Why do trees break in Louisville during ice storms? We asked an expert
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After the torrential rain, the brutal freeze, the snow, the ice and the sun blips of the last two weeks, it’s hard to remember that it wasn't that long ago that we experienced the first of the ice storms of the season. It wasn’t much — just about one-quarter-inch of freezing rain on the power lines and tree branches, but it was a doozy just the same. Power lines down, branches crashing onto parked cars. ... I’m always amazed by what that little bit of ice can do to disrupt our 21st century way of life.
So the obvious question is: Why so much damage from what amounts to just an overly healthy dose of Jack Frost? Why do some tree species come through with shining colors while others are turned unceremoniously into large piles of future wood chips? The answer is both wood strength and tree architecture.
Wood strength is pretty straightforward. The faster they grow, the faster they come crashing down. Fast-growing species produce wood that is significantly less dense. The torque produced by wind, snow and ice loads on a branch generates a tremendous strain, and when that strain exceeds the strength of the wood, we all know what happens.
Fast-growing and soft-wooded species include our friends the pines (Pinus species) and many other conifers (which is why we sometimes refer to the group as “softwoods”), the silver maples (Acer saccharinum), the Callery pears (Pyrus calleryana, including "Bradford") and the dreaded Siberian elm (Ulmus pumila.) We saw plenty of their branches on the ground after the ice storm.
But beyond wood structure, it is wood health that can also impact branch strength. Any internal decay, no matter the tree species and how strong its wood, can drastically reduce load tolerance and lead to branch failure. Of course, the biggest problem with internal decay is that it’s, well, internal and hard to see, much less assess.
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But tree/branch architecture can lessen or intensify the wood strength calculation. We all know the problem with our much maligned "Bradford" pear. Aside from weak wood, it produces characteristically narrow branch angles that form what’s called included bark. What that means is that as the trunk and branch increase in diameter over the years, the very narrow angle between the two is so sharp that bark gets caught in between.
Instead of a big old branch having a big old chunk of continuous wood connecting branch to trunk, there’s precious little actual connecting wood and much more separation by bark. The result is a very week union, one that is susceptible to breakage.
Conversely, a big old white oak with branches that emerge at almost 90-degree angles from the trunk can have huge areas of continuous wood and as a result, can support literally thousands of pounds of weight.
But there’s more to tree architecture. Look at a nicely shaped white pine after an ice storm. Rarely do you see the short, top branches broken. But down low is where you normally see damage on the longer branches. The reason? It’s pretty simple when you think about some basic physics.
A 30-foot-long branch weighs a whole lot more than a 1-foot branch. A 30-foot-long branch also catches a whole lot more ice than a 1-inch branch. The combined weight can be mind-boggling. Take a mature oak tree that might have up to about 20,000 square feet of branch surface area. Now coat that whole surface with one-quarter-inch of ice. The result is more than 3,500 pounds of water clinging to the branches — or something akin to parking your Ford F150 pick-up truck up in the tree!
And the physics gets scarier from there. According to my physics research department (aka my astrophysics Ph.D. candidate son), if you have 30 pounds of ice on a branch that is 1-foot long and you move that ice instead out to the end of a 30-foot-long branch, the force exerted by the 30 pounds of ice is more like 900 pounds! (Actually, according to that same research department the force remains the same. It is the torque that increases with distance — for you physics-minded folks out there who are still paying attention.)
And to make matters potentially much worse, a full crop of leaves about doubles the surface area of a mature tree. If we get one of those ice storms while the trees are still in full leaf, you can park both “his” and “hers” pick-ups in the tree hanging over your house.
Yew Dell Botanical Gardens, 6220 Old Lagrange Road, yewdellgardens.org.