Moss Walk at Cook Forest, PA   Edward Frank
  Jul 16, 2006 19:57 PDT 

Moss Walk - Cook Forest

Saturday, July 15, 2006, I attended a "Moss Walk" at Cook Forest State Park.
The program blurb read: "Ever wonder what kinds of mosses and liverworts
grow in the Forest Cathedral? Can these often overlooked bryophytes tell us
about the health of the environment? Join our local molds, spores, and
fungus expert, Dr. Harold "Hank" Webster, Penn State Dubois biology
professor, on an exploratory walk through the Forest Cathedral to identify
these tiny members of the forest community. Program starts at the Log Cabin
Inn Environmental Learning Classroom; magnifying glasses will come in

DSCN0454.JPG (2584868 bytes)
Dr. Harold Webster

I had first met Dr. Webster at the Wapiti Audubon Society meetings, but had
not know he was an expert in mosses. ENTS were well represented at the talk
wit Dale Luthringer, Anthony Kelly, (and Jacqui Noel), Carl Harting, and
Myself. I knew very little about mosses aside from what I had read in some
field books. Most of the references I had found when discussing this topic
earlier this spring were either too simplistic or too complicated to
understand without an adequate background.

Several of us arrived early and Dr. Webster and us scouted several areas
near the Log Cabin Inn nature center. The program started at 3:00 with
those of us listed above and three other people from the general public -
this was really a better response than Hank had expected. He went over the
life cycle of the mosses and liverworts briefly. They both can reproduce
sexually and asexually. The characteristics of various reproductive stages
and growths are often important in the differentiation of different species.

DSCN0485b.JPG (535865 bytes)
Coral Fungus against a backdrop of moss

After a brief introduction we all headed out into the field. We didn't
travel far because mosses and liverworts grow everywhere. With everyone
using hand lenses, Dr. Webster showed us examples of a number of different
species of mosses and liverworts and pointed out distinctive
characteristics. Many could only be broken down to genera in the field
using hand lenses. Microscopic work would be needed to specifically
identify the species. It was really useful to have someone point out in the
field using actual specimens various physical characteristics present in the
specimens. The entire walk embraced a few hundred yards and we looked at a
couple dozen different mosses and liverworts.

After the talk, all of the ENTS accompanied Dr. Webster to the vicinity of
the Seneca Pine in the Forest Cathedral area to look at the thick moss mats
present there. There are boulders in the area covered by moss. It is a
larger species, and if I read my notes correctly, Polytrium ohioense   I had
chosen this destination for the group because of the large amount of moss
present, the fact that it was in an old growth area, and the existence of
large sandstone blocks, and fallen logs in various states of decay. I
wasn't sure about the diversity of species, just that there was quite a bit
of moss.

DSCN0487.JPG (8606857 bytes)
Moss topping boulders under the Seneca Pine

I learned a many things and have a better perspective on mosses and
liverworts. Still, as with all trips of this nature, more questions were
raised than answered. Someone ask "Howe old were these moss mats atop the
boulders?" Good question. Mosses are non-vascular, spore producing
organisms. They do not have true roots. If you look at an individual plant
you can see the evidence of annual growth. These examples were perhaps
three inches in length. The living portion of the organism was perhaps 8 to
ten years old. The thing is however, that the bottom portion of the moss
was in the process of decaying at the same time as the upper portion was
growing. The actual height of the moss was dependant of the balance of the
rate of decay and growth.    This particular organism may have been alive
for decades, hundreds of years, or even thousands, but the portions of this
organism living at those times have since decayed. So how old are they?

The rock likely was not covered by moss 10,000 years ago because of the
periglacial climate present in the area. However soon after the glaciers
began to retreat moss could have first grown on this rock. Has it died and
repopulated the rock since that time? Maybe. periods of drought may have
hurt the moss, but are unlikely to have killed it. The tops of the boulders
are typically a xeric environment, but moss that has been dried for years
will start to grow again when wetted. Forest fires could have burned the
moss at some point in the past. Clearly the environment and moss growths
seem to be in some form of a long-term steady state. This is an area of the
forest undisturbed by logging. The little moss growing on that rock could
be a colony of individuals who are each older than the oldest pines in the
forest. The Seneca Pine giving shade to the rock could be nothing but a
fledgling with respect to the ages of the moss. Hank observed that the
mosses are generally not subject to browsing by insects and deer. That is
true they are not commonly eaten. Over the 500 million years of their
existence they have evolved a series of nasty chemicals that make them taste
bad. He said there was one chemical formally known only to occur in
spiders, that was found to occur is some moss species.

