A primary characteristic of the genetic of the general
population is homogeneity. Most trees in this population would contain
almost all of the more common genetic traits, with an occasional tree
bearing one of the more unusual genes. A random sample of this
population is likely to not include many of the less common genetic
materials. This common or typical material should be sampled, but
should not make up the entirety of the collection.

There are a number of mechanisms that that in general tend to affect the
genetic diversity of species of plants and animals. These include: 1)
Geographic Isolation or Physical barriers to migration or interbreeding;
2) Chemical Specializations, and 3) Climatic Extremes, 4) Latitudinal
variations.

Geographical isolation is a powerful mechanism in the process of
speciation and can enhance the genetic variability between the isolated
population and the general population. An uncommon gene in the general
population may be physically expressed only rarely. In most cases its
contribution to the overall gene pool is lost in the wash of the more
common genetic populations. In an isolated population, if an unusual
genetic trait is present, it will be given a greater weight in the
overall gene pool. And if this trait is somehow advantageous, the
number of trees in the population carrying this trait will increase over
time. It is very difficult to shift the genetic balance of a large
population and this process is very slow, while an area with an isolated
smaller population will shift rapidly to whatever characteristics work
best for that particular environment. An example of this are the two
populations of fox squirrels on either side of the Grand Canyon. When
the plateau was continuous the entire population freely interbred. With
the formation of the Grand Canyon the two populations diverged, each
concentrating a different characteristics gene set, so now they are two
distinctly different populations with different physical
characteristics. (There is some argument about whether they have
speciated or not.)   

Other types of barriers are filter barriers – those are where the
specific conditions, environmental or other, only allow individuals with
specific characteristics to cross the barrier. For example if there was
a dryer stretch of territory between two wetter areas, only those
species or individuals that were able to tolerate the drier climate of
the intervening area would be able to migrate past this filter barrier.
Therefore populations that established themselves on the far side of the
barrier would have a different genetic character than the populations on
the initial side. This may become a more important process as global
warming forces the northward migration of many tree species.

Distance itself is a form of a filter barrier. The pollen or seeds of a
particular plant only travel so far, so the area that is directly
influenced by a particular individual plant is limited. If there is a
population of tree that spreads across a thousand miles, it is clear
that the trees at the far end are not freely interbreeding with trees at
the near end. There often is a gradual change in the characteristics of
a particular population over distance. The populations at either
extreme may be quite different from each other in physical
characteristics and genetic characteristics.

The second category are populations that have differentiated because of
their ability to grow in an environment that is chemically different
from the rest of the overall population. I am not sure if this is
applicable to hemlocks or not. There are a number of areas called
barrens because of the “barrenness” of the land surface where only a
few species of plants have been able to adapt to these chemically harsh
settings. Examples include some of the serpentine barrens, grown on
bedrock primarily derived from serpentine, gypsum outcrops that are only
populated by a few gypsophile plants, there are shale barrens that are
barren because of the weathering mechanism of the bedrock, and pine
barrens, populated mostly by pines. Are there examples of hemlocks that
have been able to adapt to some of these unusual chemical environments?


The third category of mechanisms are those areas with harsh climates.
Examples might include some of the stunted forests atop Appalachian
mountains, populations in the edge of swamps, populations in extensive
scree slopes, populations, often stunted, by dry soils. To the extent
that hemlocks are found in these extreme areas, these likely represent a
concentration of less common genes or other genetic variations.

The fourth category is latitudinal variations. Populations tend to vary
in characteristics from north to south. The trees from southern
climates tend to be more heat tolerant, while those from the north tend
to be more cold tolerant. In deciduous species there are differing
times when the trees flower, leaf out, and drop their leaves that vary
with latitude. That is one reason why it is not good to plant a tree
from too many degree latitude north or south of the planting site.
Southern trees in a northern area will flower or leaf out before the
last frost and be damaged, they may not change color or drop their
leaves before they are killed by the frosts of autumn. Trees from the
north planted southward may not tolerate the heat well. So clearly
there is a genetic variation that is latitude dependant. Latitudinal
variations and heat tolerance will become a more important issue as
global warming occurs. The question is whether or not the remaining
populations can migrate north as fast as the climate warms

I am sure there are other mechanisms that can be identified, but this
gives some basis for developing a sampling strategy.   

