Eastern Hemlock (Tsuga Canadensis)
The Eastern Hemlock or Tsuga Canadensis can be found throughout much of the eastern United States as well many parts of southern Canada. Because of this proliferation and survival, I feel it is an important and essential part of our forest ecosystems. The Eastern Hemlock is a Coniferous tree that can live to be over 800 years old. The tree is a perennial meaning that it grows year after year and drops its seeds each year. The eastern Hemlock can reach height of 175 feet and have a diameter of approximately 76 inches. The tree is quite common throughout much of the eastern United States with pockets of Eastern Hemlock often seen in southern Ohio. The tree is of particular importance at this time due to the fact that it is under a great threat of extinction. In the past decade scientist have seen a steady advance of Hemlock Woolly Adelgid (HWA). This is an insect that was introduced from Asia. It attacks the Eastern Hemlock and causes a high rate of mortality among the species.
The Eastern Hemlock flowers from April to June and its pollen is spread by the wind. Then after pollination the cones close up. The hemlock then dispenses its seed through small cones. The seeds fall out of the cones in the fall and have small wings which allow them to move small distances away form the parent tree. The Eastern Hemlock is found in clusters which suggest that their seeds do not disperse a great distance. The clusters of Eastern Hemlock often create small microclimates which differ greatly from the deciduous hardwoods which often surround the Hemlock. This creates a crucial nesting and bedding area for many birds and mammals. The threat posed by Hemlock Wolly Adelgid can have many unforeseen consequences to other species besides the eastern Hemlock.
Eastern Hemlock Hemlock Wolly Adelgid
Image of Eastern Hemlock Example of HWA on an Eastern Hemlock
(Plants.USDA.gov) (Plants.USDA.gov)
Habitat and Threats
The eastern hemlock is a tree commonly found throughout the eastern and central sections of the United States as well as much of Canada. The eastern hemlock is found primarily in the Temperate Broadleaf and Mixed Forest habitat type, although sub-species of the eastern hemlock are found throughout North America. The eastern hemlock often grows on the sides of hills or in the bottoms of valleys. This particular species clusters together in pockets which are often surrounded by deciduous forest such as maple and oak. The eastern hemlock creates a micro-climate around the areas where it grows which help to sustain its own species as well as provide a unique habitat for many others within the forest. It is widely believed that these communities of hemlock are in great danger of being killed either by disease or through human actions. Therefore it is important to understand that hemlock affect many other species besides themselves. The climate created within a hemlock stand is quite unique, and provides habitat for a wide array of species. If the hemlock is truly in trouble, then these species also face possible loss of habitat and must learn to adapt if they have any hope of surviving. Some species which utilize hemlock stands do so without any harm to the hemlock itself, but others such as the white-tail deer have been blamed for the reduction in the number of hemlock trees. It is unknown how many species may be affected should the eastern hemlock be removed.
The white-tailed deer is a common species found throughout North America. The deer often inhabits many of the same regions as the eastern hemlock. The deer depends on the eastern hemlock for a source of shelter during the winter months, but also and more importantly the deer uses the eastern hemlock as a source of food. The deer may enjoy browsing on eastern hemlock, but many experts believe that deer browsing is a great threat to the survival of the eastern hemlock and its surrounding habitat. A study conducted by Anderson and Loucks titled White-tail deer influence on structure and composition of tsuga canadensis forest. The study looked at the damage that was being done to eastern hemlock forests by the White-tail deer. To conduct this study the authors set up control sites in the Flambeau Scientific Area located in Wisconsin. Some of the study areas were fenced so that deer would be denied entrance into the study area. Other areas were left open so that the deer could come and go at will. After the elapsed time of the study the author concluded that there was a significant increase in eastern Hemlock within the enclosures. The only hemlock saplings seen in the open control sites had been protected from browsing by fallen trees or some other form of natural protection. The study found that deer browsing had a detrimental effect on the ability of eastern hemlock trees to regenerate themselves. The opinion of the authors of this study is that hunting of deer should be increased in order to save the eastern hemlock forests from depletion. The past few decades have seen the populations of deer increase greatly; this is most likely part of the reason for the decline in eastern hemlocks. It is difficult to say how great of an impact deer have on the eastern hemlock, but deer browsing is an important factor that needs to be considered when looking at problems facing the eastern hemlock.
