Influences of eastern hemlock mortality on nutrient cycling

Abstract

Mortality of eastern hemlock (Tsuga canadensis (L.) Carriere) may be caused by a variety of agents, but hemlock trees of all sizes over a large geographic area are currently threatened by an outbreak of the hemlock woolly adelgid (HWA: Adelges tsugae Annand) in the eastern United States. In this paper, we review what is currently known about changes in nutrient cycling due to hemlock mortality, identify gaps in current knowledge, and describe research being conducted to better understand the influences of hemlock mortality on nutrient cycling. Recent research clearly demonstrates that hemlock mortality can strongly influence nutrient cycling rates. Jenkins et al. (1999a) concluded that hemlock mortality due to HWA infestation is likely to result in accelerated nitrogen (N) mineralization and nitrification rates. Yorks, Leopold, and Raynal have observed elevated nitrate and cation (e.g. ammonium, Al, Ca, Mg) leaching in soil water during hemlock decline and mortality. Such nutrient losses can result in reductions in site nutrient capital and future forest productivity. Nitrate and cation leaching is also likely to cause reductions in surface water quality near impacted sites. Despite significant progress regarding our understanding of hemlock mortality and nutrient cycling, several important questions remain. For example: 1) What processes drive the accelerated N cycling rates found at infested hemlock stands?; and 2) How long will changes in nutrient cycling (e.g. elevated nutrient losses to soil water) continue after hemlock mortality? To further understand relationships between hemlock decline and nutrient cycling rates, our current research includes a search for relationships between soil and foliar nutrient status and susceptibility to HWA damage, continued soil water sampling in healthy and dying hemlock stands, stream water sampling from watersheds with a wide range of hemlock abundance, and measurement of soil characteristics in HWA-infested stands.

Parent Publication

• Proceedings: symposium on sustainable management of hemlock ecosystems in eastern North America

Citation

Yorks, Thad E.; Jenkins, Jennifer C.; Leopold, Donald J.; Raynal, Dudley J.; Orwig, David A. 2000. Influences of eastern hemlock mortality on nutrient cycling. In: McManus, Katherine A.; Shields, Kathleen S.; Souto, Dennis R., eds. Proceedings: Symposium on sustainable management of hemlock ecosystems in eastern North America. Gen. Tech. Rep. NE-267. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station. 126-133.