Genetic Diversity
  Dec 22, 2005 07:11 PST 

I think that the structure of a tree is based more on environmental conditions than genetics. The amount of light, and nutrients. Trauma to the tree as it grows changes the course of which way and how it grows. Ultimately the tree that is damaged in it's younger years seems to have less of a chance of survival to a great age. Here at work, in the woods, I get to watch things happen. Often a small tree will be beat up by a male deer. Sometimes it will die outright, other times it is disfigured and continues to grow. It is no longer a single stem specimen, but may have an odd shape. If a limb falls from a larger tree and strips branches off a smaller tree, then that tree seems to focus energy to the healthy parts of it's system. This too will result in an atypical form. Often times in nursery stock the leader will die back to a point, but not kill the tree. It seems that the tree will cut it's losses and send out another leader when the conditions improve. There are millions of scenarios that change the structure of a tree that are environmental.

If a tree was grown in a controlled space, without outside interference, I wonder if all tulip poplars would have the same habit (or any species)

I do see genetic differences in trees also. Just yesterday a saw a red oak that was about 10"DBH that had a solid line of buds around about 1/3 of the root flare. I looked up at the tree to see many clusters of odd buds that didn't seem to be growing. I wondered if it was a genetic or environmental oddity.

How diverse do you think the genetic makeup of a species is in a location? Is it more harmful or helpful to introduce plants of the same species from a different genetic pool? I think it is better for humans to be less genetically similar, but what about trees? Is it a case where over tens of thousands of years a genetic strain becomes dominant due to survival of the fittest? Does that in turn make them more susceptible as a group when a new threat is introduced (HWA)?

I hope some of the science-types will chime in, as I would love to hear about research that has been done in these areas. Questions I have had for some time.

RE: Genetic Diversity   Darian Copiz
  Dec 23, 2005 10:46 PST 
Scott, ENTS,

Although I don't qualify as a science-type, I find the topic very
interesting, particularly in regard to genotypes. It is an issue that
has occasionally come up at the Maryland Native Plant Society. For rare
or endangered species it has been pointed out that one should not
introduce plants of the same species from a different location because
it may negatively impact the local genotypes adaptability to the site.
It may also diminish aspects of the local population that make it
unique. Of course with animals there are problems with inbreeding in an
isolated population. I also wonder is this a concern with plant species
as well? The question of whether it is better for a local population of
a species to die out or for it to be augmented through additions of a
"non-native" population is something that appears to be an area of
current debate.

For species that are common and not isolated, perhaps the issue is not
as important. However, it does still have implications on how we use
tree species from different locations. Many of us have seen planted
trees that change leaf color or lose their leaves much later (or sooner)
than the trees of the same species growing naturally in the area. The
use of cultivars is very popular in the landscape industry. Although,
as far as I know, it is not common in the forestry industry I imagine it
is something that could occur. I think the results on ecological
processes and the natural heritage of areas could be disastrous, or at
the least saddening.

In the landscape industry there is increasing homogenity - everything is
becoming the same. I for one, would like to see plant and landscape
diversity preserved. Whereas people used to plant a red maple from the
local genepool, now everyone plants 'Red Sunset' or 'October Glory' or a
few other cultivars. Although these may be fine trees, I think it is
unfortunate that we lose the potential for the fine qualities that other
trees may posess and the loss of uniqueness that every location has. As
a landscape architect I try to avoid using cultivars when possible or
try to use a cultivar that comes from the region that I am designing

A phrase I sometimes like to quote from Aldo Leopold is "If the biota,
in the course of aeons, has built something we like but do not
understand, then who but a fool would discard seemingly useless parts?"
Although this does not pertain directly to introductions it does relate
to the question of how much we should tinker with natural processes. My
opinion is that we should keep meddling to a minumum. On the other
hand, we have altered some things so greatly that it might be necessary
for us to actively meddle.

Other thoughts on the issues? I think it's a great topic for


Genotypes and adaptability   Edward Frank
  Dec 25, 2005 15:54 PST 


A general population of a species contain a high degree of variability, only
some of which is expressed in the general population. In an isolated
population certain genetic characteristics tend to become concentrated.
Those are the characteristics that represent the features that allow the
best fit of the plant to the particular environmental characteristics of the
site. If there is a free gene flow across the entire area then the plants
populations tend to be more generalized in their general nature. Speciation
occurs when the genetic characteristics of a particular population have
become too different to allow viable interbreeding with the remainder of the
population.   This may be a result of simple concentration of certain
genetic characteristics or mutations or both. In strict terms a Chihuahua
and a St. Bernard are the same species, but they can no longer interbreed
because of concentrations of genes responsible for the size of the organism.

