Arboriculture & Urban Forestry 32(4): July 2006 133 Silver Birch (Betula pendula) Pollen and Human Health: Problems for an Exotic Tree in New Zealand Ian F. Spellerberg, Nils E. Eriksson, and Vincent St. A. Crump Abstract. Silver birch (Betula pendula) is commonly used as a street tree in temperate climatic regions. However, the medical literature contains a wealth of reports on the health effects of pollen from silver birch. In many countries such as New Zealand, silver birch is the main tree that causes allergic symptoms, including seasonal hayfever, asthma, and other health conditions such as food allergies (the oral allergy syndrome). Exposure to pollen from silver birch is more likely to occur in cities because of the numbers of the trees and the human population density. Even if there were doubts about the extent of the problem and the costs associated with the problem, the precautionary principle should apply. The health- related problems of silver birch should be promulgated and trees should be removed. Key Words. Allergies; human health problems; silver birch pollen. In the arboriculture literature, there are many articles that include lists of trees that are suitable for urban environments. Similarly, in the research literature, there are many papers that deal with selection of trees for urban environments (see for example Ware 1994; Kristoffersen 1999; Saebo et al. 2003). There are many factors to consider, including suitabil- ity of the site, aesthetic and design factors, and nuisance factors. Nuisance factors include propensity to drop limbs, shed bark, have unpleasant odors, and have poisonous prop- erties. The fact that some plants and plant pollen cause allergies (allergenic) has been known for some time. Indeed, there are some species such as privet (Ligustrum spp.) and ragweed or mugwort (Ambrosia spp.) and, more recently, olive (Olea europaea) that pose allergy-related problems worldwide and are troublesome for agriculture (Baldo et al. 1992; Wickens 2001). Some plant species that cause allergies are so wide- spread that it is difficult to eradicate them. However, some species of trees that are introduced species may be much more manageable by way of some eradication and by educa- tion. Allergy-free gardening has been addressed by several authors such as Ogren (2000). However, management of plants that cause allergies may have to be modified country by country because of local conditions. Silver birch (Betula pendula) is native to Europe and Asia Minor. It has been introduced to Australia and New Zealand. Ecologically, silver birch is a fast-growing and short-lived species. It has male catkins on the ends of small shoots, visible all winter. The female flowers are typically on branched stalks and some appear at the base of the male catkins. The pollen is dispersed by wind over several weeks in the summer. The pollen can be spread over considerable distances, and in our experience (Sven-Olov Strandhede, pers. comm.) in Europe, the pollen can even cross the Baltic Sea. In temperate climatic regions, silver birch is, with few exceptions, considered to be a suitable species as a street tree. Many authors extol its universal appeal as an ornamental tree suitable for private gardens and streets and parks. The main reasons for its popularity include its whitish/silver-colored bark, graceful form, and autumn colors. However, some civic authorities (e.g., Palmerston North, New Zealand) list silver birch as being unsuitable because of the seeds and the roots. In New Zealand, pollen from introduced species is a com- mon cause of hayfever and asthma. A major contributor is the pollen from perennial rye grass. Among tree species, the pol- len from pines has not been found to be a problem. Of all introduced species in New Zealand, the introduced silver birch is the main problem. The health-related problems of silver birch pollen, al- though well documented in the medical literature, appear not to have reached arboriculture literature or landscape architec- ture literature. Conversations with arboriculturalists and land- scape architects have confirmed this conclusion. The purpose of this article is therefore to promulgate the issue and prompt discussion. METHODS While undertaking research on the criteria used to select trees for urban environments, the health problems caused by silver birch pollen were noted in the medical literature. Conse- quently, a detailed review of the medical literature was un- dertaken and this was supplemented with discussions with medical researchers, arboriculturalists, and landscape archi- tects. ©2006 International Society of Arboriculture
July 2006
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