Allellochemical competition w/neighbors

> From: Stephen.Pushak at saudan_HAC.COM
> When you talked about root competition, you didn't mention any
> chemical mechanisms (aside from pH) such as allelochemicals
> in the soil or substrate and I was curious if that might be a
> factor. In other words, some plants may inhibit their less
> competive neighbours to different degrees chemically.

Sorry!  In my long-winded drivel, I guess I still missed a few

The "allelochemical" issue is a very real one, and I didn't list
it because I thought it was so well supported in this group.
Soil chemical balances are very important also (you won't get
anything but buffalo grass on a site with a pH of 4.1), but you
got me going on allelochemicals so I'll talk about that!  :^>

Although potentially significant anywhere, I personally believe 
that allelochemicals is much greater a factor for terrestrial 
plants (over aquatic plants) where the potential for lower-water 
availability (during key dry periods of the year) make competition 
so much more severe and allelochemicals so much more toxic and 

Many terrestrial plants use allelochemicals so overtly you could
even call it a "predatory weapon".  The field forbs plant "kingsweed"
puts out such a high concentration of toxins, patches of the
stuff can grow and wipe out plants many inches away, before
the neighbors can even compete!  The same thing is done by sagebrush
all over the western US:  if left undisturbed by fire or plowing,
sagebrush wipes out all grass and other forbs in the area.  You
can even smell the allelochemicals for sage--it does go in the
soil, yes, but that strong smell inhibits other vegetative growth
above ground in high concentrations.  I like the smell of sage,
but I'm afraid the smell inhibits me in high concentrations too.
(So there aren't high concentrations of me around sage).  It kills 
insects, too.  Well, both the allelochemicals *and* high 
concentrations of me kill insects.

Again, both of these are examples of dry-climate competition where
allelochemicals are the biggest club (the biggest weapon) the
plant can use.  During the wet season, plants don't need it:  although
some competition exists for water, space, and nutrients, the REAL
test on whether the plant is there next year is if it can survive
the "bad times" when things get dry, and competition gets tough.
This is when the allelochemicals are most toxic and most effective.
In fact, during "wet times" the tool is just not as effective:
enough water will disperse the toxins, or at least water it down
enough that it is less effective.  Most life forms can handle many
forms of toxins if you "water it down enough" and distribute it
low enough, as compared to the developmental success (Net Primary
Productivity) of the poisoned individual.  

(Darn those complex systems!  I will point out that for VERY arid
climates, vegetation may be so far apart that competition does
not exist between individuals or among populations, and 
allelochemicals thus have no value; the vegetation is thin because 
harsh conditions "thin" the growth.  A good metaphor would be the 
figurative conflict in literature of "man against man" 
[plant/population against plant/population] or "man against nature"
[plant/population against nature].  Put them together and things
REALLY get complex.  :^> ).

Some species simply decide not to compete (would we call these
"pacifists"?  ;^> ), or specify WHEN not to compete.  Typical
arrangements are like the C3 and C4 grasses:  C3 grasses get
growing early in the year, take advantage of the high water
availability, and bloom, complete their life cycles, and bail.
(Like Bromus tectorum).  This works well for annuals, but many
perennials do this also.  C4 grasses are slower to get started,
and are more adept at competing for the efficient uptake of
water and nutrients, and dominate late in the year or during
those long dry summers.  (Like Agropyron smithii).  Each has 
developed its own niche, and actually "share" space during different 
times of the year.  One of my favorite pastimes (when I have a LOT
of time) is to watch the species shift during the year (once you 
know which species do what, it's easier to watch; and you know 
how much summer you have left!  :-)

Because our aquatic systems offer greater potential for diffusion
and equilibrium with chemicals (a medium or "universal solvent" of
water is always available), I'm guessing that allelochemicals
just aren't as effective as they are on land (although non-circulating
substrates may increase the effectiveness in our aquariums).  
The mere fact that many aquatic plants exist in moving water
suggests to me that evolution would not favor a significant
allelochemical role.  (The toxins would be dispersed too quickly
to be effective.)  However, that's not to say it *can't* be 
effective:  It can be such a strong factor on land, it only makes 
sense that the same principle is (at least minimally) active in 
aquatic systems.  For terrestrial systems, some plants put out 
more, some less.  Some have specific toxins that work more 
effectively against target species, some not.  If we follow 
"Jacob's ladder", I would suppose that some terrestrial plants do 
not exude any allelochemicals, although I'll bet they are marginally
present in small degrees for many.  However, I would point out that
I have *not* seen an aquarium plant that I would say "strongly" 
exudes allelochemicals to suppress growth/competition from 
neighbors (although I believe it can be there to a lesser degree). 
I personally believe that allelochemicals in our aquatic systems 
more provide and incremental advantage (even though that may be 
a significant incremental advantage) that a species must use in 
coordination with its other options in its competitive "bag of 

A funny story:  (Well, funny to foresters, maybe not to the rest
of you)!  :^>  There was a guy in Illinois that was re-planting
an area with black walnut.  This is a high-value tree, but it
grows "really screwy" with crooked bole (trunk) and branches all
over the place.  For many of the deciduous trees with commercial
value, foresters must "train" the plant to grow tall and straight.
This guy decided he would use white pine to "train" the black
walnut (most pines are "excurrent" with a central bole, and thus
grow straight inherently).  

The site was planted heavily with white pine intermixed with 
black walnut.  The white pine grew much faster, and would
soon overtop the walnut.  However, the walnut would grow tall,
straight, and with few lateral branches (putting all its energy
into increased height) to try and break through the canopy.  Then, 
40 years into the future, the white pine would be harvested and
that would leave a stand of tall, straight black walnut to grow
for another 50 to 70 years.

Anyway, he planted the site mighty thick and it started working 
too well.  The white pine took off and grew like crazy.  The 
black walnut was so over-topped, it was looking mighty thin (but 
was tall).  The forester became increasingly worried as he realized
that he planted the site so thick, because he thought that the
walnut (which was the desired crop) would not be able to compete 
at all and die.  Every week for three years he went out there, 
and tried to figure out how to explain to "the powers that be" that 
a half-a $million planting operation killed all of the desired 
crop.  (When you vegetate big sites, you are talking big bucks 
to plant, trim, or try to do anything).  

Then, literally almost overnight, all of the white pine was 
dead.  It was magic.  Incredible.  Prayers were answered.  Cheers 
were heard for miles around.  The guy could keep his job.  The pine 
was dead where it stood, and the black walnut *finally* broke 
through the canopy and grew out laterally.

How did this happen?  In a coordinated effort, the black walnut
pruduced a sudden onslaught of allelochemicals through its roots
into the soil that wiped out all of the white pine.  I must remind
you that the white pine had previously so obviously been the 
dominant "owner" of the site.  I think they are speculating that 
it was environmentally triggered (like some years are bumper-crops 
for seed production/reproduction, while other years are not).

This time, I really promise to shut up and not talk so much.  I 
really do.  Really.  "Edna, enough!  don't encourage that young man!"

charleyb at gr_hp.com	or	charley at nrel_colostate.edu