| Self-competition |
Edward
Frank |
|
Jun
20, 2007 18:32 PDT |
Self-Competition
Awhile back we were having a discussion about the concept of
self-competition in trees relating to the loss of branches. Lee
Frelich
characterized it as self-competition, while I thought it was
better
described as optimization. There is weight in terminology. I
don’t
think self-competition is the right way to characterize the
process. To
a degree we are bound by the terms we use to define processes.
The
boundaries of our exploration of a concept is limited by these
boundaries.
The problem in my mind is what advantage is gained in terms of
genetics
by having branches compete against each other? Are they
competing in
the same way as trees compete with each other?
What I wonder, is if the idea is to capture all of the light and
to deny
it to other trees or lower branches, why are the leaves at the
top not
opaque? Why do they not capture all of the light, which it the
logical
extension of the process growing taller to capture more light at
the
expense of its neighbors. They have had almost
70 million years to
evolve opaque leaves, why haven’t they done so?
There are two examples that almost fit this concept. Hemlock
trees
capture almost all of the light passing through their mass of
needles,
but that includes needles at all levels of the tree. They also
appear
to use allopathy – chemical releases to limit growth of other
plants in
their understory in addition to capturing most of the light. The
other
example are the palms. These are not true tree, but their leaves
are
clustered at the top of the stem rather than consisting of
branches at
all different levels of the trunk.
Is there some advantage to allowing light to pass through the
upper
leaves, or alternatively is there some limitation that prevents
them
from capturing a higher percentage of the light. Letting light
through
seems a strategy that is being pursued. Leaves at the top of
most
broad-leaf angiosperms seem to be smaller and more transparent
than
leaves lower in the canopy. Some of the giant Eucalypts in
Australia
have photosynthetic bark to capture some of the light that is
passed
through the canopy. Of what benefit is this strategy to the
trees? Did
it initially foster growth of its offspring to replace it when
the tree
dies? What other advantages could it gain the trees?
Alternatively, is
there some limit to how efficient a leaf can be in capturing
light-
maybe a maximum density of chlorophyll or something similar? Why
not
multiple layers in each leaf rather than multiple layers of
leaves?
If competition with other trees were the primary goal, then
capturing
more of the light higher would be a better strategy. Letting
light
through is a gift to competitors. Perhaps the
idea of competition is
too simplistic. We have symbiotic organism that both benefit
from a
relationship. There are those where the competition is tooth and
nail,
fight for the death of one organism or the other. What about
levels of
competition in between? Perhaps isolationism – one organism
ignores the
existence of another, coexistence where they both vie for
resources but
there are not any real adaptations to limit competition,
cooperation to
a degree with limited benefits to each other – a symbiosis
light, what
about competition, but not too hard? Are there degrees of
relationships
between organisms using the same resources somewhere between
symbiosis
and strong competition?
Ed Frank |
Ed-
You raise some worthy questions...I'll take a stab at some of
them, in the body of your text below, IN SMALL CAPS...:>}
From: Edward Frank <edfr-@comcast.net>;Reply-To: ENTST-@topica.comTo: ENTST-@topica.comSubject: Self-competitionDate: Thu,
>Self-Competition
>
>Awhile back we were having a discussion about the concept of
>self-competition in trees relating to the loss of
branches. Lee Frelich
>characterized it as self-competition, while I thought it was
better
>described as optimization. There is weight in
terminology. I don’t
>think self-competition is the right way to characterize the
process. To
>a degree we are bound by the terms we use to define
processes. The
>boundaries of our exploration of a concept is limited by
these
>boundaries.
>
>The problem in my mind is what advantage is gained in terms
of genetics
>by having branches compete against each
other? Are they competing in
>the same way as trees compete with each other?
