Hemlock Gene Pool   Edward Frank
  Apr 12, 2007 19:46 PDT 


I have been wondering about the statement that eastern hemlock resides
in a small gene pool. I have looked at the pollen viewer program
http://www.ncdc.noaa.gov/paleo/pollen/viewer/ and don't see nay point
since the last ice age that hemlock has been missing from the landscape
or which had an extremely tiny range. What indications are there that
the gene pool is somehow limited?

Ed Frank
RE: Hemlock Gene Pool   Steve Galehouse
  Apr 12, 2007 20:22 PDT 


For me, a "deep" rather than a shallow or small gene pool would imply
a number of closely related species which could potentially interbreed,
such as the three needled pines of the southeast--pitch, loblolly,
longleaf, pond etc.--a genetic soup. Eastern hemlock pretty much stands
alone--I don't think there is any evidence of it hybridizing with
Carolina hemlock--and the tree looks and acts very much the same
throughout its range, except for its possible height and girth in
different locations.
I vaguely recall and article stating that Eastern hemlock "began" as a
species of the Southern Appalachians, and expanded its range northward
after the ice age, as opposed to it using the southern mountains as a
refugium from a natural, more northern distribution. I hope there is
enough genetic diversity in the species to afford it some resistance to
HWA, but I would be surprised if that was the case.

Steve Galehouse
Re: Hemlock Gene Pool   Fores-@aol.com
  Apr 13, 2007 05:10 PDT 

I have heard/read that there was a period several thousand years ago when
eastern hemlock almost went extinct. I think that is was during the studying
of sediment cores from Lake Champlain that this was discovered and there is a
lot of debate about what caused the rapid die off...it could have been
another version of the adelgid. From what I recall the die off was extremely
sudden and hemlock pollen was absent from the Lake Champlain sediments for over
900 years afterwards. My gut tells me something like that a die off like that
could shrink the gene pool

Re: Hemlock Gene Pool   Kirk Johnson
  Apr 13, 2007 07:09 PDT 
This is an interesting paper on the subject.

-Kirk Johnson


Haas, J.N. and J.H. McAndrews. 2000. The summer drought related hemlock
(Tsuga canadensis) decline in eastern North America 5,700 to 5,100 years
ago. Proceedings: Symposium on Sustainable Management of Hemlock Ecosystems
in Eastern North America. GTR-NE-267.

Hemlock and hemlock stands were sharply reduced 5,700 to 5,100 years ago
because of summer droughts. Seedlings and young hemlock suffered and died
because of their shallow rooting system, as soil moisture and atmospheric
humidity are the most important limiting factors for hemlock. Larger trees
under moisture stress sustained a fatal, but local pathogen-pest attack,
perhaps the hemlock looper although not all trees died. Such an attack was
triggered by drought, but would not have been the reason itself for the
decline of hemlock. Understanding such complex interactions between climate,
plants and animals on a longlasting scale is important when assessing the
possible effect of future climatic change. It also shows that major
mortality of hemlock populations occurred long before today's attack by
pathogens such as the hemlock woolly adelgid. And it also indicates that
hemlock survived climatic stress and pathogen attack 5,700 to 5,100 years
ago, and that the recovery took more than 1,000 years.

Re: Hemlock Gene Pool   brown_-@colstate.edu
  Apr 13, 2007 07:36 PDT 

Hemlock did undergo a precipitous decline acroos most of its range
roughly 4500 or so years ago which has generally been attributed to an
insect defoliator. There are many papers that reference this in pollen
records. Try a search on 'hemlock decline'.
It was fairly synchrounous across much of its range but its
distribution at that tome was somewhat similar. It thus underwent a
population bottleneck and then recovered.

There are some new techniques extracting genetic information out of
fossil pollen that could shed some light on this in the future.

Roger Brown
RE: Hemlock Gene Pool   Lee E. Frelich
  Apr 13, 2007 10:00 PDT 

Bob et al:

When I was a post-doc in Margaret Davis' lab at the University of MN around
1990, she had a student named Cathy Zabinski who studied genetic diversity
in hemlock throughout its range for her Ph.D. project, and found it to be
very narrow like red pine (based on number of alleles found on foliar
samples, published in a paper in Canadian Journal of Forest Research in
December 1992). Cathy is now at the Big Sky Institute at Montana State
University in Bozeman, MT.

Models she ran population genetic models of the hemlock decline around 5000
ybp and showed that it probably was not a small enough bottleneck to cause
the observed low levels of diversity. Bottelnecks in trees have to be
incredibly small to change population genetic structure. Hemlock looper
parts are found in the sediments around the time hemlock declined and
almost disappeared simultaneously throughout its range. It is common for a
native insect to cause massive mortality at the time of climate change or
an unusual climate event like an extreme drought, and for recovery to take
several centuries after such events.

Hemlock had a glacial refuge around 20,000 ybp in the Carolinas and spread
north from there to New England and then west to the Lake States, reaching
the Porcupine Mountains about 3000 ybp. I am not sure we know where the
species originated, which would have been millions of years ago with a much
less complete fossil record.


