Dead and dying trees are one of the most exciting parts of any landscape. Like a star in supernova, trees may persist for decades or even centuries before they are consumed in a flurry of activity. Of course this analogy brings to mind some of the wildfires that happen each year. The destructive power of fire in a dry, fuel-laden forest is both terrifying and exhilarating, the ratio of which probably determined by one’s distance from the fire and the skills and tools available to control the flames.
Trees burn so well because they are dense collections of carbon-rich molecules, such as cellulose, that are created, year after year as the tree uses energy from the sun and carbon dioxide from the air to construct its architectural matrix. When terrestrial plants first arose, they could simply cover rocks and gather all the sunlight they needed. But, with the advent of stems and leaves, some plants were able to outcompete others for sunlight and so a cold war of sorts began. The individual with the largest leaves gathered the most sunlight while shading out the other plants. When one kind of plant grew taller, it was more successful. But at some point, gravity and wind would pull it back to the ground-- at best, making the race start again for that plant, at worst, killing it. But with the development of woody tissue plants could get taller and taller while withstanding the effects of gravity and wind as well as climbing vines, perching birds, and our clamoring primate ancestors.
This cellulose is tough and not very nutritive. Termites famously being one of the few animals that can, through their symbiosis with gut bacteria, eat it. Some species further protect their wood with a variety of toxins. The volatile oils found in eucalyptus are especially interesting—they contribute to the hot/cold feeling of products like Vicks and they are explosive when burned. Most animals that appear to eat wood actually only consume a thin layer of the plant found just beneath the bark called the phloem. This goes for rabbits, deer, voles, even beaver. Many wood-eating insects are also only found in the phloem, too, most infamously, the Asian long-horned beetle and the emerald ash borer. The wood portion of a tree is useless to most organisms.
Of course you can see all sorts of animals in a forest but dead and dying trees are rare, at least in a healthy forest, so there are many species that utilize them more frequently than a given living tree, dead trees called snags are like the corner bodega where it seems everybody stops briefly during the day. Lack of leaves makes snags perfect perches for birds of prey. Even more importantly, as water, ice, fungi, and bacteria invade and soften the wood, that once tough and useless superstructure becomes accessible to many more species.
Woodpeckers are the most famous cavity nesting birds. They have a range of special adaptations that allow them to be among the first animals to take him take advantage of softwood: toes and claws that can grip vertical surfaces, stiff tail feathers that help form a tripod while they’re excavating, chisel shaped beaks, tendon strengthened for necks, and shockproof brains. Less specialized birds like nuthatches also excavate dead wood and there are a lot of species that either modify existing cavities or just move in to whatever is available. As the holes grow in size, they allow more water in which promotes fungal growth. Many species of fungus only grow on the decaying wood of certain species of tree. Soft wood also allows insects like carpenter ants to construct galleries to live in and raise their brood. (They don’t eat wood, they just remove it to create living space.) This soft, punky wood also provides good bedding for animals as diverse as mice and queen hornets.
All this leads me to the elm that I stopped at this morning. The tree is just 100 yards north of the Nature Museum along Cannon drive, south of a park bench. It’s mostly dead but still has a good quantity of sap within it. This sap is slowly oozing out nutritious sugary gobs of elm candy which are being eagerly consumed by many animals. Although these gobs are the elm equivalent of maple syrup at first, natural yeast in the air have also begun fermenting it. Standing beneath the tree, you can smell warm, beer-like yeasty-ness. This betrays the presence of alcohol, as well as sugar.
Here’s a list of the animals I’ve seen on the tree just today:
Birds: starlings, robins, barn swallows, flickers, English house sparrows, a downy woodpecker, and a crow
Mammals: I only saw one grey squirrel in the tree, and he was there simply because a dog was chasing him. I suspect squirrels will avoid the tree for some time due to some of the insects that are so prevalent. I wouldn’t be surprised to see some rats eating the cicadas at the base in the evening though.
Arthropods: Here’s where you hit the jackpot. 2 species of ant, cicada exoskeletons and adult cicadas, moth egg masses, spider sacs, some cocoons (one with a dead pupae inside), flesh flies, picture winged flies, and house flies. The occasional butterfly flits past: red spotted purple, questionmark, red admiral, and painted lady. The coolest part though, are the hymenoptera galore! Of course the cicada killers are the most obvious, but there are also honeybees, bumblebees, yellowjackets, bald-faced hornets, and even an ichneumon.
There are lots of things I expect to see at the tree but have not seen there yet. Since the tree is an elm and is just dying this season, it should be able to stand safely for many years. It will be interesting to see all of the diversity that uses this tree over the next few years and seasons. I hope you’ll take the time to visit the elm and tell us what you have seen there, too.