March 05, 2012
Trees Behaving Badly
To James White, botanist and teacher.
Though you might forgivably mistake a man for a tree at the level of gross morphology, nevertheless, a tree undeniably dwells in place whereas a person’s home is born in motion. Agnes Arber, the Cambridge plant anatomist and philosopher, remarked in her 1950 classic The Natural Philosophy of Plant Form that “among plants, form may be held to include something corresponding to behaviour in the zoological field.” If by behavior we refer to the sum of all the activities of an organism, then the manner in which a plant grows – marshalling its leaves to best secure light, disposing its roots to obtain nutrients and to harness it to the earth – is comparable to the more rambunctious activities that animals deploy for analogous purpose. The behavior of plants – the punctuated rhythms of their growth – is founded on the quite simple laws of cell division and extension. This was Arber’s lesson. And this, at eighteen, was the first conceptual framework that preoccupied me. If simplicity rules the world of plants, why not also true for animals, for people, for me?
Thirty years ago I took several wintery trips out to North Bull Island, a five-mile stretch of sand in Dublin Bay which formed in response to 18th Century engineering projects at the mouth of the River Liffey. Now a site of considerable conservation interest, I went along with fellow biology student Liam Dolan to observe the curious behavior of Armeria maritima roots. The plant, commonly known as sea pink, grew profusely on the dunes. The two Liams were at that time under the thrall of Jim White from the Botany Department at University College, Dublin and were both taking his course on the architecture of trees. To those of us who studied with White in the 1980s, he seemed like a visitor from another planet, one who pointed out the strangeness of his new home to the gaping residents, most of whom had never noticed the oddness of the world surrounding them. Jim’s lectures were marvels of erudition, scientific concision, and anecdote. Many years later when I worked in a Costa Rica, a well-known tropical forest researcher told me that he had only written one “fan letter” in his life and this was to James White. Anyway, we returned to White with our sketches and observations on the architectural patterns formed by sea pink roots. He was little incredulous at first – surely we were out looking at birds? In those days it was not uncommon for hale teens to spend their days traipsing out with binoculars to Bull Island to observe birds. It was unimaginable, apparently, that the youth would be looking at plant roots. Perhaps it’s best for a teacher not to sense the full measure of his impact, as it may constrain the random suggestions with which he peppers his lectures.
Jim White, recently retired, was primarily a plant population ecologist. He did graduate work in Bangor University, North Wales, with British ecologist and Darwinian demographer John Harper with whom he wrote several influential papers. For instance, a paper on linking changes in plant size and plant abundance in populations synthesized available data to test the so-called “Yoda -3/2 power law”. Kyoji Yoda (1931–1996) – the real Yoda, for botanists at least– was a leading Japanese ecologist who developed a mathematical description of the pattern by which plant populations “self-thin”, so that the number of plants surviving in an aging population reflects not the initial density of sowed seeds but the biomass of the emerging stand of trees. Stated simply, the patterns whereby many seedlings get whittled down and eventually, indeed inevitably, become a forest with fewer big trees are mathematically easily described – as the plants get bigger their number gets fewer. What’s interesting to me in rereading White and Harper’s paper on this topic is that they dip into the management and pure forestry literature, not confining themselves to ecological experimental literature. I recall Jim complaining in lecture to us once that the library at Harvard, where he went after Bangor, lacked in the area of forest management tables. When recently I asked Jim about this recollection, he told me that I was misremembering him on this point. Nevertheless, in the sunless days of pre-globalized Ireland knowing an Irishman that hobnobbed with an international elite warmed the cockles of our uncouth hearts.
