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David A. Orwig - One of the best experts on this subject based on the ideXlab platform.
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net primary production and soil respiration in new england hemlock forests affected by the hemlock woolly adelgid
Ecosphere, 2014Co-Authors: Adrien C Finzi, Poliana C L Raymer, Marcandre Giasson, David A. OrwigAbstract:The abundance of eastern hemlock (Tsuga canadensis) in eastern US forests has declined since the 1950s owing to the introduction of the non-native insect, hemlock woolly adelgid (HWA, Adelges Tsugae). In southern New England, eastern hemlock is being replaced by the deciduous tree species, black birch (Betula lenta). To date there is little understanding of whether hemlock loss will fundamentally alter ecosystem C balance and component fluxes. In this study, we use a comparative approach to study potential changes in C fluxes and N cycling associated with HWA-induced hemlock decline and replacement. The stands include primary- and secondary-growth hemlock forests (>230 and 132 years old, respectively), recently disturbed stands (5 and 18 years old) that now have rapidly growing black birch saplings, and a mature black birch stand of age similar to the second-growth hemlock stand. We found that aboveground net primary production was higher in the aggrading black birch stand and significantly so at 18-years...
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Interactions between invasive herbivores and their long-term impact on New England hemlock forests
Biological Invasions, 2014Co-Authors: Sara Gomez, Liahna Gonda-king, Colin M. Orians, David A. Orwig, R. Panko, Laura Radville, Nicole E. Soltis, Carol S. Thornber, Evan L. PreisserAbstract:The introduction of the hemlock woolly adelgid (Adelges Tsugae) in the eastern United States has caused extensive damage to eastern hemlock (Tsuga canadensis) stands. The presence of other herbivores can affect adelgid density and hemlock health. Here, we report the results of long-term work monitoring hemlock forests in a 7,500 km2 latitudinal transect of southern New England. In biannual surveys between 1997 and 2011, we assessed change in hemlock basal area, stand-level hemlock defoliation, the density of four exotic herbivores (A. Tsugae, Fiorinia externa, Nuculaspis Tsugae and an unidentified Nuculaspis sp.) and explored relationships between biotic and abiotic variables. Hemlock basal area decreased by approximately 60 % over the 14 years of the study. Hemlock stand defoliation increased locally in some stands but on average stand defoliation did not change throughout the study period/area. Local increases in defoliation were probably driven by the northward expansion by A. Tsugae, and to a certain extent by the co-occurring presence of F. externa and Nuculaspis scales. Average F. externa density increased during the study and also expanded dramatically northward, although its density did not impact hemlock stand defoliation and only reduced hemlock basal area in some stands. Stand-level variation in F. externa densities was most strongly affected by the presence of Nuculaspis scales, which were present in about 80 % of the stands. While A. Tsugae continues to be the primary threat to eastern hemlock, its impact needs to be studied in the context of a broader herbivore community.
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a foundation tree at the precipice Tsuga canadensis health after the arrival of adelges Tsugae in central new england
Ecosphere, 2012Co-Authors: David A. Orwig, Jonathan R Thompson, Nicholas A Povak, Megan Manner, Donald Niebyl, David R. FosterAbstract:Hemlock (Tsuga canadensis) plays a unique role in Eastern forests, producing distinctive biogeochemical, habitat, and microclimatic conditions and yet has begun a potentially irreversible decline due to the invasive hemlock woolly adelgid (Adelges Tsugae; HWA) that causes foliar damage, crown loss, and mortality of host trees. Understanding the regional, landscape, site, and stand factors influencing HWA spread and impact is critical for predicting future landscape dynamics and directing effective management. Using aerial photographs, we documented hemlock distribution throughout central Massachusetts and subsampled 123 stands to examine the spatial pattern of HWA and its impact on tree vigor and mortality since its arrival in 1989. In the study region, over 86,000 ha of hemlock forest were mapped in 5,127 stands. White pine (Pinus strobus), red oak (Quercus rubra), red maple (Acer rubrum), and black birch (Betula lenta) were common overstory associates. Hemlock abundance increased from south to north, commonly on western and northwestern slopes. Average stand size was 55 ha, overstory basal area ranged from 23 to 55 m2 ha−1 and overstory stem densities averaged 993 ha−1. By 2004, 40% of sampled stands were infested, but most stands remained in good health overall; only 8 stands contained high HWA densities and only two had lost >50% overstory hemlock. Out of fifteen stand and landscape predictor variables examined, only latitude and winter climate variables were related to HWA density. Cold temperatures appear to be slowing the spread and impact of HWA at its northern extent as HWA infestation intensity and hemlock mortality and vigor were significantly correlated with average minimum winter temperature. Contrary to predictions, there was no regional increase in hemlock harvesting. The results suggest that regional HWA-hemlock dynamics are currently being shaped more by climate than by a combination of landscape and social factors. The persistence and migration of HWA continues to pose a significant threat regionally, especially in the northern portion of the study area, where hemlock dominates many forests.
