This blog post is dedicated to Hannah Silver who has always been a believer in the Lorax.
The relationship to nature and urban form are two important aspects of what defines the urban forest. But Beijing and China have showed me culture and history shape and influence the fabric of the urban forest even more. Dating back to the 13th century, the Mongolian emperor Kublai Khan mandated that street trees along major boulevards be spaced no more than two paces apart. This was to ensure plenty of shade during hot summer months and to provide snow markers during the winter (Profous, 1992). These close plantings also help retain soil moisture and protect the bole of the tree from extreme fluctuations in heat and cold which could damage the tree. You can see in the pictures below the trees boles are often painted white to increase sun reflectance preventing solar damage to the trunk. The result is a dense urban forest planting which focuses on the form and shape of the bole of the tree, rather than on height and branching patterns typical of western aesthetics. China has lost many of its great trees in history from repeated invasions, cultural revolutions and political conflicts. Examples of these are the Boxer Rebellion in the 19th century, the invasion of the Japanese in the Second World War and during the Cultural Revolution of the 1960s (Profous, 1992). The great trees of China are now often found in the courtyards of temples and palaces, where monks and citizens protected them from the violence and chaos of their times. Due to these trees advanced age they are often propped and supported to maintain their form and to prevent cracking and damage would could injure and expose them to disease.
Many tree species are selected based on cultural pride for the city, with each city having a “brother tree” and “sister flower”. The two “brother trees” for Beijing are the Japanese pagoda tree (Sophora japonica) and the Oriental arbovitae (Platycladus orientalis). The Japanese pagoda tree is a common street tree found on major boulevards but also in the hutongs, residential alleys, some of which date back over 700 years. These trees are hardy against the extreme urban environmental conditions like heat, pollution and compaction often found in Beijing. Because of the cultural significance of the Japanese pagoda tree and its hardiness, the tree is ubiquitous across the Beijing’s urban forest. The Oriental arborvitae is common in residential areas and in hutongs, prized for their compact and straight growth form, as well as the symbolism for longevity (Profous, 1992). According to a study of Beijing’s urban forest (Profous, 1992), it is composed of over 90 different species of trees, with four genera making up over 55 percent of the forest canopy. Outside of right-of-ways the urban forest is much more diverse in the hutongs where many families will diligently care for and maintain trees in courtyards, oftentimes not visible from the street. These family trees are often fruit and nut trees, providing local fresh sources of food.
Comparatively, Portland Oregon’s urban forest is composed of over 170 different tree species (DuVander, 2013); nearly double that of Beijing. The data from this article made me think of the implications for Beijing’s urban forest. While major corridors, both road and transit, are required to plant at least two different species of trees, the low diversity and close spacing run the risk of hosting and spreading diseases and pest populations throughout the city. This made me think of city planning policies in Portland surrounding their urban forest. Recently the city of Portland banned the planting of maples as street trees within the city to limit an overabundance of maples, increase the diversity of other tree species and ultimately make the urban forest more resilient. The cautious planning of Portland’s forest comes after a hard lesson learned after in the 1960s an outbreak of disease wiped out most of the mature American Elms in Portland (DuVander, 2013). Perhaps Beijing could take warning from Portland’s experience and plan the urban forest for more resiliency in design and placement.
Since nature is so highly valued in Chinese culture and tradition the urban forest in Beijing will continue to be a priority for the city and its citizens. Hopefully as Beijing continues to grow, expand, and become more dense, so too will its urban forest. I am excited to see and learn more of how Chinese planners and horticulturalists integrate nature and ecosystem services into the urban environment.
DuVander, Jenny, Dec. 4, 2013, “Tree primer: Diversity in the urban forest”, The Oregonian, link: http://www.oregonlive.com/hg/index.ssf/2013/12/tree_primer_diversity_in_the_u.html
Profous, George V., 1992, “Trees and Urban Forestry in Beijing, China” Journal of Arboriculture 18(3): May 1992, p. 145-154