City Structures: Making Sense of Trees

In “A City Is Not a Tree,” architect Christopher Alexander argues that traditional city planning often results in cities that are rigid and lacking in vitality. He proposes an alternative approach based on the concept of “living structures,” which are characterized by flexibility and adaptability.

In many ways, Alexander’s approach is a reproach to suburban tract home and master-planned communities, as he writes;

Too many designers today seem to be yearning for the physical and plastic characteristics of the past, instead of searching for the abstract ordering principle which the towns of the past happened to have, and which our modern conceptions of the city have not yet found. These designers fail to put new life into the city, because they merely imitate the appearance of the old, its concrete substance: they fail to unearth its inner nature.

In contrast, Alexander’s ideas have been on display in the development of cities like New York and Los Angeles, which have grown in fragmented and organic ways. These city’s distinct neighborhoods, each with their own unique character, can be seen as examples of living structures. Rather than being organized according to a predetermined plan, these neighborhoods have evolved in response to the needs and desires of the people who live there, resulting in vibrant and dynamic urban environments.

Living Structures

In “A City Is Not a Tree,” Christopher Alexander uses the concepts of “tree” and “semilattice” to illustrate the differences between traditional city planning and his proposed approach.

A tree is a hierarchical structure in which each element is a part of one larger whole. In the context of city planning, this would mean that a city is organized according to a single, unified plan. Each neighborhood or district would be a part of the larger whole, with clear boundaries and a defined purpose.

The structural simplicity of trees is like the compulsive desire for neatness and order that insists that the candlesticks on a mantelpiece be perfectly straight and perfectly symmetrical about the centre. The semilattice, by comparison, is the structure of a complex fabric; it is the structure of living things - of great paintings and symphonies.

Alternatively, a semilattice is a “living” structure in which each element has multiple connections to other elements. This allows for more flexibility and adaptability, as each element can be connected to multiple others in different ways. In the context of city planning, this would mean that a city is organized in a more organic and fluid way, with neighborhoods and districts overlapping and intersecting with one another.

Alexander argues that a semilattice structure is more conducive to the development of a vibrant and dynamic city. Traditional tree-like structures often result in rigid and lifeless urban environments, whereas semilattice structures allow for more flexibility and adaptability, enabling a city to evolve and grow in response to the needs and desires of its inhabitants.

What’s a semilattice anyway?

One real-world example of a semilattice structure is the internet. In the internet, each website is connected to multiple others through links. This allows users to navigate from one website to another in a flexible and adaptable way, rather than following a predetermined path.

Another example of a semilattice structure is a network of roads or highways. In a city with a well-developed network of roads, each street or highway is connected to multiple others in different ways. This allows for flexible and adaptable movement within the city, rather than requiring people to follow a predetermined route.

A third example of a semilattice structure is a group of friends. In a group of friends, each person is connected to multiple others through different relationships and connections. This allows for flexible and adaptable social interactions, rather than everyone in the group having to follow a predetermined social hierarchy.

Overall, semilattice structures are characterized by multiple connections and flexibility, which allows for adaptability and evolution. This is in contrast to tree-like structures, which are hierarchical and rigid.

Trees gone wrong

this section is directly from the essay

Take the separation of pedestrians from moving vehicles, a tree concept proposed by Le Corbusier, Louis Kahn and many others. At a very crude level of thought this is obviously a good idea. It is dangerous to have 60 mph cars in contact with little children toddling. But it is not always a good idea.

There are times when the ecology of a situation actually demands the opposite. Imagine yourself coming out of a Fifth Avenue store: you have been shopping all afternoon; your arms are full of parcels; you need a drink; your wife is limping. Thank God for taxis!

Yet the urban taxi can function only because pedestrians and vehicles are not strictly separated. The cruising taxi needs a fast stream of traffic so that it can cover a large area to be sure of finding a passenger. The pedestrian needs to be able to hail the taxi from any point in the pedestrian world, and to be able to get out to any part of the pedestrian world to which he wants to go. The system which contains the taxicabs needs to overlap both the fast vehicular traffic system and the system of pedestrian circulation. In Manhattan pedestrians and vehicles do share
certain parts of the city, and the necessary overlap is guaranteed.

What can we do to design better places?

Real estate developers and city planners can make use of the principles in Christopher Alexander’s essay by incorporating flexibility and adaptability into their planning and development processes. This could involve designing neighborhoods and districts that overlap and intersect with one another, rather than being organized according to a predetermined plan.

Additionally, developers and planners can incorporate the idea of living structures, which are characterized by adaptability and the ability to evolve over time. This could involve designing buildings and other structures that can be easily modified or expanded in response to changing needs and desires.

Furthermore, developers and planners can focus on creating a sense of community and connection within neighborhoods and districts. This could involve incorporating public spaces and other amenities that encourage social interaction and a sense of belonging.

Overall, by adopting the principles in Christopher Alexander’s essay, real estate developers and city planners can create more vibrant and dynamic urban environments.