Re: Hemlock Gene Pool   wad-@comcast.net
  Apr 13, 2007 11:56 PDT 

In my limited experience, I have found that insect infestation is usually secondary to some other form of stress. This may not be true for an introduced pest such as HWA. It makes sense to me that years of drought were followed by destruction by insects, as the trees would not have the resources to defend against the insects. HWA may be a different situation, as the trees are more or less healthy, and being killed. Unless there is some underlying stress that we don't know about that is making the Hemlock more susceptable to HWA. I was very surprised to see an indigenous population of Hemlock along the Pennypack Creek outside of Philadelphia that did not have an HWA problem. On a few trees with lower branches, I could only find one HWA. Hopefully Hemlock won't be reduced to a population that will take 1,000 years to recover, but the Earth often works in larger spans of time than we humans can fathom.

Re: Hemlock Gene Pool   DON BERTOLETTE
  Apr 13, 2007 12:37 PDT 

There is no underestimating the value of having long-term reference
conditions as comprehensive as possible...lake and bog sediment cores, for
macrofossils and palynology, ice cores, and as Ed and I have earlier
discussed, the chronological layering of cave stalagmites, stalactites, and
such. These give the big picture and reaquaints us with a broader
perspective...often we get tied up in the moment and fight insurmountable
odds, when those restoration dollars may be more effective in other
The challenge is finding the mid-term reference conditions between known
historic records, the records of dendrochronology, and the near-term
palynology records...the scales broaden as our means of measurement gets
more remote...
It wouldn't be inappropriate to include reference conditions in the
excellent trip reports ENTS have been compiling.
RE: Hemlock Gene Pool   Brandon Gallagher
  Apr 13, 2007 13:46 PDT 

Lee et al.,

Do all populations necessary have "deep" gene pools?
The common thought seems to be if we have a species with relatively
homogenous pool of alleles we determine there must have been a limiting
event sometime in that species' history. Is it possible from the onset
of the speciation event the pool was small and stable thus at no time
during that species history was the pool "drained" to the point we
observe now?

Brandon Gallagher Watson
Plant Healthcare Specialist
RE: Hemlock Gene Pool   Lee Frelich
  Apr 13, 2007 15:14 PDT 


Yes, it is possible that the species never had a very diverse gene pool.

It depends on how the speciation event occurred. If it was a founder event
where a few trees were separated from the rest of the range of some other
hemlock species, and then underwent selection to adapt to the environment
which came to differ over time from that of the rest of the species, plus
random drift, and that event did not occur long ago, then the species might
have a very narrow genetic base.

On the other hand if a species with a large range with millions of
individuals in diverse habitats was split in half by the appearance of a
mountain range, or prairie, or other unsuitable environment, and selection
took the two large pieces of the population in different directions, it may
have a diverse genetic base right from the start of the species.

Both have been observed in the fossil record and in modern speciation
events for various plant species.


Back to Bob-Hemlock Gene Pool   Steve Galehouse
  Apr 13, 2007 16:24 PDT 

Bob, ENTS,

Here is the information I vaguely recalled concerning hemlock
originating as a species of the southern Appalachians; an excerpt from
E. Lucy Braun’s “Deciduous Forests of Eastern North America”, published
1950, taken from a 1974 reprint edition, page 45, chapter 4, The Mixed
Mesophytic Forest Region, italics hers:

   “The category to which hemlock belongs is open to question. Because
it is a dominant in the Hemlock-White Pine-Northern Hardwoods region, it
is usually thought of as northern, although it does not extend far into
the adjacent boreal forest. It is just as conspicuous and abundant in
much of the lower elevation Appalachian forest as it is in the north. To
explain its occurrence in the mixed mesophytic forest by Pleistocene
migration from the north is to overlook the general mixed character of
the Tertiary forest, which by zonal segregation gave rise to the late
Pliocene and Pleistocene banding of formations we recognize today. If
hemlock was not part of the undifferentiated forest of the Tertiary, of
which the mixed mesophytic forest is a persisting remnant, how is the
distribution of its root parasite, Buckleya distichophylla, to be
explained? This rare endemic of the Southern Appalachians (p. 481) has,
as have many other Tertiary genera, two close relations in Asia. Boreal
species do not display this eastern America-eastern Asia relationships.
This feature and the contemporaneous arrival of hemlock and mesophytic
hardwood species in post-glacial northward migration, as shown by pollen
records (p. 465), point so strongly to the long indigenous nature of
hemlock in the area of the Mixed Mesophytic Forest climax that it seems
best to omit mention of it as a “northern” species. If the entire flora
of the Mixed Mesophytic Forest region be considered, developmental and
edaphic communities as well as climax communities, it is apparent that
southern plants far outnumber northern and that the endemic element is
prominent. However, the floristics of the several climaxes is a separate

I don’t know if this book is still out there and available, but it’s a
very interesting read. Its ISBN # is 02-841910-3. I’d be happy to lend
and share my copy with interested ENTS members.