It was at Harvard, it seems, that White brushed up against the revolution in plant morphology on the subject of which he lectured at UCD in the 1980s. This turn in botanical thought was the study of plant architecture, a discipline that emerged in the 1970s, but with roots in Goethe’s conception of plant morphology. It was, as I mentioned above, the study of plant architecture as interpreted by Jim White that had Liam and Liam (“Liam squared”) excavating plant roots in Dublin Bay dunes in the winter of 1982. Moreover, it was plant architecture that transformed me into a scientist. If there were relatively few principles to which the form of trees in all their prodigious shapeliness conform, then a young man might have a chance of making sense of the vanquishing world surrounding him. The chaos of existential choice, the whelming of a mysterious world where God was strategically enigmatic to his creation, the troubling enigmas of love, the derangements of Irish politics, the conundrums of origins – could all of this be made simple, constringed, reasonable? It seems silly now, but at that time the study of the shapes of trees paired with evolutionary theory, my other intellectual passion at the time, allowed me step out from the tepid bath of my youthful religiosity (admittedly a long and stuttering process) and confidently clasp onto the heterogeneousness of this world. More literally than perhaps any pantheist in history I traded a faith in God for a belief in trees – this is what the study of plant architecture made possible for me. Not consciously, of course, but trees in a very real sense both broke and repaired aspects of my soul. (That such events occur is, presumably, why Rick Santorum decries universities!)
The most significant account of tree architecture is given by Francis Hallé, Roelof Oldeman, and P. Barry Tomlinson’s in their 1978 classic Tropical trees and forests: an architectural analysis. Hallé and Oldeman are French and Dutch respectively, but Tomlinson was at Harvard where Jim White met him. I don’t have access to a copy of this book as I write this (although one may purchase a copy on Amazon for the lordly sum of $300) but I recall that Jim is acknowledged by the authors. There are several solid reviews of the concepts of plant architecture available in English, including accounts by the principal originators.
So, what’s so revolutionary about plant architectural studies, the elegance of which has been likened to Einstein’s equations? An examination of trees in tropical forests revealed their construction to emerge from relatively simple deterministic rules. In an abstract way these same deterministic rules may apply to other tree-like structures investigated by other disciplines – rivers, computer models etc. The normal pattern of growth of a plant is its “architectural model”. The observed pattern of growth from germination to flowering is what they then capture in the notion of an architectural model. It is a dynamic concept referring to the pattern of unfolding of a genetic plan. Combining these models with experimental observations on the physiology of plants helps explain some of the evident pattern found in common to all trees, for instance their “trunkiness”, that is, their possession of a main axis around which branches are arranged.
Tree architecture results from patterns of cell division in the meristems of plants. To refresh your memory, meristems are generalized plant cells capable of dividing and giving rise to the specialized cell types that make up the structure of plants. Meristems have a localized distribution in plant bodies. For instance, apical meristems are found at the tips of growing shoots and roots. If people grew like plants our bodies would develop and gain complexity as if extruded backwards from fingers, toes and the crowns of our heads. Our limbs would accrete by inward deposition from the sedulous industry of cells just beneath our skin. The behavior and life span of meristematic cells determines the direction, the rate, and the rhythmic growth of shoots, and ultimately gives rise to the distinct architectural models of trees. Given information about the programming of meristematic activity one might predict the form of the emerging tree.
A central conclusion of this analysis is that trees come in a restricted range of architectural models. Despite the enormous diversity of tropical trees, all species fit into one of only twenty-three plans (22 in some accounts). One might, for instance, sit down and sketch thousands upon thousands of different possible patterns for the growth of tree truck and limbs, but the trees of our imagination are largely impossible. There are a limited number of ways of being a tree. Plant architecture makes intricacy predictable.
Jim White’s botanical interests were chiefly concerned with population questions. He did not publish extensively on plant architecture. However, he included an account of the matter in his 1979 review – The Plant as a Metapopulation – linking the topic of plant growth and form with questions concerning the size of plant populations (there are other uses of the term metapopulation in community ecology that do not concern us here). Now this was quite an interesting paper because it posed the perplexing question: when one counts plants what precisely should one enumerate? In responding to the question White showed that morphology was central to understanding plant demographic patterns – the structure of plants and the way they are enumerated are intimately linked.
