1. Sr. Sci., Danish Ctr. for Forest, Landscape and Plng., Danish Forest and Landscape Res. Inst., Hoersholm Kongevej 11, DK-2970 Hoersholm, Denmark (corresponding author). E-mail: tbr@fsl.dk
2. Program Leader, Western Ctr. for Urban Forest Res. and Educ., U.S. Dept. of Agriculture Forest Service, c/o Dept. of Envir. Horticulture, Univ. of California, 1 Shields Ave., Davis, CA 95616.
3. Extension Spec., Extension Service, Univ. of California, Room 200, 625 Miramontes, Half Moon Bay, CA 94019.