The Tree watches. The Tree knows. The Tree understands.

World-Trees are giant bioengineered plants used for construction and land ornamentation. First invented on Earth during the Low Age, they were later perfected on extrasolar settlements such as Elora and Smyrnia. They are among the largest feats of bioengineering in human space, only rivalled by zero-gravity coral weaves. World-trees are used as scaffolds or supporting structures for arcologies, districts, and even cities. They aren't built but grown over the course of several decades.

A world-tree starts with a forest of engineered sequoias, or similar native trees with low density wood. This forest is allowed to grow naturally for twenty to thirty years, before the trees are intermingled with a transbiological weave that links them together in a single organism. As the forest continues to grow, the original canopy turns into a thick layer and the trees slowly merge within a single, dense stump of wood. Then, the world-tree keeps growing upwards, developing patches of lower density wood and buoyant sacks to keep its balance. Gardeners and engineers can then shape the world-tree as they see fit, adapting it to the arcology, space elevator, launch loop, habitat or city district the transbiological plant is used for. 

The size and height of a world-tree depends on the forest it was born from. Most world-trees are grown from forests that are a few hundred meters in diameter, and tower a hundred to two hundred meters above ground. On low gravity planets, however, kilometres-large forests can be merged to create world-trees taller than most planetary mountains. On the other end of the scale, skilled gardeners may give the world-tree treatment to miniature bonsai forests, resulting in trees that are barely a dozen meters tall. The shape of a world-tree is also a direct consequence of its size. Below two hundred meters or so, world-trees tend to look like regular trees once the stump is in place, with a canopy and a mostly cylindrical trunk. Above this height, however, world-trees are akin to artificial plateaus, forming giant stumps upon which entire districts or cities may prosper.

Illustration: Rhyzom open-source RPG art, CC-BY-NC 3.0.