Mossy Maple Polypore Identification Guide

Where You Meet It

This is a polypore you find by looking at trunks rather than at the ground. Walk a maple-rich woodland, an old street planting, or a park with mature broadleaves, and check every wound: frost cracks, pruning scars, storm-broken branch stubs, and especially the deep crotches where two big limbs part. A metre or two above eye level you will see a short row of small, overlapping shelves growing straight out of the injured bark, and their upper surfaces will be furred with green moss. That moss cap is the reason for the English name and it is usually visible before any fungal detail is.

The fungus is perennial, so the same wound produces the same cluster season after season, each year adding a new layer. Over time the brackets thicken, fuse laterally, and can form a continuous rippled crust running along the seam of an old crack. Because it is perennial you can find it in any month, which makes it one of the few polypores that is genuinely a year-round record.

The Upper Surface

Individual brackets are small, usually 1 to 5 cm across and up to about 10 cm on old fused masses, and 1 to 3 cm thick. They are sessile and very broadly attached, semicircular to shelving, and frequently tiered in overlapping ranks. When freshly grown the surface is white to ivory and finely felted; it dulls quickly to buff, then greyish, then dingy. Almost always the older, back part of the bracket carries a plush of green moss and algae, so the shelf looks like a small mossy ledge glued to the tree. The advancing margin is thick, blunt and pale, and in the growing season it stands out as a clean white rim beyond the mossy part.

The Pore Surface

Turn a bracket over, or crouch and look up at it, and the underside looks like smooth white porcelain. That impression is diagnostic: the pores are so small, about 5 to 8 per millimetre, that the naked eye cannot resolve them. Put a hand lens on it and a dense field of tiny round to angular mouths appears. Colour is white to ivory when fresh, ageing to pale straw or dull buff, and the surface does not bruise to any striking colour when pressed. If you can see the pores clearly without magnification, you are almost certainly not looking at this species.

Section, Flesh and Tube Layers

Cut a bracket vertically with a strong knife. Two things matter. First, the context is white to very pale cream, tough and fibrous, somewhere between corky and firm-elastic, and it does not change colour when exposed. Second, because the fungus is perennial, the tube layer is stratified: you will see several stacked bands of tubes, each 1 to 4 mm deep, often separated by thin sheets of white context. Counting those bands gives a rough age for the cluster.

Just as important is what you will not see. There is no thin black line separating the upper felted layer from the context, which immediately removes several common shelf fungi from consideration. The flesh is unzoned or at most very faintly zoned, and the odour is mild and unremarkable.

Getting a Spore Print

Perennial polypores are slow print subjects. Take a fresh, actively growing bracket with a clean white margin, cut it free, and set it pore-surface-down on dark card inside a covered box with a damp tissue for humidity. Leave it overnight and preferably a full day. The deposit is white and often sparse. Under a microscope the spores are broadly ellipsoid to almost globose, smooth, colourless, and very small, around 3.5 to 4.5 by 3 to 4 µm. Old weathered brackets outside the growing season may drop nothing at all.

Substrate and Ecology

Oxyporus populinus is a wound coloniser and heart-rot agent of living broadleaved trees. Maples are its classic hosts, sugar maple and Norway maple above all, but it also occurs on beech, basswood, ash, poplar, box elder and horse chestnut. It causes a white rot of the heartwood while the tree carries on growing around it, which is why the brackets emerge from wounds and crotches rather than from dead down timber. Finding it is a reliable sign of internal decay in an otherwise leafy tree.

Separating It From Look-alikes on Structure Alone

Trametes versicolor and relatives are annual, thin, leathery and strongly zoned in contrasting bands, their pores are far coarser at 3 to 5 per millimetre, and they colonise dead wood rather than living wounds.

Trametes hirsuta is greyish and coarsely hairy on top and, decisively, shows a thin black line beneath the hairy layer in section; its pores are also much larger.

Cerrena unicolor is also frequently moss-covered, but it is thin and annual, its pore surface is grey and maze-like rather than white and even, and it too has the black line under the tomentum.

Ganoderma applanatum has a brown context, a pore surface that bruises brown when scratched, and a copious brown spore deposit that dusts everything below it.

Postia and Tyromyces species are soft, watery and annual, with much larger pores and flesh that discolours when handled.

Trichaptum and Irpex species break down into teeth or flattened spines rather than keeping a smooth, uniform pore field.

In practice, the trio of moss-topped tiers, a pore surface too fine to resolve by eye, and a wound on a living maple is conclusive.