Evan L. Preisser - One of the best experts on this subject based on the ideXlab platform.
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Seasonal changes in eastern hemlock (Tsuga canadensis) foliar chemistry
Canadian Journal of Forest Research, 2020Co-Authors: Ian G. Kinahan, Colin M. Orians, Chad M. Rigsby, Suzanne K. Savage, Nick Houseman, Andrew S. Marsella, Anita A Oppong-quaicoe, Brenton Deboef, Evan L. PreisserAbstract:Eastern hemlock (Tsuga canadensis (L.) Carriere) is an eastern North American conifer threatened by the invasive hemlock woolly adelgid (Adelges Tsugae Annand). Changes in foliar terpenes and pheno...
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impact of hemlock woolly adelgid adelges Tsugae infestation on xylem structure and function and leaf physiology in eastern hemlock Tsuga canadensis
Functional Plant Biology, 2017Co-Authors: Brett A Huggett, Evan L. Preisser, Jessica A Savage, Guangyou Hao, Michele N HolbrookAbstract:Hemlock woolly adelgid (Adelges Tsugae Annand) (HWA) is an invasive insect that feeds upon the foliage of eastern hemlock (Tsuga canadensis (L.) Carriere) trees, leading to a decline in health and often mortality. The exact mechanism leading to the demise of eastern hemlocks remains uncertain because little is known about how HWA infestation directly alters the host’s physiology. To evaluate the physiological responses of eastern hemlock during early infestation of HWA, we measured needle loss, xylem hydraulic conductivity, vulnerability to cavitation, tracheid anatomy, leaf-level gas exchange, leaf water potential and foliar cation and nutrient levels on HWA-infested and noninfested even-aged trees in an experimental garden. HWA infestation resulted in higher xylem hydraulic conductivity correlated with an increase in average tracheid lumen area and no difference in vulnerability to cavitation, indicating that needle loss associated with HWA infestation could not be attributed to reduced xylem transport capacity. HWA-infested trees exhibited higher rates of net photosynthesis and significant changes in foliar nutrient partitioning, but showed no differences in branch increment growth rates compared with noninfested trees. This study suggests that HWA-induced decline in the health of eastern hemlock trees is not initially caused by compromised water relations or needle loss.
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Interactions between invasive herbivores and their long-term impact on New England hemlock forests
Biological Invasions, 2014Co-Authors: Sara Gomez, Liahna Gonda-king, Colin M. Orians, David A. Orwig, R. Panko, Laura Radville, Nicole E. Soltis, Carol S. Thornber, Evan L. PreisserAbstract:The introduction of the hemlock woolly adelgid (Adelges Tsugae) in the eastern United States has caused extensive damage to eastern hemlock (Tsuga canadensis) stands. The presence of other herbivores can affect adelgid density and hemlock health. Here, we report the results of long-term work monitoring hemlock forests in a 7,500 km2 latitudinal transect of southern New England. In biannual surveys between 1997 and 2011, we assessed change in hemlock basal area, stand-level hemlock defoliation, the density of four exotic herbivores (A. Tsugae, Fiorinia externa, Nuculaspis Tsugae and an unidentified Nuculaspis sp.) and explored relationships between biotic and abiotic variables. Hemlock basal area decreased by approximately 60 % over the 14 years of the study. Hemlock stand defoliation increased locally in some stands but on average stand defoliation did not change throughout the study period/area. Local increases in defoliation were probably driven by the northward expansion by A. Tsugae, and to a certain extent by the co-occurring presence of F. externa and Nuculaspis scales. Average F. externa density increased during the study and also expanded dramatically northward, although its density did not impact hemlock stand defoliation and only reduced hemlock basal area in some stands. Stand-level variation in F. externa densities was most strongly affected by the presence of Nuculaspis scales, which were present in about 80 % of the stands. While A. Tsugae continues to be the primary threat to eastern hemlock, its impact needs to be studied in the context of a broader herbivore community.