>
>What I wonder, is if the idea is to capture all of the light
and to deny
>it to other trees or lower branches, why are the leaves at
the top not
>opaque? WHILE IT VARIES
FROM SPECIES TO SPECIES, MANY SPECIES HAVE SMALLER
LEAVES/NEEDLES WHERE THEY GET FULL EXPOSURE, AND LARGER LEAVES
WHERE SUN ISN'T SO AVAILABLE...THERE IS A BALANCE STRUCK
THERE...IN AN IDEAL PLANT WORLD, THE CHLOROPHYLL WOULD BE
TRANSLUCENT...AS IT IS, THERE ARE SPECIES THAT PASS MORE
LIGHT THROUGH THAN OTHERS...OTHER CONSIDERATIONS COME INTO PLAY
SUCH AS MOISTURE RETENTION, STRUCTURE, ETC. Why do
they not capture all of the light, which it the logical
>extension of the process growing taller to capture more
light at the
>expense of its neighbors. They have had almost
70 million years to
>evolve opaque leaves, why haven’t they done so? LOGICALLY
IT DOES NOT FOLLOW, ORIGINAL PREMISE IS FLAWED.
>
>There are two examples that almost fit this
concept. Hemlock trees
>capture almost all of the light passing through their mass
of needles,
>but that includes needles at all levels of the
tree. They also appear
>to use allopathy –(ALLELOPATHY)
chemical releases to limit growth of other plants in
>their understory in addition to capturing most of the
light. PROBABLY MORE TO DO
WITH MOISTURE/SOIL NUTRIENT COMPETITION... The other
>example are the palms. These are not true tree,
but their leaves are
>clustered at the top of the stem rather than consisting of
branches at
>all different levels of the trunk.
>
>Is there some advantage to allowing light to pass through
the upper
>leaves, or alternatively is there some limitation that
prevents them
>from capturing a higher percentage of the light. THINK
OF SOLAR PANELS THAT YOU MIGHT PUT UP TO POWER YOUR HOUSE,
YOU WOULD WANT AS MUCH LIGHT BE CAPTURED AS POSSIBLE, OPTIMIZING
THE SOLAR ENERGY YOUR SPACE OCCUPIES... Letting
light through
>seems a strategy that is being pursued. Leaves at
the top of most
>broad-leaf angiosperms seem to be smaller and more
transparent than
>leaves lower in the canopy. I
THINK YOU'RE ON TO IT HERE. Some of the giant
Eucalypts in Australia
>have photosynthetic bark to capture some of the light that
is passed
>through the canopy. OPTIMIZATION
AGAIN... Of what benefit is this strategy to the
trees? Did
>it initially foster growth of its offspring to replace it
when the tree
>dies? What other advantages could it gain the
trees? Alternatively, is
>there some limit to how efficient a leaf can be in capturing
light-
>maybe a maximum density of chlorophyll or something
similar? Why not
>multiple layers in each leaf rather than multiple layers of
leaves? THE ARRAY OF LEAF
STRUCTURE/SHAPE/ORIENTATION STRATEGIES IS INCREDIBLY VARIED AND
INTERESTING
>
>If competition with other trees were the primary goal, then
capturing
>more of the light higher would be a better strategy. COMPETITION
WITHIN A TREE, AND BETWEEN TREES SAY IN THE SAME SPECIES WOULD
BE QUITE SIMILAR IN STRATEGY AND OBJECTIVES, COMPETITION BETWEEN
SPECIES MAY HAVE ENTIRELY DIFFERENT STRATEGIES, BUT SIMILAR
OBJECTIVE, OPTIMIZING ENERGY CAPTURE PER UNIT OF SPACE OCCUPIED Letting
light
>through is a gift to competitors. Perhaps the
idea of competition is
>too simplistic. We have symbiotic organism that
both benefit from a
>relationship. There are those where the
competition is tooth and nail,
>fight for the death of one organism or the
other. What about levels of
>competition in between? Perhaps isolationism –
one organism ignores the
>existence of another, coexistence where they both
vie for resources but
>there are not any real adaptations to limit
competition, cooperation to
>a degree with limited benefits to each other – a symbiosis
light, what
>about competition, but not too hard? THE
SYNERGY OF INTERSPECIES "ACCOMMODATION"
IS WHAT MAKES "PLANT COMMUNITIES MORE SUCCESSFUL THAN
THEY'D BE OTHERWISE, AND OVER TIME, THOSE COMMUNITIES THAT ARE
MOST FUNCTIONAL FORM CANDIDACY FOR OLD-GROWTH FOREST
ECOSYSTEMS, OR AS GARY SUGGESTS AUTOPOETIC (sp?)