I wonder if they may also have some anti-plant characteristics also. There
typically are several species of moss, liverworts, and lichen growing in a
patch on the forest floor. There are a few tiny sprouts of red maple or
hemlock among the moss, but these rarely are more than a fraction of an inch
in height. If were easy to colonize within these mats of moss on rocks, or
on the forest floor, why were they not overwhelmed by angiosperms long ago?
On some boulders hemlock has managed to sprout. On these rocks the moss has
died. The surface is covered by a thin layer of soil, followed by debris of
fine branches, hemlock needles, and cones beneath the trees. What has
killed the moss? Alleopathy is a characteristic of hemlock. It gives off
chemicals that suppress the growth of other plants. This likely has killed
the moss. The boulders have a limited amount of soil, difficult rooting,
and are subject to drying. Those trees that do survive atop the boulders
will grow to a certain height be fore stopping. They are suppressed by
these limitations. That is why trees growing atop a series of rocks all may
seem similar in height, even though they obviously vary in age. Back to the
hemlock on the boulders. This hemlock killed the moss on a particular rock.
If it is blown over or dies, the rock potentially could be recolonized by
moss again. Perhaps this cyclical process has restarted the moss again and
again, resetting the age of the mat.

There are dozens of microenvironments that affect the growth of a particular
species of moss, as well as the circumstance of first come first served. It
would be an impossible task to try to delineate each of these micro
environments that may only be a few centimeters in size. On the original
field trip there was a moss that typically grew on rotting logs, Here on a
shaded sandstone cliff was the same species covering a vertical face. Why?
Dr. Webster is retiring in a year. perhaps then he will have time to do
some needed field work in the Cook Forest State Park. Bryologists are not a
common specialty.

DSCN0501.JPG (2988187 bytes)
Fallen Cucumber Tree after 18 months

After leaving this area we hiked down to the potentially 400+ year old
fallen cucumber tree. Since its fall a year and a half ago, the decay
process has started. Moss now grows on the western side of the fallen log
almost to the top. Moss grows along some of the branch whorls near the top
of the tree. The smaller branches are beginning to decay. In the center of
a hollow in the fallen trunk, what last year was light reddish slightly
decayed wood, has now turned dark brown and become finer. Small plants are
starting to sprout in the humus. I will post photos later. Dr. Webster
identified some of the moss species as: Dicranus montanum, Pltygyrins
repens, and Mnium sp. (again my spelling may be off). Also present were
slime molds and a number of unidentified crustose lichens. Fro here we
headed back to the parking lot and went for dinner. Dr. Webster did another
program at the campground later that evening.

One website that has some nice photos of bryophytes is:

In the way of field guides for mosses, there are few worth while. A recent
book has been published called:   50 Outstanding Mosses and Liverworts of
Pennsylvania and Nearby States by Susan Munch looks to be a good starting
point for people interested in the field. This book with photos has just
been published. Dr. Munch is a professor at Albright College in Reading
Pennsylvania. Mosses are generally widely distributed. Dr. Webster talked
of one he had collected in Barrow Alaska and also in the steps of an Aztec
pyramid in Mexico City. So it would be appropriate for people across the
eastern US. It can be ordered from the author at:

I haven't ordered my copy yet, so I don't know any more info that that.

Ed Frank

RE: Moss Walk at Cook Forest, PA HWA   Will Blozan
  Jul 17, 2006 07:55 PDT 


Awesome description of the walk and new insights! I enjoy walks with Dr.
Frelich as he seems to know a ton about mosses. I know they can tell so much
about the forest, but somehow my gaze is turned upwards...

