杏吧原创

This medieval 鈥淭ree of Life鈥� works as a mnemonic memory aid (Image: Beinecke Rare Book and Manuscript Library, Yale University)

From studying the bible to visualising computer storage, Manuel Lima鈥檚 sumptuous The Book of Trees explores the tree diagram鈥檚 appeal for showing information

IN THE early 1990s, 14 computer scientists at the University of Maryland were sharing an 80-megabyte hard drive. The drive was often overloaded, with expendable files taking up space in neglected sub-directories. Finding anything was like blindly reaching along all the branches of an overgrown tree.

There had to be a better way, thought departmental professor Ben Shneiderman. So he wrote a six-line algorithm that visualised the drive as a rectangle. Vertical divisions split the rectangle into smaller ones, representing directories, which then subdivided horizontally to show subdirectories. Each of the smallest rectangles corresponded to a megabyte of storage space, so free space was visible at a glance.

He called his invention a 鈥渢reemap鈥�, and it was adopted by computer labs around the world. It soon found other uses, such as in an interactive chart of stocks and shares, still popular today.

These hierarchical treemaps 鈥渆pitomize the recent growth of information visualization鈥�, writes Manuel Lima in The Book of Trees: Visualizing branches of knowledge. And as big data engulfs labs and lives, the need for such powerful visualisations will only increase.

Lima, a digital designer and information guru, thinks visual literacy, including the ability to express ourselves graphically, is as important as reading and writing. True to this visual orientation, he has provided us with a fine field guide to tree forms past and present in a sumptuous book that places pictures firmly in the foreground. Full-colour reproductions of charts from many of the world鈥檚 great library collections graphically connect 2014 with the 1000-year evolution of using trees as a mode of visual communication.

Tree of Jesse, a 15th-century genealogy of the Virgin Mary (Image: Biblioth猫que Nationale de France)

Trees were symbolically important for most ancient cultures, often worshipped and frequently present in art. Their association with immortality and their branching structure made them natural scaffolds for genealogies, showing, for example, the lineage of Christ and of royalty. They visually established pedigree and, equally crucial in medieval societies, helped to control inbreeding by showing how closely people were related to a potential spouse.

Yet, as Lima鈥檚 book shows, the greatest impact of trees was in the realm of taxonomy, as visual representations of abstract religious and scientific concepts. Religion illuminated the way, with 13th-century scribes drawing trees to show relationships between scriptural texts, to aid memory and encourage exegesis 鈥� the practice of critical interpretation of texts common in monasteries.

聯The greater impact of trees was in taxonomy, as visual representations of abstract concepts聰

According to Lima, these tree illustrations supported 鈥渃ombinatorial invention and creativity鈥�. His idea of exegesis is overly modern (monasteries were not tech start-ups) but it鈥檚 easy to see how visualisation nurtured more systematic thinking. And, in turn, more systematic thinking nurtured more elaborate visualisation.

Lima convincingly singles out 13th-century Spanish philosopher Ramon Llull as a key figure, whose encyclopedic Arbor Scientiae (Tree of Science) presented a unified vision of knowledge. His 16 domains of science, from the moral to the celestial, are each represented by a branch, and all are supported by a single trunk fed by 18 roots. The roots are also labelled, with nine bearing divine attributes such as wisdom, and nine signifying logical principles, including contrariety.

Over the next five centuries, the roots were pruned, but the tree of knowledge flourished as a metaphor 鈥� think 鈥渂ranches鈥� of science 鈥� and evolved as a visualisation model.

In fact, the French 贰苍肠测肠濒辞辫茅诲颈别, the Enlightenment鈥檚 foremost encyclopedia, was prefaced by a tree diagram. This schematised its contents with as many as eight levels of branching. Interestingly, in the 1751 edition, the tree was abstract, rendered entirely in type with nested brackets as branches.

The powerful combination of taxonomic complexity and visual simplicity in this tree foreshadows many of the contemporary ones in Lima鈥檚 book. Here, the typographic tree is one branch of a tree diagram, reaching from medieval drawings to Shneiderman鈥檚 treemaps.

Lima has skilfully organised his book to reveal these developments. In addition to his section on rectangular treemaps, his chapter on radial trees is especially absorbing, all the more so because there鈥檚 nothing overtly arboreal about them.

For instance, a 鈥渟pecies-level supertree of mammals鈥� shows rodents, monotremes and the rest of us as nodes on a circle. Connective brackets sequentially link up species, genera, orders and families to a common ancestor at the centre. The radius is a 166-million-year timeline, and a smaller concentric circle is drawn at the 65-million-year-mark.

Originally published in , this supertree revealed how little mammalian diversification was affected by the dinosaur mass extinction. In The Book of Trees, it exemplifies the potential of visualisation 鈥渢o explain and educate; to facilitate cognition and gain insight; and, ultimately, to make the invisible visible鈥�, as Lima writes in his preface.

This is all fascinating. But Lima鈥檚 book lacks balance. Reading his intellectually sparse introductory text and captions, you would never guess that tree diagrams have been criticised by various experts for half a century.

This omission is all the stranger given that Visual Complexity, his previous book, drew on several critiques, including some by French philosophers Gilles Deleuze and Felix Guattari. There, Lima summarised their views approvingly: trees are 鈥渁uthoritarian, unidirectional, and stagnant鈥�, contrasting them unfavourably with network maps.

At that time, Lima鈥檚 contrast between trees and net maps was too extreme and his distinction between them too stark. Now he seems to have shifted to the opposite position.

His examples in The Book of Trees show that tree diagrams, especially interactive ones, can be remarkably dynamic.

However, for any diagram to be properly interpreted, its limitations must be fully grasped so that we know which qualities it can鈥檛 represent, or, at least, can鈥檛 represent effectively. For example, the Mercator map of the world is good for sea navigation, but bad for judging the relative size of continents: one of its key limitations is the representation of landmass. Likewise, suppose you wanted to use a tree to show the World Wide Web鈥檚 structure. This would represent it very poorly, since its structure is a network, not a hierarchy.

The problem of limitations is especially true for tree diagrams that don鈥檛 resemble trees 鈥� and fewer and fewer do. Lima鈥檚 guidance is sadly absent where it鈥檚 needed most.

Despite these reservations, there鈥檚 much to be gained by exploring Lima鈥檚 trees. What he lacks as commentator, he makes up for as a curator, and his subject couldn鈥檛 be more apposite.

Eclipse Voronoi treemap, a representation of the hierarchical file structure of the multilanguage software development system Eclipse (Image: Oliver Deussen)

As data visualisation becomes ubiquitous, we typically look at diagrams as simple infographics. Being reminded of the complex, old-growth forests of medieval scribes or Enlightenment savants cultivates our appreciation of contemporary trees 鈥� and may inspire us to combine old forms with new in creative ways.

This article appeared in print under the headline 鈥淏ranching out everywhere鈥