Life on Trees

"How many organisms inhabit a tree?" — a question that seems simple at first glance, but is surprisingly difficult to answer.

At the base of the climb, one can observe mosses clustered on the more humid side of the trunk, and lichens on the drier side. Mosses are non-vascular plants, and on a surface no larger than a cigarette pack, a dozen or more species can be found — whether on branches or leaves.

The same goes for lichens, which are a symbiotic association between a fungus and an alga, with the alga providing energy through photosynthesis.

Large fungi are rare in trees, appearing mainly on dead wood. However, microscopic fungi inhabit the living wood and leaves. Very little is known about their way of life, yet up to a hundred species may be found within a single tree.

Reaching the semi-horizontal branches, one finds a wide variety of so-called epiphyte plants, which grow on other plants — in this case, the tree. In South America, there is a particularly great variety of orchids, sometimes over 100 species in a single tree, often hard to spot due to their tiny flowers.

Bromeliads, plants from the pineapple family, are also abundant. With their rosette-shaped leaves, they act as natural reservoirs, collecting and holding rainwater.

These small reservoirs also host invertebrates, like dragonfly larvae and other aquatic insects. Some frogs use them for part of their life cycle. Birds also visit to drink or bathe, as shown by DNA traces left behind in the water.

Near Rio de Janeiro, another large tree is under close observation (Araçá project). It is located in one of the last remnants of the Atlantic Forest, which has lost nearly 92% of its area. If the forest disappears, the entire water supply for the large city will be threatened.

We found more than a thousand species living on a single Amazonian tree, with even more yet to be revealed. Cutting down a tree means losing this vast, interconnected community vital for the forest’s resilience to climate change and other disturbances.

To safeguard forests, local communities must be supported and benefit from biodiversity research. The Life on Trees project engaged park rangers and schools in Peru, and worked with the Humboldt Institute’s Social Appropriation team in Colombia.