When a population from an outside area is introduced into a different area
it can have an adverse effect. Essentially the native species have adapted
to flourish best in that area, and introducing outside plants can dilute the
gene pool. The concentration of characteristics that allow for the optimum
characteristics for that specific area becomes lower as the native genes
intermix with the external gene pool. A second effect is that an isolated
population my not reproduce as rapidly as the introduced population. A
lower reproduction rate may have an adaptive benefit for the local
population. Introducing outside populations into an area, may not only
dilute the local gene pool, but if they reproduce more rapidly they may
replace the local population almost entirely. Faster reproduction may seen
to be a good thing, but if the area is subject to stress, like drought,
freezing weather, poor soils, a stressful situation may eradicate an
introduced more rapidly reproducing population, whereas the local specimens
may have survived the stress.

Mono-culture is not a problem in natural populations like they are in a
planted population. In a planted population the genes from a single
specimen or small group of specimens is reproduced over and over again
thousands of times. In a natural population the genetic variation is still
retained, even within an isolated population. The general traits found in
the general population are still present, although they be represented in by
less frequent expression. Genetic characteristics or physical
characteristics are not lost because they are useless or serve little
function, they are lost because they have a detrimental effect. For example
the eyes of the blind cave crayfish were not lost directly because they were
useless in the dark, they were lost because it cost the organism large
amounts of energy to grow and provided no benefit. In caves with restricted
food supplies, eyeless organisms are more common. In caves, such as many in
the tropics, where the food supply is not a limitation, there are fewer
organisms without eyes. It doesn't cost them much to keep them if there is
food available (Although genetic eye defects may be higher in troglobitic
populations than the general surface populations). Back to the main point
of discussion here.   The genetic characteristics typical of the general
population tend to be retained in isolated populations because they cost the
isolated populations little to retain. Therefore the isolated populations
retain almost all of the genetic variability of the overall population.

Hemlock from a variety of sites have been transplanted to farms and areas
outside the areas affected by Hemlock Wooly Adelgid. One of my concerns I
have expressed to Will and others working on studying these hemlock
populations before they are destroyed, is that specimens and information
from isolated populations or populations in extreme environments may not be
being sampled as they should be. These populations have concentrated
certain genetic characteristics that are more diffusely spread amongst the
general population. Therefore even a systematic sampling of the general
population may not include the genes and certainly not the genotypes
represented by trees in these isolated and extreme environment populations.
There needs to be a systematic sampling and collection of trees from these
unique populations if there is to ever be a hope of reintroducing hemlock
into its former range with specimens that are representative of the
diversity of the species as a whole.

I don't know under what circumstances that reintroduction might take place.
Certainly the booming populations of the adelgid would need to be controlled
if not eradicated. But if this diversity is to be preserved, even for
research farms down the road, these populations need to be sampled now, when
they are still alive.

Ed Frank
RE: plant genotypes   Lee E. Frelich
  Dec 27, 2005 06:22 PST 


I think we should do reciprocal transplants between neighboring populations
of plants in fragmented landscapes--whether they are rare or not. It is
one way of making the populations behave as if they are not in a fragmented
environment. Landscape scale gene flow was an important process that we
have eliminated on many landscapes for 100-300 years in the eastern U.S.

I don't agree with those who think every little variation in genotype is
worth saving. Many of the unique local genotypes could be artifacts of
fragmentation caused by people.

We need to restore landscape-scale gene flow or many species will go extinct.

Re: plant genotypes   Edward Frank
  Dec 27, 2005 18:41 PST 


I have re-read my original comments after reading Lee's post. I still
believe what I wrote previously but I could have been more articulate.
Essentially I was talking about populations that are genetically different
from the general population because of geographic or environmental
isolation. The local plants are specialized for that environment, because
for generations they have selected for characteristics best suited for
growth in that environment. Plants form a longer distance away may grow if
transplanted in that locality, but will not do as well in the long term as
plants from the local area. The best option would be to try to encourage
growth and somehow increase the rate of reproduction of the local plants.
The second best would be to transplant plants from nearby locations to
supplement the plants at the specific locality. Plants from the nearby area
are more likely to have the same adaptive characteristics of the local
plants and should do well. Plants from longer distances may present
problems as I outlined in the initial post. If a species has become
functionally extinct from an area, I see no problem from replanting the area
with specimens from other locations. I don't believe, as Lee sugests, that
the unique local genotypes are an artifact of human cause forest

