Why Does the Slash Pine Prune Itself?

Pinus elliottii

Pinaceae

John and I botanized and photographed today at Halpatioke Park in Stuart, Florida.  Great place for pines, and for thinking about why they shed their lower limbs.   Such self-pruning is not limited to  Slash Pines, but they are mighty good at it.  The trunk becomes bare below the crown as the crown rises.

The standard explanation is that discarding those flammable lower branches is protection from ground fires.    Ain’t sayn’ it ain’t so, but then again,  the party line strikes me as more intuitively easy to surmise than based on data.

I’ve heard other related notions, such as the lower limbs being too costly to maintain relative to their contribution to overall photosynthesis.   Also I’ve heard speculation that symbiotic fungi may facilitate severance.  Again, maybe, but those untested ideas don’t rock my world.

One more thought:  have you ever noticed how loaded with vines Slash Pines become?  Smilax, Grapes, Poison Ivy, Virginia Creeper, and more.  Those vines must sometimes weight a ton, especially when wet, and snag thunderstorms and hurricanes.  And they shade branches.  I wonder if branch shedding might help with de-vining.

When leaves, and in some species twigs,  fall from trees they break off cleanly at a preset fracture point called an abscission zone.   Not so in Slash Pines.  The doomed branches seem to die slowly and decay on the parent tree, until they are sufficiently rotten to fall, blow, fragment, or be knocked off.  The breakage point can be anywhere from the trunk to 5 or 6 feet out.

Instead of worrying why the tree discards branches from a standpoint of what good it may do the tree, let’s shift our gaze to how it happens, and how a Slash Pine is vastly more prone to it than its broad-leaved neighbors.  Time for comparison.

Before we dare to compare, a useful fact:  as a woody stem ages its central region loses functionality, and the outer younger wood and associated tissues are where the vital action is. Remember that, inner regions kaput, outer layers lively.

Now  look closely in the photos below at the broadleaf Florida Privet disinclined to self-pruning.    The two photos show the main stem (left) with a branch diverging to the right, split open down the middle.     Notice that the  white fibrous wood of the branch is continuous with the young outer material of the parent main  stem.   The branch and the active wood of the main stem are the same wood.  Wood from the main stem arches out into the branch and sustains its life.

Florida-Privet keeps its branches.  The wood extending into the cut-off branch on the right is a continuation of the wood of its parent.  When one stops and the other starts is not clear.

Florida-Privet closeup showing the parent stem outer wood bending out and becoming one with the branch.

Now, by contrast, look below at the pine main branch and its side-branch, likewise split open.  The side-branch is mostly separate from the outer layers of the parent stem.   The side-branch resembles a spike driven into the core of the parent.  Its main connection to the parent is the parent’s aging inner tissues, declining and  choked by the expanding girth of the parent.    The hollow center of the parent stem is contiguous with the decaying center of the side branch.     Anchored in decline, choked,  and mostly independent from the lively outer layers of its parent,  the side-branch fails.

The pine side-branch, circled, plunges to the center of the parent-stem whose outer layers are not much integrated into the branch.   The parent is becoming hollow at the core (the dark regions) showing insect damage…the  death tunnel extends directly into the side branch. In the upper right corner the branch is separating from the outer layers of the parent-stem. In the lower right, the outer parental layers narrow down and mostly stop short of becoming a major component of the branch, looking like their expansion may even help gag the branch base.

If the increasing diameter of the parent stem helps doom the side branch,  in cases of all else being equal, such as two adjacent trees or two forks of one tree, you’d expect the branches to fail at about the same parent-stem diameters.   Notice that in the two photos below.

If the parent-stem dies and thus quits thickening, a side-branch below the damage  may then be spared and live on.  Examine the photo below.

Looks like the death of the parent stem gave the side-branch a stay of execution.

Let’s come back now to the notion that the self-pruning is an adaptation to rise safely above ground fires.  Having branches rot because of an anatomical quirk seems a roundabout way to achieve fire avoidance.   But we could turn the beat around…maybe pines thrive in places with ground fires because they lose their lower branches.   Perhaps the chicken came before the egg.