Effects of Hemlock Mortality on Streams in the Southern Appalachian Mountains

Article excerpt

Abstract. -

The death of eastern hemlock (Tsuga canadensis) trees in response to infestation by the introduced hemlock woolly adelgid (Adelges tsugae) may affect ecosystem processes and structure of streams. Prior to hemlock mortality, we documented the conditions of eight small streams and their associated riparian forests within the Appalachian Mountains of North Carolina, U.S.A. Hemlock was the dominant tree species on all riparian sites and was always associated with rhododendron (Rhododendron maximum). Significant trends of increasing canopy openness, increasing light to the streams and increasing annual temperature range were observed. Contributions of hemlock to litterfall, in-stream wood, and benthic organic matter were important at the beginning of the study, suggesting that the loss of hemlock may significantly modify the trophic dynamics and physical structure of southern Appalachian streams. Increased growth of rhododendron in response to hemlock mortality may compensate for the trophic influences of hemlock loss. However, because of rhododendron's negative effect on growth of seedlings of other tree species, the greatest ecosystem impact of hemlock wooly adelgid may be more extensive rhododendron thickets within the riparian corridors of southern Appalachian streams.

Introduction

Historically, eastern hemlock (Tsuga canadensis (L.) Carr.) has been considered a foundation species throughout much of eastern North America (Ellison et a?, 2005). But stands of eastern hemlock and Carolina hemlock (T. caroliniana Engelm) are currently declining rapidly due to infestation by the hemlock woolly adelgid (HWA, Adelges tsugae Annand) and little regeneration is expected (Orwig and Foster, 1998; Preisser et al, 2011). While in the Northeast, hemlock typically occurs along ridge tops, on steep slopes, and in narrow valleys (Orwig et al, 2002), in the soudiern Appalachians, eastern hemlock is the only conifer abundant along streams and may play an important role as a riparian species. These regional differences in distribution could influence the consequences of hemlock mortality. Ehrenfeld (2010) contrasted the structure of terrestrial and aquatic ecosystems and concluded that structural differences do not produce fundamental differences in the mechanisms by which invasive exotic organisms alter ecosystem function. But, in the case of the adelgid, differential effects on forests and streams of the soudiern Appalachians may produce such fundamental differences. In forests, the adelgid causes deadi of a single tree species, which, on a regional basis, may be relatively unimportant (Albani et al, 2010). But, because hemlock is one of the dominant riparian species along small mountain streams, its loss may have larger effects on stream organisms and in-stream processes (Siderhurst et al, 2010).

In addition, while hemlocks killed by the adelgid are typically replaced by hardwoods in the Northeast (Batdes et al., 2000; Orwig and Foster, 1998; Jenkins et al., 1999; Kizlinski et al., 2002; Small et al, 2005), riparian forest regrowdi in the Southeast may be very much influenced by the presence of rhododendron, especially Rhododendron maximum L. (Kincaid, 2007; Kincaid and Parker, 2008; Nuckolls et aL, 2009; Roberts et al., 2009; Spaulding and Rieske, 2010; Krapfl et al, 2011; Ford et al, 2012). The co-occurrence of hemlock and rhododendron in riparian areas was noted in die earliest studies of southern Appalachian vegetation (e.g., Pinchot and Ashe, 1897). Rhododendron is known to inhibit seed germination and seedling growth of many trees (Beckage et al, 2000; Nilsen et ai, 2001; Beier et al., 2005; Lei et ai, 2006). Hemlock is one of the few tree species that can propagate under a dense rhododendron canopy (Phillips and Murdy, 1985; Van Lear et al., 2002). Consequently, we might expect hemlock death to be followed by expansion of dense thickets of rhododendron along low-order steams (Roberts et al., 2009). …