In-Stream Hemlock Twig Breakdown and Effects of Reach-Scale Twig Additions on Appalachian Headwater Streams

Abstract

Eastern hemlock (Tsuga canadensis) is a prominent tree in the forests of eastern North America, where it commonly grows along headwater streams. It is experiencing widespread mortality due to infestations of an introduced insect, the hemlock woolly adelgid (Adelges tsugae). Eliminations of tree species are known to have ecosystem-level effects, and one consequence of hemlock death is a change in allochthonous inputs to headwater streams. I predicted that hemlock twigs' dendritic structure, abundance, and resistance to decay currently make them highly effective retainers of leaves in headwater streams, with consequences for nutrient uptake. To understand the role of hemlock twigs in streams and to compare their functions to those of a potential replacement species, I (1) quantified the decomposition and microbial colonization of twigs and (2) manipulated twig standing crops to quantify effects on leaf retention and nutrient uptake. Hemlock twigs provide a poor-quality substrate for microbial colonization and growth relative to birch (Betula lenta) twigs and are more resistant to breakdown than birch. Although hemlock twigs appear to be effective in retaining leaves, they do not substantially affect reach-scale uptake of ammonium, which is much more strongly influenced by the timing of leaf inputs. Although hemlock death may subtly change patterns of organic matter accumulation and breakdown, the potentially important effects of hemlock death include changes in large wood inputs, changes in hydrologic regime, and increases in rhododendron cover.