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Home , Arboretum, Cornus controversa, Dendrology

Gestalt Dendrology: Looking at the Whole

Peter Del Tredici he most remarkable thing about is they develop and change over time. howEvery year they add height and girth in a flush of new growth. They are forever expand- ing, from the bottom to the top and from the inside to the outside-a tree that is not expand- ing is a tree that is dying. The growth of trees is totally different from that of vertebrate animals, which tend to reach their full developmental potential relatively early in life, and then main- tam themselves in the mature stage for as long as possible. To put it another way, animals are closed and entire in their development while trees are open and expansive. The easiest way to visualize what is meant by open development versus closed is to look at the different approaches to dealing with bodily injury. In trees, if a limb is broken off, then so be it; the trunk will grow around the break and attempt to cover over the dead branch by pro- ducing callus tissue that grows inward from the outer edges of the wound. In many cases, the tree will also produce a new branch just below the location of the old one. Some people call this response wound-healing, but in reality the tree is simply walling off the damaged or dead tis- sue-compartmentalizing it-in an effort to protect the undamaged portions of the trunk. Wound healing revealed in the trunk of an Dead tissue embedded within the trunk is of umdentified comfer that is supportmg a porch m downtown Alaska. The naked little consequence so long as rot does not spread Skagway, shows the that into the living wood (Shigo 1986). Mammals, of clearly complex mterlockmg gram forms where the branch is attached to the trunk, cannot tolerate the of dead tis- course, presence as well as where wound-healmg callus tissue has sue within their bodies. If they are to survive, been produced after the branch died. they must repair the injured body part; growing a new one is not an option. three have defined the field, tree archi- Understanding what trees are and how they tecture describes the processes that regulate grow is central to the discipline known as tree the growth and development of trees. Contrary architecture, which was developed durmg the to the meaning of the term architecture when 1970s by a Frenchman, Francis Halle, a Dutch- applied to buildings, tree architecture is about man, Roulof Oldeman, and an Englishman (and dynamic change in tree form over time; it is not Harvard professor), Barry Tomlinson. As these about the static, geometrical shapes of mature

The distmctme, "layered" architecture of the pagoda dogwood, controversa, growmg at the Arnold . This type of model is displayed by many understory trees found m temperate, deciduous . 4

trees seen in field guides. To put it another way, tree archi- tecture describes how trees develop their shapes, not what shapes they display. The discipline is particularly exciting because it deals with trees holistically, as intact, well-integrated organisms that are greater than the sum of their parts. Compared to most modern with its fixation on DNA sequencing, tree architecture takes a refreshingly nonreductionist approach to development. The Meristem is the term used for the specialized tissue that allows trees to continue expanding throughout their entire life span. This mer- istematic tissue produces the , the stems, the flow- ers, the bark, and the - the differentiated tissues of the -while remaining undifferentiated itself. What makes meristematic tissue unique is that it exists in a perpetually embryonic state that allows the tree to be reborn every spring through- out its entire life. Four different types of mer- istems are produced by trees. The most obvious one is the Tree is with meristematic centers meristem, or growing growth contmuously embryonic, locahzed m the shoot and and m the vascular and cork point, which is located at the tips cambmms. Clockmse from top left: heart of on every every Expandmg of Fraser’s magnoha, frasem, showmg tree. On a mature specimen promment, fohaceous stipules that protect the expandmg leaves. All , for example, there can be tissue, as well as floral, is produced by the shoot menstem. thousands, if not tens of The pmmary and lateral roots of the red mangrove, Rhizophora thousands of buds, each with mangle. The brackish water the tree grows in makes it easy to observe the the root a tiny dome of branchmg of system embryonic The cambium the tissue that is the meristem. layer of japanese , Acer palmatum, made msible by the growth of wound-mduced callus a botched Shoot followmg produce attempt at graftmg. and leaves, , primary The dramatic, exfohatmg bark of the paperbark maple, Acer gmseum. twigs. Located at the tip of The more extensive the cambium growth, the more extensive the exfohation. 5