The idea of ancient communities of moss mats on boulders (and logs) is
interesting to think about. Their presence in the forest should be perceived
no less important than that of the ancient tree canopy. The interplay and
shade/moisture relationship may be more key to the function of the system
than we now know.

I have great concern for the eastern hemlock forests and the role that
species plays in so many ecosystems. The loss of the deep shade to hemlock
woolly adelgid will undoubtedly disrupt the balance of the ancient moss
communities and disturb the seeding beds of those species that tend to root
on shaded, moist organic material, be it moss mats or wet logs (yellow and
black birch, rosebay rhododendron, eastern hemlock, etc.).

In the wake of hemlock woolly adelgid in the southern Appalachians the sheer
mass of the giant trees will lead to mechanical destruction of many of the
remaining non-hemlock trees as the huge snags snap and fall. In some areas,
the forest floor will then be a sunny, desiccated and disturbed environment-
a rich seedbed for exotic and early successional plants. The old-growth
system will be lost forever- human disturbance leaving a permanent impact.

Finding out more about the ecological significance of these moss ecosystems
could help with funding to preserve the hemlock canopy harboring the
conditions for their perpetuation.

Re: Moss Walk at Cook Forest, PA
  Jul 17, 2006 08:15 PDT 

It never ceases to amaze me how much a thick carpet of moss adds to enjoying
the woods. In addition to all of its incredible water holding capacity and
providing breeding and living habitat for dozens of amphibians least
once a year I find myself taking off my boots on a hot midsummer day just to
massage my feet in a moss "shag" carpet 3 or 4 inches thick. There is some
sort of guilty pleasure in savoring something in the woods that could never be
reproduced indoors.

Re: Moss Walk at Cook Forest, PA   Lee E. Frelich
  Jul 17, 2006 09:10 PDT 

Sounds like an interesting hike. I noticed many of the same species when I
was there.

The correct spellings are:
Polytrichum ohioense (tallest moss in the park, with some stems reaching 6 inches).
Dicranum montanum
Platygyrium repens
Mnium sp.

Regarding the moss mats on rocks, I believe there would be a lichen
successional stage that may preceed the moss in many cases. In a forest
with white pine, fires were probably common and would burn many of the moss
mats once or twice per century, animals may strip the moss off the rocks,
and not all moss species are extremely shade tolerant and heavy shade can
kill some species, as can smothering from litterfall (especially broadleaf
litterfall). If a dense clump of smaller trees occurs directly over a rock,
the moss thickness will decline, or the moss may disappear. A given rock
may have an old tree above it for several centuries, and the shade from the
high canopy may not be so dark, allowing moss mats to thicken. Moss mats
on rocks are probably more dynamic over time than you seem to think. In
northern MN moss mats a foot thick come and go on a time scale of 1-2
centuries as the forest responds to fires.

Re: Moss Walk at Cook Forest, PA   Michele Wilson
  Jul 18, 2006 06:08 PDT 

Nice report, Ed;
I would've enjoyed that walk & learning experience. I always stop for a
moment to admire mats of moss, etc. as I do my forest work and do speculate
on the effects of the forest floor microsites via thinning out the forest.
I wonder if there are any invasive species of mosses and liverworts also
interrupting the matted flow.


RE: Moss walk at Cook Forest   Edward Frank
  Jul 18, 2006 20:14 PDT 


Thanks for your reply. It would be nice to talk with you in person in the field while we looked at some of these things. These comments are certainly worth further discussion. I am not sure I agree with your comments however.

Forest Fires: Clearly this is most likely the limiting factor with respect to the maximum age of the moss mats. The age of those fire events could be quite variable however. These mats in Cook Forest for example are in an area that is not frequently subject to fire events, unlike some other ares where frequent fires dominate the ecology. There haven't been any fire recorded in this area for at least 100 plus years. There may have been some fires at Cook in some area in the late 1800's if I recall comments Dale made one time, but those were not in this area.

I don't know when the last fire was in the area of the Seneca Pine. There are trees like beech and cucumber which are less likely to survive forest fires, compared to the white pines, which are quite old, perhaps in excess of 30o to 400 years in the area. This does not mean that a fire did not occur during that time, but certainly there was not any major conflagration like you see on the news occurring in the western United States.

