Cottonwood and the River of Time: On Trees, Evolution, and Society

22. The Essence of Trees

22 / the eSSenCe of tReeS m ost of this book has been devoted to a discussion of trees in the plural, in the aggregate, and in natural populations, forests , and plantations—as a consequence of its genetic and evolutionary bias and the emphasis on underlying processes. But let us not ignore the tree as an individual, with the ingredients that give it such an iconic presence among organisms in our daily life. what is it that makes a tree a tree and captures our fascination? what image could more powerfully convey a sense of this than the above picture of that colossal oak at ludwigsruhe on a hill above the Jagst valley in southern Germany?1 what more impressive statement of bold individuality against the collective anonymity of the neighboring hardwood stand? no words can do justice to the singular silhouette of this giant. no knowledge of genetics or physiology is needed to feel awed by this 227 228 The essence Of Trees living monument that has faced the elements for hundreds of years and survived. even the untrained eye will perceive a repetitive theme that reverberates through the towering hierarchy of stem, branches, branchlets, and twigs. every unit on this tree is unmistakably part of the whole. The little twists and angles in the twigs add up at an ever increasing scale to shape the ultimate form, the unique profile, the Gestalt of the tree. tree growth is modular and iterative. its basic unit, as in all plants, is a phytomer, a unit composed of a shoot segment with a leaf and an axillary bud (fig. 22.1). The bud contains a meristem with another phytomer in primordial form. once this primordium elongates into another full-fledged phytomer, the process repeats itself, adding building block after building block to develop a simple plant body. Yet plants are more complex than that. even small annuals like Arabidopsis have a stem with many small branchlets. and since each branchlet is subtended by a leaf, it evidently developed from an axillary meristem, whereas the stem originated from a terminal or apical meristem. and in order for the plant to gain an erect stature, the apical meristem had to exert a regulatory , dominating function over the axillary meristems, a process called apical dominance. a gradient in apical dominance would then allow a hierarchal architecture to emerge, with branchlet length diminishing toward the apex. finally, allowing successive phytomers to rotate their orientation along the stem’s axis would result in a three-dimensional plant body. add some roots and turn some shoots into flowers and you have what it takes to build a dandelion, a tulip, or a corn plant. But not quite a tree. tree growth adds one more dimension to those of an annual plant, namely that of secondary growth. not only does the tree preserve the first year’s structure, it bulks it up through radial growth. This occurs through the activity of another meristem, the vascular cambium. in contrast to the point-like apical and axillary meristems, it is a laminar or sheet-like meristem that connects the shoot’s vascular bundles and turns them into a vascular cylinder, adding xylem on the inside and phloem on the outside. Through stepped-up lignification of this sheath, this turns the plant into a woody body, capable of bearing the added weight and staying upright. Then, year after year, it keeps adding The essence Of Trees 229 one annual sheath after another, gaining height and volume, reaching toward the light and shading out smaller neighbors. This outer, living sheath is carried by an ever-increasing core of dead wood. in fact, by the time a tree reaches the size of that magnificent oak above, as much as 90 percent of it will be dead tissue. in the same way, one of those 4,000-year-old bristlecone pines may in essence be a dead, woody sculpture sustaining a tiny strip of peripheral living matter that connects a few needle bundles with a root below: leaves hanging on to mummified ancestral cells that even in their death provide essential support for life in the sun. in this, the tree is a perfect example of a lasting legacy in a very physical sense. Plant growth is a celebration of decentralization, and trees push it to its extreme. The dispersed meristems in this open growth system provide a mechanism to deal flexibly with...