"A City is Not a Tree" by Christopher Alexander

Part of sustainable urbanism is the development of a scalable, efficient city model which would allow for flexible interrelationships, while still providing structure and hierarchy. Christopher Alexander challenges the more common tree model, instead suggesting that a more flexible semi-lattice is required to properly understand the relationships within the city. 

The essay "A City is Not A Tree" by Christopher Alexander discusses how people visualise the world through tree structures, rationalising the natural sphere to simplify our understanding of complex inter-relationships. 

Alexander describes a “compulsive desire for neatness and order” and how “for the human mind, the tree is the easiest vehicle for complex thoughts”. However, while it is tempting to use the tree as a universal model, he notes that truly natural cities have the organisation of a semi-lattice. He further asserts that the semi-lattice is the “structure of living things”. It is this level of complexity and vibrant inter-relationships that allow a city to be successful, something that is created through the semi-lattice. 

He continues by challenging the idea that the tree model is the more orderly model, arguing that the semi-lattice presents a stronger and more robust system. Alexander concedes the difficulty of designing a city as a semi-lattice, stating that “designers… cannot achieve the complexity of the semi-lattice in a single mental act.” He further notes that “when we organise a city artificially, we organise it as a tree”, commenting on the failure of the built environment to design successful cities. 

In conclusion, a key part of sustainability is being able to understand the complex interrelationships within a city, and to create a framework capable of supporting the flexible, multi-layered, and organic relationships necessary to achieve a more sustainable built environment. Christopher Alexander suggests that the semi-lattice model is a more accurate representation of these complex relationships, as opposed to the more traditional tree model.