Hemlock woolly adelgid (Adelges tsugae) infestation affects water and carbon relations of eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tsuga caroliniana)

Abstract

SummaryHemlock woolly adelgid (HWA) is an exotic insect pest causing severe decimation of native hemlock trees. Extensive research has been conducted on the ecological impacts of HWA, but the exact physiological mechanisms that cause mortality are not known.Water relations, anatomy and gas exchange measurements were assessed on healthy and infested eastern (Tsuga canadensis) and Carolina (Tsuga caroliniana) hemlock trees. These data were then used in a mechanistic model to test whether the physiological responses to HWA infestation were sufficiently significant to induce changes in whole-plant water use and carbon uptake.The results indicated coordinated responses of functional traits governing water relations in infested relative to healthy trees. In response to HWA, leaf water potential, carbon isotope ratios, plant hydraulic properties and stomatal conductance were affected, inducing a reduction in tree water use by > 40% and gross primary productivity by 25%. Anatomical changes also appeared, including the activation of traumatic cells.HWA infestation had a direct effect on plant water relations. Despite some leaf compensatory mechanisms, such as an increase in leaf hydraulic conductance and nitrogen content, tree water use and carbon assimilation were diminished significantly in infested trees, which could contribute to tree mortality.

Influence of Hemlock Woolly Adelgid Infestation Levels on Water Stress in Eastern Hemlocks within the Great Smoky Mountains National Park, U.S.A.

“…The lowest average monthly rainfall was documented in January (8.02 cm), suggesting that transpiration rates rather than precipitation patterns were the primary driver of seasonal water potential fluctuations in this species. Even under favorable conditions, the reduction in water potential we observed to occur with increasing infestation may have resulted in a higher incidence of mid-day stomatal closure and reductions in photosynthesis, as has been previously observed in this species [15]. Accelerated mortality in eastern hemlocks is likely to occur at high hemlock woolly adelgid infestation levels.…”

“…It now ranges as far north as Massachusetts, south to North Carolina and north Georgia, and west into Tennessee and Kentucky. Hemlock mortality is caused by reduced carbohydrate reserves in the tree as result of adelgid feeding in the ray parenchyma cells [14] and reduced photosynthetic rates [15][16][17], affecting trees of all size and age classes [18]. Damage to the tree from heavy adelgid infestation reduces the ability of the hemlock to transport imidacloprid, one of the primary systemic chemicals used to treat infestations of hemlock woolly adelgid, throughout the tree [1,13].…”

The Natural Evolutionary Potential of Tree Populations to Cope with Newly Introduced Pests and Pathogens—Lessons Learned From Forest Health Catastrophes in Recent Decades

“…Over the last few decades, the species has invaded vast areas in North America where it caused extensive mortality of eastern (Tsuga canadensis [83]) and Carolina hemlock (Tsuga caroliniana [84]). The damage in northeastern USA has been severe with adelgid-induced mortality exceeding 95 % and 50-75 % defoliation in surviving trees [85,86].…”

Terpene Chemistry of Eastern Hemlocks Resistant to Hemlock Woolly Adelgid

“…Infestation by HWA has been shown to alter xylem growth and water relations in eastern hemlock (Domec et al 2013;Gonda-King et al 2012). This effect is associated with a hypersensitive response (Radville et al 2011), reduced photosynthetic productivity (Domec et al 2013), and mobilization of nitrogen to new-growth tissues (Gomez et al 2012).…”

“…Infestation by HWA has been shown to alter xylem growth and water relations in eastern hemlock (Domec et al 2013;Gonda-King et al 2012). This effect is associated with a hypersensitive response (Radville et al 2011), reduced photosynthetic productivity (Domec et al 2013), and mobilization of nitrogen to new-growth tissues (Gomez et al 2012). In addition, infested hemlocks display a local increase in phenolic compounds (Pezet et al 2013), and a simultaneous increase in monoterpene volatilization and decrease in stored mono-and sesquiterpenes (Broeckling and Salom 2003;Pezet et al 2013).…”