If you walk along Pine Street in downtown Philadelphia, from 7th Street to 6th Street, you’ll see a number of trees planted there: various species, of various sizes and ages – with different kinds of leaves and bark, and different kinds of fruits and flowers and seeds – there’s a nice little bit of diversity along that city block, in a number of different kinds of ways, many of which are characteristics of diversity that are commonly noted when people look at trees in cities. But there is also another kind of variability in the trees on that block of Pine Street, a kind that we don’t often think of when we look at urban trees, and that variability is in the basal scars.
What are basal scars? Well, as Tom Wessels sagely defines them in his book Forest Forensics, they are “scars at the base of tree trunks created by the removal of bark from fire or some form of impact”. Basically, they are areas of a tree, towards the base, where the bark was once stripped away and you can see directly to the wood that had been underneath that bark, and the bark has grown up scar tissue around the sides of the wound.
Why does Tom Wessels define them in his book? Well, in Forest Forensics, as well as an earlier book of his, Reading the Forested Landscape, he discusses how basal scars can be created, and therefore how they (basal scars, that is) can be used to interpret prior land use history. For example, if there are trees growing on a slope, and there are basal scars on the uphill sides of the bases of those trees, this implies that there was a forest fire there at some point. Why is this? As Mr. Wessels explains, the uphill side of a tree trunk is an excellent tool for catching sticks and twigs and leaves as they tumble down a hill – to a botanist, these are plant parts, but to a forester, however, especially one interested in fire ecology, this is debris that is fuel for fires. And as this debris, this fuel, is piled up against the trunk, and moreso on the uphill side, then that uphill side will generally burn hotter than the other sides of the trunk – and possibly hot enough to burn away the bark, and then to leave a scar. Therefore, scars on the uphill sides of trees on a slope implies a forest fire at some point in their history.
A certain basal scar orientation can also imply logging – if there are basal scars that are facing each other, that is, if trees that face each other have basal scars directly across from each other, this implies a logging history. Why is this? Again, as Mr. Wessels explains, when loggers were dragging out logs from trees they had cut down, they would have taken them along logging roads, and since trees are frequently found in forests, one might expect that there would be trees on either side of those logging roads. And, since those cut logs would quite likely have bumped up against those trees on either side of a road, well, one would then expect logging to cause there to be basal scars on trees on either sides of logging roads, and therefore opposing basal scars are considered one line of evidence to support a history of logging in a forest stand.
OK, so on Pine Street between 6th and 7th Street, in downtown Philadelphia, there has not been logging any time recently, and so these basal scars are most likely not from some Philadelphian Paul Bunyan’s lugging of logs through the city streets. And, while there may well be house fires on that block with a reasonable frequency, those fires are unlikely to spill onto the street, and therefore unlikely to scar the trees there.
So what is it? What left those scars? Vandals? Construction workers as they build or mend houses and accidentally bump into the trees with their equipment? Cars as they drive down the street and occasionally veer onto the sidewalk? Car doors as they let people out? Errant bicyclists careening into arboreal blockades? Which of these is it?
Let’s look at the evidence…
As one walks east from 7th Street, towards 6th, one sees first a willow oak, and then another one – that first one has a pretty large scar on it, on the west side of the tree; the second does not have any major scarring. The next tree is an elm, somewhat small (less then 3″ dbh [=diameter at breast height]), with no scars, and then the next tree is a sweetgum, with a scar on the west side of the tree. This is followed (going eastward), by two Norway maples, both with no scars, and then these two are followed by a Norway maple with a scar on the street side of the trunk of the tree. This is followed by a ginkgo with a street side scar, and then finally, right up towards 6th Street, there is a sweet gum with a mild scar (not so strong that one can see through to the wood, but a noticeable change in the texture of the bark), and that light scar faces roughly westward, and is somewhat angled towards the street.
