“There certainly have been many new things
in the world of visualization; but unless
you know its history, everything might seem novel.”
— Michael Friendly
It’s tempting to consider information visualization a relatively new field that rose in response to the demands of the Internet generation. “But,” argues Manual Lima in The Book of Trees: Visualizing Branches of Knowledge, “as with any domain of knowledge, visualizing is built on a prolonged succession of efforts and events.”
This book is absolutely gorgeous. I stared at it for hours.
While it’s tempting to look at the recent work, it’s critical we understand the long history. Lima’s stunning book helps, covering the fascinating 800-year history of the seemingly simple tree diagram.
Trees are some of the oldest living things in the world. The sequoias in Northern California, for example, can reach a height of nearly 400 feet, with a trunk diameter of 26 feet and live to more than 3,500 years. “These grandiose, mesmerizing lifeforms are a remarkable example of longevity and stability and, ultimately, are the crowning embodiment of the powerful qualities humans have always associated with trees.”
Such an important part of natural life on earth, tree metaphors have become deeply embedded in the English language, as in the “root” of the problem or “branches” of knowledge. In the Renaissance, the philosophers Francis Bacon and Rene Descartes, for example, used tree diagrams to describe dense classification arrangements. As we shall see, trees really became popular as a method of communicating and changing minds with Charles Darwin.
In the introduction Lima writes:
In a time when more than half of the world’s population live in cities, surrounded on a daily basis by asphalt, cement, iron, and glass, it’s hard to conceive of a time when trees were of immense and tangible significance to our existence. But for thousands and thousands of years, trees have provided us with not only shelter, protection, and food, but also seemingly limitless resources for medicine, fire, energy, weaponry, tool building, and construction. It’s only normal that human beings, observing their intricate branching schemas and the seasonal withering and revival of their foliage, would see trees as powerful images of growth, decay, and resurrection. In fact, trees have had such an immense significance to humans that there’s hardly any culture that hasn’t invested them with lofty symbolism and, in many cases, with celestial and religious power. The veneration of trees, known as dendrolatry, is tied to ideas of fertility, immortality, and rebirth and often is expressed by the axis mundi (world axis), world tree, or arbor vitae (tree of life). These motifs, common in mythology and folklore from around the globe, have held cultural and religious significance for social groups throughout history — and indeed still do.
The omnipresence of these symbols reveals an inherently human connection and fascination with trees that traverse time and space and go well beyond religious devotion. This fascination has seized philosophers, scientists, and artists, who were drawn equally by the tree’s inscrutabilities and its raw, forthright, and resilient beauty. Trees have a remarkably evocative and expressive quality that makes them conducive to all types of depiction. They are easily drawn by children and beginning painters, but they also have been the main subjects of renowned artists throughout the ages.
Our relationship with trees is symbiotic and this helps explain why it permeates our language and thought.
As our knowledge of trees has grown through this and many other scientific breakthroughs, we have realized that they have a much greater responsibility than merely providing direct subsistence for the sheltered ecosystems they support. Trees perform a critical role in moderating ground temperature and preventing soil erosion. Most important, they are known as the lungs of our planet, taking in carbon dioxide from the atmosphere and releasing oxygen. As a consequence, trees and humans are inexorably intertwined on our shared blue planet.
Our primordial, symbiotic relationship with the tree can elucidate why its branched schema has provided not only an important iconographic motif for art and religion, but also an important metaphor for knowledge-classification systems. Throughout human history the tree structure has been used to explain almost every facet of life: from consanguinity ties to cardinal virtues, systems of laws to domains of science, biological associations to database systems. It has been such a successful model for graphically displaying relationships because it pragmatically expresses the materialization of multiplicity (represented by its succession of boughs, branches, twigs, and leaves) out of unity (its central foundational trunk, which is in turn connected to a common root, source, or origin.)
While we can’t go back in time it certainly appears like Charles Darwin changed the trajectory of the tree diagram forever when he used it to change minds about one of our most fundamental beliefs.
Darwin’s contribution to biology—and humanity—is of incalculable value. His ideas on evolution and natural selection still bear great significance in genetics, molecular biology, and many other disparate fields. However, his legacy of information mapping has not been highlighted frequently. During the twenty years that led to the 1859 publication of On the Origin of Species by Means of Natural Selection, Darwin considered various notions of how the tree could represent evolutionary relationships among specifics that share a common ancestor. He produced a series of drawings expanding on arboreal themes; the most famous was a rough sketch drawn in the midst of a few jotted notes in 1837. Years later, his idea would eventually materialize in the crucial diagram that he called the “tree of life” (below) and featured in the Origin of Species.
