The second life of a dead tree

Sometime in the past six or eight decades, a carbon dioxide molecule entered one of the stomates on a leaf of a paper birch across the meadow outside our dining room window. Once inside, a photon from the sun split the carbon from the two oxygens and sent the carbon onward into the green machinery of the cell. There, it was bound with five other carbon, twelve hydrogen, and six entirely different oxygen atoms into a glucose molecule. This molecule joined millions of others in the great river of sap flowing to the rest of the tree where they were linked into the long chains of cellulose and the great sheets of lignin comprising most of the wood and the bark.

A few years ago, this tree died and became what foresters call a snag, from the Norwegian snage or Icelandic snagi, “a sharpened spike”. While drinking my coffee at breakfast, I’ve been watching how new food chains assemble themselves from the remains of this tree, and thinking about the diaspora of its carbon atoms. Like any dead plant, half the weight of the snag is composed of carbon. That’s a lot of carbon atoms.

Sapsuckers discovered the birch even as it was in the process of dying, stitching parallel lines of tiny matchstick-sized holes around its circumference, slurping the glucose-rich sap oozing from the holes. This loss of sap alone could have killed the tree. But insects entered the holes and, if the sap flow did not flush them out, began laying thousands of eggs which quickly develop into larvae. These larvae grew and molted, grew some more and molted again, all the while boring through the cambium beneath the bark and leaving behind galleries of tunnels on the surface of the wood.

A year or two later, teal green and yellow ochre lichens began expanding, slowly and patiently but remorselessly, across the chalky white bark, looking like the jeweled crusts on a Faberge egg. Each species of lichen is a unique symbiosis of an algae and a fungus. Sending its hyphae into the bark and wood, the fungal partner separates the carbon from the nutrients there; it then transfers the nutrients onto the algal partner and sends the carbon back into the atmosphere as carbon dioxide. The algal partner, like any live tree, captures carbon dioxide from the atmosphere and turns it into simple sugars which it shares with the fungus. How much of this carbon dioxide was previously released by the fungus? As far as I am aware, this is an open question. If the answer is “almost all”, then the lichens are nearly closed systems where the carbon dioxide released by a fungal cell would be almost immediately taken up by an adjacent algal cell.

Fomes conkThe fruiting body of another species of fungus, Fomes fomentarious, is swelling like a horse’s hoof from the surface of the bark. This Fomes may have begun life as a parasite on the living tree, perhaps when a spore entered one of the sapsucker holes. Now, it is continuing its life as a decomposer, sending its hyphae deeper into the dead wood, breaking the snag’s cellulose into smaller glucose molecules which it then takes up and rejoins into the cellulose in its own woody body.

The snag is falling piece by piece down to the ground. Downy and hairy woodpeckers followed the sapsuckers, punching larger round holes and scattering splinters of wood onto the forest floor while searching for the larvae. These holes make the snag a sponge for rainwater. One spring, a pileated woodpecker patiently began chiseling a rectangular cavity the size of several playing cards just beneath a fork in the trunk, casting large wood chips down to the base of the snag. Later, two chickadees began cleaning out the cavity for their nest. But the snag at the fork was already so weakened that it could not withstand any further loss of support. One day, one branch of the fork broke and fell. Two gentle chickadees, responsible for the delivery of a big chunk of woody debris to the forest floor.

On the forest floor, a staggering diversity of microbes, fungi, and insects are consuming the wood, building their bodies from some of its carbon and respiring the rest as carbon dioxide. In Sweden, about 50 species of fungi and close to 100 species of beetles alone have been found in birch, aspen, and spruce snags and downed logs. As much as 20% of the beetles are listed on the International Union for Conservation of Nature’s Red List of Threatened Species. Although the number of fungal and beetle species occupying any one snag or log is much lower, usually fewer than 10 or 20, this means that each snag and log has its own distinctive suite of species. Fungal diversity peaks in the intermediate stages of decay but the beetle diversity increases continually into the more advanced stages. The more fungal species in any one snag or log, the more beetle species are there also. Is each beetle consuming a particular fungus so that a greater variety of fungi provides food for a greater variety of beetles? No one seems to know: we don’t know enough about the natural history of each beetle and fungal species to answer this question.

