The notion of scale and the effects of Hemlock Woolly Adelgid (Adelges tsugae) infestation on Eastern Hemlock (Tsuga canadensis) forest ecosystems: Is there a future for our beloved Hemlock?

Standing-dead hemlocks at George W. Childs Park (Delaware Water Gap National Recreation Area).  Photo by Carl Figueiredo

The Eastern Hemlock (Tsuga Canadensis) is an evergreen, coniferous tree which often grows in pure stands on acidic soils and is characteristic of moist cool valleys, rock outcrops and ravines (Little, 1980). Its range encompasses most of the north east region of the United States going from Southern Ontario east to Cape Breton Island, south to Northern Alabama and west to Eastern Minnesota (Little, 1980). The eastern hemlock is the most shade tolerant tree species in North America and is capable of surviving underneath a shaded forest canopy for as long as 350 years (Quimby, 1996). It is a climax or late successional species which can dominate a forest for hundreds of years.

The uniqueness of this tree is its ability to form various microclimates within the forest. The microclimates, in turn, create an abundance of habitat for many forms of fauna, both terrestrial and aquatic. Studies show the exclusive use of hemlock bench and ravine habitats (as opposed to equivalent hardwood sites) by three species of wood warbler and a vireo (solitary vireo, black-throated green warbler and blackburnian warbler) and mixed use by two additional species (red-eyed vireo, and ovenbird) (Young, et al. 1998). Hemlock forests can also be a critical factor in supporting native brook trout populations, by maintaining cool stream temperatures and stable stream flows (Evans et al. 1996, Snyder et al. 1998).

Hemlocks are under attack by the aphid-like insect called the Hemlock Woolly Adelgid (Adelges tsugae), from here forth referred to as HWA. It is native to Japan and was accidentally introduced to North America (Oregon) in 1924 (McClure, 1987). HWA is dispersed by wind, birds, deer and humans, and continues to move farther north at a rate of 30 kilometers per year (McClure, 1990). Evidence of HWA infestation can be detected by noticing its white egg sacs, "woolly" in appearance, on the underside of the branch, especially on new growth. Adelgids feed on twigs and new growth by piercing at the base of the needles which results in desiccation and eventual loss of needles (Shields et al. 1996). Infested trees usually die within 9 years; some have been killed in as little as 4 years (McClure, 1987). HWA populations, comprised mainly of females, grow at an exponential rate due to the female’s ability to reproduce asexually (without fertilization by males) and to produce up to 300 eggs at a time (McClure, 1987). HWA populations are extremely vigorous and are an ongoing threat to the future of the delicate hemlock forest ecosystems of the North-East.

Now, a question arises: What type of significant impact will the loss and/or disturbance of the eastern hemlock have on the biotic and abiotic components of the hemlock forest ecosystem? This is not an easily answered question due to the fact that the ecosystem responses of these events will occur at a variety of different organizational levels and scales.  Such disturbances may manifest themselves as changes in energy inputs, microclimate environments, or physical habitat structure available to birds, fish and aquatic macro-invertebrate communities (Snyder et al. 1998). One recent study found that there was a considerable increase in ammonium-N availability in hemlock stands experiencing infestation; mainly due to increased litter production (Jenkins et al. 1999). Jenkins et al. 1999, found that nitrification rates were 30 times as high in forests experiencing hemlock mortality than under healthy stands. This large level effect has some serious implications at various organizational levels and scales. Since hemlocks occur mostly in riparian (streamside) zones or wet areas the issue of nitrate leaching becomes a major factor. The excess nitrogen being exported into streams can lead to freshwater pollution. This has grave implications for much aquatic and terrestrial life. At a larger landscape scale the slight increase in available nitrogen its relatively insignificant.  However, at a smaller scale one can notice the influential effects of this event.  As nitrogen levels increase in an aquatic, stream-side forest area, algal growth in aquatic systems may be increased altering habitat and resource availability for much of the species which rely on the streams. Small streams in hemlock forests are three times more likely to support native brook trout populations than similar streams in hardwood forests (Snyder et al. 1998). The brook trout, which reproduces naturally in many of the North East’s hemlock streams, will undoubtedly be negatively affected. Not to mention the multitude of avian and terrestrial species which rely on aquatic insect life as a food source.