I must agree fully with Lee concerning the need to transfer species between
areas that have become fragmented by human activities. Human driven change
often occurs rapidly. With intervening highways, suburban areas, farms, and
cities the plants and animals can no longer travel or migrate between these
forested fragments. Over time the populations will become increasingly
depauperate in fragmented forest pockets. I am not one who believes that
the best way to manage a forest for posterity is to leave it completely
alone. I think humans must intervene in some ways to help offset the
effects of human activities, invasive species, and forest fragmentation. I
would support the reintroduction of species that are lost or are all but
gone from the original forest setting.

With global warming there may be a need in the future to do more. Lee
presented arguments at the last Forest Summit that the rate of global
warming may shift the local climate at a faster rate than the tree can
migrate northward, especially in light of the fragmented state of the
forests throughout much of the country. We may need to develop seed banks
and replant southern species farther northward. Otherwise these species may
be all but lost in the future forests.

I must strongly disagree with Lee about the importance of these "little
variations" with the genotype. We have a chance to preserve these
variations within the hemlock populations before they are destroyed by the
Hemlock Adelgid, and we should do so. I think these pockets represent an
important element of the genetic make-up of the entire species. Currently
we as a planet are undergoing a major extinction episode. The greatest
extinction ever on the planet was not the death of the dinosaurs but the
extinction at the end of the Permian period between the Permian and Triassic
Periods. During this extinction over 90% of the species on Earth went
extinct. How many species of vertebrates do you think made it through this
extinction event? In the late Permian there were literally thousands of
species of fish, amphibians, reptiles, and therapsids.   At the beginning of
the Triassic Period only twenty vertebrate SPECIES had survived. Most of
these were fish species. Some may have survived because of chance of
location, but most survived because there was something in the genetic
make-up of each of their sub-populations that enabled them to survive. The
key to the eventual survival of a species, like hemlock, may very well be in
these sub-populations.   In any case they should be preserved, just because
we can, if for no other reason.

Ed Frank

Re: plant genotypes    Lee E. Frelich
   Dec 28, 2005 06:09 PST 


We probably don't disagree as much as you might think. I meant my comments
to apply mostly to areas that had been heavily impacted by humans for a
long time, but was in a hurry and sloppy with the explanation, so it didn't
sound that way. Sorry about that.

I support efforts to save natural genotypes that have developed in areas
with unique climate or physiography, as well as populations near the edge
of the range that are isolated as a result of natural processes, where we
are sure we are saving genetic variation useful to future generations, and
not meaningless founder effects caused by human impact. The officially
sanctioned (by the state government, timber companies and environmental
groups) guidelines for timber harvest (also called best management
practices, or BMPs) have incorporated my thoughts on this issue. While
writing the Generic Environmental Impact Statement on Timber Harvesting in
MN, which guides forest management in the state through the year 2040, I
listed all the species that have populations near the edge of their range
in MN, and listed that rare forest types, and said that special care should
be taken to make sure these populations are regenerated and maintained over
time. Next thing you know, the new BMP book showed the range maps for all
these species, and also listed the rare plant communities. Now any timber
company or agency that wants to claim they are managing their lands in an
environmentally acceptable way, and/or wants to get FSC certification, has
to follow these guidelines. Of course, I had no idea that what I said was
going to become policy and be carved in stone for eternity.

You have done a great job of summarizing the issue in your post below. The
only thing I would add is that we should see natural selection as more
dynamic than most people think it is. Trees actually evolve faster than
other organisms in terms of generation numbers. That is because for most
trees, a million or more seeds fall on the forest floor for each seed that
becomes a successful tree, leading to very strong selection. This means
that trees can adapt to changing climate through evolution, rather than
migration as they have in the past. The only reason trees migrate when the
climate changes is that other trees more efficient in the new climate
out compete them before they have a chance to evolve (2 or 3
generations). Also, trees have a very large proportion of total genetic
variability for the species within individuals and small populations.
Therefore, it is possible to allow mixing of genes from different
populations, and build a lot of variability into one population, and let
natural selection decide which genes should be expressed on a given site.
This may be a good strategy given that we don't know exactly what the
climate will do (although there should still be a series of stands that are
kept genetically pure, so to speak, that represent the variations of each
species throughout its range).