Trees are a significant part of our urban green space, and they offer irreplaceable ecological and social values. The field of urban forestry has been expanding in Canada with the urbanization and population growth in Canadian cities. Native and hybridized trees have thrived on urban landscapes because of intentional planting and natural hybridization.
However, it remains debatable whether hybridized trees should be planted in place of native tree species in urban forests. There are arguments revolving around the benefits and risks of tree hybridization to urban forests. One argues that tree hybridization can provide resilience to urban forests in the storm of climate change and exotic diseases. The other side may argue that the planting of hybridized trees can influence the gene pool of native species and jeopardize the urban ecological integrity.
Climate change has huge impacts on urban forests, expressed in the more frequent occurrences of extreme temperature events and weather conditions. Climate change also encourages diseases and pests, such as the emerald ash borer forest pest, to escape their range into new areas. The hybridization of trees is designed to adapt to these challenges that are less favourable for some native trees. For instance, butternut hybrids in North America have better tolerance against the butternut canker disease. Hybridizing trees with tolerance to a particular environmental condition, like drought, flood or heat stress, may potentially improve the tolerance of urban forests to such stressors.
It is not unusual to think about hybridizing native with non-native tree species, or backcrossing hybrids in order to fight the stresses in a rapidly urbanizing environment. At times, hybrids can be better adapted to the local environment than either side of the parental species. Judy Loo, a researcher with the Canadian Forest Service pointed out that planting hybrids can be justified, as sometimes hybridization is the only way to save a species from a full-scale disease like the beech bark canker. In addition, in the long run, the biodiversity in urban forests may increase once the hybrids are adapted to the environment and gradually proceed to speciation, the formation of a new species. This is especially with multiple backcrossing resulting in a hybrid that is similar in characteristics to the native tree, and thus minimizing the impact on the wildlife food sources obtained from the native tree.
On the other hand, the planting of hybridized trees in urban areas is criticized due to the uncontrollable nature of crossbreeding between hybridized and native trees, which may lead to the loss of ecological integrity. There are concerns that the overabundance of hybridized trees would have negative effects on the gene flow between urban forests and adjacent areas. Barb Boysen from the Forest Gene Conservation Association revealed that the overplanting of hybrids in the urban environment raised the question of whether the native species are still “native.” The reason is that hybrids are often mistaken or confused as native trees. Hybridized species with exotic genes may probably be resistant to specific environmental conditions or diseases at the time, but they can also invade native communities through hybridizing and disrupt ecological functions.
Hybrids in an urban setting can become the invasive species in the wild. In the city of Columbia, Missouri, Callery pear (Pyrus calleryanna), a hybridized species of ornamental trees, became problematic after they interbred and aggressively invaded adjacent fields, parks, and open areas. Hybrid Callery pear displaced native plants and resulted in economic costs due to vegetation management issues that arose with the fast-growing branches affecting the power lines in settled areas. In Toronto, the city actively promotes the planting of native trees, such as sugar maple and silver maple, to enhance the genetic diversity and genetic stock of native species.
Essentially, what we want is to have native species with high resistance to stresses. Nevertheless,
strategically planting hybridized species at specific locations can be beneficial. Implementing techniques like multiple backcrossing and avoid planting beside a river, a natural area or in a park can reduce the risk of spreading hybridized species. The benefits and risks of planting hybridized trees in urban forests may vary geographically and are needed to be seriously evaluated.
By Jacob Ke
City of Columbia. (2016). “Stop the spread!” of Invasive Callery Pear Tree Hybrids.
City of Toronto. (2016). How to Select and Buy Native Plants.
Guiasu, R. C. (2016). Chapter 3: Speciation, Biodiversity, and Introduced Species. In Non-native
Species and Their Role in the Environment.
Hoban, S. M., McCleary, T. S., Schlarbaum, S. E., Anagnostakis, S. L., & Romero-Severson, J.
(2012). Evolutionary Applications, 5(7), 720-731.
Kerr, P. (2009). Hybrid or Native. ISA Ontario.
OFAH/OMNR Ontario’s Invading Species Awareness Program. (2012). Emerald Ash Borer.
Please note that opinions expressed are the author’s own. They do not necessarily reflect the views and values of The Blank Page.