As noted earlier, the hemlock is a highly shade tolerant species which causes mature hemlock stands to produce a thick canopy cover. This affects the amount of available light which penetrates the canopy. Streams cool by as much as 5° centigrade during the summer while flowing through hemlock ravines (Evans, et al. 1996). The reduction in ambient temperature due to reduced light creates a unique microclimate which greatly affects species composition and biodiversity. Hemlock forest streams typically support about 65 species of aquatic insects with only about 35 species occurring in hardwood streams; with about 15 species of aquatic insects occurring almost exclusively in hemlock streams (Snyder et al. 1998). With increasing numbers of dead hemlocks in the forest matrix the amount of incident light hitting the forest floor will increase. This seemingly large level effect sets the stage for a myriad of smaller organizational level effects. Mature, canopy level hemlock trees would eventually be replaced primarily by deciduous trees, especially black birch (Betula lenta) and red maple (Acer rubrum) (Orwig and Foster, 1998). With increased light availability the forest floor would quickly begin to change. Saplings would soon dominate the open forest floor and form a dense thicket for many years. With the increased resource availability of available light hitting the forest floor for new colonizers the potential for more invasive exotic species is escalated. Invasive exotic plants, such as "tree-of-heaven" (Ailanthus altissima), Japanese barberry (Berberis thunbergii) and Japanese stilt-grass (Microstegium vimineum) spread through affected forests (Orwig and Foster, 1998). These species can establish themselves rather quickly and can eventually choke out other native species from the forest floor. Ultimately, the extent of the change in ecosystem processes following hemlock mortality will depend on the differences between hemlock and its successors in terms of their influence on ecosystem function (Jenkins et al., 1999).

This alteration in forest composition would also have detrimental effects on many populations of avian species as well as their prey and/or predators. The alteration in temperature would also cause streams to be warmer, in turn, affecting the amount of water flow which would alter the species densities and presence of many aquatic insects. Snyder et al. 1998, concluded that the number of aquatic species in hemlock streams would probably decline by 35 percent or more. The seemingly insignificant increase in available light, even if only slightly, on the forest floor demonstrates a domino type effect of disturbances or disruptions on several smaller scales.

Now the question remains: How will large level effects of hemlock decline affect humans? The resource planners at the Delaware Water Gap National Recreation Area, part of the National Park’s Service, located in NW New Jersey and NE Pennsylvania, have taken this question into much consideration. How can such a large scale effect such as the severe decline of hemlock forests and ecosystems affect the individual human? Dead and fallen trees will aggregate on the forest floor and in stream channels causing debris dams, which will destabilize stream channels and increase the chance of severe flood damage, due to debris flows and scouring, to roads, trails and structures (Evans, 2000). Build up of debris in streams can be very harmful not only to aquatic life but humans as well. Piles of debris can certainly break free and damage places inhabited by humans. Another factor is that most hemlocks do not fall immediately after their demise. The number of hazard trees now present due to the HWA infestation is enormous. Aside from the eminent danger of hazardous standing and downed trees a loss of a cultural resource is also occurring. Every year thousands of people from all over the country come to places like the Delaware Water Gap National Recreation Area to enjoy the beautiful and relaxing settings of the many hemlock lined ravines and streams. The short-term temporal scale effect of the HWA infestation on humans is the closure of many beloved and visited natural areas due to hazardous conditions provided by the abundance of dead hemlocks.  Without the hemlock the flocks of people would no longer come to the park to fish for the prized brook trout, relax and enjoy the natural and serene settings.  As a matter of fact, Pennsylvania will even have to find itself a new state tree if the hemlock leaves us forever.

As you can see, the loss of our natural hemlock forest ecosystems is obviously an important issue encompassing many aspects of the natural and human centered worlds. At this point the future of the eastern hemlock remains unsure. The effectiveness of the use of bio-control beetles (Pseudoscymnus tsugae) on a regional scale has not been promising. However, much research is currently underway to determine an effective way to save the natural and cultural resource for generations to come. It is the dedicated work of many biologists that will allow us a better understanding of the puzzling ways of the HWA.

Standing-dead hemlocks at George W. Childs Park (Delaware Water Gap National Recreation Area).  Photo by Carl Figueiredo

Canopy gaps caused by demise of HWA infested hemlocks, Donkey's Corners (Delaware Water Gap National Recreation Area).  Photo by Richard Evans.

HWA infestation.  Photo by Robert L. Anderson, USDA Forest Service

Ode to the Hemlocks

Marion Kloster

Great Smokey Mountains National Park

They tower like giants,

above the forest floor,

as high as some mountains,

where the red-tails soar.