In some ways, the answer to the question of what gets counted as the unit in plant population studies seemed obvious enough – individual plants. If you and I are individuals, what shows up as our counterparts in the vegetable world? Individual stems, buds, all productions from a single seed? Unlike the case for animals, plants of the same age are often of disparate size. Indeed, many plants are clonal, so that an entire plant derived fro m a single seed (termed a genet) can consist of many stems each of which may be usefully counted, depending upon why you want the number. Quaking aspen, for example, can produce colonies of thousands of trees, all genetically identical. If you are counting genets those thousands of trees add up to a grand total of one! On the other hand, plants are clearly a collection of subunits; shoots, leaves and buds, and these repeating subunits can all be individually counted. So, questions concerning the population of plants are not separate from questions about their anatomy and morphology – their structure, shape, and form. It was here that White had intersecting interests with Hallé, Oldeman, and Tomlinson. Depending upon the nature of your question you might count meristems (White cites a study by Oldeman where the latter had counted the buds and other meristems in the canopy of a forest), shoots or other aspects of the modular composition of a plant that are expressed in the architectural model. At the very least, an architectural appreciation of trees called attention to their structural subunits. One can’t count the entities that comprise vegetation without a nuanced understanding of the morphology of plants.
The revolution in plant morphology represented by the identification of a limited number of architectural models meant that the complexity of world became a little simpler – reality is constrained, not everything is possible. White’s proposed “rapprochement” between morphology and population had, in some ways, the reverse effect. What counts as the unit for plant population studies became less apparent – reality is less constrained, new perspectives are possible.
There was a time in my life when my primary preoccupation was precisely to make the complex simple – the search, in other words, to have mental models to live by. Learning from Jim White all those years ago I got my first taste of the power of conceptual thought. Profusion becomes reducible to principle. Additionally, conceptual frameworks can make unity diverse. For all of that, I learned surprising things about plant architecture and metapopulation theory in preparing this post: Although I had faithful memories of some aspects of architecture and metapopulation, I had also misremembered important aspects of both. I felt confident that Hallé and Oldeman had wrung their models out of theoretical assertions concerning the behavior of meristems, though, in fact, the models were based upon patient observation of trees growing in potting soil in concrete boxes. Hallé and Oldeman had simply spent a lot of time being with trees – as much world-enchantment as conceptual framework or theory in the work. Additionally, in their work, and White’s writing, there is more openness to plasticity, to chance and happenstance than I recalled. Morphology and demography may coalesce around models, but both recognize that trees do not conform to simple patterns. I had, almost exclusively, recalled the deterministic aspects of morphological laws. But, the architecture of trees is individuated and unpredictable. In response to damage or in certain environmental circumstances trees reiterate, piling one expression of its inherent model on top of the next, so that the tree becomes a colony of repeating patterns. Moreover, trees may be found to grow along gradients between the models. Sometimes trees contravene the rules, sometimes trees behave badly.
I learned more models than I had immediately realized all those years ago. Every time I know I am lecturing well, I feel as if I channel Jim White. Apparently, it is not only trees that reiterate. But perhaps the allure of concepts, both those that unify and those that fructify, is a young person’s game. If the challenge of early life is to tamp down the complex to the simple, the challenge of middle life – for me, at least – is to open up to beings, acknowledging that for every increment in our knowing there are more things that lie beyond our concepts. The next tree I come across may not be the tree I predicted; it may be the tree I dreamt of.
Arber, A. (1950). The Natural Philosophy of Plant Form. Cambridge.
Hallé, F., R. A. A. Oldeman, and Tomlinson, P. B.. (1978). Tropical trees and forests : an architectural analysis. New York, Springer-Verlag.
Tomlinson, P. B. (1987). "Architecture of Tropical Plants." Annual Review of Ecology and Systematics 18: 1-21.
White, J. (1979). "The Plant as a Metapopulation." Annual Review of Ecology and Systematics 10: 109-145.
White, J. and J. L. Harper (1970). "Correlated Changes in Plant Size and Number in Plant Populations." Journal of Ecology 58(2): 467-485.
Thanks to James White for responding to a long set of questions on metapopulations and plant architecture (residual errors in understanding remain mine). Thanks also to Vassia Pavlogianis for several of the photographs. Finally, thanks to Professor Will McNeill, DePaul University Department of Philosophy, for a surprising conversation on this topic as we beveraged-forth on March 1st 2012, and for reminding me what Nietzsche might say about all of this.
Photo Credit: Banner Picture (Heneghan), James White from Trinity College, Dublin.All other photos are from Vassia Pavlogianis.
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