One group of populations that need to be sampled are those found in
various outliers and refugia around the edges of the current population
distribution. In the case of hemlock there are many places to consider.
In the southern boundary several outlier populations are defined in the
article: "Disjunct eastern hemlock (Tsuga canadensis) stands at its
southern range boundary" by Justin L. Hart and David Shankman, in
Journal of the Torrey Botanical Society 132(4), 2005, pp. 602–612. The
article deals primarily with Alabama, but some other outliers are
marked. In North Carolina is an outlier east of the general population
range at Hemlock Bluffs Nature Reserve in Cary, NC. In Virginia there
was a population in the southeastern portion of the state, but the
forestry people there report that it was cut. It would be worthwhile to
see if there is any regrowth from this site. There are a number of
smaller pockets in central Ontario and other Canadian Provinces that
mark the northward boundaries of the hemlock populations. In Indiana
there are several outlier pockets. One of these is at Hemlock Cliffs
area of Hoosier National Forest. In Minnesota there are a couple of
pockets that mark the westward extent of the hemlock populations. The
largest of these is at Jay Cooke State Park and the Hemlock Ravines
Natural Area. These should be sampled. Brandon Gallagher Watson a certified
Arborist from Minnesota reported several large (25-40" DBH) E. Hemlocks
in Minneapolis' Theo Wirth Park. It is unclear if these are naturally
occurring or one planted historically. They could be sampled. There
may be other locations of isolated or disjunct populations that should
be sampled.

I have not identified any, but any populations adapted to unusual
chemical environments should be sampled, as well as any adapted to
extreme climatic conditions. Trees from various different latitudes
north to south should be sampled to obtain the range of variation from
north to south. Likewise samples from the eastern, central and western
extent of the range should be sampled because of the filter barrier
nature of distance. It was suggested in another forum that perhaps
samples should be taken from areas whre there is a mixed hemlock and
deciduous forest, rather than focusing on just hemlock dominated sites.
I don’t really see that the mixed forest hemlocks should be any
different genetically from those in the hemlock dominated forests. I am
not opposed to sampling them, but do not believe they will add anything
new to the overall genetic sampling process.

Sampling should focus on older trees and any trees that express a
distinct physical difference from those in the general hemlock
population.   When sampling an old tree, you know what you are getting
in terms of size and longevity. When sampling a younger tree you are
buying a pig in a poke. You do not know if this tree is going to be a
viable long term resident of that forest, whether its genes are “good”
or not. You may be getting a genetic sample just an average tree rather
than an extraordinary tree. Most of the samples collected will consist
primarily of the most common genetic materials, so I don’t see any
reason to specifically sample for those traits. The focus should be on
the exceptional or unusual traits as much as possible.

-------------------------------------

What should be collected?

There are several ways to collect genetic material. One is to collect
seeds from cones and save them., another would be to collect small
sprouts and replant them somewhere else where they could be monitored
and treated fro infestations, material could be collected fro cloning,
and a final option to consider is to not collect the material at all,
but to treat the trees where they grow with chemicals, such as
imidacloprid, so they are not killed by the adelgid and will continue to
grow and produce seed for as long as they live.

Camcore http://www.camcore.org/projects/hemlock.php is actively
collecting seeds from Carolina hemlock to help preserve the genetics of
the species. Seeds are being collected from various ample sites and
planted, in one of several locations. Details on their efforts are
provided at the website link above.

Clearly the Carolina hemlock is critically threatened with effective
extinction in its native range if something is not done. I applaud the
efforts of Camcore as it is one approach that can be taken. There is
debate about whether the money would be better spent chemically treating
groves of the species, rather than the expensive process of collecting
seeds and growing the trees in a nursery. I question that also, but any
effort that may help preserve the species is a plus from my standpoint.
The methodology used by this group for the collection and growth of
hemlocks from seeds would be a model if that was the option chosen to
preserve the genetic diversity of the species. I see no reason to
reinvent the procedures they have developed. I might however suggest a
independent assessment of where they sampled and try to identify other
potentially worthwhile sample sites they did not sample.

Trees could be transplanted as sprouts or samplings to a single
location, or series of locations. The trees in this nursery grove could
then be treated on a regular basis and monitored for HWA or other pests
that might kill them. This would allow trees to be replanted into the
wild once a viable permanent solution to the HWA was found to serve as
seed trees for future generations.    Multiple sites would be better as
it would prevent a single event, such as a fire from wiping out the
entire genetic repository.

Samples of living tissue could be frozen for potential cloning at some
future time. There generally is a limited time in which a tissue sample
will survive before it dies and is useless for cloning purposes. Recent
attempts to clone the Methusala Bristlecone pine failed because the
samples could not be processed in time to be successfully cloned. A
better methodology might allow you to flash freeze the samples in the
field using liquid nitrogen for future cloning. The disadvantage of
this methodology is the cost involved in keeping the specimens froze for
an extended period of time. The benefit would be that the specimens
could last for hundreds of years and still be viable to work with at
some future time. They would be in a laboratory setting and not subject
to natural disasters that could destroy other growing populations.

Even if samples are collected, I would strongly recommend that a
treatment program be set up with long term funding, an endowment for
example, to allow chemical treatment of at least a representative number
of the trees in any given population. These would then continue to
produce seeds for their lifetime and serve as seed trees for future
reestablishment of the hemlock populations.

Thank you,

Edward Frank