every root-and again, there are thousands of root tips on a mature tree-is a root meristem that, over time, produces the tree’s massme, underground root system. The third type of meristem is the vascular cambium, a of tissue that sheaths the trunk, the branches, and the roots of the tree, and is responsible for the secondary increase in girth m all these parts. The vascular cam- bium is basically a gigantic cylin- drical meristem that outlines the periphery of the entire tree and produces the wood that forms the bulk of the tree. When as that broken branch mjury-such A fence post trom lodgepole pme, Pmus contorta, m Yellowstone mentioned the earlier-exposes National Park, clearly showmg the vamation m annual growth rmgs. cambium layer to the elements, it is the vascular cambium that produces the cal- prints of vascular cambium activity, with each lus tissue that overgrows the wood. The last ring accurately recording the amount of growth a type of meristem found m trees is the cork cam- tree makes in any given year. Because tree bium, which produces the bark that protects growth is mostly a function of rainfall, the width and insulates the tree. In general, the growth of of an annual growth ring provides an indirect the cork cambium keeps pace with the growth measure of the moisture available to the tree of the vascular cambmm, sloughing off the old that year. Scientists have used the information layers of bark as it produces new ones. embedded m the width of growth rings to recre- Whereas the specialized cells that compose ate past rainfall patterns that go back, quite liter- meristems retain their full developmental ally, thousands of years (Cohen 1998). Similarly, capacity throughout the life of the tree, cells in for trees that shed portions of their bark in dis- mammals can express their full developmental crete plates, such as london planes, stewartias, potential only in very young . As the and ponderosa , the size of the plates that mammalian ages, a liver cell can only are sloughed off each year serve as an mdirect produce a liver cell and a brain cell a brain cell. measure of the trunk’s expansion: the greater the This closed system of development, which lim- cambium expansion the larger the plates. Bark its the potential of any given cell early in its life patterns are distinctive for each . span, stands in contrast to the open system of Growth and Goethe , in which cells located at the periphery of Rings the plant body retain their full developmental Growth rings and bark plates are just some of potential throughout the life of the tree. Theo- the more obvious indicators of the principle that retically speaking, a tree’s life span is limited everything that happens to a tree over the only by environmental disaster or predation by course of its life is embedded in its form. To put other orgamsms (Kaplan and Hagemann 1991).(. it another way, both the external and internal A direct consequence of the meristematic structure of trees are manifestations of basic structure of trees is that everything that has ever physiological processes. It was the German happened to them over the course of their long poet, philosopher, Johann Wolfgang lives is embedded in the very fiber of their being, von Goethe who developed this concept and which is to say, the structure of their wood. The pioneered its use in science. The word he coined growth rings in nontropical trees are the foot- for this type of analysis-morphology-is still Tree growth is structurally optmnzed to promote stability and to reduce mechanical stress on all parts Left, bnstlecone pme, Pmus amstata, interacting mth a rock on Mt. Evans m the Colorado Rockies Above, a close-up of the at left, showmg the adaptme growth that the tree produced in order to deal with the immovable rock.

in use today. Morphology, literally, is the study wind, and that these responses are embedded in of the development of form. Indeed, it was the tree’s external form. For trees, the problem Goethe who, in 1790, first published the revolu- is balancing the need to expand its surface area tionary idea that the various components of as it searches for light and water with the need flowers are actually modified leaves-an idea to remam stable m relation to the force of grav- that modern genetic analysis has come around ity. In the real world, where destabilizing forces to supporting some two hundred years later abound, the necessity for expansion is often in (Arber 1950; Kaplan 2001).~. conflict with the necessity to remain upright. Goethe’s ideas about morphology have Trees have resolved this dilemma by employing mostly been forgotten by modern science, but a process known as adaptive growth, which they can be powerful analytical tools for anyone allows trees to add extra tissue (i.e., wood) to who takes the time to learn how about them. reinforce those parts of the trunk that are over- Indeed, within the field of mechamcal engineer- loaded, while ignoring those parts that are ing Goethe’s concept of "reading " is underloaded. To put it another way, adaptive making a comeback thanks to the development growth allows trees to structurally optimize of sophisticated computer programs that can their trunks, branches, and roots in order to accurately model the dynamic growth processes reduce the chances of mechanical failure under of living organisms. conditions of extreme loading such as wind, What these models demonstrate is that the snow, and ice. growth of a tree is responsive to external By carefully studying the growth of trees, stimuli, especially light, water, gravity, and Claus Mattheck, a professor of at 7

the University of Karlsruhe, and his colleagues have developed a sophisticated computer pro- gram-known as Computer-Aided Optimiza- tion, or CAO-that accurately describes the growth of real trees in real situations. By adapt- ing this program to industrial design problems, engineers are now able to analyze the stresses experienced by various machine parts that tend to break frequently and to "grow" new parts that add extra material only to locations where it can have the greatest impact in terms of reducing mechamcal stress. In other words, they reinforce only those sections of the part that typically fail as opposed to making the entire part heavier. It is a remarkable and important engineering breakthrough that specifically mimics the adaptive growth processes that trees use to minimize the stresses they experience in their natural habitats.

