Signs of Summer 8: Among Hemlock Trees

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I am standing in the middle of an old growth hemlock stand that is tucked away off a hiking trail in Laurel Hill State Park. This four acre plot probably escaped logging because of confusion about the exact location of a surveyor’s line. Cutting a tree on someone else’s property was an expensive mistake (fines and reimbursement of the owner of three times the value of the logged trees and, possibly, even penalties for trespassing!). So, to avoid doing all of the work and only getting the privilege of paying many times over for it, disputed property lines often generated safe zones for fragments of primal forest.

Many of the hemlocks around me are impressively large. A few measure nearly four feet in diameter and stand one hundred feet tall. According to Kershner and Leverett  (2004) these trees range between 200 and 300 years of age. So, they are not even half way along on their potential life spans of 800 to 1000 years. There is a range of tree diameters and heights within the stand, which, I assume, reflects a distribution of ages.

The forest floor is covered with a thick layer of shed hemlock needles. Deep in the litter layer a slow process of decomposition is going on. The very top needles are relatively dry, but layers just below and on down to the soil surface are increasingly moist. The soil is black and wet. The thick, acidic mulch layer prevents many understory species from growing here. There is an open quality to this part of the forest. It is very easy to walk around. The mulch underfoot is soft and springy. Wood sorrel, wood fern, club moss, and polytrichium moss grow in scattered patches, and radiating out away from the hemlocks is a three dimensional cone of scattered and ever smaller hemlock seedlings.

The reality of this hemlock stand is an emotional experience. Intellectually, though, I know that these slopes of Laurel Hill were once densely covered with often pure stands of hemlocks trees, and I also know that these hemlocks were cut down in the late 1800’s and early 1900’s for their wood and to harvest their bark tannins. The clear-cut slopes, thanks to the resilience of Pennsylvania’s vegetative communities, reforested themselves in fast growing hardwood trees like red maple and black cherry and stump sprouting red oak but not, immediately, in hemlocks. Standing among these few surviving trees humbles my imagination to try to visualize what this primal forest was like. Contemplation of the immensity of the individual trees, the impact of the stand on the very climate of the site, and the overwhelming continuity of the former forest is awe inspiring. This a beautiful spot, but what a place it once was!

So, why were hemlock trees here? Why did this forest grow, and, very significantly, why is it growing back so slowly? To try to answer these questions, let’s start at the beginning of a single tree and imagine its life over several hundred years:

To get a hemlock, you have start with a hemlock (to paraphrase Schleiden and Schwann’s Cell Theory statement, “omnis hemlockae et hemlockae” (“hemlocks always come from hemlocks”)). So, a mature, seed-producing hemlock had to have been right here because hemlock seeds do not travel very far from their parental tree source (Goldman and Lancaster 1990). Therefore, the parental trees of the hemlocks around me themselves stood very close to this very same spot. And, the parental trees of those trees, and their parents’ parents’ parental tree, and so on, were also right here or at least very close by. The generations of trees were separated by centuries or even millennia of time, but all of these trees stood trunk to trunk and branch to branch in this very space.  Where we see a hemlock forest there has been a hemlock forest for a very long time.

The parental tree of our example hemlock could have started to make cones when it reached 15 years of age, but if it were growing in the deep shade, reproduction might actually have begun much later on in its life. A hemlock can grow so slowly for one or two hundred years when light deprived that it scarcely makes growth rings (Tubbs 1977). In this suppressed state, energy is used to maintain existence with little left over for growth or reproduction. Removal of the shade suppression (say, when an older, nearby tree succumbs to disease, is thrown over in a wind storm, gets struck by lightning, or simply reaches the end of its possible life span) stimulates the young tree to rapidly grow and eventually reach its reproductive maturity..

Each hemlock tree has both male and female cones. The male cones’ pollen is dispersed by the wind and mostly falls in places where it will dry up and inconsequentially fail. A very small percentage of these shed pollen grains falls into the open scales of the upwardly directed ovulate cones and are thus given the opportunity to fertilize an ovum. Most of these pollen grains, though, even when they are “lucky” enough to be blown into an ovulate cones, also dry up and fail to accomplish fertilization (Nienstaedt and Kriebel 1955).

