Mossy Maple Polypore

The Mossy Maple Polypore (Oxyporus populinus) is a modest-looking perennial bracket that most people notice not because of the fungus but because of what is growing on it. The upper surface is rough and moisture-retentive, and it is almost always colonised by green algae and by true mosses, so a cluster of brackets can look like a small green shelf garden protruding from a tree wound.

The genus Oxyporus is characterised by white, soft-fibrous to corky flesh and very fine white pores, and this species is the most frequently encountered member in the temperate Northern Hemisphere. Fruit bodies are perennial, adding a new tube layer each year, and they grow slowly, so a large cluster may represent many seasons.

It is a fungus of living trees, particularly maples, colonising through wounds, branch scars and old pruning cuts. Because it fruits on the trunk of an outwardly sound tree, it is a familiar sight in parks and street plantings as well as in natural woodland.

Cap

Perennial, small, 2-8 cm across and 1-3 cm thick, forming semicircular to shelf-like brackets that are almost always fused into overlapping clusters or tiers along a wound. The upper surface is white to cream when very young, quickly becoming buff, grey and rough, and typically covered with green algae and a dense growth of moss. The margin is thick, blunt and white while actively growing.

Pores

The pore surface is white to cream, ageing pale buff, and it does not stain appreciably when bruised. The pores are extremely fine, roughly 5-7 per millimetre, so the surface looks smooth and almost porcelain-like to the naked eye. Because the fruit body is perennial, a vertical cut reveals several stacked tube layers, often separated by thin bands of white flesh.

Flesh and spore print

The flesh is white to cream, soft-fibrous when fresh and becoming firm and corky, and it is clearly demarcated from the layered tubes below. It is homogeneous and unzoned. The spore print is white.

The Mossy Maple Polypore fruits on living hardwood trees, above all maples, and also on beech, poplar, ash, elm, lime and horse chestnut. Fruit bodies emerge from wounds, branch stubs, frost cracks, old pruning cuts and the crotches between major limbs, usually within a few metres of the ground, and clusters build up in the same spot year after year.

It is widespread throughout Europe, temperate Asia and North America and is common in urban parks, avenues and gardens as well as in natural woodland. Because the brackets are perennial they can be found at any time of year, though the white growing margin and fresh pore surface are most evident during the summer growing season.

Oxyporus populinus causes a white rot of the heartwood of living hardwoods. Mycelium enters through bark wounds and colonises the central non-living wood, degrading lignin and cellulose and leaving the heartwood pale, soft and spongy while the surrounding sapwood continues to function.

Infected trees can survive for many decades with a progressively hollowing trunk, and the fungus is one of the principal agents that turn mature maples into veteran hollow trees. Those hollows, and the soft decayed wood inside them, are the foundation of a whole community of cavity-nesting birds, roosting bats, and specialised invertebrates. The mossy caps themselves add a further microhabitat, holding moisture and supporting mosses, algae, tardigrades and mites on the trunk face.

The Mossy Maze Polypore (Cerrena unicolor) also carries algae and moss on a hairy cap, but it is annual, thinner and more flexible, has a maze-like to toothed greyish underside rather than very fine round white pores, and shows a thin black line beneath the cap surface in section.

Trametes species such as Turkey Tail may become algal green on old caps but are thin, leathery, strongly zoned and grow on dead wood rather than from wounds on living trunks. Rigidoporus and Perenniporia species have similarly fine pale pores but differ in habit and in the absence of a perennial mossy cap cluster. Fomes and Ganoderma brackets on living hardwoods are much larger, with hard crusted caps and thick woody flesh, and their pore surfaces are far coarser or bruise brown. When in doubt, look for the stacked tube layers plus fine white pores plus moss-covered top on a living maple: that combination is diagnostic.