shields the sunny urban fence posts
- Size
- Width: 5–20 cm
- Lifespan
- 20–100 years
- Diet
- Photosynthetic. Obtains nutrients via symbiotic relationship between fungus and alga. Requires clean air, stable bark or rock surfaces, and good light.
- Habitat
- Grows on the bark of trees, on rocks, and occasionally on old fence posts throughout New Zealand. A creature of the bark and the stone. Found from sea level to the subalpine zone.
- Range
- Found throughout the North and South Islands on bark of native and introduced trees, rocks, and fence posts. Most common in open, sunny locations in urban, rural, and natural areas.
- Endemism
- Native
- Main Threats
- None significant. This species is one of the most common and widespread lichens in the world. Localised threats include air pollution, removal of old trees, and bark disturbance.
- Population
- Not Threatened. This is one of the most common and widespread lichens in the world, found on every continent except Antarctica. In New Zealand, it is abundant on the bark of native and introduced trees.
- Conservation Status
- Not Threatened
- Human Risk
- harmless
- Handling Note
- foliose lichen, safe to handle
- Conservation Note
- Native lichen; not assessed by NZTCS as lichens are generally outside the scope of current threat classifications.
- Te Ao Māori
- No recorded Māori name distinguishes the common grey shield lichen from other lichens. Lichens were generally called pūkohu (mosses and lichens) or pukorokoro (crustose lichens). The grey, leafy lichen on the bark would have been noticed. But it was not a major resource. Too small. Too tough. Too bitter. The presence of grey shield lichen on a tree was a sign of clean air. And an old, stable environment. It meant that the forest was healthy.
The one that looks like old paint peeling off a fence has a body that is flat, leafy, and grey. Pale grey to bluish-grey. Sometimes with a hint of green. The lobes are broad and rounded. Spreading across the bark like a map of a foreign country. The surface is wrinkled and pitted. Covered in a network of white lines called pseudocyphellae. Tiny breathing holes that give the lichen its characteristic, freckled appearance.
What makes it special is its ubiquity. The common grey shield lichen is everywhere. It grows on the trunks of fruit trees in suburban orchards. It grows on the branches of pōhutukawa along the coast. It grows on the rocks in the city park. It is the grey ghost of the bark. The living paint that covers the old wood. You have seen it a thousand times without knowing its name.
The underside of the lichen is black. Covered in tiny, root-like structures called rhizines. These anchor the lichen to the bark. Holding on through wind and rain. The margins of the lobes often have small, cup-like structures called soralia. Which produce powdery granules (soredia) that break off and grow into new lichens. It is a master of fragmentation. A plant that reproduces by crumbling. Scattering its dust across the bark.
Biologically, the common grey shield lichen is a partnership. A fungus and an alga living together. The fungus provides structure and protection. The alga provides food through photosynthesis. It is a self-contained ecosystem on the bark of a tree. A tiny world of cooperation and mutual dependence.
The common grey shield lichen is also a bioindicator. It is sensitive to air pollution. Particularly sulphur dioxide. In cities, it is less common near industrial areas. In clean air, it thrives. It is the canary on the bark. The one that tells us whether the air is healthy. Where the grey shield grows, the air is clean enough to breathe.
To find common grey shield lichen is to find the grey map on the bark. It is flat. Leafy. Wrinkled. A living painting on the trunk. It is the lichen of the familiar. The one you have seen a thousand times without knowing its name. It is the grey shield. The common one. The one that makes the old trees look older.