So, an infinitesimally small percentage of the pollen grains produced actually fertilize ova inside the female cones and make viable embryos and seeds! When fertilization does occur, the ovulate cone closes its scales around the seed and turns from its upward orientation to a downward, dangling position. The cone grows and then, as it and its seeds mature, turns from an initial yellowish green to purplish brown and then, as it dries, to a deep, dark brown. In mid-October the cone opens, and the seed, with its short, terminal wing, flutters downward to the litter surface probably just beneath or at least very close to the parental tree (USDA Forest Service 1974).

Eastern hemlocks make cones frequently (two out of every three years) and a single tree can continue to make cones well into its fifth century of life (USDA Forest Service 1974). That’s a lot of cones and a lot of seeds! Very few of these seeds, though, ever get the chance to become a tree. Many of the seeds are eaten (by rodents, by insects, by squirrels)(Abbott 1962), many of the seeds dry out on the dry surface of the litter, and many others rot away when the litter layer is too wet (Le Madeleine 1980). Conditions have to be just right for seed survival. Many surviving seeds then don’t germinate because the ten week rest period at freezing temperatures didn’t occur so their dormancy was never released. Many other seeds don’t germinate because the temperatures after the dormancy release are too cold or too warm (optimally it should be 59 degrees F, but a range of 44 to 65 degrees F is tolerable). A very precise micro-environment on the forest floor is required to give the hemlock seed its shot at germination!)(Goldman and Lancester 1990). So, a massive number of seeds are needed to get just a couple of them through this dense ecological filter of catastrophic probabilities.

The seeds that germinate might have landed on a mound of soil and litter or maybe on a rotting stump or a rotting tree trunk. Its overstory must keep it shaded, though. Any increase in sunshine on the germination site will increase the rate of moisture loss and will quickly kill the seed or the seedling (Jordan and Sharp 1967).

After a year of growth the seedlings would be about one inch tall with a half an inch of roots extending down into the moist layers of the litter. The upper half inch of litter is very vulnerable to drying so even short period of drought can kill these young seedlings. Any seedling not in the deep shade, or any seedling not growing on a moist enough pile of litter will die (Goldman and Lancaster 1990).

By the second year, the seedling’s roots have extended deep enough into the litter layer that moisture delivery is more assured. The seedling’s chances of survival and ability to grow have greatly increased. Hemlock seedlings, as I mentioned before, can survive in deep shade. Even in conditions in which only 5% of full sunlight is available to the seedling, enough photosynthesis can occur to keep the seedling alive if only barely growing, and the seedling can persist in this state of suppression for one or two centuries (Tubbs 1977)!

When the seedling is 3 to 5 feet tall it is able to withstand the impacts of full, direct sunlight (Gladman and Lancaster 1990). Hemlocks an inch in diameter can be a hundred years old and some that are two or three inches in diameter can be 200 years old (Tubbs 1977)! The tiny trees we see growing around the parental hemlocks in our Laurel Hill site may have been growing and waiting for a very long time.

These young trees are very vulnerable to tree predators. White tailed deer readily browse young hemlocks as do cottontail rabbits and snowshoe hares (Abbott 1962, Anderson and Loucks 1979, Euler and Thurston 1980, Goldman and Lancaster 1990). The probabilities are very high that in the many decades the young trees sit well within reach of these browsing animals they will be consumed. And hemlocks are not capable of root sprouting (USDA Forest Service 1965), so destruction of the above ground mass results in the death of the tree.

But some trees, somehow and against all probabilities, survive all this, and are present when a light gap opens (a large, shading tree (maybe the young tree’s parental tree?) is wind thrown, struck by lightning or killed by defoliators or diseases). The opening of the overstory then allows young tree to rapidly grow.

Centuries pass. Our tree has steadily grown and is now nearly four feet in diameter and one hundred feet tall. It dominates its space in the forest and rains down seeds onto the forest floor around it every two out of three years. As mentioned previously, a very small number of these seeds ever germinate, and an even smaller number of the seedlings ever grow, but there is a slow, steady wave of hemlock seedlings growing out from around each of the seed producing trees. The limited, seedling nurturing micro-environments under the parent trees slowly expand as the parent trees grow. The seedlings are also encouraged or destroyed by the temperature and rainfall fluctuations of their growth site’s weather and climate patterns.

