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Journal American Rhododendron Society

Current Editor:
Dr. Glen Jamieson ars.editor@gmail.com

Volume 62, Number 2
Spring 2008

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Mapping Rhododendron Planting Zones Across North America - You Can Help
Willem Morsink
Toronto, Ontario, Canada

Dan McKenney, Ph.D.
Sault Ste. Marie, Ontario, Canada

Dear Members:
Hundreds of ARS members are or have been involved over the years with establishing a continent-wide treasure trove of information on the culture of the genus Rhododendron. This includes the ARS district-wide lists of numerous "Proven Performers". The experience of ARS members is invaluable and can now be further captured to help develop more refined potential planting or growing zone range maps across all of North America.
        We are asking for your experiences to be input to the "Going Beyond the Zones" website (http://planthardiness.gc.ca; McKenney et al, 2007a). Presently there are very few observations and maps for rhododendrons. These maps/models will provide higher resolution maps of potential growing zones than the broad-scale district designations currently available. Your participation as ARS rhododendron experts and gardeners will provide the observations needed to generate climatic profiles and hence maps. Your input will be a permanent contribution that will be kept forever by the system and will help provide a stronger scientific basis to our understanding of climatic tolerances and range potential of rhododendrons for North America.

How Does It Work?
The general aim of the Plant Hardiness project is to develop potential range maps for individual species of trees, shrubs and perennial flowers. To date this project has compiled about 2,000,000 location-specific observations for thousands of species across the United States and Canada. The location coordinates are linked to new climate models that allow for a climate profile to be generated. Climatic profiles can now be mapped much more rigorously than previously possible thus giving a better indication of the possible range of individual species.
        The climate profile for a species, variety, or the hybrid/cultivar is basically the distribution of climate values taken from all locations where the plant is known to occur and survive. By compiling many locations a set of climate values can be estimated (e.g., things like annual mean temperature, precipitation, etc.). As these values are accumulated a better understanding of the climate-tolerance-survival profile is generated and mapped for each species.

Species and Hybrids Suggested for Input
Which rhododendrons to be input to the plant hardiness mapping system is a difficult issue given the huge number of possible species and hybrids. A balance is required between a reasonably comprehensive list and practicality. Experience suggests a wide range of potential users including expert horticulturalists, academics, gardeners and even government agencies interested in examining potential distributions of hosts for alien species. After considerable discussion we are suggesting input from four rhododendron subgenera:
        Elepidotes-(large-leaf): Subgenus Hymenanthes rhododendrons
        Lepidote (medium/small-leaf): Subgenus Rhododendron rhododendrons
        Deciduous azaleas: Subgenus Pentanthera and other deciduous rhododendrons
        Evergreen azaleas: Subgenus Tsutsusi rhododendrons
        These names have been added to the plant hardiness web site. They are primarily based on the "Proven Performer" Lists for ARS Districts 1-12. Others names may be added in the future.

How Can You Help?
To generate these profiles and maps for rhododendrons we need participation from ARS members from all over North America. It is actually quite a simple process - contributors need only to identify which rhododendron species and/or hybrids have survived at least 3-5 years at their location. Once about 30-50 observations are entered the potential range maps are generated. These models are continually updated as more data are submitted. So the project is very much a work in progress. If you want to help by entering data, please register for an account at http://planthardiness.gc.ca. Once you are registered, log into your account to enter data (you may enter non-rhododendron observations as well!). You do not have to be a registered user to view maps. Simply select a genus and then a species and/or hybrid from the list and view. Maps will be generated once 30 to 50 locations have been received.

Determining Your Location Coordinates
Besides accurate species' identifications the models require specific locations to be provided—this means the longitude and latitude coordinates (in decimal degrees) of where each species are known to survive. The usual locations are house lots, special purpose gardens such as an arboretum, parks and so on. For example: "The Botanical Gardens at Ashville, North Carolina, ten-acre site, at 151 W.T. Weaver Boulevard, has coordinates 82.550 longitude and / 35.600 latitude." This location may have a list of a dozen or more rhododendron species, as well as many other plant species. The more precise the locations the better because the climate estimates will be more accurate - at least 3 decimal places is preferred. All locations are kept confidential and coarsened so it is impossible to trace any locations down to an actual location on the ground.
        You can use a global positioning device or get coordinates from topographic maps showing coordinates.

Thinking About Climate Change and Rhododendrons
Climate change is predicted to strongly impact plant distributions (McKenney et al 2007b). The Plant Hardiness project also models where the climate habitat for that species could be expected to move under several climate change scenarios. The climate change scenarios have been developed using the Intergovernmental Panel on Climate Change climate change scenarios and represent different possible greenhouse gas emission paths this century. At the website you can view results based on several climate change scenarios and for different time periods in the coming century. No one knows what scenario will actually happen because much depends on the path of future greenhouse gas emissions. Nevertheless the models give a general indication of where some species might be suited in the future.
        We hope you find the opportunity to contribute to this project exciting and useful. Your participation can help provide much needed scientific data and provide a mechanism to put rhododendrons even more on the "map"!

McKenney, D.W., Pedlar, J., Lawrence, K., Campbell, K., Hutchinson, M. 2007a. Beyond traditional hardiness zones: using climate envelopes to map plant range limits. Bioscience. 57:929-938.
McKenney, D.W., Pedlar, J., Hutchinson, M., Lawrence, K., Campbell, K. 2007b. Potential impacts of climate change on the distribution of North American trees. Bioscience. 57: 939-948.
        Additional useful references for the classification and identification of rhododendrons.
Chamberlain, David, Roger Hyam, George Argent, Gillian Fairweather and Kerry S. Walter, 1996. The Genus Rhododendron, Its classification & synonymy. Edinburgh: Royal Botanic Garden, Edinburgh.
Galle, Fred C, 1987. Azaleas: revised and enlarged edition. Timber Press, Portland OR. 519 p.
Greer H.E. 1996. Greer's Guidebook to Available Rhododendrons-Species and Hybrids. Printed in Thailand, Eastern Printing, 3rd Edition, 227 p
Leach, D.G., 1961. Rhododendrons of the world. Charles Scribner's Sons, NY.

Willem Morsink is a member of the ARS and past president of the RSC Toronto Chapter, ARS District 12.

Volume 62, Number 2
Spring 2008

DLA Ejournal Home | JARS Home | Table of Contents for this issue | Search JARS and other ejournals