There is a long history of problems associated with the overuse of specific plants in agricultural monocultures. The Irish potato famine of 1845 to 1850, which was caused partly by dependence on a single variety of potato (Solanum tuberosum), called “the lumper,” is a well-known example of this (Fraser, 2003). The term “monoculture” is also now used for large expanses of nonagricultural crops, such as street trees (Flemer, 1981; Frank et al., 2006). The extensive loss of trees in Europe and North America to dutch elm disease, which was first described in 1921, is a well-known example of a problem resulting from this type of monoculture (Wilson, 1975). This article presents further results from a study of the wholesale nursery industry (Polakowski et al., 2011) that noted some lack of awareness of the problems that arise from monocultures and the potential role of educators in overcoming this lack of awareness.
Risks associated with overplanting come from lack of genetic variation, which increases susceptibility to pest damage and environmental stresses. Genetic variation occurs across species and within species. “Biodiversity” is the term generally used for the range of diversity across different species in an area (Lankau and Strauss, 2007). “Genetic diversity” typically refers to the range of different genetic characteristics within a single species. Both, as explained below, play a role in reducing risks associated with monocultures.
There are many examples of pest problems that have spread through landscape plantings with low biodiversity. These pests, which attack more than one species within a commonly planted genus, include dogwood anthracnose (Discula destructiva), hemlock woolly adelgid (Adelges tsugae), and ash decline disease (Chalara fraxinea or Hymenoscyphus pseudoalbidus) (Bakys et al., 2009; Daughtrey et al., 1996; Spaulding and Rieske, 2010). Another example is emerald ash borer, which attacks at least four different species of ash (Fraxinus sp.). In 2002, the borer was found in Michigan and Ontario (Poland and McCullough, 2006). Ash is widely used as a street tree. Only two years later, 15 million ash trees were estimated to be dead or dying from emerald ash borer injury in Michigan alone. Positive examples of efforts to reduce these potential problems can be found. For example, only three trees of the same species may be planted in a row in downtown Chicago (Heimlich et al., 2008). However, insufficient action has been taken to reduce the risks associated with low biodiversity (Muller and Bornstein, 2010).
Genetic diversity within individual species of landscape trees has unwittingly been reduced because of the use of clonal propagation for preferred clones (Iles and Vold, 2003). A recent study highlighted the issue (Morton and Gruszka, 2008). About 100 years ago, 200 seedlings of london plane tree (Platanus ×acerifolia) were planted in Schenley Park, Pittsburgh, PA. Since then, about half have died from sycamore anthracnose (Apiognomonia veneta) or canker stain (Ceratocystis platani). Researchers analyzed remaining park trees and trees from nurseries in Pennsylvania and Oregon. They found much greater genetic diversity in the park trees than in the nursery trees and concluded that this was because few seedling trees are sold today. Nurseries want disease-resistant london plane trees, so most clone ‘Bloodgood’. Park managers, who had wanted to replant with london plane trees with expanded genetic diversity, were unable to do so. Instead, the researchers provided local nurseries with propagation material from the remaining Schenley Park trees, which presumably are resistant to both sycamore anthracnose and canker stain (Morton and Gruszka, 2008).
Little has been published about what the green industry knows regarding this problem or how they are responding or could respond to the risks associated with low biodiversity and genetic diversity in landscapes. A survey of people making plant inventory decisions in wholesale nurseries in Washington State was conducted to determine current awareness of diversity issues, focusing primarily on biodiversity (Polakowski et al., 2011). Respondents indicated some awareness of the issues: 85% agreed that “increasing the number of different plant species used in an area is important for biodiversity.” Many appeared to lack an in-depth understanding of the issue; only 45% agreed that “planting more than 10% of the same plant species in a region greatly increases the risk of insect or disease outbreaks.” Most apparently did not feel that the nursery industry played a role in the problem, because 78% agreed that “most wholesale nurseries currently offer an adequate range of genetically different plants for their customers to choose from.” A majority of the respondents (60%) were educated about plants at higher educational institutions. The survey also found that education was an important factor in respondents’ knowledge of the issue: those who had learned about plant diversity issues in “school or college classes” scored significantly higher on a composite score of diversity knowledge than those who did not. To further investigate who the nursery industry thinks may be responsible for this issue and what role education could play in contributing to action on this issue, additional analyses were performed with the data from the study.
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