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failure under stress the effect of the exotic herbivore adelges Tsugae on biomechanics of Tsuga canadensis
Annals of Botany, 2014Co-Authors: Nicole E. Soltis, Evan L. Preisser, Sara Gomez, Gary G Leisk, Patrick Sherwood, Pierluigi Bonello, Colin M. OriansAbstract:†Background and Aims Exotic herbivores that lack a coevolutionary history with their host plants can benefit from poorly adapted host defences, potentially leading to rapid population growth of the herbivore and severe damage to its plant hosts. The hemlock woolly adelgid (AdelgesTsugae) is an exotic hemipteran that feeds on the long-lived conifer eastern hemlock (Tsugacanadensis), causing rapid mortality of infested trees. While the mechanism of this mortality is unknown, evidence indicates that A. Tsugae feeding causes a hypersensitive response and alters wood anatomy. This study investigated the effect of A. Tsugae feeding on biomechanical properties at different spatial scales: needles, twigs and branches. †Methods Uninfested and A. Tsugae-infested samples were collected from a common garden experiment as well as from naturally infested urban and rural field sites. Tension and flexure mechanical tests were used to quantify biomechanical properties of the different tissues. In tissues that showed a significant effect of herbivory, the potential contributions of lignin and tissue density on the results were quantified. †Key Results Adelges Tsugae infestation decreased the abscission strength, but not flexibility, of needles. A. Tsugae feeding also decreased mechanical strength and flexibility in currently attacked twigs, but this effect disappeared in older, previously attacked branches. Lignin and twig tissue density contributed to differences in mechanical strength but were not affected by insect treatment. †Conclusions Decreased strength and flexibility in twigs, along with decreased needle strength, suggest that infested trees experience resource stress. Altered growth patterns and cell wall chemistry probably contribute to these mechanical effects. Consistent site effects emphasize the role of environmental variation in mechanical traits. The mechanical changes measured here may increase susceptibility to abiotic physical stressors in hemlocks colonized by A. Tsugae. Thus, the interaction between herbivore and physical stresses is probably accelerating the decline of eastern hemlock, as HWA continues to expand its range.
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factors affecting settlement rate of the hemlock woolly adelgid adelges Tsugae on eastern hemlock Tsuga canadensis
Agricultural and Forest Entomology, 2007Co-Authors: Elizabeth Butin, Evan L. Preisser, Joseph S. ElkintonAbstract:We assessed the importance of several factors potentially affecting the settlement rate of the invasive hemlock woolly adelgid Adelges Tsugae (Hemiptera: Adelgidae) on uninfested foliage of the eastern hemlock, Tsuga canadensis . We conducted our experiments in Massachusetts (U.S.A.) with overwintering sistens adelgids, and applied standard densities of infested foliage to uninfested branches in a planned multiple-comparison design. 2 Settlement rates of progrediens crawlers produced by the overwintering sistens were highest when adelgid-infested foliage was loosely attached to uninfested foliage and both branches were then enclosed in a mesh sleeve. 3 Early-emerging crawlers settled at a higher rate than did late-emerging crawlers. 4 Increasing the density of infested branches did not affect settlement rates. 5 We also tested whether less severe winter conditions improved settlement, and found that overwintering infested foliage in a refrigerator decreased settlement rate relative to foliage overwintered outdoors. 6 Our results suggest a protocol for adelgid inoculations that could substantially increase the success rate of experimental manipulations and encourage additional research on the population dynamics of this pest.
Lynne K. Rieske - One of the best experts on this subject based on the ideXlab platform.