COMMUNITIES... Are there degrees of relationships
>between organisms using the same resources somewhere between
symbiosis
>and strong competition? I'LL
APPROACH THIS QUESTION IN A LATER
Don
|
| Re:
Self-competition |
Lee
Frelich |
| Jun
21, 2007 16:10 PDT |
Ed:
Leaves are very inefficient at capturing light, and (except for
extremely
suppressed trees in deep shade), they can capture more light if
it can go
through the surface of the leaf into the interior of the leaf,
where
several layers of cells with chloroplasts may occur, and through
to other
leaves below. If leaves were more efficient at capturing solar
energy, then
it might be a good strategy to have opaque leaves at the top of
the tree.
However, like most things in nature, leaves are poorly designed
because
during evolution they had to make do with what was available,
rather than
having a design for optimum efficiency.
Regarding branches, the competition among branches might
sometimes lead to
optimization, but that's only because they are connected to the
same stem
down below, and they brush against each other and sort things
out so that
they fill the available space, not because the tree has some
strategy for
optimizing the use of space or resources.
Lee
|
| Re:
Self-competition |
Edward
Frank |
| Jun
21, 2007 18:23 PDT |
Lee,
It is a form of speech I guess. Of course the tree does not have
an
intelligent strategy, but things that have been successful in
the past
become programmed in as part of their genetic code. So there is
a
cumulative genetic strategy in how the tree deals with varying
amounts of
light at different levels.
Ed
|
| Re:
Self-competition |
Thomas
Diggins |
| Jun
21, 2007 19:15 PDT |
Evolution is often driven by trade-offs, in addition to the
constraints
of "working with what you have" rather than what might
be best. I can
speculate on a few trade-offs in terms of leaf anatomy
adaptatations...
An opaque leaf might have to be thicker, so gas exchange might
not be
sufficient to keep up with the greater amount of intercepted
light
(i.e., too little CO2 or too much O2 inside the leaves for
maximum
photosynthetic efficiency). Unless, of course, there were more
and /or
bigger stomata, but then water loss would be more of a problem,
and so
on... Also, opaque leaves might be substantially heavier,
requiring
concommitantly more architectural support (especially at the top
of the
tree), perhaps more than could be generated by any additional
photosynthetic productivity. Evolution doesn't necessarily
produce the
best, but more often merely the good enough, especially within
the
constraints of phylogeny, physiology, etc. I have a feeling a
lot of
the academics on ENTS were probably assigned in some grad class
somewhere Stephen Jay Gould's "Spandrels of San Marco/Panglossian
Paradigm" article, which argues creatively against
over-reliance on
adaptationist explanations for ALL biological traits. Cool
discussion;
I'm glad I checked email tonight!
Tom
|
| Re:
Self-competition |
Jess
Riddle |
| Jun
21, 2007 19:48 PDT |
Ed,
Interesting questions. You touch upon several issues that are
basic
to plant biology and ecology that I'm interested in too. I don't
have
answers to most of your questions, but I have a few guesses that
might
shed light on some areas.
Leaves may be more efficient at gathering light than they appear
at
first glance because plants are not concerned with the same kind
of
light humans are. Some wavelengths allow for much more efficient
photosynthesis than others, and I would guess much of the light
we see
transmitted through leaves is of poor quality as far as the
plants are
concerned. Capturing all light energy could actually be
detrimental
to the plant due to heat stress damaging the leaves. Additional
layers of photosynthetic cells could also hurt a leaf's energy
budget;
they would probably capture much less light, but respiration and
nutrient requirements would be almost as great as the other
photosynthetic layers. The upper photosynthetic layers would
always
shade the lower layers, but the optimum position for a leaf
would vary
with the position of the sun.
Jess
|
| RE:
Self-competition |
Edward
Frank |
| Jun
22, 2007 00:02 PDT |
Lee, Don, Jess, Tom,
OK, lets suggest two propositions. The first is that individual
branches suceed or fail in a particular tree based upon simple
competition. If the chance is favorable and they get enough
light they
will survuve. If they are not recieveing sufficient light they
will
hang on until the end trying to compete and survive, until they
die.
(Perhaps that is overly dramatic - but please allow some poetic
license.) The alternative proposition is that different branches
will
each have a random chance of having a favorable position or not.
