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 faciitate severance. Again, maybe, but those untested ideas don’t rock my world.
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.
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.
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.
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.