Effects of Eastern Hemlock Decline on Large Wood Loads in Streams of the Appalachian Mountains

Abstract

Evans, Daniel M., C. Andrew Dolloff, W. Michael Aust, and Amy M. Villamagna, 2012. Effects of Eastern Hemlock Decline on Large Wood Loads in Streams of the Appalachian Mountains. Journal of the American Water Resources Association (JAWRA) 48(2): 266‐276. DOI: 10.1111/j.1752‐1688.2011.00610.x Abstract: Eastern hemlock (Tsuga canadensis), a foundation species important to riparian forests of the Appalachian Mountains, is in decline due to the hemlock woolly adelgid (HWA) (Adelges tsugae). The effect of this insect on large wood (LW) production and recruitment to streams is largely unknown. We measured LW load, riparian forest composition, and T. canadensis health in 47 streams from Maine to Alabama and developed a conceptual model that frames the expected forest stand and instream LW response to the disturbance caused by HWA. In stands with HWA, 24.3% of the overstory T. canadensis basal area was stressed and 7.5% was dead, whereas only 3.0% was stressed and 2.0% was dead in stands without HWA. Mean T. canadensis effective LW load was 0.20 m3/100 m2 in streams with adjacent stands infested by HWA, almost three times higher than in streams flowing through uninfested stands (p = 0.048). Higher frequencies of LW were found at streams with HWA present and there was a positive association between the percent of dead T. canadensis and T. canadensis LW load (ρ = 0.4468; p = <0.0001). Greater total LW loads were associated with higher stream gradients, lesser bankfull widths, and streams at lower latitudes. Our research supports the hypothesis that HWA infestation of T. canadensis in riparian forest stands has increased LW loads in streams. This episodic disturbance to the riparian forest overstory may have lasting impacts on aquatic ecosystem structure and function.

Large wood in central Appalachian headwater streams: controls on and potential changes to wood loads from infestation of hemlock woolly adelgid

“…Distribution of LW diameters appeared to be similar to Evans et al . () where HWA‐infested sites located throughout the eastern USA hemlock range tended to have higher proportions of large diameter LW; lengths were not reported in their study. Relative volume by diameter size class was fairly uniform for small to intermediate size classes (e.g.…”

“…Evans et al . () quantifies LW loads throughout the native range of the eastern hemlock extending from Maine to Alabama, representing a broad range of hydroclimatic regions compared to the Central Appalachian region, which may account for LW loads that are approximately 50% lower than those found in this study. However Evans et al .…”

“…The presence of jams, three or more pieces of LW in contact with one another, was documented and the dimensions of all LW pieces associated with the jam accumulation that met the length and width criteria were recorded. The volume ( V ; m 3 ) of each LW piece and LW jam accumulation was approximated using the formula for the volume of a cylinder where d is the midpoint diameter and L is the length of the LW piece within the bankfull channel, which is a conservative estimate in order to evaluate the effective LW within the channel (Evans et al ., ).…”

“…However, in the headwater regions of these streams, reach scale (10 1–2 m) controls on wood load appear less consistent across different hydroclimatic settings (Comiti et al ., ; Mao et al ., ; Cadol et al ., ; Wohl et al ., ). In addition, natural and anthropogenic disturbances such as fire (Jones et al ., ; King et al ., ), storms (Kraft et al ., ; Wohl et al ., ), disease (Evans et al ., ; King et al ., ), debris flows (May and Gresswell, ), and timber harvest (Gomi et al ., ; Hassan et al ., ; Magilligan et al ., ) play critical roles in LW characteristics at reach and network scales. LW characteristics at the headwater reach scale are important because they can strongly affect water, sediment, carbon, and nutrient transport processes to the downstream network (Harmon et al ., ; Warren et al ., ; Battin et al ., ) and critically influence instream habitat conditions for important freshwater fauna (Dolloff and Warren, ; Benke and Wallace, ).…”

“…Within the context of the large‐scale disturbance associated with HWA infestation, this study provides baseline information on LW loads in eastern hemlock forest headwater streams in the central Appalachian Mountains, USA, an under‐studied region (but see Evans et al ., ), across a gradient of HWA infestation from uninfested to those in severe decline. The objectives of this work were to: (i) quantify LW loads in central Appalachian headwater streams; (ii) assess variables that predict reach‐scale LW loads to determine the relative importance of recruitment and retention controls on LW load; and (iii) identify potential early impacts of HWA on LW characteristics in headwater streams of the central Appalachian Mountains.…”