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Brian K. Maynard, Ph.D., Associate Professor of Horticulture
University of Rhode Island, Department of Plant Sciences, Kingston, RI 02881
A paper from the Proceedings of the 11th Metropolitan Tree Improvement Alliance (METRIA) Conference held in Gresham, Oregon, August 23-24, 2000, and cosponsored by the Landscape Plant Development Center.
Additional index words: pinus, turpentine beetle, blue stain fungus, sustainable plants
Owners of ocean front property on the eastern seaboard have always had to work with a limited pallet of salt-tolerant landscape plants (Harris et al., 1998, Fig. 1). Particularly important in this mix are evergreen trees and shrubs, which function as windbreaks, screens and foundation plants. The list of evergreens that can tolerate near-coastal conditions is fairly extensive, including cedars, junipers, larch, cypress, holly, andromeda, spruce and pines (Flint, 1997; see also the URI Sustainable Plant List). However, as the landscape edges toward the shore the pallet thins considerably - to rugosa rose, bayberry, shore juniper, and a few others.
|Figure 1. Salt-tolerant plants on beach.||Figure 2. Japanese black pine (left) and Austrian Pine (right).|
Two pine species that landscapers have come to rely on heavily include the highly salt-tolerant Pinus thunbergii, Japanese black pine and P. nigra var. austriaca, Austrian pine (Dirr, 1998; Fig. 2).Yet, over the past decade these trees have undergone a major decline due to the combined effects of turpentine beetle (Dendroctonus terebrans) and bluestain fungus (Leptographium spp.) (Fig. 3).
Figure 3. Damaged pines.
Pines affected by this pest complex yellow and die within a few years of infestation (see also Cornell factsheet). A major decline of Japanese black pine is occurring this year on Block Island (New Shoreham, RI). Thousands of trees, often all of the trees on a property, are dead or dying, in part because of the added stress of a record-breaking drought in 1999.
An equally lethal pest is pinewood nematode (Bursaphelenchus xylophilus), transmitted by long-horned borers (pine sawyer) which feed on healthy trees and breed in logs or dying trees. Austrian pine, as well as red pine (Pinus resinosa), is also susceptible to diplodia tip blight (Diploidia pinea). Heroic efforts are made to salvage these pines, typically through the use of potent insecticides (Dursban or Lindane), yet most are lost. For now, we recommend against planting any black pines in the Northeast, which leaves a big blank spot in coastal landscapes from New Jersey to Massachusetts. Alternate plants that will survive and grow in sandy soil and salt-impacted coastal areas are sorely needed.
The greatest attraction of Japanese black and Austrian pines in the Northeast lies in their salt-tolerance. However, little research has been conducted on the salt-tolerance and culture of other conifers in coastal landscapes. Locating substitutes for these pines is a top research priority of the University of Rhode Island (URI) Sustainable Landscapes Program, which has as its focus the reduction of pesticide use in the landscape. Among alternative species we have identified, three pines stand out as being salt-tolerant. Two of these, P. cembra, Swiss stone pine, and P. heldrechii var. leucodermis, Bosnian pine, possess stiff needles, like the black pines, and upright habits useful for evergreen screening. Although Swiss stone and Bosnian pine are not as common in the landscape, URI plant protection specialists have yet to see turpentine beetles or pinewood nematodes on these species. The culture and use of these alternative pines needs to be studied in more detail.
Another promising alternative to black pine is the Southwestern white pine, Pinus strobiformis (Fig 4).
Figure 4. Alternative species.
Southwestern white pine is not well known outside its native range of Arizona and New Mexico. It has stiff blue needles, similar in length to those of Japanese black pine, but the needles occur in bundles of 5, like Pinus strobus, Eastern white pine. Research by Townsend and Kwolek (1987) found this species to be the most salt-tolerant of the 5-needled pines (Table 1).
Table 1. Salt tolerance of pines.
Like Japanese black pine, Southwestern white pine also is tolerant of high ozone levels, which occur several times each year in the northeast. We are not aware of any formal studies on diseases affecting Southwestern white pine. This species appears to be susceptible to pine shoot weevil, a minor pest, but does not get turpentine beetle, pinewood nematode, or diplodia. Through URI's close partnership with the local green industry, we hope that this pine will be available locally in coming years.
Over the next five years we will research the salt-tolerance of likely substitutes for pest-prone salt-tolerant evergreens that are used in coastal areas and along roadsides in the northeast. To achieve this objective we will collect provenance seed of southwestern white pine from trees exhibiting appropriate drought/salt tolerance and cold-hardiness and have these seed grown out by regional conifer nurseries in parallel with production trials at URI (Kuser and Ching, 1980; Schopmeyer, 1974). Seedlings will be large enough to permit salt screening within one year. Blocks of seedlings will be treated with aerial salt and rated for damage. Plants exhibiting the greatest salt tolerance will be grown on in our nursery for additional testing and selection.
At the same time we will obtain liner stock (typically two year-old seedlings; 2-0 stock) of southwestern white pine, Swiss stone pine and Bosnian pine, and work with cooperating nurseries and landscapers to plant and evaluate test blocks of 20-50 plants in coastal locations (eg. Sakonnet Point, Block Island, Warwick Neck, RI). Station staff will plant, initially irrigate, fertilize and cultivate these blocks (Fig. 5)
Starting in 2001 we will evaluate salt-spray tolerance of these pines at the URI Agricultural Experiment Station. Container- and field-grown stock will be treated, in replicated blocks, with foliar sprays and irrigation water containing salt (NaCl) in concentrations representing a range of salinity from clean water through full strength seawater (~3M NaCl). Salt damage will be assessed as visible injury, reduction in growth, and winter survival. The most salt-tolerant seedlings and clones will be propagated vegetatively, by grafting and rooting of cuttings, and evaluated further. Trial blocks will be evaluated for host plant tolerance of salt spray and soil salt, as well as resistance to associated pest complexes, as appropriate.
|Figure 5. Test blocks.|
Dirr, M.A., 1998. Manual of Woody Landscape Plants: Their Identification, Ornamental Characteristics, Culture, Propagation and Uses. Fifth Ed. Stipes Publishing Co., Champaign, IL
Flint, H.L., 1997. Landscape Plants for Eastern North America, Second Ed., J. Wiley & Sons, Inc., New York. 842 pp.
Harris, R.W., J.R. Clark and N.P. Matheny. Arboriculture: Integrated Management of Landscape Trees, Shrubs, and Vines (Third Ed.). 1998. Prentice Hall, Englewood Cliffs, NJ. 687 pp.
Kuser, J.E. And K.K. Ching. 1980. Provenance variation in phenology and cold hardiness of western hemlock seedlings. For. Sci. 26: 463-470.
Schopmeyer, C.S., 1974. Seeds of Woody Plants in the United States. Agriculture Handbook No. 450. USDA Forest Service, Wash., D.C.
Townsend, A.M. and W.F. Kwolek. 1987. Relative susceptibility of thirteen pine species to sodium chloride spray. J. Arbor. 13:225-228.
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