Tuliptrees Colby Rucker August 14, 2002 1:54 PM Bob, 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 species. 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 trees. 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. Colby   Home