Environmental Factors and the Spread of the Hemlock Woolly Adelgid (Adelges tsugae) in Western North Carolina

Banks, M.A. & Benson, B.

Appalachian State University, Biology Department, 572 Rivers Street,

Boone, NC 28608

I. Introduction

The Eastern Hemlock ( Tsuga canadensis ) and Carolina Hemlock (Tsuga caroliniana),are two of the most common trees found in the north east US (Rogers, Robert S., 1977). These two species and possibly other members of Tsuga are under growing threat from the hemlock woolly adelgid (HWA). Hemlocks are some of the long living trees reaching ages up to 500 years (Hett Loucks, 1976) and a leading element of old growth forests, providing habitat for a variety of biota such as Ruffed Grouse, Yellow-bellied Sapsucker, and the Great Horned Owl (Lanier, John W. et al. 2000).

The hemlock woolly adelgid (HWA) (Adelges tsugae) is an invasive insect of eastern Asia, it is found on the underside of the base of the hemlock needles. HWA feeds on the starch reserves of the Hemlock shoots and drains the hemlock of its phloem sap. Eventually Hemlocks starve and lose their needles, stop producing new growth and eventually die (Day Salom, 2010). Most of the hemlocks in the Eastern US are currently under threat from this invasive species.

II. Objectives

The objective of this survey is to assess if any basic environmental factors will have an effect upon hemlock survivorship in regards to HWA infestation. For the purposes of this study the average amount of sunlight, elevation, slope, and proximity to water were used as categorical variables.

Hemlocks are able to grow in low and high sunlight conditions such as under-story and shaded slopes (Kobe, Richard K., et al. 1995). The ability of these trees to exist and thrive under such varied light conditions suggests it may have an impact upon survivorship. Hemlocks are commonly found up to 4900 feet above sea level (Brisbin & Robert, 1970), with changes in elevation present different atmospheric pressures, wind gradient patterns, and O2 levels. Slope values are also a possible consideration as it can be used as an indicator of wind exposure. The proximity to a water source is also a possible factor as it attracts other biota to the area, which can act as transport vector for the woolly adelgid.

III. Methods

The study area took place in the Southern Appalachian Mountain Range, specifically: Moses Cone Mansion Memorial Park and Simms Creek, in Blowing Rock, NC, USA ; Grandfather Mountain and Profile Trail in Banner Elk, NC, USA; and Beacon Heights in Linville, NC, USA. Access to these sites were limited to the trails, limiting the survey methodology. The trails all had experienced various levels of HWA infestation.

A DeLorme Earthmate PN-60W, GPS receiver, was used to collect data points where hemlocks were present between the dates of April 19 and April 22, 2013. All hemlocks sighted on the trails were assessed, recorded, and categorized as alive and dead; those hemlocks categorized as dead had two or more branches displaying infestation of HWA or were visibly dead, all others were listed in the alive category.

IV. Analysis

698 individual hemlock trees were evaluated during this survey, 200 of those were categorized as alive and the remaining 498 as dead.

Slope / Avg. Sunlight / Elevation / Distance to Water
χ2 p-value / 1.12x10-05 / 2.35x10-04 / 1.07x10-18 / 0.399
Correlation / 0.866 / 0.957 / 0.948 / 0.938

All the data points collected were mapped using ESRI ArcMap 10.1 and overlaid with map data obtained from from US Geological Survey (USGS), US Department of Agriculture (USDA), and North Carolina Department of Transportation (NCDOT). Slope data was derived from NCDOT 3 meter contour maps and stored as a raster layer. All other environmental factors were located as raster layer maps, data values from these raster layers was then appended to the hemlock data points. The distance to water was calculated using the ArcGIS “Point Distance” tool to determine the distance of each surveyed hemlock to the closest stream, lake, river, pond, or other water body and appended to the data set.

Counts of the data set were then conducted according to category and environmental variable. Each environmental variable was binned according to equal intervals over the range of the variable. Chi square and one tailed correlation values were then calculated to identify possible relationships between the alive and dead categories according to environmental variable.

V. Results and Discussion

The low p values and high correlation values for slope, sunlight and elevation indicate that the occurrence of living hemlocks varies closely with dead, this indicates that these environmental factors have negligible impact upon survivorship. The only environmental factor that showed independent variation was the proximity of hemlocks to water with a p-value of 0.399, indicating a relationship between survivorship and water proximity.