However instead of these branches in less favorable positions
fighting
it out to the bitter end, perhaps some mechanism exists that
cuts them
off before they use up other resources. Energy, water, and
nutrients
are not wasted on a foregone failures. These assets are left to
branches
that will survuve.
I can see that an adaptation like this would be ferasible, but I
don't
know if it actually might work that way or not. Is there some
ideas
about how this concept could be tested? I see these as two
distinctly
different processes.
Ed
|
| RE:
Self-competition |
Lee
E. Frelich |
| Jun
22, 2007 06:57 PDT |
Ed:
Yes, I believe an experiment could be done to test this.
Basically the item
to measure would be whether the carbon balance of a given branch
is
negative or positive. If a branch is maintained even though it
is drawing
more photosynthate from the rest of the tree than it produces
(negative
carbon balance), then it could survive until the last leaf dies,
and the
determination of whether to keep the branch would be made at the
leaf
level. If the branch is cut off as soon as it develops a
negative carbon
balance, even though it has live leaves at the tip, that would
be evidence
for a response at the level of the entire tree.
It would be a difficult thing to measure, although someone has
probably
done it. There is a lot of dreadfully boring tree physiology
literature
that I don't read.
Lee
|
| Re:
RE: Self-competition |
Thomas
Diggins |
| Jun
22, 2007 09:18 PDT |
Hi folks,
Plants DEFINITELY translocate valuable nutrients, etc, from
senescing
tissues. In fact, quite a bit of energy is reabsorbed from
leaves
before they fall. Details? Not an expert on this stuff...
Tom
|
| Re:
RE: Self-competition |
Neil
Pederson |
| Jun
22, 2007 10:42 PDT |
| RE:
Self-competition |
Edward
Frank |
| Jun
22, 2007 19:27 PDT |
Don, Others
My logic isn't flawed but perhaps it was overstated. One of many
possible eveolutionary strategies would be to grow opaque leaves
near
the top of the trees. By postulating possible strategies, it can
allow
an evaluation of real life situations. Was there any indication
that
there was a trend toward this solution? If so, how much? If not,
was
there some flaw in the premise? Were other strategies more
effective?
Were there limitations that prevented the postulated end to be
achieved?
In response to a major change, such as an extinction event ot
climate
shift, there will be a wide variety of responses from the
surviving
populations. Each of the different adaptaions that result
represent a
different evolutionary strategy. By strategy, I mean a series of
changes along a specific evolutionary path. Often once a path is
taken,
say trees grow taller, further changes will tend to enhance or
augment
the existing changes along the same direction as the initial
step. This
is an observation based upon my paleontology studies. What I am
saying
that is that if a strategy is viable, and if large numbers of
organisms
are affected, and particularly if there are a variety of
different kinds
of organisms present, then most of the viable strategies will be
pursued.
Given time and a stable situation, many of the viable strategies
will be
prusued as they will give the particular organism a slight
advantage and
will promote that trait being passed on to the organisms
offspring.
I have several of Stephen Jay Gould's books in my library and I
gave him
a rave review in an email post about the ENTS bookstore. His
observations are typically on the mark, and his conclusions
demonstrate
the thought put into the questions. Most of the time I am sure
he is
right. I am also sure time will show he is mistaken sometimes.
Mostly
what will be found is that the concepts he is presenting are not
the
end-all of understanding but steps on the pathway toward
understanding.
I am throwing weird and sometimes off target ideas out into the
mix. I
would like to see others add their own ideas of what is
happening out
there into the overall mix.
Ed Frank |
| Re:
RE: Self-competition |
Thomas
Diggins |
| Jun
23, 2007 17:04 PDT |
Actually, Ed, there's NO flaw at all in any of your logic. One
of the
most useful (and fun) evolutionary exercises for both professors
and
students alike is to try to envision/design the BEST possible
solution
to a set of potential selective pressures, and then to postulate
WHY
reality may not necessarily equate to maximum potential. It is
often
these "shortcomings" in biological reality that tell
us much about the
way life has evolved. In a slightly different twist, many
ecologists
view exceptions to their models and hypotheses as more
interesting than
agreements. After all, exceptions raise NEW questions, rather
than just
reiterating the answer to an old question. Of course, if all or
most
data are exceptions to your model, then obviously your model
sucks...
unless you're a politician!
Tom
|
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