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Effect of an exotic herbivore, Adelges Tsugae, on photosynthesis of a highly susceptible Tsuga host, with notes on conspecifics
Arthropod-Plant Interactions, 2014Co-Authors: Lori A. Nelson, Dylan N. Dillaway, Lynne K. RieskeAbstract:Hemlocks are significant components of temperate forests of Asia and North America, and in eastern North America, they are threatened by an exotic herbivore, the hemlock woolly adelgid, Adelges Tsugae . The adelgid is native to Asia and northwestern North America, but is highly invasive in eastern North America where natural enemies are unable to regulate populations and eastern hemlock, Tsuga canadensis , is highly susceptible. In order to gain a better understanding of the metabolic effects of A. Tsugae on eastern hemlock, we evaluated its effects on photosynthesis and also evaluated photosynthesis on Tsuga species from various geographic origins. We measured light-saturated photosynthesis ( A _sat) and dark respiration of T. canadensis that were infested with adelgid and found a significant decrease in A _sat and a small but significant increase in dark respiration, suggesting that A. Tsugae triggers a physiological response in eastern hemlock by decreasing metabolic activity. In a separate experiment, we also measured A _sat of five different hemlock species, including eastern hemlock, the Pacific Northwestern T. heterophylla and T. mertensiana , and the Asian T. diversifolia and T. chinensis . Only weakly significant differences in A _sat were found, with the highest rate in the eastern North American T. canadensis and the lowest in the Pacific Northwestern T. mertensiana . The relatively high photosynthetic rate of T. canadensis could possibly play a role in its susceptibility to A. Tsugae . A better understanding of this metabolic response could help develop effective management strategies for combating the highly invasive A. Tsugae .
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The aftermath of an invasion: Structure and composition of Central Appalachian hemlock forests following establishment of the hemlock woolly adelgid, Adelges Tsugae
Biological Invasions, 2010Co-Authors: Heather L. Spaulding, Lynne K. RieskeAbstract:As the highly invasive hemlock woolly adelgid, Adelges Tsugae, continues to expand its distribution in eastern North America, affected forests will incur drastic changes in composition and structure. While these changes have been well-studied in dense hemlock forests in the Northeast, relatively little work is known about the effects of the adelgid at the western edge of the range of eastern hemlock, Tsuga canadensis . We evaluated the nature and extent of these changes using vegetation assessments coupled with growth simulations. The woody plant community was assessed in three strata (upper, mid- and lower) and was used to predict forest succession. Using the Forest Vegetation Simulator (FVS), we then projected the growth of hemlock forests 20 years into the future with and without the effects of the adelgid. In forest simulations lacking adelgid invasion, little change in composition or structure is forecast. In contrast, our projections predict a near complete loss of the hemlock forest type within 20 years of adelgid establishment, with widespread conversion to hardwood forest types, most notably white oak-red oak-hickory, chestnut oak-black oak-scarlet oak, and yellow poplar-white oak-red oak. Hemlock loss will result in denser deciduous forests with thinner canopies and multiple gaps, and significant alterations to terrestrial and aquatic wildlife habitat.
Preisser, Evan L - One of the best experts on this subject based on the ideXlab platform.
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A Four-Year, Seven-State Reforestation Trial with Eastern Hemlocks (\u3cem\u3eTsuga canadensis\u3c/em\u3e) Resistant to Hemlock Woolly Adelgid (\u3cem\u3eAdelges Tsugae\u3c/em\u3e)
DigitalCommons@URI, 2020Co-Authors: Kinahan, Ian G., Grandstaff Gabrielle, Russell Alana, Rigsby, Chad M., Casagrande, Richard A., Preisser, Evan LAbstract:We conducted over a decade of research into individual eastern hemlock (Tsuga canadensis; hemlock) trees that are potentially resistant to hemlock woolly adelgid (Adelges Tsugae; HWA), an invasive xylem-feeding insect that is capable of rapidly killing even mature trees. Following clonal propagation of these individuals, in 2015 we planted size- and age-matched HWA-resistant and HWA-susceptible hemlocks in HWA-infested forest plots in seven states. In 2019, we re-surveyed the plots; 96% of HWA-resistant hemlocks survived compared to 48% of susceptible trees. The surviving HWA-resistant trees were also taller, produced more lateral growth, retained more foliage, and supported lower densities of the elongate hemlock scale Fiorinia externa, another invasive hemlock pest, than the surviving HWA-susceptible trees. Our results suggest that HWA management may benefit from additional research exploring the identification, characterization, and use of HWA-resistant eastern hemlocks in future reforestation efforts
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Seasonal changes in eastern hemlock (\u3cem\u3eTsuga canadensis\u3c/em\u3e) foliar chemistry