From Meristems to Modules Another consequence of the meristematic nature of trees is their modular growth, a concept that refers to the construction of a tree through the repetition of uniform structural units. For every tree, these basic modular units remain consistent A stmkmg example of reiteration m tuhp poplar, Liriodendron tuhpifera, growmg at the Arnold through its life and are composed of a segment Arboretum. It was probably mduced a dramatic of stem that by produces mcrease m hght following the death of a nearby and- leaves, branches, specimen of the same species. when sexually mature- flowers. When trees are both tropical and temperate-based on the young, the modular units branching pattern of their twigs and on the that they produce are rela- position of their flowers. While somewhat theo- tively large in size and retical in concept, these architectural models few in number. As trees are useful in categorizing the known array of age, however, the modular growth forms displayed by trees. Relatively few units become smaller and of the twenty-three models are required in order more numerous, resulting to describe the vast majority of temperate trees. in an increasingly finer One corollary of the modular nature of tree and more ramified net- growth is that most trees have the capacity to work of branches, as seen repeat their basic architectural model during on the cover of this issue. the course of their lives. Such repetition of the Every tree species has basic model can happen either because of trau- its own characteristic matic injury to the tree’s framework, or because module that helps to the tree is experiencing conditions that are par- define its architecture. ticularly favorable to its growth. Regardless of The basic architectural Halle, Oldeman, and the cause, such reiteration, as it is called, is an "module" in frangzpam, Tomlinson have identi- of tree in in a world Plumena rubra, conszstmg important part growth fied models with ofa terminal znflorescence twenty-three fraught diseases, insects, snow, ice, and four subtending that describe the architec- drought, and hurricanes that threaten the health branches ture of all known trees- and stability of trees. 8

At the risk of sounding anthropomorphic, one might say that the shape of an individual tree is analogous to the personality of a human, being the product of the complex interaction between genetic endowment (nature) and environmental pressures (nurture). Quite literally, everything that ever happens to a tree in the course of its long life is embedded in its form, even the little things that might have happened to the tree when it was just a sapling. The body language of trees speaks not only to the influence of the past in the present, but also to the promise of the future. Bibliography Arber A. 1950. The Natural Philosophy of Plant Form Cambndge : Cambndge University Press Bell, A. D. 1991. Plant Form Oxford: Oxford University Press. Cohen, M. P. 1998 A of Bmstlecones Tales of Change m the Great Basin Reno: University of Nevada Press. Del Tredici, P. 2000. Aging and Rejuvenation m Trees. Arnoldia 59(4): 10-16. - - . 2001. Sprouting in temperate trees: A morphological and ecological review. A tmsted white , Picea glauca vnr albertiana, Botanical Review67: 121-140. m Canada’s Banff National Park, showmg the effects Femmger, A. 1968. Trees New York: Viking Press, New of several episodes of snow-loadmg and recovery York (reprinted 1991 by Rizzoli Books, New York) At its most basic the of reitera- level, process Goethe, J. W. v. 1952. Goethe’s Botanical Writings, trans. tion allows trees to expand indefinitely when B. Mueller. Honolulu: University of Hawaii conditions are good, or to start over after they Press (reprinted 1989 by Ox Bow Press, have been severely damaged. Following this Woodbndge, Connecticut).I. Hallc, F., R., A. A. Oldeman, and P B. Tomlinson. 1978. logic to its conclusion, one can say that trees, as Tropical Trees and Forests Berlin. Spnnger- have the to into a they age, potential develop Verlag. colony of reiterated subunits, linked together Kaplan, D. R. 2001. Fundamental concepts of by a common trunk and root system. Under leaf morphology and morphogenesis~ a extreme conditions, some trees that sucker contribution to the mterpretation of molecular from the base of the trunk genetic mutants. International Journal of literally fragment Plant Sciences 162: 465-474. into segments that their own roots sys- develop Kaplan, D. R., and W. Hagemann. 1991. The relationship tems and establish from complete autonomy of cell and organism m vascular plants. their original parent (Del Tredici 2001).~. BioScience 41~ 393-703. In order to reconstruct the history of any Mattheck, C. 1998. Design in Nature. Learmng from given tree, it is particularly important to be able Trees Berlin: Sprmger-Verlag. A. 1986 A New Tree NH: to recogmze the occurrence of reiteration, even Shigo, Biology Durham, Shigo and Trees, Assoc. if one don’t know what type of event mduced P. 2000. Trees. Their Natural History the reiterative On the Thomas, response. negative side, it Cambndge : Cambndge University Press. might have been caused by a weather event, a Zimmermann, M. H., and C. L. Brown. 1971. Trees biological attack, or some human intervention. Structure and Function Berlin: Spnnger- On the positive side, reiteration could be the Verlag. result of being grown in full sun as a specimen tree or to fol- being exposed additional sunlight Peter Del Tredici is director of living collections at the lowing the death of a nearby dominant tree. .