Birch bark is full of terpenes and tar which are not as easily broken down by microbes and fungi as the softer wood, so the bark becomes transformed into nearly hollow white pipes as the wood within decays. Red-backed voles (Clethrionomys gapperi) use these hollow logs as shelter and also prey upon the insects and the fungi consuming the wood. Fungi and insects in the log are way stations in the food chain, transferring the carbon from the birch tree to the furry bodies of voles and other small mammals.

The woodpeckers and voles create a big and important shift in the food chain. The carbon in the insects and fungi in the wood comes exclusively from the birch tree, but the woodpeckers and voles also forage away from the snag and log, so some of the carbon in their diet comes from prey living elsewhere. Carbon atoms from many sources are converging along the strands of numerous food chains into the bodies of voles and woodpeckers. The voles on the ground and the woodpeckers above them on the snag are nodes where several food chains are woven into a food web. In every food web, there is at least one species that weaves it together from several food chains by mixing carbon from several sources in its diet. If the populations of these species decline, the web begins to unravel.

When some voles leave the shelter of the log, they might be captured by great grey owls or boreal owls. The carbon atoms in the vole bodies, including some from the birch tree, will be carried to a tree some distance away where the owl roosts. There, as the owl rips the small bodies apart with its hooked beak, some of that carbon will become owl flesh, some will be respired through the owl as carbon dioxide, and the carbon in the vole’s hair will drop to the forest floor in the owl’s pellets to be decayed by a different soil food web. At each of these steps, the carbon atoms from the birch are diluted further and further as they are mixed with carbon from other sources in the bodies of voles, mice, owls, soil microbes. Finally, the carbon will be incorporated into soil humus, where it can reside for 500 years or more before being respired by microbes and fungi into the atmosphere.

So, after many centuries, all the carbon taken up by the birch tree when it was alive will be evaporated through many food webs into the atmosphere. On average, each carbon atom will remain in the atmosphere for 200 years or so, enough time for the winds to transport it anywhere on earth, making the atmosphere the ultimate mixing bowl for carbon.

A few centuries from now, a carbon dioxide molecule emanating from these food webs might be wafted far from the birch outside my window to the Antarctic Ocean, where it will be captured by a wave of ice cold water. Once dissolved into the water, it will be taken up by a diatom, which will then be eaten by a krill passing by, which will then become trapped in the baleen of a blue whale, which then sends it into her stomach with a swipe of her massive tongue. The carbon atom will make its way into a molecule of fat in the rich milk which the whale feeds to her calf, who will in a short while defecate it into the sea where microbes will release it again into the atmosphere.

The flow of carbon weaves together a tapestry of food webs that covers the planet.

For Further Reading

Bowman, J.C., D. Sleep, G.J. Forbes, and M. Edwards. 2000. The association of small mammals with coarse woody debris at log and stand scales. Forest Ecology and Management 129: 119-124.

Franklin, J.F., H.H. Shugart, and M.E. Harmon. 1987. Tree death as an ecological process. BioScience 37: 550-556.

Harmon, M.E., J.F. Franklin, F.J. Swanson, P. Sollins, S.V. Gregory, J. D. Lattin, N.H. Anderson, S.P. Cline, N.G. Aumen,J.R. Sedell, G.W. Lienkaemper, K. Cromack Jr., and K.W. Cummins. 1986. Ecology of coarse woody debris in temperate ecosystems. Advances in Ecological Ressearch 15: 133 – 302.

Hjältén, J., F. Stenbacka, and J. Andersson. 2010. Saproxylic beetle assemblages on low stumps, high stumps, and logs: Implications for environmental effects of stump harvesting. Forest Ecology and Management 260: 1149-1155.

Lindhe, A. and Å. Lindelöw. 2004. Cut high stumps of spruce, birch, aspen, and oak as breeding substrates for saproxylic beetles. Forest Ecology and Management 203: 1-20.

Lindhe, A., N. Åsenblad, and H-G. Toresson. 2004. Cut logs and high stumps of spruce, birch, aspen, and oak – nine years of saproxylic fungi succession. Biological Conservation 119: 443-454.

Smith, C.Y., I.G. Warkentin, and M.T. Moroni. 2008. Snag availability for cavity nesters across a chronosequence of post-harvest landscapes in western Newfoundland. Forest Ecology and Management 256: 641-647.

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