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Cracked Cap Polypore - Fulvifomes robiniae

Good evening, friends, and happy Lunar New Year. The year of the rabbit.

This week's mushroom is the Cracked Cap Polypore (Fulvifomes robiniae, formerly known as Phellinus robiniae). This is one of the first mushrooms I ever learned, and it's a great one to be able to nail down because it's basically the only mushroom, and certainly the most conspicuous, that you'll see growing on black locust trees (Pseudoacacia robiniae). Today we'll look at how to identify a black locust tree, how to identify the cracked cap polypore, and the relationship between the two.

Fulvifomes robiniae

One of my first forays into learning about the natural world was when I was volunteering at the Audubon in Connecticut. I remember my mentor there, Andy, pointed into the forest and said, "you see those trees with the deep furrowed bark, those are black locust, and they're not native to here but they're wood is great for making fence posts". It was February and all the trees looked the same to me, their bark not withstanding. In an attempt to prevent that from happening today, I went out this morning to try and photograph that unique bark that was invisible to me just six years ago. Below, the black locust is in the foreground on the right and you can compare it to the chalky, layered bark of a black cherry (Prunus serotina) further back on the left. They're two of the most common trees in Central Park and the tri-state area.

Fulvifomes robiniae

Notice all the vines (oriental bittersweet and japanese honeysuckle) which, in addition to the black locust, are also not native to the area. This part of the preserve in Manitou is located by train tracks and has historically been mined (so lots of ecological/soil disturbance over the past couple centuries). This type of disturbance usually suits non-native species, and the black locust thrives in spoil areas that were logged, stripped for mining, or cleared for roads. On the one hand, this human disturbance creates an imbalance which favors invasive species, but on the other-hand these species are the first on hand to help heal the earth. The locust tree has a symbiosis with bacteria that allows it to take nitrogen in the area and make it bioavailable in the soil - providing a vital nutrient for all other plants and microbes.

In my hand below is another distinguishing characteristic of the black locust. Those small spines - almost like little devil horns - are almost always found at the nodes (where branches form) on saplings. As with anything in nature, there are exceptions to the rule and some trees don't have these spines. Learning the bark and the buds, both rather distinct on the black locust, is something you can do this time of the year because you're not relying on seasonal qualities like leaves and seed pods.

Fulvifomes robiniae

In my opinion, the best way to learn any tree, plant, mushroom, or perhaps anything in general, is repetition. Once you've identified your tree (or mushroom, plant, etc.), try to see it daily, and then try to find it somewhere else. You'll pick up on the intricacies and differences of that organism relative to similar species and you'll eventually come to learn that the black locust does, in fact, have deeply ridged bark. In case you are curious, though, check out this video which gives a succinct look at all the unique characteristics of the black locust in just over 120 seconds.

Before moving on, I just wanted to look at the "native range" of the black locust, since I've mentioned multiple times that it is not native. The map below, from the US Forest Service (Reference 4) shows the tree is originally from mid-Appalachia and a few pockets in the Ozark and Ouachita Mountains. The tree has been planted for timber due to it's durable qualities and this has helped expand it's range. Not only has it been introduced to the northeastern US, where it grows in disturbed areas like the one we saw above, but it can also be found in Europe, Asia, Africa, and Australia.


Now, finally, onto the mushroom. Can you tell the weather was inclement today and I had more time to write? The mushroom is appropriately named "cracked cap" because of it's cap (pileus) that is fuzzy when young but turns woody, gnarled, and cracked with age - don't we all. This thing is a rock, it's basically as dense as the wood from which it grows. The mushroom is perennial and persists on the tree year after year. It simply puts a fresh pore layer on each spring/summer. What makes this neat is that you can then see years of growth if you're strong enough to break off a chunk (I'm very gentle and tender so the picture I used below is Gary Emberger's photo from Reference 2). If you look closely you can count 10+ years of growth in those horizontal lines - each delineating a new layer of pores and another year of growth.

Lines shown through years of growth

Moss, algae, and lichens will begin to colonize the cap of older mushrooms like the one below that can be found on a black locust atop Rota's rock in the Ramble of Central Park. Not only is the mushroom impressive for it's longevity, the black locust host is known for it's anti-fungal properties which don't seem to inhibit our F. robiniae. In a study from NC State (Reference 5), extracts from the tree's heartwood, bark, and leaves all inhibited the growth of another common northeastern mushroom, Trametes versicolor (Turkey Tail).

Fulvifomes robiniae

Alright, just a couple more mushroom facts and we'll get out of here. The fungus is saprobic, decomposing dead locust trees, but also parasitic in that it will actively decay the heartwood of living trees. They get rather large in size and can expand to half a meter in width. They share the same range as their host tree, and can subsequently have been found everywhere the black locust has been introduced. The mushroom produces rusty brown spores which you can see staining the wood below.

Fulvifomes robiniae

Another week, another interesting ecological relationship. Thank you for reading,



1) Kuo, M. (2010, February). Phellinus robiniae. Retrieved from the MushroomExpert.Com Web site:


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