It was a calm Monday evening. The
sun was riding low over Pomona College’s Frary Dining Hall as people sat outside
enjoying their dinner in the warm twilight. Then, without warning, a long,
rolling groan filled the air. It lasted about a second or two and was followed
by a sharp crack as the massive California sycamore above the outdoor patio area fell suddenly on the diners below. Luckily, the
people eating outside recognized the imminent danger, and most were able to
escape before the tree toppled.
Surveying
the leftover wreckage that evening—including a flattened wooden table and a
smashed concrete wall—I couldn’t help but wonder what had caused this
misfortune. What could possibly cause such an enormous tree to literally uproot
and drop to the ground? What’s the science behind tree stability? Could we have
foreseen this before it was too late?
While
I’m admittedly no expert on trees or tree science, I did some research on the
issue, so I hope I can inform and enlighten at least to some degree. As it
turns out, what happened to the California sycamore outside Frary is not
necessarily an uncommon event. It is important to keep in mind that trees are
living things, and their lives have different, distinct stages. Furthermore, like
animals, trees have physiological responses to environmental stress and injury
that can change how they grow and interact with the environment.
It
seems that the tree outside of Frary fell victim to what is known simply as
“tree failure” which, according to the University of California’s Tree Failure
Report Program (yes, that exists), is simply a structural failure or physical
breakage of a tree. A fairly common occurrence, tree failure is worrying not just because the tree might fall directly on people, as happened on the night of Monday, April 14, but also because it might fall over a roadway.
The U.S. Forest Service, an agency of the Department of Agriculture, has published guidelines for
identifying “hazard trees” that are likely to fail and cause damage to
property or people. Their criteria for possible defects is exhaustive, and
includes signs such as fungal infection, cracking, cankers (which are the big
bumps that trees grow after damage), and even “poor tree architecture,” which is
when a tree leans heavily, thereby changing its center of gravity and making it
more susceptible to the consequences of other defects.
If
my memory serves me correctly, the sycamore outside of Frary was forked at its
base, and both sides of the fork leaned fairly dramatically—it certainly was
not a tree that rose vertically into the air. Looking at the remnants of the tree,
it appears that the root system pulled straight out of the ground. Given that the Forest Service also points to “root damage” as a hazardous tree defect,
perhaps the root system was not entirely intact, and the tree’s lean, which adds
inherent instability, caused the entire thing to spontaneously fail.
A tree’s
roots are essential for several reasons. Aside from absorbing all of the
nutrients to power the tree’s cellular processes, the root system literally holds
trees in the ground. But
what could compromise the integrity of the root system in the first place? According
to Tivon Feeley, a forester at the University of Iowa, chronic tree stress can
cause a host of issues, including increased susceptibility to disease and
decreased root and branch growth. The stress itself can also be caused by a
range of factors, such as acute physical damage, human influence, and drought.
That’s
right—drought. Unsurprisingly, drought has a huge impact on tree growth and
mortality. Michael Ryan, publishing in Tree
Physiology, says that drought can cause “carbon starvation.” Trees need
carbon, the backbone atom of all living material, in order to grow and maintain
themselves. They can make usable carbon by taking carbon dioxide out of the air
and “fixing” it through photosynthesis, a process that requires both sunlight
and water to occur. While trees in Southern California are usually replete with
sunlight, water is a bit more difficult to come by, especially in the midst of
a historic drought. Remember, too, that the even more massive tree on Walker
Beach spontaneously failed earlier this year.
While
drought-related stress could have caused stunted or weakened roots, there’s no
way to know for sure until the independent arborists that Pomona has
hired to investigate the failure make their report. The arborists are somewhat
like the forensic teams on CSI, collecting information from the scene of the
death and taking pictures. When I talked to them, they refused to comment on
what might cause a tree to fail. Until they formally report their findings,
we’re just left with speculation. It could have been the drought, or an
undiagnosed infection, or something else. For now, the best advice I can give
for all residents of the “City of Trees and Ph.D.s” is to keep
your wits about you and remember that no tree is too big to fail.
Warren Szewczyk PO ’15 is a neuroscience major who also co-hosts the radio program Reality Check, which explores the intersection of science and the spirit. In his spare time, he is an avid writer of spoken-word poetry.