A major fire would definitely wipe out any moss mats in its path. Smaller fires jump from place to place and may skip portions of the forest. Smoldering fire in the litter zone and small underbrush potentially might not affect the mats isolated atop larger boulders. There is a group of pine trees in the Forest Cathedral Area in the 400 plus year range. Perhaps a major event opened the forest at that time. White pine require open areas to grow effectively. This may have been a major forest fire, it which case the maximum age of the moss mats would be in the same time frame as the maximum age of these trees. If it were a major blowdown that opened the forest, then that is a different story. I doubt that the blowdown would itself destroy the moss mats and any that had not since been destroyed by fire could predate the oldest of the trees in the area.

Has there been a fire event sine this 400 year old event? I have no way to tell at this point. Using Occam's Razor, in which the simplest explanation is the most likely, then lacking any evidence of a fire, I would be forced to conclude that there has not been any. Therefore, if this assumption is correct then the maximum age of the moss mats would be 400 years or greater since the last restart. I am not convinced of your assentation of "In a forest with white pine, fires were probably common and would burn many of the moss mats once or twice per century."

Lichens: You have suggested that lichens would be part of a typical succession pattern for the moss mats. Initially after glaciation the first growth on a rock would be lichens. Crustose are the first to form. I am not sure this is true for rocks that have become bare as a result fire or other cause. if you look at rocks turned over by road construction, lichens do grow, but they are accompanied in the same time frame as the mosses. These rocks are not in isolation. There is often bits of soils still attached. There is organic matter collecting in pockets from fallen pine needles and leaves. All of these give the moss a chance to grow. I am sure there is a successional sequence, and it would be interesting to figure out its pattern, but in many cases I think succession on these boulders are driven more so by opportunistic events than strict succession.

Animals stripping moss from rocks: I don't know why they would do this. But it is not impossible. So if we would consider that evidence of this stripping of the moss mat from a boulder would be detectable for say 1 year. (Actually it is likely more like a dozen years - just guessing). Then the reciprocal of the frequency of the stripped moss boulders among the total number of moss covered boulders would equal the mean time in years per animal disturbance. in effect: If 1 boulder among 500 were striped by animals in a given year, then the average time for any particular boulder to be stripped by animals would be 500 years. This does not indicate the residence time for a particular boulder, that could be much higher or lower, but the average time for the population of the boulders. Now in the area of the Seneca Pine there are hundreds of boulders. I would need to do a survey, but I have not seen any boulders that appear to have been stripped by animals. The reciprocal of that is infinity, isn't realistic, because I haven't surveyed all of the rocks in the area. But it is my impression that the time between stripping incidents by animals will be extremely large and not a limiting factor in the maximum age of the moss mats.

Variations in light: Moss grows well in the shade and it grows well in the light. One particular species seems to be the climax species of the moss mats. It is growing well in brightly lit areas and in darkly shaded areas. I would think this moss would develop a mat regardless of the light conditions.   If moss succession is involved, perhaps there would be different paths, but the widespread distribution of this moss in various areas, substrates, and light conditions suggests mats made of this moss is the end result.

Dynamic: I think the mats are dynamic, getting thicker and thinner with varying conditions, but the question is not whether they get thicker or thinner, but whether they are continuous over a long period of time, and how long is that time period?

There are a number of disturbance types that could affect the growth, death or recovery of the moss mats. There are fire disturbance - how long do they take to recover and how does that process work? Blowdowns - how does this affect existing mats and how do they recover, change, or decline as a consequence of the blowdown? How do man-made disturbances like timbering operations affect the moss mats and how does this compare to changes in the moss mats in areas affected by natural disturbances? What are the effect of wide scale overstory loss from hemlock wooly adelgid? These are all subjects for enquiry.

There are limiting factors in the age of the moss mats. Among them I see fire, death by alleopathy (hemlock example in initial post), and perhaps smothering by leaf litter as suggested in your notes. Acid rain and other pollution surely are also adversely affecting the mosses in the forest.

Ed Frank