View Full Document

Stand, Landscape, and Ecosystem Analyses



View the full content.
View Full Document
View Full Document

13 views

Unformatted text preview:

Stand Landscape and Ecosystem Analyses of Hemlock Woolly Adelgid Outbreaks in Southern New England An Overview David A Orwig and David R Foster 1 Abstract Hemlock woolly adelgid HWA Adelges tsugae an introduced aphid like insect from Asia is expanding across the northeastern United States through the range of eastern hemlock Tsuga canadensis L Carr and has the potential to severely reduce or eliminate this important latesuccessional species While infestation and unimpeded migration of HWA presents a tremendous management problem it also has provided an unusual opportunity to examine the impacts of an introduced pest as it spreads In order to develop insights into these management issues we have developed a multi faceted research effort that examines various forest responses to HWA outbreaks in Connecticut including stand and community reorganization dynamics landscape patterns of HWA infestation damage and hemlock mortality microenvironmental changes and their impact on ecosystem processes and the effect of cutting infested forests on successional and ecosystem dynamics This paper presents an overview of this research conducted by Harvard Forest scientists Introduction Hemlock woolly adelgid HWA Adelges tsugae Annand an introduced aphid like insect from Asia is expanding across the northeastern United States through the range of eastern hemlock and has the potential to severely reduce or eliminate this important late successional species McClure 1995a Orwig and Foster 1998 Despite the growing knowledge about the biology of HWA McClure 1987 1989 1990 1991 1995a Salom et al 1996 Young et al 1995 and increasing efforts to find raise and release natural or exotic biological controls Gouli et al 1997 McClure 1995b McClure and Cheah 1998 Sasaji and McClure 1997 Others this volume we still know very little about how HWA infestation will affect forest and ecosystem processes Therefore this paper will present results from past and current research by Harvard Forest scientists on the various forest responses to HWA outbreaks in southern New England Research Approaches Stand level Dynamics As part of a large study investigating stand to landscape level forest dynamics resulting from HWA infestation we examined the initial community response of hemlock stands varying in mortality levels in south central Connecticut Orwig and Foster 1998 Since 1995 mortality of overstory and understory hemlock has risen to over 60 in half of the stands and continues to increase 5 to 15 per year The health and vigor of remaining trees has deteriorated in all stands with the majority of trees containing less than 25 of their foliage We have observed no sign of tree recovery on these sites over the last 4 years and predict that all sampled trees will die within the next few years A rapid recolonization of these forests with seedlings of black birch Betula lenta L red maple Acer rubrum L and oak Quercus as well as opportunistic herbaceous species has continued to occur following additional hemlock mortality Seedling densities have increased in moderately damaged sites and have thinned but increased in cover in heavily damaged areas We will continue to examine the ongoing dynamics in these forests to gain detailed information on the mechanisms and rate of vegetation recovery Landscape level dynamics Data and observations from these original eight stands were limited to southern Connecticut and we were interested in how representative the results were compared to a much broader study region To meet this objective we established a 5900 km2 transect surrounding the Connecticut River Valley extending from Long Island Sound north to the Massachusetts border Orwig et al unpublished data Within the transect we mapped hemlock stands from aerial photographs and then visited and compiled information on stand composition and structure presence of HWA degree of overstory and understory mortality seedling densities and site characteristics from 114 stands HWA presence was observed in nearly 90 of all stands visited and hemlock sapling and overstory trees have experienced much higher rates of mortality 20 to 100 in the southern part of the state compared to rates in the northern part of the state 0 to 15 Hemlock mortality exceeded 20 on most topographic aspects not just the drier southwest or west facing slopes The health of remaining trees exhibits a pattern similar to mortality with healthier trees located in the northern part of the transect These data suggest that except latitude site factors play little if any role in the susceptibility of forests to adelgid and that most stands will suffer heavy or complete mortality following infestation Forest composition data from infested stands is useful in predicting the species that will eventually replace hemlock Currently black birch red oak Quercus rubra L and red maple are present in the overstory of most stands and are starting to become established in the understory We have found that most forests contained few hemlock seedlings but modest to high densities of hemlock saplings 200 to 800 ha 1 However saplings are currently experiencing high levels of adelgid infestation and mortality and seeds do not remain viable for more than a year or two Therefore we predict a complete change in cover type from hemlock to hardwood dominated forests across broad geographical areas 1 Harvard Forest Harvard University Petersham MA 01366 Proceedings Symposium on Sustainable Management of Hemlock Ecosystems in Eastern North America GTR NE 267 123 Ecosystem level Impacts Results from prior ecosystem level research conducted on sites we have been following for several years Jenkins et al 1999 Yorks et al this volume suggest that HWA infestation can lead to dramatic changes in nitrogen cycling Jenkins et al 1999 found large increases in N mineralization and nitrification rates in infested versus healthy hemlock forests However we still do not know how rapidly these changes occur how long they persist or to what degree changes in soil temperature or biotic uptake affect nutrient cycling In order to answer these questions we have initiated a study examining N availability mineralization and nitrification rates in a subset of Connecticut sites that are infested with HWA but have experienced little to no hemlock mortality Orwig et al unpublished data These measurements will continue for at least 3 years to investigate the temporal dynamics and potential mechanisms driving the changes Additional


Access the best Study Guides, Lecture Notes and Practice Exams

Loading Unlocking...
Login

Join to view Stand, Landscape, and Ecosystem Analyses and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Stand, Landscape, and Ecosystem Analyses and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?