If we then walk back towards 7th, from 6th Street, the first two trees we see are sweet gums without any scarring, followed by a London plane with a pretty big scar facing towards the street, and another scar facing east. This eastward facing scar is at the end of a stumpy branch; it is not on the main trunk of the tree. After that somewhat mangled London plane, as we go westward now, there is a sweet gum without a scar, followed by a somewhat small London plane (less than 10″ dbh), also without scarring. The last two trees on the block, before we get to 7th Street, are both willow oaks, and neither have scars.
There is only one tree with scarring on the east side, and that one is the London Plane whose scar looks to have been derived from a branch having broken off (this is because the stump of that branch is still there, providing evidence of its own existence). All the other scars are either on the west sides of the tree trunks, or facing in to the street.
Of 16 trees on this block, none of them have damage directly on their trunks on their east sides, nor on the sides facing in to their respective sidewalks. Two of them have basal scars on their west sides, three of them have damage on the street side, and one of them has evidence of light damage at an angle between the street side and the west side.
Traffic goes east on Pine Street, and north on 7th Street – therefore, it makes sense to me to come to the conclusion that the westward facing scars are from cars running into the trees. The one eastward facing scar, the one that would have come from a broken branch, doesn’t disagree with this pattern, I think, and I’ll get back to that in a minute (depending on how fast you read, that is). The tree on the southeast corner of 7th and Pine would be especially vulnerable, as someone making a turn there could easily hop the sidewalk and run right into that willow oak there.
The street facing scars are most likely from car doors – and not just any car doors, but more often from passenger side car doors than drivers’ sides. Note that there are two street side scars (possibly three, if we count the faint one on the sweetgum) on the south side of the street, which is the right side (= passenger side) of the street in the direction one would be driving on this block, while there is only one street side scar on north side of the street, and that one on the north side is accompanied by evidence of a broken branch, and a relatively large branch it is (it’s over an inch across), which implies that the strike that did this had some force to it – perhaps more than an opening door would have caused, and perhaps more likely from it having been sideswiped by a car. This broken-branch line of evidence also suggests that, as I mentioned above, the eastward facing scar on this London plane doesn’t disagree with the idea that basal scars here are due to collisions with cars coming from the west – breaking off that branch could easily have happened from a collision from a car coming from the branch’s opposite side and snagging that low lying branch right off (it’s only a foot or so off the ground).
But back to the car doors for a moment – one would expect there to be more damage from passenger side doors than driver side doors simply because the driver can see out his-or-her side more easily than that of the passenger’s, and so would be more likely to avoid hitting a tree with their own door, while a passenger would be more likely to pop out the door and, inadvertently, hit a tree with it as it swings out. And so our evidence fits what makes sense, which doesn’t always happen, and so I like to point out when it does.
To summarize a bit here – the evidence strongly suggests that these basal scars are from cars colliding with the trees, and also from passenger side doors hitting them (hitting the trees, that is). If these scars were from vandals or sturdy bicyclists, I’d expect there to be scars on all sides of the trees. There are not. And if these scars were from construction or maintenance of the houses on this street leading to tree damage, I’d expect there to be scars on the sidewalk side of the trees. There are not. And so, it appears as though the main source of damage to these trees is cars.
This is only one city block of course, and it will be interesting to see if this pattern holds elsewhere. It also would be interesting to see if we can find evidence of traffic direction changes, etched in the scars of street trees – for example, if a street’s traffic direction changed, but its trees did not, we would expect to see some scarring on the older trees on the opposite side of current oncoming traffic. Or if there’s places where we might see evidence of a street getting changed from a two-way to a one-way street – older trees would mark the past all the way up to today, the current trees would only record the present and the more recent past. Or, looking forward, as more bike lanes are put in place, for example as we now have in the south lane of Pine Street (including between 6th Street and 7th Street, the area we’re talking about here), maybe there will be fewer passenger side door bumpings with trees, and we’ll see a reduction in street side basal scars where bike lanes are put in. Or perhaps we won’t. One never knows, but eventually we will, if we keep on looking.