Darwin was cognizant of the significance of the tree figure as a central element in representing his theory. He took eight pages of the chapter “Natural Selection,” where the diagram is featured, to expand in considerable detail on the workings of the tree and its value in understanding the concept of common descent.
A few months before the publication of his book, Darwin wrote his publisher, John Murray: “Enclosed is the Diagram which I wish engraved on Copper on folding out Plate to face latter part of volume. — It is an odd looking affair, but is indispensable to show the nature of the very complex affinities of past & present animals. …”
The illustration was clearly a “crucial manifestation of his thinking,” and of central importance to Darwin’s argument.
As it turned out it was the tree diagram, accompanied by Darwin’s detailed explanations, that truly persuaded a rather reluctant and skeptical audience to accept his groundbreaking ideas.
Coming back to the metaphor, before we go on to explain and show some of the different types of tree diagrams, Lima argues that given the long-lasting nature of the tree and its penetration into our lives as a way to organize, describe, and understand we can use the tree as a prism to better understand our world.
As one of the most ubiquitous and long-lasting visual metaphors, the tree is an extraordinary prism through which we can observe the evolution of human consciousness, ideology, culture, and society. From its entrenched roots in religious exegesis to its contemporary secular digital expressions, the multiplicity of mapped subjects cover almost every significant aspect of life throughout the centuries. But this dominant symbol is not just a remarkable example of human ingenuity in mapping information; it is also the result of a strong human desire for order, balance, hierarchy, structure, and unity. When we look at an early twenty-first-century sunburst diagram, it appears to be a species entirely distinct from a fifteenth-century figurative tree illustration. However, if we trace its lineage back through numerous tweaks, shifts, experiments, failures, and successes, we will soon realize there’s a defined line of descent constantly punctuated by examples of human skill and inventiveness.
Types of Tree Diagrams
Trees have been not only important religious symbols for numerous cultures through the ages, but also significant metaphors for describing and organizing human knowledge. As one of the most ubiquitous visual classification systems, the tree diagram has through time embraced the most realistic and organic traits of its real, biological counterpart, using trunks, branches, and offshoots to represent connections among different entities, normally represented by leaves, fruits, or small shrubberies.
Even though tree diagrams have lost some of their lifelike features over the years, becoming ever more stylized and nonfigurative, many of their associated labels, such as roots, branches, and leaves, are still widely used. From family ties to systems of law, biological species to online discussions, their range of subjects is as expansive as their time span.
The transition from realistic trees to more stylized, abstract constructs was a natural progression in the development of hierarchical representations, and a vertical scheme splitting from top or bottom was an obvious structural choice. … Of all visualization models, vertical trees are the ones that retain the strongest resemblance to figurative trees, due to their vertical layout and forking arrangement from a central trunk. In most cases they are inverted trees, with the root at the top, emphasizing the notion of descent and representing a more natural writing pattern from top to bottom. Although today they are largely constrained to small digital screens and displays, vertical trees in the past were often designed in larger formats such as long parchment scrolls and folding charts that could provide a great level of detail.
With the adoption of a more schematic and abstract construct, deprived of realistic arboreal features, a tree diagram could sometimes be rotated along its axis and depicted horizontally, with its ranks arranged most frequently from left to right.
Horizontal trees probably emerged as an alternative to vertical trees to address spatial constraints and layout requirements, but they also provide unique advantages. The nesting arrangement of horizontal trees resembles the grammatical construct of a sentence, echoing a natural reading pattern that anyone can relate to. This alternative scheme was often deployed on facing pages of a manuscript, with the root of the tree at the very center, creating a type of mirroring effect that is still found in many digital and interactive executions. Horizontal trees have proved highly efficient for archetypal models such as classification trees, flow charts, mind maps, dendrograms, and, notably, in the display of files on several software applications and operating systems.
The Book of Trees: Visualizing Branches of Knowledge goes on to explore multi-directional, radial, hyperbolic, rectangular, Voronoi, and circular tree maps as well as sunbursts and icicle trees.