When most people think about Bwindi Impenetrable National Park’s vegetation, they think of trees — the towering canopy species, the strangler figs, the ancient mahogany. But the forest floor and the surfaces of every tree in Bwindi are covered by an entirely different category of plant life: the mosses, lichens, and ferns that collectively constitute some of the oldest and most ecologically important organisms in the entire ecosystem.
Mosses: the forest’s moisture regulators
Mosses are non-vascular plants that absorb moisture directly through their surfaces. Bwindi’s mosses are extraordinarily diverse — from the flat bright-green sheets that carpet the forest floor to dense cushion mosses on rotting logs, from feathery pleurocarpous mosses hanging from branches to upright star-shaped acrocarpous mosses on exposed bark. A thick moss mat on the forest floor can hold many times its own weight in water — functioning as a sponge that absorbs rainfall during wet periods and releases it slowly during dry periods, moderating the hydrology of streams and springs that supply drinking water to surrounding communities. For insects, mosses provide microhabitat of enormous complexity — the spaces between moss stems constitute a miniature three-dimensional landscape in which mites, springtails, nematodes, and vast numbers of other invertebrates live out entire life cycles.
Lichens: the forest’s partnership organisms
Lichens are not a single type of organism but a symbiotic partnership between a fungus and a photosynthetic partner — usually an alga or cyanobacterium. Bwindi’s lichen diversity is remarkable: crustose lichens cover vast areas of tree trunk in intricate grey, green, orange, and white patches; foliose lichens hang from branches and encrust mossy surfaces; fruticose lichens dangle from the highest branches in conditions of high humidity. Lichens are among the most sensitive biological indicators of air quality available — in pristine environments like Bwindi, where the air carries nothing but moisture and the chemical byproducts of the forest itself, lichen communities reach extraordinary diversity. Several species at Bwindi fix atmospheric nitrogen, contributing an important nutrient subsidy to the forest ecosystem.
Ferns: ancient vascular plants of the forest floor
Ferns have a fossil record extending over 360 million years — predating both the dinosaurs and the flowering plants that today dominate most terrestrial ecosystems. Bwindi has been estimated to support over 100 fern species, ranging from tiny filmy ferns whose fronds are only a single cell thick to large tree ferns whose arching fronds reach several metres in height. Epiphytic ferns — species that grow on the surfaces of trees — colonise bark, mossy branches, and even large leaves in the canopy. Staghorn ferns and bird’s nest ferns are common on large horizontal branches throughout Bwindi. Several fern species are consumed by gorillas in Bwindi, particularly young crosiers rich in protein. Above 2,000 metres, trees become festooned with epiphytic mosses and lichens to an extent that gives the forest an almost ethereal appearance — every branch coated in several centimetres of dense moss growth. On your trek through Bwindi, take a moment to look down and closer: the miniature forest growing on the surfaces all around you is as old as the mountains themselves.
