Species name: Quercus macrocarpa
Common name: bur oak (sometimes burr oak or even burl oak), mossycup oak
Location: Western University campus
Burr oak trees (that's how I spell it since that's how I was taught to spell it, but Bur oak is just as common a spelling) are native to the Carolinian forest of eastern North America, and are still present throughout much of their native range. Unlike most oak species, the burr oak does best in open areas like savannahs (depending on the area, they can sometimes be the only oak species in a traditional Oak Savannah habitat) and actually does rather poorly in forested areas. It is very shade-intolerant but grows very quickly, and these two characteristics have made it one of the most popular boulevard trees (or landscaping tree). The burr oak is also incredibly long-lived, with some individuals reaching ages of 350 years or more! That's one old tree!
Like most oak species, the burr oak is known for its masting. Masting is when a tree over-produces fruit in an attempt to overwhelm seed predators so that some of their offspring (fruits are the structures produced by plants to carry and protect their seeds, which are their babies) make it to the next generation. Burr oak seeds are some of the most prized nuts in the eyes of wildlife (like squirrels, deer, and in some places even bears), and they will selectively eat burr oak acorns over any other type of acorn. There are a few reasons why this occurs: first, the acorns are the largest of any oak species ("macrocarpa" means "big fruit"). Second, the acorn wall, or that tough shell you have to crack through to get to the yummy meat of the nut, is quite thin compared to other oak species so it's much easier to eat. Third, there are fewer tannis in the acorns of the burr oak, which make it much more palatable than other oak species. They are rumoured to even be choice edibles to humans, although I've never tried one. As long as you roast it a bit I could see them being at the very least edible, but tasty? I don't know if I'd go that far. I would make a terrible forager! When tree seeds are the most eaten food item of an animal species that has co-evolved with the tree, they tend to use really odd yearly cycles to over-produce fruits. These masting years, depending on the species, are every 7, 9, or 11 years. Our red oak in the front yard had a masting year a couple years ago. It sounded like it was raining acorns every time the wind lightly blew through the tree! It was incredible. The squirrels didn't know what to do with themselves :) To put it into perspective just how many burr oak acorns are eaten by wildlife, there are reports of certain trees in Tennessee and Kentucky in "bear country" near the Great Smokey Mountains having a 0% reproductive rate in some years (non-masting years, that is). When you set up video cameras to check out what's going on with these trees reproducing less than their neighbours, you catch bears climbing the trees and eating every single acorn they encounter. They'll even rip off branches or shake the trunk of the tree to get the ones higher than they can climb (bears are heavy, you know!), and then scramble down the tree to eat what dropped. Something tells me you don't want to come between a burr oak and a bear...
Unfortunately, the burr oak that I took a picture of and its nearest equally old neighbour are on their way out as far as tree lifetimes go. The third picture above shows where a branch broke off many, many years ago and a massive hole has formed in its place. I have no idea if the hole is occupied, but it's large enough to be an ideal place for a Great Horned Owl to build a nest. And those birds are huge! It would also be a good nesting place for a raccoon family, or the equivalent of a Hollywood mansion for a family of squirrels. The fourth picture shows where on that same tree the bark is starting to come off the tree. Unlike in my previous blog post about the butternut (which you can read HERE), this is not a direct result of a fungal infection. There is now likely a fungus that has exploited the tree's old age and exposed wood, but that bark fell off because the tree is just getting old and nearing the end of its natural life. The last picture shows where more bark is coming off the tree near the base, and you can see that it, at one point, was heavily infested with insects. Again, that's likely not what caused the bark to come off in the first place, but once the old rotting wood was exposed the insects took advantage. The tree is also likely hollow, because the old inner wood of trees is often decayed by fungi before the tree is completely dead; this is called heart rot. This also doesn't directly kill the tree, but rather creates a standing hollow tube. Doesn't take much of a wind storm to knock the tree down once the structural integrity is compromised. I personally have no idea how old this tree is (I might be "old" according to my students, but I'm not THAT old!), but I'm guessing upwards of 250 years. It will be a shame to see this tree come down in a storm, and I'm sure it's not that many more years before it does.
Aside from being some of the oldest trees that can be found around southern Ontario and the eastern half of the United States, burr oaks can also be some of the largest. They grow to be very, very tall trees but also grow to an enormous girth. It's hard to tell from the above pictures, but the burr oak I was taking pictures of is one of the largest trees on campus. To put it into perspective, I took a picture of the width of the tree at "breast height" (about 4 feet off the ground; this is traditionally a way to determine if a tree is ready for logging) with the arboretum label for size:
Still doesn't really look like much, but that arboretum label measures 2.5 by 5 inches (or about 6 x 14 centimetres). So if you figure out what the diameter of the tree is (by my rough estimation, 63 inches or 160 centimetres), that's...enormous. That would make it about 196 inches in circumference (or almost 5 meters!). You would need three people joining hands to comfortably circle the tree. That's bigger than enormous! There are two trees of this size that are burr oaks, and a couple hackberries (look for this species to be covered in my next blog), all in the same general location. Standing here looking at the trees, I often wonder what these trees have "seen". They would have survived the entire outwards growth of the city I live in, and would have been here long before London, Ontario actually became a city. I wonder who planted them? Or was this an acorn that sprouted there? If it was, where was the tree it came from? It would be long gone by now. Did anyone care for this tree when it was a wee seedling? If so, who were they? What did they do? Did the Kingsmill family, the ones that owned the property that Western University is built on for many generations before Western purchased all of the land in 1919, ever use this tree for a rope swing? When there were bears living in southwestern Ontario, did bears used to climb this tree to get the acorns? So many questions and no one left to ask...
When I helped Jane Bowles with an arboretum tour back in June, she said that the four trees in this small patch of campus were "some of" the largest, oldest trees on campus. I never thought to ask her where the biggest or the oldest trees were. I guess I'll have to go exploring to find them myself now (and learn a bit about how to determine the age of a really big tree depending on the species). Until then, I'll just have to be content with the geezer trees I know :)
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