DigitalCommons@URI, 2020Co-Authors: Kinahan, Ian G., Rigsby, Chad M., Orians, Colin M., Savage, Suzanne K., Houseman, Nick L., Marsella, Andrew S., Oppong Anita, Deboef, Brenton L., Preisser, Evan LAbstract:Eastern hemlock (Tsuga canadensis (L.) Carriére; hemlock) is an eastern North American conifer threatened by the invasive hemlock woolly adelgid (Adelges Tsugae Annand). Changes in foliar terpenes and phenolics were evaluated in new (current year growth) and mature (1-year old growth) hemlock needles during the growing season and into plant dormancy. From April through September, foliar concentrations of non-volatile soluble phenolics, condensed tannins, lignin, mono- and sesquiterpenes α-pinene, camphene, isobornyl acetate, and diterpene resin were quantified. After September, additional analyses of metabolites that continued to differ significantly in new versus mature foliage were carried out. Total soluble phenolic concentration and condensed tannin concentration in new foliage remained low relative to mature foliage throughout the growing season and converged in December. Lignin concentration in new foliage converged with that of mature foliage by July. Concentrations of α-pinene, camphene, isobornyl acetate, and diterpene resin in new foliage converged with mature foliage within one month of budbreak. The convergence of terpene concentrations in new and mature foliage suggests that these metabolites may play a role in herbivore defense during the peak growing season. Conversely, soluble phenolics, including condensed tannins, may defend foliage from herbivory outside of the spring growth period
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Facilitation between invasive herbivores: hemlock woolly adelgid increases gypsy moth preference for and performance on eastern hemlock
DigitalCommons@URI, 2020Co-Authors: Kinahan, Ian G., Rigsby, Chad M., Shoemaker, Emma E., Orians, Colin M., Savage, Suzanne K., Baranowski, Alex K., Whitney, Elizabeth R., Preisser, Evan LAbstract:Interactions between invertebrate herbivores with different feeding modes are common on long‐lived woody plants. In cases where one herbivore facilitates the success of another, the consequences for their shared host plant may be severe. Eastern hemlock (Tsuga canadensis), a canopy‐dominant conifer native to the eastern U.S., is currently threatened with extirpation by the invasive stylet‐feeding hemlock woolly adelgid (Adelges Tsugae). The effect of adelgid on invasive hemlock‐feeding folivores remains unknown. This study evaluated the impact of feeding by hemlock woolly adelgid on gypsy moth (Lymantria dispar) larval preference for, and performance on, eastern hemlock. To assess preference, 245 field‐grown hemlocks were surveyed for gypsy moth herbivory damage and laboratory paired‐choice bioassays were conducted. To assess performance, gypsy moth larvae were reared to pupation on adelgid‐infested or uninfested hemlock foliage, and pupal weight, proportional weight gain, and larval period were analysed. Adelgid‐infested hemlocks experienced more gypsy moth herbivory than did uninfested control trees, and laboratory tests confirmed that gypsy moth larvae preferentially feed on adelgid‐infested hemlock foliage. Gypsy moth larvae reared to pupation on adelgid‐infested foliage gained more weight than larvae reared on uninfested control foliage. These results suggest that the synergistic effect of adelgid and gypsy moth poses an additional threat to eastern hemlock that may increase extirpation risk and ecological impact throughout most of its range
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Conifer Responses to a Stylet-Feeding Invasive Herbivore and Induction with Methyl Jasmonate: Impact on the Expression of Induced Defenses and a Native Folivore
DigitalCommons@URI, 2019Co-Authors: Rigsby, Chad M., Shoemaker, Emma E., Mallinger, Mary M., Orians, Colin M., Preisser, Evan LAbstract:Trees attacked by multiple herbivores need to defend themselves against dynamic biotic challenges; appropriate responses to one stressor can elicit hormonal responses that are antagonistic to another. Hemlock (Tsuga canadensis) infestation by hemlock woolly adelgid (HWA; Adelges Tsugae) results in the accumulation of the defensive hormone salicylic acid. We explored the potential for HWA infestation to interfere with anti‐folivore‐induced defence signalling and its implications for a native folivore (hemlock looper; Lambdina fiscellaria). Hemlocks were infested with HWA and/or sprayed with methyl jasmonate; foliar defences were analyzed and foliage quality for looper larvae was assessed. Both treatments activated foliar defensive traits, including a HWA‐mediated increase in peroxidase activity and an accumulation of cell wall‐bound phenolics and lignin, as well as a methyl jasmonate‐mediated increase in lipoxygenase activity. The two treatments had an additive effect on other defensive traits and both treatments negatively affected looper performance. These results suggest that salicylic acid and jasmonic acid are not strictly antagonistic in conifers and that both have a role in anti‐folivore defence signalling. The present study illustrates the need for a better understanding of hormone signalling, cross‐talk and induced responses in conifers
Colin M. Orians - One of the best experts on this subject based on the ideXlab platform.