If only they could talk,

the stories we’d hear

of generations gone by,

the changing seasons through the years.

They murmur with the wind,

as it rustles their boughs,

they crack and creak,

in a more forceful attempt to speak.

Take notice! Look at me!

they seem to say,

I’m the oldest of a great species,

please take the time to glance my way.

Is it God’s plan for the Adelgid to destroy these beauties?

Is it time for Mother Nature’s cycle to complete her duties?

Or will their strength see them through,

to weather their toughest storm,

will they still be here?

when our great-grandchildren are born?

What is the answer?

no one knows their fate,

so far for now,

take the time to walk amongst them,

enjoy, appreciate.

Links

Forest Ecosystem Response to Hemlock Wooly Adelgid in Southern New England

Hemlock Woolly Adelgid Newsletter, Issue 5

Research & Resource Planning Division 2000 Annual Report - Delaware Water Gap National Recreation Area

Environmental Assessment for the release of Bio-Control beetles (Pseudoscymus tsugae)

USDA Forest Service Hemlock Woolly Adelgid Website

Biological Control of Hemlock Woolly Adelgid in the Eastern United States--McClure (FHTET-2000-08, Jan. 2001)

NJ Dept of Agriculture - HWA Information

Literature Cited

Evans, R.A., E. Johnson, J. Shreiner, A. Ambler, J. Battles, N. Cleavitt, T. Fahey, J. Sciascia, and E. Pehek. 1996. Potential impacts of hemlock woolly adelgid (Adelges tsugae) on eastern hemlock (Tsuga canadensis) ecosystems. In: S.M. Salom, T.C. Tigner, and R.C. Reardon, eds. Proceedings of the First Hemlock Wooly Adelgid Review, Charlottsville, VA, 1995. USDA Forest Service Forest Health Technology Enterprise Team-Morgantown, WV. FHTET 96-10. pp. 42-57

Evans, R.A. 2000. Eastern hemlock ecosystems and hemlock woolly adelgid. Delaware Water Gap National Recreation Area. National Park Service. Research & Resource Planning Division Annual Report. 8pp.

Jenkins, J.C., J.D. Aber, and C.D. Canham. 1999. Hemlock woolly adelgid impacts on community structure and N cycling rates in eastern hemlock forests. Canadian Journal of Forest Research 29:630-645.

Little, E.L. 1980. National Audubon Society Field Guide to North American Trees. Alfred A. Knopf, Publisher, New York.

McClure, M.S. 1987. Biology and control of hemlock woolly adelgid. Connecticut Agricultural Experiment Station Bulletin 851, pp. 3-9.

McClure, M.S. 1990. Role of wind, birds, deer, and humans in the dispersal of hemlock woolly adelgid (Homoptera: Adelgidae). Environmental Entomology. 19:36-43

Orwig, D.A. and D.R. Foster. 1998. Forest response to the introduced hemlock woolly adelgid in southern New England, USA. Journal of the Torry Botanical Society. 125(1):60-73.

Quimby, J. 1996. Value and importance of hemlock ecosystems in the United States. In: S.M. Salom, T.C. Tigner, and R.C. Reardon, eds. Proceedings of the First Hemlock Woolly Adelgid Review, Charlottsville, VA, 1995. USDA Forest Service Forest Health Technology Enterprise Team-Morgantown, WV. FHTET 96-10. pp. 1-8.

Shields, K.S., R.F. Young, and G.P. Berlyn. 1996. Hemlock woolly adelgid feeding mechanisims. Proceedings of the First Hemlock Woolly Adelgid Review, Charlottsville, VA, 1995. USDA Forest Service Forest Health Technology Enterprise Team-Morgantown, WV. FHTET 96-10. pp. 36-41.

Snyder, C., J. Young, D. Smith, D. Lemarie, R. Ross, and R. Bennett. 1998. Influence of eastern hemlock decline on aquatic biodiversity of Delaware Water Gap National Recreation Area. Final Report of the USGS Biological Resources Division, Leetown Science Center. Aquatic Ecology Laboratory. 66p (http://ael.er.usgs.gov/groups/gis/hemlock/dewa.html)

Young, J., F. Van Manen, and R. Ross. 1998. Modeling stand vulnerability and biological impacts of the hemlock woolly adelgid. Study Plan Number 2055. USGS, Leetown Science Center, Kearneysville, WV 25430. 32pp.