Tuliptrees Colby Rucker
August 14, 2002 1:54 PM


Another day in the mid-nineties with no rain in sight. Did Kermit the frog
say, "It's not easy being green?" The same goes for trees. Although
Maryland springtimes promote a exuberance of new growth, our summers are
often cruel, seemingly bent on reclaiming what has been produced.

Of course, there are a few deep coves where the sun hardly penetrates, the
drying winds are broken, and the multiple canopies conserve the moist
fragrant air, and fresh green moss hangs in strips from old logs. The
trunks of tuliptrees rise straight from these havens, their tops vertical,
supported by strong fibers and non-compressible water.

Elsewhere, the tuliptrees will begin dropping yellowed leaves. While oaks
retain all their foliage, inviting scorch and even the fatal exaustion of
all available moisture from the entire structure, tuliptrees are survivors.
By shedding excess leaves, less precious water is expended, and the
remaining leaves continue to function.

More importantly, the tuliptree prevents its woody structure from the
killing effects of drought. Still, drought has an effect on the structural
architecture of trees. Although we say, "as the tree is bent, so grow the
tree," the structural evolution of each tree is more complex, and drought is
but one of many influences that creates the structural identity of each

A straight line being the shortest distance between two points, some trunks
become straighter with time. The black locust has no terminal bud, and
young stems are usually somewhat crooked. As the trunk thickens, growth is
most rapid along concave curves, that being the shortest path between crown
and roots. A locust post split to butterfly the curve is flat, while one
split at right angles is irregular. Concave curves of the limbs of
sassafras, basswood and some junipers can fill rapidly, producing flattened
cross-sections, not unlike the flaring buttress root formations on some

Of course, as trunks thicken, forks are raised, and limbs leave chevrons on
the bark, like the wake of a boat, each spreading at precise angles
according to the angle of the limb and growth rate of the trunk. Bark on
concave surfaces must be shortened, and crumples, slowly forming corrugated
surfaces, the attached cambium so influencing the contour of the wood
beneath. Roots increase in diameter, easily elevating the turf, like moles,
and adding most of their diameter on the upper surfaces.

Competition for sunlight constantly changes the structure of trees, Of
course, lower and inner branches die as essential sunlight is captured by
new growth of the expanding crown, but other changes take place. Some trees
produce numerous limbs growing upward at a considerable angle. Due to
competition, most do not greatly increase in diameter, but they grow longer,
and gravity bends them downward. This bending is beneficial to the
individual limb, causing it to move outwards, access more sunlight, and
thereby thicken and stabilize. As gravity brings the most slender limbs
below horizontal, they lose sunlight and die.

Resistance to bending is the result of the tensile strength of fibers, and
the compressive strength of cell walls. These forces are often apparent in
white mulberry branches, where snow loads cause the wood to separate between
the two forces, the lower half buckling downward, leaving a space large
enough to put your hand into. Sections of wood exposed by a split fork are
flat in cross-section, and easily broken, like a ruler, because the two
forces are close together. Hollow trees are strong, but fail when twisted,
or when the sides are forced outwards, allowing tensile and compressive
forces to move closer together.

Heavy woods are hard to compress, but lighter woods, such as tuliptree, are
strengthened by non-compressible water, and weakened when that moisture is
reduced by drought. The multiple-arched structure of tuliptrees is
influenced by several factors. Fast growing vertical branches may be bent
by the load of precipitation on the broad leaves. This also occurs in
sycamore and sweet gum, and is most likely after a drought. This bending
moves a lateral branch upward, and it becomes the new leader. In time, a
series of leaders so produced bend, the lower portions die from lack of
sunlight, and a multiple-arched structure is created.

Although the intolerance of tuliptree leaves causes new growth to move
outward, gravity is a constant factor, causing large structural members to
bend over the years. The development of a tuliptree crown creates a
structure where members move from one another, more than simply radiating
from a central point. In winter, the outline usually shows curvatures in
the upper crown arranged in a given direction, allowing one to describe a
tree as being "left-handed" or "right-handed."

On dry-mesic soils with mixed hardwoods, tuliptrees grow more slowly. If
there is no abundance of available moisture, the supply may be more uniform
through the seasons, producing trees of excellent quality. The logs are
light in weight, with a distinct yellow heartwood. Poplar growing on wet
soils, especially old-field sites, produces much heavier logs with less
heartwood, which is somewhat greenish. Often called "blue poplar," not
"yellow poplar," the lumber is more prone to warp and decays easily.

Tuliptrees are noted for their straight trunks. The tallest trees are those
that maintain a vertical stem completely to the top. Such trees are rare,
and usually occur in the deepest, coolest coves, protected from the extremes
of climate, especially during drought. A handsome grove of eight or ten
trees in a deep ravine near here has several trees nearly 150 feet in
height, and the trunks are straight and devoid of branches to considerable
height, 98 feet in one case. These vertical lines terminate suddenly, with
several divergent and crooked leaders forming the upper fifty feet. It
appears these trees were released by logging nearly 100 years ago, which
stimulated faster top growth on slender stems prone to bending, while
logging allowed the once-sheltered cove to dry out, resulting in extremely
awkward subsequent top structures.

Very old tuliptrees growing on dry-mesic sites with little competition often
develop large crowns with numerous growing tips, none having a distinct
advantage. Such trees are usually about 114 feet tall, and display multiple
arching down to the twig level. At this stage, it seems that vertical
growth does little more than offset the influence of gravity in bending the
larger non-vertical structures.

So, we have another drought, and plants will suffer, but the tuliptrees will
endure, though constantly changing their structure, adding greater interest
to our natural world.