Hemlock forests grow from hemlock forests. The old trees are needed not only for seeds but also for the control of the forest floor environment. The slow recovery of a logged hemlock forest reflects the very slow return of the influence of the mature trees on the seedling nursery of its forest floor.

Are small hemlock stands like this one in Laurel Hill State Park sustainable? Can they continue to extend themselves at their edges and begin to knit together their old range and scope? Can they be “ecological crystals” that reform the old growth, hemlock forest? Looking at the edges of this site, I see hemlock needles mixing with fallen maple leaves and the slowly growing, outward curve of spreading hemlock seedlings.  I see the beginnings of some possibilities. Hemlocks live in time frames that are so very different from short-lived species like humans. What will happen here? I’ll get back to you in five hundred years and let you know how it’s going.

References for Eastern Hemlock:

Abbott, H. G. 1962. Tree seed preferences of mice and voles in the Northeast. Journal of Forestry 60 (2): 97-98.

Anderson, R. G. and O. L. Loucks. 1979. White-tail deer (Odocoileus virginianus) influence on structure and composition of Tsuga canadensis forests. Journal of Applies Ecology 16: 855-861.

Eckstein, R. G. 1980. Eastern hemlock in north central Wisconsin. Wisconsin Department of Natural resources, Report 104. Madison. 20 p.

Euler, D. and L. Thurston. 1980. Characteristics of hemlock stands related to deer use in east central Ontario. Journal of Applied Ecology 17:1-6.

Eyre, F. H. (ed) 1980. Forest cover types of the United States and Canada. Society of American Foresters, Washington, D.C. 148 p.

Goldman, R.M. and K. Lancester. 1990. Tsuga canadensis (L.) Carr: Eastern Hemlock. pp 604-612, In, Burns, R. M. and B. H. Honkala (tech coord) “Silvics of North America: Volume 1, Confers.” U.S. Department of Agriculture, Agriculture Handbook 654. Washington, D. C.675 p.

Hepting, G. H. 1971. Diseases of forest and shade trees of the United States. U.S.Department of Agriculture, Agriculture Handbook 386. Washington, D.C. 658 p.

Hough, Ashbel F. 1960. Silvical characteristics of the eastern hemlock. USDA Forest Service, Station Paper NE-132. Northeast Forest Experiment Station, Upper Darby, PA. 23 p.

Jordan, J.S. and W.M. Sharp. 1967. Seeding and planting hemlock for ruffed grouse cover. USDA Forest Service, Research Paper NE-83. Northeastern Forest Experiment Station, Upper Darby, PA. 17 p.

Kershner, B. and R. T Leverett. 2004. The Sierra Club Guide the Ancient Forests of the Northeast. Sierra Club Books. San Francisco.

LeMadeleine, Leon. 1980. Seed-borne pathogens of hemlock seed from Argonne Experiment Forest. Evaluation Report NA-FB/U-8, January. U.S.Department of Agriculture, Forest Service, Northeastern Area State and Private Forestry, St. Paul, MN. 4 p.

Nienstaedt, H. and H.B.Kriebel. 1955. Controlled pollination of eastern hemlock. Forest Science 1(2): 115-120.

Rogers, R.S. 1980. Hemlock stands from Wisconsin to Nova Scotia: transitions in understory composition along a floristic gradient. Ecology 61(1): 178-193.

Tubbs, C. H. 1977. Manager’s handbook for northern hardwoods in the north Central States. USDA Forest Service, General Technical Report NC-39. North Central Forest Experiment Station, St Paul, MN. 29 p.

U.S.Department of Agriculture, Forest Service. 1974. Seeds of woody plants in the United States. C.S.Schopmeyer, tech. coord. U.S. Department of Agriculture, Agriculture Handbook 450. Washington, D.C. 883 p.

Willis, G.L. and M.S.Coffman. [n.d.] Composition, structure, and dynamics of climax stands of eastern hemlock and sugar maple in the Huron Mountains, Michigan. Michigan Technical University, Department of Forestry, Technical Bulletin 13, Houghton, 43 p.