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Seasonal changes in eastern hemlock (Tsuga canadensis) foliar chemistry
Canadian Journal of Forest Research, 2020Co-Authors: Ian G. Kinahan, Colin M. Orians, Chad M. Rigsby, Suzanne K. Savage, Nick Houseman, Andrew S. Marsella, Anita A Oppong-quaicoe, Brenton Deboef, Evan L. PreisserAbstract:Eastern hemlock (Tsuga canadensis (L.) Carriere) is an eastern North American conifer threatened by the invasive hemlock woolly adelgid (Adelges Tsugae Annand). Changes in foliar terpenes and pheno...
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Interactions between invasive herbivores and their long-term impact on New England hemlock forests
Biological Invasions, 2014Co-Authors: Sara Gomez, Liahna Gonda-king, Colin M. Orians, David A. Orwig, R. Panko, Laura Radville, Nicole E. Soltis, Carol S. Thornber, Evan L. PreisserAbstract:The introduction of the hemlock woolly adelgid (Adelges Tsugae) in the eastern United States has caused extensive damage to eastern hemlock (Tsuga canadensis) stands. The presence of other herbivores can affect adelgid density and hemlock health. Here, we report the results of long-term work monitoring hemlock forests in a 7,500 km2 latitudinal transect of southern New England. In biannual surveys between 1997 and 2011, we assessed change in hemlock basal area, stand-level hemlock defoliation, the density of four exotic herbivores (A. Tsugae, Fiorinia externa, Nuculaspis Tsugae and an unidentified Nuculaspis sp.) and explored relationships between biotic and abiotic variables. Hemlock basal area decreased by approximately 60 % over the 14 years of the study. Hemlock stand defoliation increased locally in some stands but on average stand defoliation did not change throughout the study period/area. Local increases in defoliation were probably driven by the northward expansion by A. Tsugae, and to a certain extent by the co-occurring presence of F. externa and Nuculaspis scales. Average F. externa density increased during the study and also expanded dramatically northward, although its density did not impact hemlock stand defoliation and only reduced hemlock basal area in some stands. Stand-level variation in F. externa densities was most strongly affected by the presence of Nuculaspis scales, which were present in about 80 % of the stands. While A. Tsugae continues to be the primary threat to eastern hemlock, its impact needs to be studied in the context of a broader herbivore community.
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failure under stress the effect of the exotic herbivore adelges Tsugae on biomechanics of Tsuga canadensis
Annals of Botany, 2014Co-Authors: Nicole E. Soltis, Evan L. Preisser, Sara Gomez, Gary G Leisk, Patrick Sherwood, Pierluigi Bonello, Colin M. OriansAbstract:†Background and Aims Exotic herbivores that lack a coevolutionary history with their host plants can benefit from poorly adapted host defences, potentially leading to rapid population growth of the herbivore and severe damage to its plant hosts. The hemlock woolly adelgid (AdelgesTsugae) is an exotic hemipteran that feeds on the long-lived conifer eastern hemlock (Tsugacanadensis), causing rapid mortality of infested trees. While the mechanism of this mortality is unknown, evidence indicates that A. Tsugae feeding causes a hypersensitive response and alters wood anatomy. This study investigated the effect of A. Tsugae feeding on biomechanical properties at different spatial scales: needles, twigs and branches. †Methods Uninfested and A. Tsugae-infested samples were collected from a common garden experiment as well as from naturally infested urban and rural field sites. Tension and flexure mechanical tests were used to quantify biomechanical properties of the different tissues. In tissues that showed a significant effect of herbivory, the potential contributions of lignin and tissue density on the results were quantified. †Key Results Adelges Tsugae infestation decreased the abscission strength, but not flexibility, of needles. A. Tsugae feeding also decreased mechanical strength and flexibility in currently attacked twigs, but this effect disappeared in older, previously attacked branches. Lignin and twig tissue density contributed to differences in mechanical strength but were not affected by insect treatment. †Conclusions Decreased strength and flexibility in twigs, along with decreased needle strength, suggest that infested trees experience resource stress. Altered growth patterns and cell wall chemistry probably contribute to these mechanical effects. Consistent site effects emphasize the role of environmental variation in mechanical traits. The mechanical changes measured here may increase susceptibility to abiotic physical stressors in hemlocks colonized by A. Tsugae. Thus, the interaction between herbivore and physical stresses is probably accelerating the decline of eastern hemlock, as HWA continues to expand its range.