Steve Galehouse
Re: Hemlock Gene Pool   orw-@fas.harvard.edu
  Apr 13, 2007 19:36 PDT 

To ENTS concerning the topics of hemlock genetics and the historical
hemlock decline. First, in terms of genetics, there has been very
little field testing of hemlock genetic variation. A study by Zabinski
(1992) examined isozyme variation in disjunct populations of eastern
hemlock and found a very low level of genetic variation. Another study
by Schaberg and others (2003) examined rare alleles in eastern hemlock
associated with silvicultural treatment and results suggest that rare
alleles decreased when trees were selectively cut and increased when
diamter-limit cutting was used. I am no expert when it comes to hemlock
and genetics, but a few other related articles are: Wang et al. 1997
examined chloroplast DNA polymorhpisms in eastern and Carolina hemlocks
and suggested that there was little evidende of chloroplast DNA
differentiation among eastern hemlock populations. There is current
research being done on determining the genetic structure of hemlock
populations by examining the microsattelite markers by scientists at
the USGS Center in Wellsboro PA.
In terms of hybridization between eastern and Carolina hemlock, it
rarely occurs. the only evidence I have is a study in the 2002 HWA
proceedings (Bentz et al. 2002), that a single hyprid of eastern x
Carolina hemlock was identified in a larger breeding program aimed at
resistance to HWA.
Lastly, much paleo work and studies have been written on the historical
decline of hemlock across much of its range ~ 5000 years ago. Certainly
several factors have been implicated and the consensus seems to be a
climatic trigger that may also have lead to defoliation of hemlock by a
pest similar to our native hemlock looper. A paper just came out
disussing this (Foster et al. 2006).   thanks much, DAVE ORWIG

Zabinski, C. 1992. Isozyme variation in eastern hemlock. Can. J. For.
Res. 22: 1838-1842.

Scaberg, P.G.,G.J. Hawley, D.H. DeHayes, and S.E. Niensohn. 2003.
Silivicultural management and the manipulation of rare alleles, pp
67-74. IN proceedings of the symposium of the North American forest
commission, Forest genetic resources and silviculture working groups,
and the International Union of Forest research organizations (IUFRO).

Wang C. M. H. Perlin, R.R. van Stockum, Jr., S.H. Hamilton, and D.B.
Wagner. 1997. Chloroplast DNA polymorphisms in Tsuga canadensis and
Tsuga caroliniana. Can J. For. Res. 27: 1329-1335.

Bentz, S.E., L.G.H> Riedel, M.R. Pooler, and A.M. Townsend. 2002.
Breeding hemlocks for resistance to the hemlock woolly adelgid. pp.
127-128. in HWA proceedings from the 2002 meeting.

Foster, D.R., W.W. Oswald, E.K. Faison,E.D. Doughty, and E.C.S. Hansen.
2006.   A climatic driver for abrubt mid-Holocene vegetation dynamics
and the hemlock decline in New England. Ecology 87: 2959-2966.

Re: Hemlock Gene Pool   Edward Frank
  Apr 16, 2007 18:33 PDT 

Dave, and everyone else who replied,

Thank you for your comments. The specifics of hemlock genetics is something
I know little about and I appreciate the information.

Ed Frank

TOPIC: Hemlock Genetic Samples

== 1 of 1 ==
Date: Mon, Dec 31 2007 10:40 pm
From: "Edward Frank"


As I understand, there was no collection of seeds or other genetic materials from the biggest hemlock you discovered on the Tsuga Search Project. Most of the largest hemlocks discovered as part of the project are dead from HWA. I am wondering how dead are the hemlock groves in the vicinity of these larger trees? Are there any still alive or is there 100% mortality?

These largest specimens represent basically the best of the genetics of a species. There may be some happenstance that helped them along the way, but to great degree they achieved their large size due to being able to resist diseases, insects, drought, competition, mul;tiple lightning strikes and all of the other stresses that killed less strong trees. Ideally these should have been the ones sampled to obtain genetic material. When sampling smaller and younger trees, it is kind of guesswork. You can not tell how good the genetic potential of a tree is until hundreds of years later. But in light of the death of the largest specimens (Do they seem to die early from the adelgid? Perhaps they are already stressed by their own age and large size?) The next best thing would be to collect seed from their offspring. If there are smaller trees in the immediate vicinity of the great dead ones, there is a good chance they may be the offspring of the larger ones, and carry much of the genetic code of the dead trees. They may even be back bred for several generations of the genetics of the larger trees. If there are any left alive it that are producing viable cones and seeds they should be collected. If they are too young to produce seed, they should at least be treated until they grow old enough to do so.

I would encourage the donation of these seeds to some group like camcore, or even to a commercial nursery that is growing them for sale to preserve their genetic line. Some could be frozen for some potential future time in which they could be grown again also.

The other areas that should be sampled are these various outlier populations of the species in order to best capture the genetic variability of the species as a whole, and not just sample the core populations as has been done by Camcore. Similarly Carolina hemlock needs special attention because of its limited range and likely effective extinction in the wild within in the next decade.

Ed Frank
(I am trying to remember to delete old posts from my replies and be conscious of changes of topics)