle growth information in extreme detail has either never been
done or is effectively lost to current memory; the hot topic has become
cell structure and operation in terms of biochemistry, as opposed to the
basics numbers related to yield. Also, scientists may have this sort of
basic information about specific cultivars and not publish it, as many
scientists are often loathe to publish useful raw data they collect lest
someone else get a derived publication out of their hard work. :-)

--Paul Fernhout