51.5% of the uninfected hemlocks were within 70 meters of a water feature, 34% of those were found alive, 5% above the expected survivorship rate for the survey sample at large, 29%. Survivorship decreases with distance from water, for example it drops to 23% beyond 250 meters to the closest water source. This is partially due to the water requirements of the trees, with a larger portion of the sample size found in close proximity to a water source however, the survivorship differences are still significant.

All uninfected hemlocks had been tagged as pesticide treated, approximately 10% of the dead hemlocks in the survey had been marked as treated. This indicates that the pesticide used is not completely effective, but this factor still introduces doubt regarding the findings of this study.

Taking these factors into account it appears that there is a relationship between water proximity, survivorship, and infection by HWA.

VI. Works Cited

1.  "Geospatial Data Gateway." USDA. <http://datagateway.nrcs.usda.gov/>.

2.  "Maps, Imagery, and Publications." USGS <http://www.usgs.gov/pubprod/>.

3.  Day, Eric R., and Scott Salom. "Hemlock Woolly Adelgid." Virginia Cooperative Extension. Virginia Tech., 2010. <http://pubs.ext.vt.edu/3006/3006-1451/3006-1451.html>.

4.  Hett, Joan M., and Orie L. Loucks. "Age structure models of balsam fir and eastern hemlock." The Journal of Ecology (1976): 1029-1044.

5.  Jenkins, Jennifer C., John D. Aber, and Charles D. Canham. "Hemlock woolly adelgid impacts on community structure and N cycling rates in eastern hemlock forests." Canadian Journal of Forest Research 29.5 (1999): 630-645.

6.  Kobe, Richard K., et al. "Juvenile tree survivorship as a component of shade tolerance." Ecological Applications 5.2 (1995): 517-532.

7.  Rogers, Robert S. "Forests dominated by hemlock (Tsuga canadensis): distribution as related to site and postsettlement history." Canadian Journal of Botany 56.7 (1978): 843-854.

8.  Skinner, Margaret, et al. "Regional responses of hemlock woolly adelgid (Homoptera: Adelgidae) to low temperatures." Environmental entomology 32.3 (2003): 523-528.

9.  USDA. "Risk, Detection, and Spread: Hemlock Woolly Adelgid - Invasive Species - Managing with Disturbance - Northern Research Station - USDA Forest Service."USDA. N.p., 2010. Web. 03 May 2013. <http://www.nrs.fs.fed.us/disturbance/invasive_species/hwa/risk_detection_spread/>.

10.  USDA. Eastern Hemlock. Rep. N.p.: USDA, n.d. Forest Products Labratory. Web. <http://www.fpl.fs.fed.us/documnts/usda/amwood/239hemlo.pdf>.

11.  Yamasaki, Mariko, Richard M. DeGraaf, and John W. Lanier. "Wildlife habitat associations in eastern hemlock-birds, smaller mammals, and forest carnivores." Proceedings: symposium on sustainable management of hemlock ecosystems in eastern North America. Newtown Square, 2000.

Appendix I. Data tables

Slope
Degrees / Alive / Dead
0 / 9 / 6
10 / 68 / 124
20 / 69 / 276
30 / 53 / 89
40 / 1 / 1
50 / 0 / 2
Average Sunlight
Hours / Alive / Dead
4 / 1 / 3
6 / 2 / 0
7 / 5 / 1
8 / 22 / 59
9 / 75 / 168
10 / 59 / 194
11 / 36 / 62
12 / 0 / 11
Distance from Water
Meters / Alive / Dead
0 / 48 / 85
35 / 33 / 59
70 / 22 / 56
105 / 16 / 56
140 / 21 / 53
175 / 12 / 36
210 / 18 / 52
245 / 8 / 27
280 / 7 / 29
315 / 7 / 26
350 / 2 / 2
385 / 3 / 11
420 / 1 / 4
455 / 1 / 1
490 / 1 / 0
560 / 0 / 1
Elevation
Feet above Sea Level / Alive / Dead
3500 / 11 / 41
3600 / 38 / 51
3700 / 4 / 4
3800 / 11 / 11
3900 / 22 / 2
4000 / 10 / 92
4100 / 71 / 128
4200 / 4 / 47
4300 / 10 / 49
4400 / 6 / 40
4500 / 10 / 33
4600 / 3 / 0