Re: allelochemical v. nutrient competition

> George Booth <booth at hpmtlgb1_lvld.hp.com> wrote:
> I think all of us at one time or another have stated that it's good to
> densely plant because "plants outcompete algae for nutrients".
> I've always felt uncomfortable about this.  Does anybody know the
> basis of this statement?  Can plants use nutrients faster, thus
> removing them from the water before algae can use them?  This seems a
> little far fetched.  Nutrients should be evenly distributed in the
> water column, easily usable by anything nearby.  
> I prefer the "allelochemcal" reasoning for dense planting.
> Any opinions?

A standard unit of measurement in my field of Forestry is "basal 
area".  Simply stated, an acre (or hectare, or any unit of land) will
support only so much "basal area", which is merely the total sum
of all the cross-sections of trees at 4.5' from the ground (called
"Diameter at Breast Height", or DBH).  For example, one acre on one 
site may support only 20 square feet of tree basal area.

As foresters, we have options:  we can grow a zillion trees with 1"
diameters, or (conceivably, limited by species morphology) we can
grow one huge tree on that acre with a six foot trunk diameter.
Regardless, that's all there is.  One site can support only so
much basal area.

A site at capacity (where basal area on the site equals that basal 
area the site will support), net productivity is low.  By cutting the
population back, you open the door for faster (and far higher net) 
nutrient fixation into biomass.  This is an argument in support
of cutting old-growth (mature) forests to favor younger, more
efficient, more productive growth.  We do that in our aquariums 
and gardens all the time.

Of course, aquatic plants are somewhat different from woody 
terrestrial plants, in that they are often able to fix biomass 
far more efficiently without competing for basal area (which is 
often primarily important because of competition for light).  
Trees can't get taller just because nutrients and light is 
available, unlike anacharis.  Stem plants in the aquarium can 
grow and add biomass much more opportunistically than can woody 
terrestrial plants.  Thus, stem plants are much better able to 
continually fix nutrients with a limited (and even constant) 
access to square inch of substrate (a key factor in "basal area" 

I think I agree with George primarily.  While a dense vegetative
population might most EFFECTIVELY remove additional nutrients from
the water column (and possibly be able to react/buffer changes most
quickly), we are really dealing with an asymptotic decrease in
vegetative biomass production the moment competition enters into
play.  A densely planted tank will certainly *not* remove nutrients
most EFFICIENTLY (competition is a far too great a factor).  I would 
propose that a well-planted thank where the species do not choke 
each other out is equally able to compete successfully with algae 
for the nutrient base; more plants may provide incremental levels of
buffering potential, but this will largely not be realized unless
a sudden nutrient surplus is available and the species density
allows net productivity despite the heavier competition.  For 
steady-state systems, I would think that densely planted verses
moderately planted aquaria is much less an issue.

Of course, if the tank is planted such that no competition is
occurring among your "desired" vegetation, the "undesirable" form
will rear its ugly head (algae).  We *want* some level of competition
in our tanks, which can be done at either end of the sigmoidal
curve.  I conclude we need only a minimal level of competition,
and a moderately (not heavily) planted aquarium is sufficient.

Also, you don't have to prune and throw your cuttings away as 
often!    :^>

--charley                            Fort Collins, Colorado USA
charleyb at gr_hp.com	or	charley at agrostis_nrel.colostate.edu