There is no denying that this fall has brought drought conditions to Western North Carolina. The North Carolina Drought Management Advisory Council shows all 17 Western North Carolina Counties where native hemlocks occur currently range from abnormally dry in the north eastern part of the mountains to extreme drought in the south western counties. The North Carolina Forest Service reports that at least 30 wildfires have burned upwards of 46,000 acres on private and state-owned land as of Wednesday November 16th.
These dry conditions spell additional stress for our region’s already threatened hemlock population. Researchers agree that while the precise relationship between the effects of hemlock woolly adelgid (HWA) and drought on hemlocks is difficult to quantify, their combination causes trees to succumb to these stressors at a much faster rate.
According to Havill (et al.), “among conifers, hemlocks are the most susceptible to drought which may explain why the decline of eastern hemlock is more severe than other tree species experiencing the same environmental changes.” In fact, according to a study published by the US Forest Service, the pollen record shows that there were two other periods of rapid decline that occurred in eastern hemlock, approximately 9,800 and 5,300 years ago respectively. While the most recent decline was likely due, in large part, to insect feeding, both periods of decline can be attributed to increased temperature variation and drought (HWA and its Hemlock Hosts, Havill et al. pg. 1).
Researchers Kelly Oten and colleagues describe a “drought-like physiological reaction” in hemlocks after HWA infestation (Stylet Bundle Morphology, Oten et al. pg. 1). In the case of both HWA and water stress, hemlocks rapidly decline in health “exhibited by needle drop, bud abortion and a lack of new growth”. Given the similar ways these trees react to both stressors it is no surprise that many researchers believe that warm dry conditions, like the ones we are currently experiencing, dramatically accelerate hemlock decline due to HWA.
What that means for the Hemlock Restoration Initiative is that we have a lot of work to do in extending the lives of our hemlocks this fall. In response, we are busy working with natural resource managers and conservation partners to identify and scout out potential “hemlock conservation areas” in preparation for hemlock treatment workdays when the rain returns. In the meantime, we have been using drought-tolerant CoreTect tablets to proceed with previously scheduled volunteer days.
November is our driest month in North Carolina. In spite of or because of this fact HRI volunteers have been hard at work. On Tuesday November 1st, celebrated by many as the Day of the Dead or All Souls Day, a team of 10 dedicated volunteers took to DuPont State Forest armed with drought friendly hemlock treatment supplies. The team managed to save 157 beautiful, endemic Carolina hemlocks from an early grave, transforming the Day of the Dead into the day of the living Carolina Hemlock. On November 13th an additional 10 committed hemlock helpers descended on Holmes Educational State Forest, to complete the hemlock treatment begun there in September. Because of these hardworking volunteers and drought appropriate CoreTect tablets, nearly all of the viable hemlocks at Holmes have received chemical protection that will help them stand up to the dry conditions for years to come.
Some researchers are now saying that the simplest, most affordable, soil drench treatment technique may also be appropriate for use in drought conditions. Futhermore, a new study by Dr. Elizabeth Benton and colleagues shows that a much lower dose than previously thought of the chemical used to treat HWA in hemlocks is appropriate for achieving optimal results. These findings together show that, despite the serious additional stress of drought on our trees, chemical treatment remains an accessible, viable option for keeping our hemlocks alive while scientists develop a more comprehensive solution to the problem of HWA. This is excellent news because many researchers believe that we are likely to see more, not less, of these unseasonably warm and dry conditions in future years. Arial Shogren of the Cary institute of Ecosystem Studies in Millbrook New York believes that “understanding the links between drought and infestation will be critical for predicting future forest response to a changing climate.” It is our job to make sure our trees hang on long enough to see the fruits of this research.
If you would like to get involved in please visit our website: savehemlocksnc.org or contact us directly at firstname.lastname@example.org
Havill, Nathan; Montgomery Michael; Keena, Melody; Hemlock Wooly Adelgid and its Hemlock Hosts: A Global Perspective; US Forest Service, Northern Research Station Hamden ,CT.
Oten, Kelly L. F.; Cohen, Allen C.; Hain, Fred P.; Stylet Bundle Morphology and Trophically Related Enzymes of the Hemlock Woolly Adelgid (Hemiptera: Adelgidae. Entomological Society of America, Annals of the Entomological Society of America, 107(3):680-690. 2014.
Benton, Elizabeth; Cowles, Richard S.; Lagalante, Anthony; Assessing relationships between tree diameter and long-term persistence of imidacloprid and olefin to optimize imidacloprid treatments on eastern hemlock. Forest Ecology and Management · June 2016
Shogren, Arial, Ladeau, Shannon; Lovett, Gary; TREE-LEVEL RESPONSE TO DROUGHT AND HEMLOCK WOOLLY ADELGID INFESTATION IN EASTERN HEMLOCK TREES ARIAL SHOGREN Vassar College, Poughkeepsie, NY 12604, Cary Institute of Ecosystem Studies, Millbrook, NY 12545 USA