The role of Hemlock organic matter in headwater streams.

Eastern hemlock (Tsuga canadensis) is a common tree throughout much of eastern North America and is currently experiencing widespread mortality due to infestation by an introduced insect, the hemlock woolly adelgid (Adelges tsugae). In the southern Appalachians, eastern hemlock is the only coniferous tree that commonly grows along streams. Organic matter inputs (such as leaves, needles, and wood) from riparian trees are the major energy source in headwater stream food chains and play important roles in the cycling of carbon and nutrients in streams. Relative to inputs from hardwoods, conifer organic matter is slow to break down in fresh water and can serve different functions. A shift from hemlock to hardwood dominance in riparian areas may have significant impacts for transformations and transport of particulate and dissolved materials in streams. The loss of small wood inputs, or twigs, from hemlock trees may be especially important. We hypothesize that hemlock twigs influence organic matter and nutrient dynamics in headwater streams because they have unique physical and chemical characteristics that make them effective retainers of organic matter. We added hemlock twigs to one stream and hardwood twigs to a second stream and are measuring their effects by quantifying organic matter standing stocks, ammonium uptake, and leaf transport distance.