The most serious threat facing the habitat of the eastern hemlock is known as HWA. The affects of Hemlock Woolly Adelgid (HWA) can be seen throughout the eastern part of the United States. There is a vast array of systems which are affected when HWA infests a Hemlock forest. HWA has a detrimental effect on the entire forest ecosystem not just on the eastern hemlock. Hemlock forests are a micro habitat, some species of birds and plants can only exist in these specific environments. When the Hemlock forests are killed by HWA a new type of vegetation takes over the area. This new vegetation brings in different birds and different plants which are able to survive in the altered environment. This is a threat that could control the entire future of the eastern hemlock. No one knows if the eastern hemlock will be able to move back into habitats which have been decimated by the spread of HWA. Currently there is no solution to the spread of HWA and scientists can only study the spread and hope that an answer is found.
Recently another problem has arisen which could pose a danger to the future of the eastern hemlock; this is the prospect of climate change. Warming of the climate could severely limit where the eastern hemlock will be able to survive. Also warmer temperatures could cause HWA to spread at an increased rate allowing it to enter into Canada. The eastern hemlock currently occupies a large area and is able to survive in many different temperature ranges, however; it is not suited for extremely warm temperature so it is very possible that the future will show less and less hemlock in the south and a steady migration in a northern pattern.
The Eastern Hemlock reaches as far north as northern Minnesota and south central Ontario. The Eastern Hemlock extends south down the Appalachian Mountains and into parts of Georgia and Alabama. The tree can also be seen as far west as Indiana, Missouri, and Minnesota. The Eastern Hemlock overlaps some other sub species such as the Carolina Hemlock.
Distribution of the Eastern Hemlock
Map shows the extent by state of the Eastern Hemlock (Eastern Hemlock updated 2008)
(Plants.USDA.gov)
Eastern Hemlock is generally restricted to regions with cool humid climates (Godman 2006). The areas which the Eastern Hemlock occupies generally receive between 29 to 50 inches of precipitation per year. The Hemlock is able to survive in a wide array of temperature ranges. The unique climates which are created by the Eastern Hemlock help to keep the temperatures lower and also retain moisture in the soils. They accomplish this by creating such a dense canopy that it is difficult for sunlight to reach the forest floor. This creates a climate which usually humid and cool.
The Eastern Hemlock is used by wildlife as well as humans. Hemlock stands provide bedding areas for whitetail deer and nesting for many species of birds. They are also cut for use as pulp and other lumber uses. Hemlocks create unique climates which shelter animals during the winter. Multiple studies have shown that the average temperature is higher inside of a hemlock stand during the winter, and snowfall is also less (Bergdahl 2007). This provides a crucial area for winter time habitat for many mammals.
The threats that currently face the Eastern Hemlock are very serious; the future can only tell what will happen to the Eastern Hemlock. The spread of Hemlock Wolly Adelgid is being monitored and scientists are hopeful that some trees may be able to survive the disease and prevent a total loss of the tree. The hope is that these trees will be able to reproduce and maintain a healthy number of Eastern Hemlock. The eastern hemlock occupies many different areas most of which do fall into the temperate broadleaf and mixed forest category but there are exceptions to this. The hemlock can stand a wide array of temperature and rainfall differences in its many different habitats. Despite the fact that the eastern hemlock is a strong and adaptable species, it can’t overcome the increased populations of white-tail deer or the spread of HWA. Areas of habitat need to be set aside and protected from human intrusions so that the eastern hemlock can be monitored. It will be up to humans to try and find solutions to things like HWA and global warming. Finding an answer will be crucial to the survival of the eastern hemlock
References:
Anderson R, Loucks O, 1979. White-tail deer influence on structure and composition of tsuga Canadensis
forest. Journal of Applied Ecology 16, 855-861
Bergdahl DR, Costa SD, Lishawa SC. 2007. Winter conditions in eastern and mixed-hardwood
Deer wintering areas of Vermont. Can J.For. Res 37: 697-703.
Cobb R, Muller T, Orwig D, Stadler B, 2005. Hemlock Woolly Adelgid in New England Forests: Canopy
Impacts Transforming Ecosystem Processes and Landscapes.
Ecosystems 8, 233-247
Godman R.M. Lancaster K. Eastern Hemlock [Internet]. [Cited 2008 Feb 17] 4:30. Available from:
http://www.na.fs.fed.us/spfo/pubs/silvics_manual%20/volume_1/tsuga/canadensis.htm.
Mladenoff D, Stearns F,1993. Eastern hemlock regeneration and deer browsing in the northern great lakes
region: A re-examination and model simulation. Conservation Biology 7, 889-898.
Tsuga Canadensis, Eastern Hemlock [Internet. [Updated 2007] USDA, Natio0nal Resource
Conservation Service [cited 2008 Feb17] Available from:
www.plants.USDA.gov
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