Fungal Jubilation in the Death of a Pine

© Else C. Vellinga
Lab
Original publication: Mycena News, January 2004

The big, old Monterey pine at the corner of our street has slowly died this year. The first signs of upcoming death were clearly visible in March, when the central crown showed signs of a beetle infestation and started to brown; a brownness which spread through all the branches and needles over the summer. By September, when the beetles had killed the tree, the main stem and the bigger branches became covered with small yellowish bumps: Cryptoporus volvatus, the pouch fungus or veiled polypore. There were as many as 50 fruitbodies per linear foot on the trunk and branches. On the entire tree there must have been several thousand. Now, in mid-December, the ephemeral fruitbodies have already disappeared from the trunk, but linger on some of the branches. Last year, there were no indications of impending doom, and I don’t know why the tree was vulnerable to the beetles’ attack. Fire weakened trees are notorious for bursting out in this mycological rash, the fire having made them beetleprone. But, fortunately there has not been a fire on our block!

Arora’s comments on Cryptoporus in Mushrooms Demystified are a beautiful example of his writing:

This bizarre evolutionary anomaly looks like a cross between a confused puffball and a bemused oak gall. The smooth, warmly tanned exterior is quite attractive (often reminding me of a small loaf of bread) and gives no hint of the tube layer within. Slicing it open, however, reveals a hollow interior with a “ceiling” of tubes. The “floor” eventually ruptures and tiny bark-boring beetles enter the “trap door” in search of tasty tube tissue and spores. After feasting they depart to construct brood tunnels in old or dying conifers, and the spores they carry with them gain entry to a new host. Later, fruiting bodies may emerge through the very holes bored by the beetles!

One of the questions I had, when seeing an entire tree covered with fruitbodies, was whether they arose from just one fungal individual which had entered the tree somewhere and then raced its hyphae through the inside of the bark, emerging in multiple places with many fruitbodies, or whether every fruitbody belonged to a separate individual, meaning that lots of spores had germinated all over the tree. I also wondered, how do the spores move from one dead tree to another, are the spores carried around by beetles, and if so, are those the same species of beetles that kill the trees? And of course, I wondered what the fungus was doing to the tree, and how extensive its mycelium was.

As always, it is easier to ask questions than to answer them. Here are some attempts.

Already in 1892, it was suggested that insects play a role in the life history of the pouch fungus, as lots of species were observed nesting and crawling around in the annual, leathery fruitbodies. But for successful transport, these insects should be within the fungus during sporulation, leave the fungus as winged adults (in other words, nesting insects don’t qualify), fly far to infect new trees, and be associated with trees attacked by bark beetles. That rules out most species; in a study in British Columbia, the only one left was a predator of the bark beetles. However, an elegant study in the late ‘70s revealed a second dispersal mechanism. This study looked at the number of spores released through the ruptured hole in the pouch, and found that, just as in all polypores, masses of spores were released – on average 4.5 billion spores per fruitbody! – and all these billions were left free to wander with the air currents. However, spores were also isolated from flying and tunneling Douglas-fir beetles, which do not live in Cryptoporus and were free of fungal spores prior to flight. Furthermore, the fruitbodies appeared through the holes the beetles made in the target trees: entrance, exit, or ventilating holes were all exploited. Whether the spores arrive internally or externally doesn’t seem to matter so long as their host penetrates the bark. It is rather unlikely that an ambient spore can penetrate the bark on its own.

In any case, Arora’s “floor” serves more as a way to keep the sporulating surface moist, than to keep the insects in the pouch until the spores are ripe. It is rather surprising that the fruitbodies appear in the summer when moisture is often scarce.

The evidence points to one conclusion: many spores landing on one tree, and producing many small individuals, rather than a single big one. Circumstantial evidence from a study of another early colonizer of dead wood, Trichaptum abietinum, bears this out. On one tree stump lots of individuals were found, but there was no genetic structuring of the local population, indicating that dispersal over great distances is the rule. The wind is in this case the most obvious vector; those Douglas-fir beetles did not carry Trichaptum spores because this species sporulates much later in the year than the beetles take wing. Trichaptum abietinum is found all over the Northern Hemisphere, but oceans impose barriers since the populations on the different continents cannot interbreed when put together on a petri dish. Cryptoporus volvatus is less widespread, it occurs in North America and east Asia , but has not been found in Europe.

Back to my last question – what is the fungus doing to the tree? Trees slowly and carefully build up a beautiful strong woody trunk, and successfully resist most organisms that threaten this structure. But as soon as the tree dies, its active resistance ceases and the wood is fair game. Wood is still wood though, and its structure is not easy to attack. There is lots of carbon, but it is sequestered in complicated and not easily available compounds like lignin and cellulose, and there are adverse conditions with little nitrogen, low oxygen levels, and nasty resins and other compounds. Nevertheless, many fungal species are adapted to the task of decomposition and there are specialists for different parts of the wood. Cryptoporus volvatus only attacks and breaks down the cellulose and hemicellulose of cell walls in the outer few centimeters of the trunk, in the socalled sapwood; this leaves the lignin intact, as well as the inner part of the branches and trunk (the heartwood). As soon as the tree falls, other species get to work.

The extremely common red-belted polypore, Fomitopsis pinicola, is often an early colonizer in the process, but again it cannot do anything to the lignin. The Red-belted polypore, just like Trichaptum abietinum, is an outcrosser, and populations that are far apart can be quite similar, genetically speaking. Wind is its main vector for dispersal, and again the spores have been found on flying bark beetles and in their tunnels. It causes the remains of the tree to crumble into brown, cubical chunks of lignin shell and specialized lignin decomposers have to finish them off.

What will be next on the neighbor’s pine depends on many things; the place where it will end up, how big the pieces are, and of course which fungi arrive there first. But in the end, nothing visible will remain, and the tree’s components will have served many different organisms and communities, and eventually, somewhere, a new tree will use them to start the cycle anew.