Eastern hemlock (Tsuga canadensis Carriére) and the Carolina hemlock (Tsuga caroliniana Engelmann) are ecologically important tree species in eastern North America forests that are currently threatened by the hem...Eastern hemlock (Tsuga canadensis Carriére) and the Carolina hemlock (Tsuga caroliniana Engelmann) are ecologically important tree species in eastern North America forests that are currently threatened by the hemlock woolly adelgid (HWA, Adelges tsugae Annand, Hemiptera: Adelgidae). HWA has spread rapidly from its original introduction site into new areas. Once present, HWA kills its hosts over a period of 4 to 10 years leading to a phenomenon that is known scientifically and colloquially as hemlock decline. To date, quarantine, chemical management, and biocontrol efforts have failed to curb the spread of the HWA. As such, forest management efforts are now being redirected towards developing an understanding of the effects of hemlock removal on vegetation dynamics, changes in forest composition, and changes in ecosystem function. In this study, we parameterize a spatially explicit landscape simulation model LANDIS II for a specific forested region of the southern Appalachians. Parameterization involves defining the life-history attributes of 37 tree species occupying 11 ecological zones and is based on knowledge of: current vegetation composition data, recent historic management and fire regimes, and life-history traits of each species. The parameterized model is used to explore a simple scenario of catastrophic hemlock mortality likely to occur as a result of HWA herbivory. Our results emphasize that hemlock is an important foundation species. When hemlock is removed from the system, forest composition changes considerably with a greater presence of shade intolerant pine and oak species. Additionally, hemlock removal leads to a period of transient, relatively unstable vegetation dynamics as the forest communities restructure.展开更多
Hemlock woolly adelgid (Adelges Tsugae Annand, HWA) outbreaks are posing a major threat to eastern hemlock (Tsuga canadensis L. Carr.) and Carolina hemlock (Tsuga caroliniana Engelm.) forest landscapes in the eastern ...Hemlock woolly adelgid (Adelges Tsugae Annand, HWA) outbreaks are posing a major threat to eastern hemlock (Tsuga canadensis L. Carr.) and Carolina hemlock (Tsuga caroliniana Engelm.) forest landscapes in the eastern USA. As foundation species, hemlocks play a variety of functional roles in forest landscapes. These species usually occur as isolated canopies and mixed species in landscapes where variation in topography is extreme. Spatially explicit inventory information on HWA induced hemlock mortality at landscape scale does not exist. High resolution aerial imageries enable landscape scale assessment even at the individual tree level. Accordingly, our goal was to investigate spatial pattern and distribution of HWA induced hemlock mortality using a high resolution aerial image mosaic in the Linville River Gorge, Southern Appalachians, western North Carolina. Our study objectives were: 1) to detect dead trees within the Lower Linville River watershed;2) to estimate the area occupied by dead trees in the forest canopy surface;3) to investigate the relationship of dead hemlocks and topography;and 4) to define the spatial pattern of the dead trees. We found ca. 10,000 dead trees within the study area, occupying over 7 ha of the canopy surface with an average area of 36 m2 per dead tree. The density of the dead trees was higher in proximity to the Linville River, at higher elevations, and on northern and northwestern aspects. Spatial pattern of the dead trees was generally clustered at all spatial scales. We suggest that although the reduction in plant biomass resulting from herbivory within the landscapes is modest, impact of the clustered distribution of hemlock mortality, especially in the riparian zones, is noteworthy. Our analysis of the pattern of hemlock decline provides new means for projecting future impacts of HWA on the range of hemlock distribution in eastern North America.展开更多
文摘Eastern hemlock (Tsuga canadensis Carriére) and the Carolina hemlock (Tsuga caroliniana Engelmann) are ecologically important tree species in eastern North America forests that are currently threatened by the hemlock woolly adelgid (HWA, Adelges tsugae Annand, Hemiptera: Adelgidae). HWA has spread rapidly from its original introduction site into new areas. Once present, HWA kills its hosts over a period of 4 to 10 years leading to a phenomenon that is known scientifically and colloquially as hemlock decline. To date, quarantine, chemical management, and biocontrol efforts have failed to curb the spread of the HWA. As such, forest management efforts are now being redirected towards developing an understanding of the effects of hemlock removal on vegetation dynamics, changes in forest composition, and changes in ecosystem function. In this study, we parameterize a spatially explicit landscape simulation model LANDIS II for a specific forested region of the southern Appalachians. Parameterization involves defining the life-history attributes of 37 tree species occupying 11 ecological zones and is based on knowledge of: current vegetation composition data, recent historic management and fire regimes, and life-history traits of each species. The parameterized model is used to explore a simple scenario of catastrophic hemlock mortality likely to occur as a result of HWA herbivory. Our results emphasize that hemlock is an important foundation species. When hemlock is removed from the system, forest composition changes considerably with a greater presence of shade intolerant pine and oak species. Additionally, hemlock removal leads to a period of transient, relatively unstable vegetation dynamics as the forest communities restructure.
基金possible by financial aid from Graduate School in Forest Sciences(GSForest),Finnish Academy project“Centre of Excellence in Laser Scanning Research”(CoE-LaSR,decision number 272195)by the US Forest Service through USDA Forest Service cooperative agreement SRS-12-CA-11330129-077.
文摘Hemlock woolly adelgid (Adelges Tsugae Annand, HWA) outbreaks are posing a major threat to eastern hemlock (Tsuga canadensis L. Carr.) and Carolina hemlock (Tsuga caroliniana Engelm.) forest landscapes in the eastern USA. As foundation species, hemlocks play a variety of functional roles in forest landscapes. These species usually occur as isolated canopies and mixed species in landscapes where variation in topography is extreme. Spatially explicit inventory information on HWA induced hemlock mortality at landscape scale does not exist. High resolution aerial imageries enable landscape scale assessment even at the individual tree level. Accordingly, our goal was to investigate spatial pattern and distribution of HWA induced hemlock mortality using a high resolution aerial image mosaic in the Linville River Gorge, Southern Appalachians, western North Carolina. Our study objectives were: 1) to detect dead trees within the Lower Linville River watershed;2) to estimate the area occupied by dead trees in the forest canopy surface;3) to investigate the relationship of dead hemlocks and topography;and 4) to define the spatial pattern of the dead trees. We found ca. 10,000 dead trees within the study area, occupying over 7 ha of the canopy surface with an average area of 36 m2 per dead tree. The density of the dead trees was higher in proximity to the Linville River, at higher elevations, and on northern and northwestern aspects. Spatial pattern of the dead trees was generally clustered at all spatial scales. We suggest that although the reduction in plant biomass resulting from herbivory within the landscapes is modest, impact of the clustered distribution of hemlock mortality, especially in the riparian zones, is noteworthy. Our analysis of the pattern of hemlock decline provides new means for projecting future impacts of HWA on the range of hemlock distribution in eastern North America.
